thermochemical properties of methylol phenol monomers and

Thermochemical Properties of Methylol Phenol

Monomers and Phenol Formaldehyde Resoles

A Thesis Submitted for the Degree of Doctor of Philosophy

By

Livia Tonge, B.Sc., B.Eng. (Hons.)

Faculty of Engineering and Industrial Sciences Swinburne University of Technology

September 2007

ii

Abstract

The principal aim of the present research is to investigate the thermochemical

characteristics of individual methylol phenol monomers, which are the first

addition products in the making of phenol formaldehyde (PF) resoles, in the

temperature range up to 250°C. The second aim of the research is to study the cure

properties of PF resoles as a whole with a particular focus on the dependence of the

reaction kinetics on the degree of the cure up to 250°C. Differential scanning

calorimetry (DSC) and the model-free kinetic analysis approach were used to

monitor the thermochemical properties of both the monomers and PF resoles as a

function of concentration of sodium hydroxide, a common basic catalyst used in

the making of the resoles.

A. The cure properties of methylol phenol monomers

A key mechanism that has been suggested to operate during the cure of the

monomers in the presence of NaOH is the formation of the sodium ring complex

that diminishes the capacity of the monomers to participate in condensation

reactions, particularly those involving ortho-methylol groups. At a particular

NaOH level, the monomer molecules may have a range of reactivity, depending on

whether they are associated with Na+. Such variation in the reactivity and the

different condensation possibilities of the monomers are critical factors governing

the cure behaviour.

Another important mechanism that has been suggested to operate during the cure is

the limitation on molecular diffusion that has the effect of slowing down the

condensation reactions of the monomers. The effect of the diffusion limitation

mechanism is more pronounced with increases in the amount of the methylol

groups in the monomers and in the levels of NaOH. The advancement in the extent

of cross-linking is another factor that exacerbates the significance of this

mechanism as the cure proceeds.

iii

Differences in the effects of these mechanisms between different samples are

manifested in differences in a number of parameters including the shape of the

DSC curves, the dependence of apparent activation energy Ea on the degree of

conversion and the heat of reactions ΔHT. These differences, together with the

established chemistry of condensation reactions, are used to elucidate possible

pathways that condensation reactions may proceed. In particular, the partial

contributions of reactions to form para-para and ortho-para linkages, as well as

ortho-ortho linkages in rare occasions, at different stages of the cure have been

proposed for each monomer at different NaOH levels.

B. The cure properties of PF resoles

The outcomes of both studies of the monomers and the resoles are complementary

to each other and provide a consistent overall picture of relevant mechanisms

operating during the cure process. In particular, the sodium ring complex

mechanism that has the retardation effect on the cure kinetics of the resoles is

demonstrated independently by both gel time measurements and DSC data. It is

suggested that the operation of this mechanism is not confined to 2-mono-methylol

phenol, but also applies to other methylol phenols present in the resoles.

On the basis of the data on the dependence of Ea on the extent of conversion, it is

suggested that the cure of the resoles proceed through two stages. The first stage is

characterized by an ascending trend of Ea up to conversion of 0.6 – 0.7, followed

by the second stage which exhibits a descending trend of Ea to the end of the cure

process. It is proposed that the partial contribution of reactions to form the para-

para linkages are dominant at low conversions and that contribution of the ortho-

para linkage reactions become more significant as the cure proceeds. The

descending trend of Ea is attributed to the increasing importance of the diffusion

limitation mechanism in the second stage of the cure. The effect of this mechanism

is more extensive for the resoles having higher NaOH / P ratio. This is attributed to

higher degree of methylol substitution and higher amount of NaOH present in these

resoles, both of which are shown in the study of monomers to have the effect of

exacerbating the severity of the diffusion limitation mechanism.

iv

The findings in the present study have practical implications in the development of

PF resole adhesive systems capable of curing faster at lower temperatures. Clearly,

for PF resole formulations with a particular F / P molar ratio, there is an optimal

level of NaOH / P molar ratio where the cross-linking reactions are encouraged and

the diffusion mechanism is minimised. The present results indicate that for a

system with a F / P molar ratio of 2, which is commonly used in the industry, a

NaOH / P ratio of 3 is sufficient to produce resoles with fully cross-linked

networks. Higher NaOH / P ratios would slow down the cure reactions due to

increasing importance of both the sodium ring complex and the diffusion limitation

mechanisms.

It is suggested that future work should involve the use of complementary

techniques such as NMR and FTIR to investigate the chemical structure of the

products at different stages of the cure of different monomers and PF resoles. This

is necessary to confirm the possible pathways for condensation reactions proposed

in the present study. As well, the issue of the effects of F / P molar ratio on the cure

properties of PF resoles should be revisited using the model-free DSC method,

given the effectiveness of this method in revealing possible complex sequences of

the cure reactions.

These additional data would add to the knowledge obtained in the present study

and aid in the development of PF resole systems capable of bonding under a wide

range of gluing conditions and curing faster at lower temperatures.

v

Acknowledgements

I wish to acknowledge and thank the Forest and Wood Products Research &

Development Corporation for their financial sponsorship of this PhD project.

I would like to thank my supervisors – Mr Aaron Blicblau, from Swinburne

University of Technology, Dr Jonathan Hodgkin from CSIRO Molecular Science,

and Dr Yoshi Yazaki, from CSIRO Forestry and Forest Products. I am particularly

indebted to Aaron for the enormous help and scientific guidance he extended to me

during the course of this project, especially his patience and willingness to assist

when problems arose.

I am grateful to CSIRO staff – Ms Mary Reilly, Mr Peter Collins, Mrs Touba

Nikpour, Dr Russell Varley for their assistance throughout this project. In

particular, I would like to highlight Mary for her dedication and very special

support.

I thank Dr Jim Gonis from Perkin Elmer for his considerable help and advice

regarding the commissioning and operation of the DSC.

My deep gratitude also goes to Gerry Scheltinga for your friendship, practical

assistance, encouragement, and steadfast interest in my progress.

To my family, Anyu, Johnnybacsi, and to Duy, I extend my eternal gratitude for

your enduring love, patience and encouragement over the years. Without Duy’s

unwavering caring guidance and support, this thesis would not have eventuated.

This thesis is in loving memory of my dad, Eric.

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Declarations

The work described in this thesis has never previously been submitted for a degree

or diploma in any University and to the best of my knowledge and belief contains

no material previously published or written by any other person except where due

reference is made in the thesis itself.

Parts of the work described here have previously been reported in the following

publications:

“Effects of Initial Phenol-Formaldehyde (PF) Reaction Products on the Curing Properties of PF Resin”

L. Y. Tonge, J. H. Hodgkin, A. S. Blicblau and P. J. Collins

in Journal of Thermal Analysis and Calorimetry, 64 (2), 721-730 (2001).

“Thermal Behaviour of Phenol-Formaldehyde (PF) Compounds”

L. Y. Tonge, Y. Yazaki and A. S. Blicblau

in Journal of Thermal Analysis and Calorimetry, 56 (3), 1347-1352 (1999).

“Cure Kinetics of Phenol-Formaldehyde (PF) Resins”

L. Y. Tonge, Y. Yazaki, A. S. Blicblau and J. H. Hodgkin

in Proceedings of the 8th Asian Chemical Congress, November 1999.

vii

Table of Contents

Title page i

Abstract ii

Acknowledgement v

Declaration vi

Table of Contents vii

List of Tables xii

List of Figures xiv

Chapter 1 Introduction 1

1.1 Background 1

1.1.1 General 1

1.1.2 The production of PF resoles 2

1.2 The Issues 3

1.3 The Objectives 4

1.4 Structure of the Thesis 5

1.5 References 6

Chapter 2 Literature Review of Thermochemical

Behaviour of PF Resole and Its Monomers 10

2.1 PF Resoles – Background 10

2.1.1 History 10

2.1.2 Application of PF resoles in the wood industry 11

2.2 PF Resole Chemistry 12

2.2.1 Formaldehyde addition to phenol to form monomers 12

2.2.1.1 General 12

2.2.1.2 Reactivity of methylol phenols with formaldehyde 12

2.2.2 Condensation reactions to form resole 14

2.2.2.1 Condensation reactions 14

viii

2.2.2.2 Effects of alkalinity on the condensation reactions 20

2.2.3 Cure reactions of resole 22

2.2.3.1 General 22

2.2.3.2 Reactions during the cure of resole 23

2.3 Effects of Formulation Parameters on Properties of PF Resoles 25

2.4 The Use of DSC to Study the Cure Behaviour of PF Resoles 26

2.5 Concluding Remarks 29

2.6 References 31

Chapter 3 Methodology and Experimental Details 38

3.1 Methodology 38

3.1.1 System parameters 38

3.1.1.1 Methyl phenol monomers 38

3.1.1.2 Reaction conditions 38

3.1.1.3 Additional experimental parameters 40

3.1.2 Thermal analysis by DSC 40

3.1.2.1 General 40

3.1.2.2 Principle of DSC 41

3.1.2.3 Analysis of DSC experimental data 43

3.1.2.4 “Effective” activation energy Eα obtained from the

model-free method 48

3.2 Experimental Details 51

3.2.1 Materials 51

3.2.1.1 Synthesis of 2,4-DMP 51

3.2.1.2 Synthesis of 2,6-DMP 54

3.2.1.3 Synthesis of TMP 56

3.2.2 Characterisation of 2,4-DMP, 2,6-DMP and TMP 57

3.2.3 DSC runs 63

3.3 References 64

ix

Chapter 4 Cure Properties of Mono-Methylol Phenols 67

4.1 Introduction 67

4.2 Effects of Scan Rate on DSC Thermograms 67

4.2.1 Peak temperature Tp 67

4.2.2 Fractional conversion αp at Tp 69

4.2.3 Heat of reactions ΔHT 70

4.3 Effects of NaOH on DSC Thermograms 71

4.3.1 Peak temperature Tp 71

4.3.2 Fractional conversion αp at Tp 73

4.3.3 Enthalpy of reactions ΔHT 74

4.4 Effects of NaOH on the Evolution of Activation Energy Ea 76

4.4.1 2-MMP 78

4.4.2 4-MMP 82

4.5 Summary 85

4.6 References 86

Chapter 5 Cure Properties of Di-Methylol Phenols 88

5.1 Introduction 88

5.2 Self-Condensation Reactions of DMP 88

5.3 DSC Thermograms 90

5.3.1 2,4-DMP and 2,6-DMP at molar ratios equal or less than 0.15 92

5.3.2 2,4-DMP at molar ratios higher than 0.15 93

5.3.3 2,6-DMP at molar ratios higher than 0.15 95

5.4 Enthalpy of Reaction ΔHT 96


5.5.1 2,4-DMP 100

5.5.2 2,6-DMP 103

5.6 Summary 107

5.6 References 108

x

Chapter 6 Cure Properties of Tri-Methylol Phenols 110

6.1 Introduction 110

6.2 Self-Condensation Reactions of TMP 110

6.3 DSC Thermograms 112

6.4 Enthalpy of Reactions ΔHT 114


6.6 Summary 124

6.7 References 125

Chapter 7 Comparison of Effects of NaOH on the Cure Properties of Mono-, Di- and Tri-Methylol Phenols 127


7.2 MMP 128

7.2.1 2-MMP 128

7.2.2 4-MMP 129

7.3 DMP 130

7.3.1 2,4-DMP 130

7.3.2 2,6-DMP 133

7.4 TMP 135

7.5 Summary 137

7.6 References 138

Chapter 8 Cure Properties of PF Resoles 140


8.2 Experimental 141

8.2.1 Resole synthesis 141

8.2.2 GPC 141

8.2.3 Gel time 142

xi

8.2.4 DSC experiments 142

8.3 Results and Discussion 143

8.3.1 GPC 143

8.3.2 Gel time 144

8.3.3 DSC curves 145

8.3.4 Enthalpy of reactions ΔHT 148

8.3.5 Effects of NaOH / P molar ratio on the evolution of activation energy Ea 150

8.4 Summary 157

8.5 References 158

Chapter 9 Conclusions and Future Work 162

xii

List of Tables

Table Page

Table 2.1: Reaction products from the self-condensation reactions of monomers as observed by Yeddanapalli and Francis 17

Table 2.2: Reaction products from the self-condensation reactions of monomers as observed by Grenier-Loustalot et al 19

Table 3.1: Reaction models used to describe thermal decomposition in solids 47

Table 3.2: 1H-NMR chemical shifts 58

Table 3.3: 13C-NMR chemical shifts 59

Table 4.1: Effects of scan rate on: the peak temperature of cure (Tp); total enthalpy of reactions (ΔHT); and the extent of the cure reactions at the peak of the exotherm (αp) for NaOH : 2-MMP molar ratios of 0.0 and 0.45 70

Table 4.2: Effects of scan rate on: the peak temperature of cure (Tp); total enthalpy of reactions (ΔHT); and the extent of the cure reactions at the peak of the exotherm (αp) for NaOH : 4-MMP molar ratios of 0.0 and 0.45 70

Table 4.3: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2-MMP molar ratio 0.45) and the corresponding values for the dependent and independent variables for equation 3.7 77

Table 5.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2,4-DMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation 3.7 99

xiii

Table 6.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2,4,6-TMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation 3.7 117

Table 8.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH / P molar ratio of 0.30) and the corresponding values for the dependent and independent variables for equation 3.7 151

xiv

List of Figures

Figure Page

Figure 2.1: Reaction paths for the addition of formaldehyde to phenol 13

Figure 2.2: Formation of dimethylene ether and methylene bridges 15

Figure 2.3: Condensation reactions of TMP from pH 3 to pH 5 21

Figure 2.4: Condensation reactions of TMP from pH 5 to pH 10 21

Figure 2.5: Condensation reactions of TMP above pH 10 22

Figure 2.6: Three- dimensional cross-linked state 23

Figure 3.1: The five initial intermediate monomers 39

Figure 3.2: Power compensated DSC 42

Figure 3.3: DSC dynamic scan peak 43

Figure 3.4: A dynamic DSC thermogram in the scanning mode depicting an exothermic reaction 45

Figure 3.5: Reaction steps for the synthesis of 2,4-DMP 52

Figure 3.6: Schematic for the synthesis of compound II 52

Figure 3.7: Schematic for the synthesis of 2,4-DMP 53

Figure 3.8: Reaction steps for the synthesis of 2,6-DMP 55

Figure 3.9: 1H-NMR spectra of 2,4-DMP 60

Figure 3.10: 1H-NMR spectra of 2,6-DMP 60

Figure 3.11: 1H-NMR spectra of 2,4,6-TMP 61

Figure 3.12: 13C-NMR spectra of 2,4-DMP 61

Figure 3.13: 13C-NMR spectra of 2,6-DMP 62

Figure 3.14: 13C-NMR spectra of 2,4,6-TMP 62

xv

Figure 4.1: Dynamic traces for 2-MMP at varying scan rates in the absence of NaOH 68

Figure 4.2: Dynamic traces for 4-MMP at varying scan rates in the absence of NaOH 69

Figure 4.3: Dynamic traces of 2-MMP in the presence of varying NaOH : 2-MMP molar ratios at 10 °C min-1 scanning rate 72

Figure 4.4: Dynamic traces of 4-MMP in the presence of varying NaOH : 4-MMP molar ratios at 10 °C min-1 scanning rate 73

Figure 4.5: Fractional conversion αp as a function of NaOH : MMP molar ratio for 2-MMP and 4-MMP 74

Figure 4.6: ΔHT as a function of NaOH : MMP molar ratio for 2-MMP and 4-MMP 75

Figure 4.7: Graph of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α =

0.95 and the corresponding square of the correlation coefficient (r2) values for 2-MMP sample with NaOH : 2-MMP molar ratio of 0.45 78

Figure 4.8: Effects of NaOH on the evolution of apparent activation energy Ea for 2-MMP as a function of the degree of conversion 79

Figure 4.9: Condensation reactions of 2-MMP 80

Figure 4.10: The sodium ring complex 81

Figure 4.11: Effects of NaOH on the evolution of apparent activation energy Ea for 4-MMP as a function of the degree of conversion 82

Figure 4.12: Self-condensation of 4-MMP 83

Figure 4.13: Addition reaction of CH2O to 4-MMP 83

Figure 5.1: Condensation reactions of 2,4-DMP 88

Figure 5.2: Minor condensation reaction of 2,4-DMP 89

Figure 5.3: Condensation reaction of 2,6-DMP 89

xvi

Figure 5.4: Para and ortho quinoid structures of 2,6-DMP and 2,4-DMP 90

Figure 5.5: Dimethylene ether linkage formation 90

Figure 5.6: DSC thermograms for the self-condensation reactions of 2,4-DMP in the presence of varying NaOH : 2,4-DMP molar concentrations obtained at 10 °C min-1 scan rate 91

Figure 5.7: DSC thermograms for the self-condensation reactions of 2,6-DMP in the presence of varying NaOH : 2,6-DMP molar concentrations obtained at 10 °C min-1 scan rate 92

Figure 5.8: ΔHT as a function of NaOH : DMP molar ratio for 2,4-DMP and 2,6-DMP 97

Figure 5.9: Graph of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α = 0.95

and the corresponding square of the correlation coefficient (r2) values for 2-MMP sample with NaOH : 2,4-DMP molar ratio

of 0.45 100

Figure 5.10: Effects of NaOH on the evolution of apparent activation energy Ea for 2,4-DMP as a function of the degree of conversion 101

Figure 5.11: Effects of NaOH on the evolution of apparent activation energy Ea for 2,6-DMP as a function of the degree of conversion 104

Figure 6.1: Condensation reactions of TMP 111

Figure 6.2: Chemical structure of trimer following condensation reactions of TMP 111

Figure 6.3: DSC thermograms for the self-condensation reactions of TMP in the presence of varying NaOH : TMP molar concentrations obtained at 10 °C min-1 scan rate 113

Figure 6.4: ΔHT as a function of NaOH : TMP molar ratio for TMP 115

Figure 6.5: Graph of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α = 0.93

and the corresponding square of the correlation coefficient (r2) values for 2,4,6-TMP sample with NaOH : 2,4,6-TMP molar ratio of 0.45 118

xvii

Figure 6.6: Effects of NaOH on the evolution of apparent activation energy Ea for TMP as a function of the degree of conversion 119

Figure 6.7: Fractional conversion as a function of temperature for TMP samples with various NaOH molar ratios 122

Figure 8.1: The weight-average molecular weight (Mw) and the polydispersity (Mw/Mn) of PF resoles as functions of NaOH / P molar ratio 143

Figure 8.2: The gel time of PF resoles as a function of NaOH / P molar ratio 144

Figure 8.3: DSC thermograms of the PF resoles having different NaOH / P molar ratios obtained at 10 °C min-1 scan rate 146

Figure 8.4: Fractional conversion of the cure reactions of the resoles as a function of temperature 147

Figure 8.5: ΔHT as a function of NaOH / P molar ratio for PF resoles 149

Figure 8.6: Graphs of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α =

0.95 and the corresponding square of the correlation coefficient (r2) values for a resole sample having NaOH / P molar ratio 0.50 153

Figure 8.7: Effects of NaOH / P on the evolution of apparent activation energy Ea for the resoles as a function of the degree of conversion 155

1

Chapter 1

Introduction

1.1 Background

1.1.1 General

The production of reconstituted wood products has become increasingly important

as the demand for wood and wood-based products continues to increase and the

availability of high quality large diameter logs continue to lessen due to logging

restrictions and environmental concerns. Reconstituted wood products consist of

products where wood chippings, shavings or course saw dust are bonded together

by adhesives to form a larger piece of solid wood product such as wood flake

boards, particle boards, wood fibre boards and plywood.

The viability of the reconstituted wood product industries greatly depends on the

understanding of and the use of suitable wood adhesives. In fact, wood bonding is

one of the largest-volume uses of adhesives, particularly in the softwood plywood

industry. Some of the commonly used wood adhesives in Australia are, phenol

formaldehyde (PF), resorcinol formaldehyde (RF), melamine urea formaldehyde

(MUF), and urea formaldehyde (UF).

Of these products, PF resoles are preferred especially for external and structural

applications because they are structurally the most durable and can provide high

quality wood bonding suitable for all climatic conditions. They are also

environmentally more acceptable due to negligible formaldehyde emission. Apart

from timber applications, PF resoles have also been used extensively because of

their high temperature resistance, high char yield and moderate flame resistance in

many areas, especially in coating applications, carbonless copy paper, air and oil

filters and in other composites.

2

Despite these advantages, conventional PF resoles have relatively slow cure rates,

require high cure temperature and are less tolerant of variations in anatomical

features and wood substrate properties such as moisture content and density, which

limit their allowable gluing conditions. Extensive research and development over

the last few decades have gone into making better performance PF resoles. One of

the most important research fields is the investigation of the mechanism and

kinetics of the cure behaviour because PF has a very complicated cure process that

involves many reactions that occur simultaneously, each of which is profoundly

influenced by reaction conditions. Detailed information on the applications,

advantages and general issues regarding the limitations of PF resoles can be found

in a number of references [1-10].

1.1.2 The production of PF resoles

Detailed information on the production of PF resoles can be found in references [2-

6]. Generally, PF resoles are produced by base-catalysed reaction between phenol

and formaldehyde and step-growth polymerisation. The resoles produced consist of

low to medium molecular weight “reactive intermediates” which are stable at room

temperature, but are thermo-sensitive and can readily be transformed into three

dimensional, cross-linked, insoluble, and infusible polymers by the application of

heat during the curing process. The first isolable products of the reaction between

phenol and formaldehyde are methylol phenols. The position of the methylol

groups on the phenol ring and the ratio of various methylol derivatives formed

largely determine the rate of polymerisation, as well as the structure and properties

of the subsequent higher molecular weight products.

It is generally agreed that the first phase, methylolation, involves the addition of

methylol groups exclusively at the active ortho and para positions of the phenol

ring to form two mono-methylol phenols, which further react with formaldehyde to

form two di-methylol phenols, followed by one tri-methylol phenol. Molar ratio of

formaldehyde to phenol, type and concentration of catalyst, temperature and pH are

important factors that influence the nature and composition of the methylol phenols

formed during this phase [11-14]. As the process advances to the second phase

with the application of heat, condensation reactions of the methylol phenols occurr

3

to form methylene and/or ether bridges. The condensation reactions of individual

methylol phenols vary considerably, leading to a large number of reaction products

with varying reactivity and thermal properties. As the process proceeds further to

higher temperatures during the cure phase, more complex condensations and

rearrangements of the pre-polymer intermediates occur, leading to highly

condensed infusible network structures. The formation and nature of this network

structure determine the properties of the fully cured product.

1.2 The Issues

Various studies have been carried out to investigate the thermochemical

characteristics of the entire PF reaction cycle that involves the addition of

formaldehyde to phenol, the condensation reactions of methylol phenol monomers

and the subsequent curing reactions. However, the kinetics and mechanisms

governing the entire PF reaction cycle remain relatively unclear, not only due to the

complexity of the system, which involves many consecutive or integrated

processes, but also the profound effect of temperature, pH conditions and the molar

ratio between phenol and formaldehyde on the reaction system [see, for example,

15-20].

Extensive efforts have gone into elucidating the reaction pathways by simplifying

the system and starting with the first addition products. For the purpose of

simplification, the use of individual methylol phenol monomers, rather than the

complex PF resoles as a whole, has been advocated as a legitimate approach to the

mechanistic study and can be very useful in providing empirical parameters for

modelling and controlling the PF reaction cycle. However, these efforts generally

concerned themselves with the mechanisms and kinetics of the reactions occurring

during individual stages, rather than with the entire PF process. Hence, there is

very limited published information regarding the thermochemical properties of

methylol phenols for the entire PF cycle [see, for example, 21-31].

Apart from the approach of using individual methylol phenol monomers, many

research efforts have been dedicated to the investigation of PF resoles as a whole.

Whilst differential scanning calorimetry (DSC) is often used to study the cure

4

properties of the resoles, these studies were often limited to the interpretation of the

DSC curves, rather than focusing on kinetic analysis to obtain relevant kinetic

information [see, for example, 32]. Where kinetic analysis was carried out, it was

often mistakenly assumed that the activation energy of the thermal reaction was

constant and did not change with the extent of the cure. A number of studies have

addressed this issue and demonstrated the complex dependence of the reaction

kinetics on the degree of the cure. Despite these encouraging efforts, the use of

DSC to obtain insights into mechanisms of the cure of PF resoles is still limited

[see, for example, 17, 18, 33-38].

1.3 The Objectives

The principal aim of the present research is to investigate the thermochemical

characteristics of the individual monomers in the temperature range up to 250°C.

As opposed to the common approach of focusing on individual curing stages, this

temperature range captures the kinetics throughout the entire PF cure cycle which

is identified to be the least well understood. The experiments incorporate the initial

lower temperature cross-linking reactions of the monomers to form the pre-polymer

compounds, through to the fully cure reactions that lead to solid network structures

occurring at higher temperatures.

The second aim of the research is to study the cure properties of PF resoles as a

whole with a particular focus on the dependence of the reaction kinetics on the

degree of the cure up to 250°C. This focus aims to address the problems created by

the common mistaken assumption in the published literature, that the activation

energy of the cure reactions did not change with the extent of the cure. It also

recognises the importance of a changing reaction medium as the cure proceeds that

may induce significant variations in the reaction kinetics of the resoles.

The thermochemical properties of both methylol phenol monomers and PF resoles

are monitored as a function of concentration of sodium hydroxide, a common basic

catalyst used in the making of the resoles. DSC is employed as the major analytical

tool to obtain relevant kinetic information using isoconversional analysis. The use

of the isoconversional method allows the activation energy to be determined as a

5

function of the extent of the cure and/or temperature without making any

assumptions about the reaction model, thus eliminating the uncertainties involved

in the traditional model-fitting approach. These kinetic data, together with relevant

established chemical information, form the basis upon which the reaction pathways

throughout the entire cure cycle will be elucidated.

The outcomes of the research serve as a contribution to efforts aiming to improve

the understanding of the cure mechanism of PF resoles, and from here, to aid in the

development of PF resole adhesive systems capable of bonding under a wide range

of gluing conditions and curing faster at lower temperatures.

1.4 Structure of the Thesis

The body of the thesis is presented in 9 chapters. Following the current chapter

which introduces the background to the research, chapter 2 is a literature review of

the chemistry and thermochemical behaviour of PF resoles and their monomers.

The effects of formulation parameters on the properties of the resoles, as well as the

use of DSC to study their cure behaviour, will also be briefly reviewed in chapter 2.

Chapter 3 presents the methodology and experimental details for the study of the

monomers. The experimental results and discussion for mono-methylol phenols, di-

methylol phenols and tri-methylol phenol are presented separately in chapters 4, 5

and 6, respectively. Chapter 7 provides a summary of the findings and compares

the thermochemical properties of individual methylol phenols in an effort to

provide a consistent overall picture of relevant mechanisms operating during the

cure process. Chapter 8 focuses on the study of PF resoles as a whole and the

effects of sodium hydroxide concentration on the properties and cure behaviour of

the resoles. The outcomes of the monomers study are used in the interpretation of

the results. Chapter 9 concludes the thesis and proposes directions for future

research.

6

1.5 References

1. T. Sellers Jr., “Wood Adhesive Innovations and Applications in North

America”, Forest Prod. J. 51, 12-22 (2001).

2. A. Pizzi, Wood Adhesives, Marcel Dekker, New York, 1983.

3. A. Knop, and L.A. Pilato, Phenolic Resins – Chemistry, Applications and

Performance, Springer-Verlag, Berlin, 1985.

4. A. A. Whitehouse, E. G. K. Pritchett, G. Barnett, Phenolic Resins, Iliffe:

London, 1967.

5. A.A. Marra, Technology of Wood Bonding: Principles in Practice, Van

Nostrand Reinhold, 1992.

6. Y. Yazaki and P. J. Collins, “Adhesion Science and Technology”, in

Proceedings of the International Adhesion Symposium, Japan, 1994, p. 607.

7. N. J. L. Megson, “Unsolved Problems in Phenol Resin Chemistry”, Chem.-

Ztg. 96(1-2), 15-19 (1972).

8. A. Pizzi, in “Handbook of Adhesive Technology”, A. Pizzi, K.L. Mittal

(ed.), Marcel Dekker, New York, 2003.

9. A. Gardziella, L.A. Pilato, A. Knop, “Phenolic Resins: Chemistry,

Applications, Standardization, Safety, and Ecology”, 2nd ed., Springer-

Verlag, New York, 2000.

10. M.F. Grenier-Loustalot, G. Raffin, B. Salino and O. Païssé, “Phenolic

resins Part 6. Identifications of Volatile Organic Molecules During Thermal

Treatment of Neat Resols and Resol Filled with Glass Fibers”, Polymer

41(19), 7123-7132 (2000).

11. J. Bouajila, G. Raffin, H. Waton, C. Sanglar, J.O. Paisse, M-F. Grenier-

Loustalot, “Phenolic Resins - Characterizations and Kinetic Studies of

Different Resols Prepared with Different Catalysts and

Formaldehyde/Phenol Ratios”, Polymers & Polymer Composites 10, 341

(2002).

7

12. G. Astarloa-Aierbe, J. M. Echeverria, A. Vazquez, I. Mondragon,

“Influence of the Amount of Catalyst and Initial pH on the Phenolic Resol

Resin Formation”, Polymer 41, 3311 (2000).

13. L.B. Manfredi, C. C. Riccardi, O. de la Osa, A. Vazquez, “Modelling of

Resol Resin Polymerization with Various Formaldehyde/ Phenol Molar

Ratios”, Polymer International 50 (7), 796-802 (2001).

14. I. Poljangek, B. Likozar, M. Krajnc, “ Kinetics of Hydroxymethyl Phenols

Formation by In-Line FTIR Spectroscopy”, J. Appl. Polym. Sci. 106 (2),

878-888 (2007).

15. L. Gollob, “The Correlation Between Preparation and Properties in

Phenolic Resins”, in Wood Adhesives – Chemistry and Technology Vol. 2,

A. Pizzi (ed.), Dekker, New York, 1989, p. 121.

16. A. Pizzi and A. Stephanou, “Phenol - Formaldehyde Wood Adhesives

Under Very Alkaline Conditions - Part I: Behaviour and Proposed

Mechanism”, Holzforschung 48, 35-40 (1994).

17. Y-K Lee, D-J Kim, H-J Kim, T-S Hwang, M. Rafailovich and J. Sokolov,

“Activation Energy and Curing Behaviour of Resol- and Novolac-Type

Phenolic Resins by Differential Scanning Calorimetry and

Thermogravimetric Analysis”, J. Appl. Polym. Sci. 89, 2589-2596 (2003).

18. M.F. Grenier-Loustalot, S. Larroque and P. Grenier, “Phenolic Resins: 5.

Solid-State Physicochemical Study of Resoles With Variable F / P Ratio”,

Polymer 37(4), 639-650 (1996).

19. T. Halopainen, L. Alvila, P. Savolainen, T.T. Pakkanen, “Effect of F/P and

OH/P Molar Ratios and Condensation Viscosity on the Structure of Phenol-

Formaldehyde Resol Resins for Overlays - A statistical study”, J. Appl.

Polym. Sci. 91(5), 2942-2948 (2004).

20. R. Banerjee, K. Patil, K.C. Khilar, Canadian Journal of Chemical

Engineering 84, 328 (2006).

21. M.M. Sprung and M.T. Gladstone, “A Study of Some Condensations of o-

Methylolphenol”, J. Am. Chem. Soc. 71, 2907 (1949).

8


Mechanisms and Kinetics of Phenol and of the First Polycondensates

Towards Formaldehyde in Solution”, Polymer 35(14), 3045-3054 (1994).

23. M. Higuchi, T. Urakawa and M. Morita, “Condensation Reactions of

Phenolic Resins. 1. Kinetics and Mechanisms of the Base-Catalyzed Self-

Condensation of 2-Hydroxymethylphenol”, Polymer 42, 4563 (2001).

24. J.H. Freeman and C.W. Lewis, “Alkaline-catalyzed Reaction of

Formaldehyde and the Methylols of Phenol; A Kinetic Study”, J. Am.

Chem. Soc. 76, 2080-2087 (1954).

25. L.M. Yeddanapalli and D.J. Francis, “Kinetics and Mechanism of the Alkali

Catalysed Condensation of o- and p-Methylol Phenols by Themselves and

with Phenol”, Die Makromolekulare Chemie 55, 74-86 (1962).

26. D.J. Francis and L.M. Yeddanapalli, “Kinetics and Mechanism of the Alkali

Catalysed Condensations of Di- and Tri-Methylol Phenols by Themselves

and with Phenol”, Die Makromolekulare Chemie 125, 119-125 (1969).

27. R.T. Jones, “The Condensation of Trimethylol Phenol”, J. Polym. Sci. 21,

1801 (1983).

28. Grenier-Loustalot, S. Larroque and P. Grenier, “Phenolic Resins: 4. Self-

Condensation of Methylolphenols in Formaldehyde-Free Media”, Polymer

37(6), 955-964 (1996).

29. N. Kamo, M. Higuchi, T. Yoshimatsu, T. Yoshimatsu, Y. Ohara, M.

Morita, “Condensation Reactions of Phenolic Resins III: Self-

Condensations of 2,4-Dihydroxymethylphenol and 2,4,6-

Trihydroxymethylphenol”, Journal of Wood Science 48(6), 491-496 (2002).

30. N. Kamo, M. Higuchi, T. Yoshimatsu, M. Morita, “Condensation reactions

of phenolic resins IV: self-condensation of 2,4-dihydroxymethylphenol and

2,4,6 trihydroxymethylphenol (2)”, Journal of Wood Science 50(1), 68-76

(2004).

31. N. Kamo, J. Tanaka, M. Higuchi, T. Kondo, M. Morita, “Condensation

reactions of phenolic resins VII: Catalytic Effect of Sodium Bicarbonate for

9

the Condensation of Hydroxymethylols”, )”, Journal of Wood Science

52(4), 68-76 (2006).

32. J. Monni, L. Alvila, J. Rainio, T.T. Pakkanen, “Novel Two-Stage Phenol-

Formaldehyde Resol Resin Synthesis”, J. Appl. Polym. Sci. 103 (1), 371-

379 (2007).

33. P.W. King, R.H. Mitchell, and A.R. Westwood, “Structural Analysis of

Phenolic Resole Resins”, J. Appl. Polym. Sci. 18, 1117-1130 (1974).

34. A.W. Christiansen and L. Gollob, “Differential Scanning Calorimetry of

Phenol-Formaldehyde Resols”, J. Appl. Polym. Sci. 30, 2279-2289 (1985).

35. G. Carotenuto and L. Nicolais, “Kinetic Study of Phenolic Resin Cure by

IR Spectroscopy”, J. Appl. Polym. Sci. 74, 2703-2715 (1999).

36. B.D. Park, B. Riedl, Y.S. Kim and W.T. So, “Effect of Synthesis

Parameters on Thermal Behaviour of Phenol-Formaldehyde Resol Resin”,

J. Appl. Polym. Sci. 83, 1415-1424 (2002).

37. G. He, B. Riedl and A. Ait-Kadi, “Model-Free Kinetics: Curing Behavior of

Phenol Formaldehyde Resins by Differential Scanning Calorimetry”, J.

Appl. Polym. Sci. 87, 433-440 (2003).

38. J. Monni, L. Alvila, T.T. Pakkanen, “Structural and Physical Changes in

Phenol-Formaldehyde Resol Resin, as a Function of the Degree of

Condensation of the Resol Solution”, Industrial & Engineering Chemistry

Research 46(21), 6916-6924 (2007).

10

Chapter 2

Literature Review of Thermochemical Behaviour of

PF Resole and Its Monomers

2.1 PF Resoles –Background

2.1.1 History

In 1910, synthetic resins formed by the condensation of phenols with formaldehyde

were the first resinous products to be commercially produced entirely from simple

compounds of low molecular weight. They remain one of the more important

products of the plastics industry as moulding and impregnated products and

insulation materials, particularly for electrical insulation. Early difficulties were the

tendency for the product to be brittle, crack, blister easily, and the violent nature of

the condensation reaction made it difficult to control. However, in 1907, Baekeland

provided the real solution of making quick-curing mouldings under controlled

conditions without the problems of cracking and blistering [1]. He showed that

acids and bases were chiefly catalytic in action and could be used in very small

proportions, whereas previously equi-molar or even larger amounts had been used.

With small proportions of an acid catalyst and a low molar ratio of formaldehyde to

phenol, permanently fusible resins soluble in common solvents, such as alcohol and

acetone, were obtained and called “novolaks”. This type of adhesive resin is not

important as a wood adhesive because the faster cure of the novolak compounds

can only result in linear molecules which result in a permanently fusible resin [2].

On the other hand, resinous compounds obtained with a basic catalyst and high

molar ratio of formaldehyde to phenol were different in character and were called

“resoles”. Once fully cured, they have the ability to form infusible, insoluble, three

dimensional cross-linked network structures which provide highly desirable

performance properties such as high modulus and tensile strength, good

11

dimensional stability and solvent resistance as well as being relatively low cost [3].

For these reasons, the ability to characterise the cure of the PF resole is of great

benefit from an application standpoint, since the degree of cure will significantly

influence the properties of the cured resin.

In the 1930s, resole adhesives became widely used in the wood products industry

for the manufacture of particleboard and plywood and then for the manufacture of

oriental strand board (OSB) since its introduction in the 1970s. Today, the PF

resoles continue to dominate composite wood adhesives and are a major cost factor

in the industry [4].

2.1.2 Application of PF resoles in the wood industry

Generally, PF resoles are produced and applied in the wood industry in three stages

[4]:

Stage A: Is obtained by reacting phenol and formaldehyde with basic catalyst. The

resin may be solid, liquid or semi-liquid, and is soluble in solvents. It can be stored

until applied to the wood components.

Stage B: The wood components and resin are then placed in a hot press, with

temperatures ranging between 130°C to 140°C and high pressures between (300 to

700) kPa. During this stage the PF resin becomes solid and insoluble, but may

swell in common solvents such as acetone or alcohol.

Stage C: On further heating, the resin in Stage B is converted to the final Stage C,

which is infusible and insoluble in organic solvents. This cure stage is normally

effected in 5 to 10 minutes. Volatiles, mainly water and insignificant amounts of

formaldehyde are eliminated during the cure process.

Major advances have been made in clarifying the mechanisms of each of the three

stages, particularly when methods were developed for simplifying the systems by

using pure phenol alcohols in place of the complex mixtures found in typical

resoles. There were some criticisms of early workers using the model phenol

alcohols because it was contended that the results might not be applicable to

12

commercial resins. However, this approach was later recognised to be sound as it

was accepted that functional groups generally undergo the same reactions in

monomeric and polymeric systems [see, for example, 4-6].

2.2 PF Resole Chemistry

Three reaction sequences must be considered in relation to PF resole production

and application: formaldehyde addition to phenol to form monomers, condensation

reactions to form resole, and finally the cross-linking reactions or cure of the

resole.

2.2.1 Formaldehyde addition to phenol to form monomers

2.2.1.1 General

The first step in the formation of resole is the addition of formaldehyde to phenol to

form monomers. This reaction is carried out at around 60°C using molar excess

formaldehyde and in the presence of alkaline metal hydroxides, commonly sodium

hydroxide, at pH 8 - 13. The reaction paths are shown in Figure 2.1. Essentially,

the formaldehyde attacks exclusively at the active ortho and para positions of the

phenol ring, adding methylol groups to these sites to form two mono-methylol

phenols (MMP), then two di-methylol phenols (DMP), followed by one tri-

methylol phenol (TMP) (compounds 1 to 5 respectively) [5, 7, 8]. Meta substitution

does not occur. The objective during this step is to react as much of the phenol with

the formaldehyde to obtain as many methylol groups attached as possible, which is

important for structural as well as environmental reasons. The methylol functional

groups on the monomers tend to react by condensation. However, at 60°C and

below, condensation reactions are negligible, thus giving the phenol an opportunity

to react relatively completely with the formaldehyde [9, 10].

2.2.1.2 Reactivity of methylol phenols with formaldehyde

Freeman and Lewis [11] first performed the most complete study to determine the

reactivity of individual methylol phenols with formaldehyde by reacting them at

30°C with an amount of formaldehyde equivalent to the total number of reactive

13

phenolic sites, so that complete conversion to TMP could be achieved in each case.

Using paper chromatography technique, they followed the reaction paths of

individual monomer compounds until full conversion to TMP occurred and

determined their individual rate constants. As most commercial resoles are

prepared at higher temperatures and lower formaldehyde concentrations, the

findings of Freeman and Lewis may not be strictly applicable to commercial

resoles. However, their results provided a starting point for a discussion of the

problem and in fact were used as a basis by later researchers [see, for example, 9,

12-16].

OH

OH

CH2OH

OH

CH2OH

OH

CH2OH

CH2OH

OH

CH2OHHOCH2

OH

CH2OH

CH2OHHOCH2

P

+CH2O

1

2 3

4

5

+CH2O +CH2O

+CH2O+CH2O

+CH2O

Figure 2.1: Reaction paths for the addition of formaldehyde to phenol [11].

They found that the reactions are second-order and that there are significant

differences in both positional and molecular reactivity. In particular, whilst an

ortho position in the phenol is slightly less reactive than the para, the introduction

of an ortho-methylol group on to the phenol enhances the reactivity of the

remaining active positions. An introduction of a methylol group in the para

position of a phenol retards further activity. These effects are multiplied in the di-

methylol analogs with 2,6-DMP being the most reactive and therefore readily

converted to TMP, whereas 2,4-DMP has very low reactivity. The observed

14

differences in reactivity between the para and ortho methylol compounds are

attributed to the effect of hydrogen bonds in the ortho methylol compounds. With

these results, Freeman and Lewis predicted that in a reaction between phenol and

formaldehyde, 4-MMP and 2,4-DMP are the major components, 2-MMP is a minor

component, 2,6-DMP will be below the limits of detection and the relative amounts

of TMP and residual phenol are determined by the amount of formaldehyde

available.

More recently, Grenier-Loustalot et al. [17] conducted a series of studies to

determine the reactivity of individual methylol phenols with formaldehyde in

conditions of resole synthesis (60°C, catalysed by NaOH at pH 8). Using a range

of techniques including HPLC, 13C NMR, FTIR and chemical assays, they

monitored the kinetic and mechanistic changes in each monomer as a function of

time and obtained rate constants by simulating kinetic curves during the first hours

using a second-order equation of the type dx/dt = kC0(1-x)2. Their results supported

the reaction path of each monomer as found by Freeman and Lewis. They also

confirmed that 2,6-DMP is the most reactive compound and 2,4-DMP the least and

will likely to accumulate in the mixture. However, the results for the reactivity of

2-MMP and 4-MMP seemed to contradict those of Freeman and Lewis. In the

experimental conditions chosen, they classified the reactivity of each monomer as:

k 2,4-DMP < k 2-MMP < k 4-MMP < k 2,6-DMP. Besides the addition of formaldehyde to the

phenol, some condensation reactions occurring between the monomers to form

dimers and trimers were also observed.

2.2.2 Condensation reactions to form resole

2.2.2.1 Condensation reactions

As heating is continued in the range from above 60°C to 100°C, the reaction

advances to the second stage of the process, which involves the condensation

reactions of the methylol phenols [6]. This may occur via three possible reaction

mechanisms to form ether and / or methylene linked chains as shown in Figure 2.2

[17]. In the case of the ether bridge, the mechanism involves the reaction between

two methylol groups and the release of one molecule of water with the creation of a

15

dimethylene ether bridge (Scheme IV, Figure 2.2). Ether formation is favoured

under neutral or acidic conditions. The formation of the methylene bridge involves

the reaction of a methylol group either with another methylol group with the

simultaneous release of one molecule of water and one molecule of formaldehyde,

or with a proton on the aromatic ring (ortho or para) with the release of one

molecule of water (Schemes V and VI, respectively, Figure 2.2). The resultant

resole has low degrees of polymerisation and consists of a complex mixture of

species such as unreacted phenol, formaldehyde, water and various monomers and

dimers with a substantial proportion of reactive methylol groups reacted.

OH OH

+ HO CH2 + H2OOH

CH2OH CH2OH

CH2O IV

OH OH

+ HO CH2 + H2OOH

CH2OH CH2OH

V+ CH2O

OH OH

+ HO CH2 + H2OOH

CH2OH

VICH2OH

CH2OH

Figure 2.2: Formation of dimethylene ether and methylene bridges.

The kinetic and mechanistic aspects of the condensation reactions of the individual

methylol phenols have been investigated in a number of studies [14, 18-23]. Whilst

the reaction conditions and the method of analysis between these studies are

different, there are differing results, but also some similarities. One of the early

experiments to shed an insight into the mechanism was conducted by Reese in a

series of experiments by individually heating the five monomers in alkaline

16

solutions at 70°C [21]. The monomers were heated alone for a predetermined

period following which the products of the reaction were separated by two-

dimensional chromatography. Reese found that for both 2-MMP and 2,6-DMP, the

condensation involved the reaction of a methylol group with a free para hydrogen

on the ring of the coupling monomer to form an (o,p) methylene link. No loss of

formaldehyde was observed and hence the reaction proceeded via Scheme VI

(Figure 2.2). On the other hand, 4-MMP and 2,4-DMP and 2,4,6-TMP, coupled

preferentially at the para position and formed (p,p) methylene links with the loss of

formaldehyde as in Scheme V (Figure 2.2). He also observed small quantities of

2,4-DMP formed when 4-MMP was condensed, which may be attributed to the

addition of the released formaldehyde to a free nuclear position.

Yeddanapalli and Francis [20, 22] carried out a series of studies to determine the

kinetics and mechanisms of the self-condensation reactions of the five monomers

in alkaline solution. They heated the reaction mixture in a reaction vessel

isothermally at (70, 80, 90) °C and samples of the mixture were removed at regular

intervals and the course of the reaction was followed by quantitative paper

chromatography to analyse and identify the reactants and products (Table 2.1). It

was noted that other reactions also appeared in minor amounts, but could not be

identified. In regard to the relative reactivity, their results indicated that the para-

position of 2-MMP appeared to be twice as reactive as the ortho-position of 4-

MMP. This is in agreement with the generally recognised fact that the ortho

position is less reactive than the para in electrophilic substitution reactions. They

also obtained values for the activation energy from the plot of the first-order log

rate constants against reciprocal of temperature (Table 2.1). The self-condensation

reaction of the 2-MMP in the absence of a catalyst was observed to be second-order

with an activation energy of 83.7 kJ mol-1. Similar to Reese, small quantities of

2,4-DMP formed when 4-MMP was condensed.

17

Table 2.1: Reaction products from the self-condensation reactions of monomers as observed by Yeddanapalli and Francis [20, 22].

Monomer Reaction Product Linkage / Mechanism

Rate constant*

(k) s-1

Ea kJ mol-1

OH

CH2OH

OH

OHCH2

CH2OH

+ H2O

ortho - para

VI 1.6x10-5

77.5

OH

CH2OH

OH

OH + H2OCH2

CH2OH

OH+CH2O

+ H2OCH2HO

+ 2,4-MMP

ortho-para

VI

para-para

V

1.67x10-5

72.4

OH

CH2OH

CH2OH

OH

+ H2OCH2HO

CH2OHHOCH2

CH2OH

OH OH

+ H2OCH2

CH2OH

CH2OH

CH2OH

ortho-para

VI

ortho-ortho

VI

6.23x10-5 __

OH

CH2OHHOCH2

OH

+ H2OCH2HO

CH2OHHOCH2

HOCH2

ortho-para

VI 8.56x10-5 81.6

OH

CH2OH

CH2OHHOCH2

OH + H2OCH2HO

CH2OH

CH2OH+CH2O

HOCH2

HOCH2

para-para

V __ __

*Rate constant for the disappearance of the monomer.

18

A more recent study carried out by Grenier-Loustalot et al. [17] simulated the

condensation reactions for each of the five substituted phenol monomers as alkaline

solutions without formaldehyde and in similar conditions of resole synthesis

(60°C, catalysed by NaOH at pH = 8) in order to determine the reaction

mechanisms and the reactivity during condensation of each of the monomers.

Using 13C-NMR and HPLC, they followed the changes during the self-

condensation reaction of these monomers in the absence of formaldehyde. Their

results, similar to those in the work of Reese and Yeddanapalli, showed only the

formation of methylene bridges under these particular experimental conditions

(Schemes V or VI, Figure 2.2) and that two parameters affecting the reactivity of

the monomers were the position and the number of methylol groups on the

aromatic ring. Table 2.2 summarises their observations in terms of the mechanism

and type of linkage formed during the self-condensation of the monomers.

The study by Grenier-Loustalot et al. showed that no ortho – ortho linkage was

formed and that a methylol group in the para position preferentially reacted with

another para methylol, rather than with an ortho methylol, to form para–para

methylene bridges. This may be due to intra-molecular interactions between

methylol groups in ortho position and the hydroxyl group of the aromatic ring, or

to steric hindrance preventing the sites from reacting [17, 22]. The reactivity of the

monomers towards themselves was also shown to increase with increasing

methylol substitution. Furthermore, the reactivity of 2-MMP toward itself was

about five times less than that of 4-MMP. These results mostly corroborate with

those of Yeddanapalli and Francis.

19

Table 2.2: Reaction products from the self-condensation reactions of monomers as observed by Grenier-Loustalot et al. [17].

Monomer Reaction Product Linkage Mechanism

OH

CH2OH

OH

OHCH2

CH2OH

+ H2O

ortho - para VI

OH

CH2OH

OH

OH + H2OCH2

CH2OH

OHHO+CH2O

+ H2OCH2

ortho-para

para-para

VI

V

OH

CH2OH

CH2OH

OH+ CH2O

+ H2OCH2HO

CH2OHHOCH2

OH

OH

+ H2OCH2

CH2OH

CH2OH

para-para (major component) ortho-para

V

VI

OH

CH2OHHOCH2

OH

+ H2OCH2HO

CH2OHHOCH2

HOCH2

ortho-para

VI

OH

CH2OH

CH2OHHOCH2

OH + H2OCH2HO

CH2OH

CH2OH+CH2O

HOCH2

HOCH2

OH

+CH2O

+ H2OCH2

CH2OH

CH2OH

HOCH2

HOCH2

HO

para-para ortho-para

V

V

20

2.2.2.2 Effects of alkalinity on the condensation reactions

It is well established that ether linkages resulting from the self condensation

reactions of the monomers only occur in slightly acidic or neutral reaction

conditions and that ether formation is essentially, if not completely, eliminated

under alkaline conditions [19, 21, 24-26]. Various studies have also been published

on the kinetics and mechanisms of the condensation reactions of the monomers,

mostly over narrow ranges of temperature and alkalinity [see, for example, 12, 27,

28]. This limitation has also been identified by Poljangek et. al. [9]. Sprung and

Gladstone [25] studied the condensation reactions of 2-MMP with itself in the

presence and absence of a basic catalyst (triethanolamine). They reported that the

self-condensation of 2-MMP is second-order without the catalyst, but is first-order

in the presence of the catalyst, and that the rate constant was independent of the

basic strength. Yeddanapalli and Francis also observed that the base-catalysed self-

condensation reactions of the mono-, di- and tri-methylol monomers in an aqueous

system were first-order, but that the reactivity of the ortho-methylol group

decreased considerably at higher concentrations of alkali [20, 22].

More recently, Kamo et. al. [14, 30, 31] studied the order of reaction of self-

condensation of each of the mono-, di-, and tri-methylol phenol monomers as

solutions under varying NaOH / monomer molar ratio. They noted that the above

mentioned earlier authors derived their kinetic results by graphing the reactant

concentration against reaction time and they ignored the possible effect of the

evolved condensation reaction products on the reaction. Kamo at. al. by using

HPLC, LC-MS and NMR analysis techniques, found that the reaction mechanism

of the condensation reactions of the methylol phenol monomers changes in a

complex manner with the evolution and further reactions of reaction products

during the condensation reaction.

Jones [19] carried out the self-condensation reaction of TMP in an aqueous

solution at 40°C over a range of pH 3-11 and monitored the disappearance of the

TMP by HPLC. Contrary to previous investigators, he proposed that the self-

condensation reactions of TMP are best explained by mechanisms that involve the

21

formation of quinone methide intermediate. In particular, from pH 3 to pH 5, the

following reactions between unionized molecules are predominant:

RR

CH2OH

O

CH2

RROH

+ H2O

quinone methide intermediate

RR

CH2OH

OH O

CH2

RR

+ CH2

R

R

R

R

OHHO + CH2O

Figure 2.3: Condensation reactions of TMP from pH 3 to pH 5.

From pH 5 to pH 10, the major reactions are between ionized and unionized

molecules:

RR

CH2OH

O

CH2

RROH

+ H2O


RR

CH2OH

O- O

CH2

RR

+ CH2

R

R

R

R

O-HO + CH2O

Figure 2.4: Condensation reactions of TMP from pH 5 to pH 10.

Above pH 10, the reactions between ionized molecules predominate:

22

RR

CH2OH

O- O

CH2

RR

+ OH-


RR

CH2OH

O- O

CH2

R

+ CH2

R

R

R

O-HO + CH2OR

R

Figure 2.5: Condensation reactions of TMP above pH 10.

In a more recent study, Higuchi [18] supported the quinone methide hypothesis and

found that the self-condensation of 2-MMP is first-order. Furthermore, he

ascertained that the reaction rate of the self-condensation of 2-MMP increases with

increase in NaOH : 2-MMP molar ratio until it reaches the maximum at around the

molar ratio of 0.10. Thereafter, it decreases as the molar ratio increases. The

activation energy was found to be 103 kJ mol-1 obtained at (80, 90 and 100) °C

with NaOH : 2-MMP molar ratios of 0.05, 0.10, 0.50 and 0.75. This value is

greater than those reported by Sprung and Gladstone (77.5 kJ mol-1) [25] and

Yeddanapalli and Francis (66.7 kJ mol-1) [20].

2.2.3 Cure reactions of resole

2.2.3.1 General

Cure is a thermally activated process, by which one or more reactants are

transformed from low-molecular weight materials to a highly cross-linked network.

During the cure, heat is applied and the cross-linking is established through the

reactive methylol groups with the occurrence of gelation at some intermediate stage

in the polymerisation process. At the gel point, the system loses fluidity since the

gel is insoluble in all solvents even at elevated temperatures due to molecular

entanglement by branching and some cross-linking. The non-gel portion of the

polymer remains soluble.

23

As the polymerisation proceeds beyond the gel point, the amount of gel increases at

the expense of the soluble portion [6]. The reaction is continued until the final

infusible, insoluble, three-dimensional cross-linked state is reached as shown in

Figure 2.6 (Scheme VII):

CH2

OH

HO

CH2 CH2

OH VII

CH2OH

CH2OH

CH2OH

CH2OH

OH

HOH2C

HOH2C

Figure 2.6: Three-dimensional cross-linked state.

Whilst it is preferable to fully methylolate the phenolic molecule to provide site for

three-dimensional cross-linking, not all reactive sites are accessible to

formaldehyde as the oligomer increases in size, due to steric reasons or molecular

shielding. These processes have been described theoretically and empirically by

the works of Flory [32] and Gan et al. [33].

2.2.3.2 Reactions during the cure of resole

The cure of a PF resin is extremely complex, involving a number of competing

reactions each of which may be profoundly influenced by reaction conditions [9,

13, 29]. A further complicating factor is introduced by the possibility of reaction at

either or both the ortho and para positions of the phenol. This not only leads to

large number of isomeric products, but also to products of varying reactivity,

depending on the location of the functional group. Knowledge of the chemistry of

cure depends greatly on studies of model systems and studies of the products of

degradation.

Previous studies have revealed amongst other things that the nature of cure

reactions is dependent on temperature. Below about 170°C, reactions characterised

by molecular extension predominate. The primary reactions in this temperature

24

range are those which form methylene and ether linkages. Methylene linkages are

the most stable as well as the most important linkages established during the cure.

It may be formed either directly or indirectly according to Schemes IV – VI

(Figure 2.2). Although the para position is favoured for condensation over the

ortho position on a per site basis, the proportion of ortho-para linkages is higher

than para-para linkages since there are twice as many ortho as para sites [5, 21-

24].

The formation of ether linkages as per Scheme IV (Figure 2.2) is another important

reaction under acidic or neutral conditions at temperatures below about 170°C.

Ether formation is essentially, if not completely, eliminated under alkaline

conditions. It appears that phenols with ortho-methylol groups are generally more

susceptible to ether formation than those with para- methylol groups [38, 39]. The

ratio of methylene to ether linkages formed also depends on the number of

methylol groups as compared to the number of free ring positions in the resole.

With a resole of high methylol content, or conversely with a resole with a few free

ortho and para ring positions, ether formation becomes increasingly important [5,

40]. The ether linkages are unstable at higher temperatures and may undergo

further reactions [41]. On the other hand, the methylene linkages are very stable

normally until the point of complete decomposition of the cured resole.

As the cure proceeds beyond about 170°C, many complex changes may occur.

Ether linkages may undergo further reactions, for instance, to form methylene

linkages with further loss of formaldehyde [3, 5, 40]. Further reactions may also

arise from monomers that have not already reacted at lower temperatures. At higher

temperatures, above about 200°C, thermal and oxidative decomposition of the

resin, together with simultaneous reactions involving the formation of quinone

methides and their polymerisation may occur, leading to extremely complex

products [3, 5, 6, 40, 63, 64].

25

2.3 Effects of Formulation Parameters on Properties of PF

Resoles

Generally, PF resoles in the cured state are insoluble infusible materials which, by

their very nature, are difficult to examine by many analytical techniques. Yet,

owing to the advances in thermal and spectroscopic methods of investigation in

polymer chemistry, progress has been made to improve the understanding of these

intricate processes. The main objectives of the majority of the studies were to

correlate the effects of the formulation parameters on: (i) the chemical structural

features of the pre-polymer compounds comprising the resole; and (ii) relate these

to cure characteristics and cured resin performance for optimisation purposes [9,

10, 13, 23, 41-44].

A key parameter that has been extensively studied is the formaldehyde / phenol

molar ratio (F / P) [12, 15, 23, 44]. Generally, increasing the F / P ratio has the

effect of increasing the molecular weight of the resole [37, 42, 45, 46]. In a study

using solid-state 13C-NMR, FTIR, spectroscopy with cross-polarization and magic

angle spinning (CP/MAS) Grenier-Loustalot et al. [47] reported that the extent of

methylol substitution in the phenolic ring increases with increasing F / P ratio.

Similarly, Holopainen et al. [45] suggested that increasing F / P value enhances the

concentration of methylol groups in methylol phenols, resulting in increasing

amounts of methylene and ether linkages and in rigid structure. Park et al. [46]

observed that as the F / P molar ratio increased, the viscosity of the resole also

increased. This was attributed to the higher degree of methylolation and more

cross-linking at higher F / P ratios. The study also showed that the gel time of the

resole decreased with increasing F / P molar ratio, suggesting that a higher F / P

molar ratio makes the resole cure faster than a lower F / P molar ratio does. This

increased reactivity of the resole was attributed to the higher amounts of reactive

mythylol groups formed in the resole with higher F / P ratios.

Another parameter that greatly influences the cure properties of the resole is the

NaOH / phenol molar ratio (NaOH / P) [48]. Park et al. [46] reported that the

molecular weight of the resole increases with increase in NaOH / P molar ratio.

26

This is in agreement with the results reported in a previous study by Gollob [42].

Pizzi and Stephanou [49] studied the cure behaviour of PF resoles under neutral

and alkaline conditions using IR, UV, 13C-NMR, paper chromatography techniques

and gel times. They found that gel times increased at around pH 9 – 10, indicating

that the rate of cure of the PF resole slows down markedly at high pH, instead of

accelerating as commonly thought. They postulated that a ring complex holding

Na+ is formed between the phenolic ring and the ortho-methylol group and

explained the progressive retardation of the cure with increase in pH on the basis of

this ring complex mechanism. Park et al. [46] also found that gel times of PF resole

increased as NaOH / P ratio was increased from 0.20 to 0.50 and attributed the

retarding effect of NaOH on the reaction kinetics to the formation of the sodium

ring complex. Likewise, Haupt and Waago [50] investigated the effect of varying

NaOH / P ratio (0.05 to 0.95) on the relative rate constants of condensation

reactions of PF resoles using gel permeation chromatography (GPC) and

viscometry. They found amongst other things that the condensation rate increased

as NaOH / P was increased from 0.05 to 0.30. However, further increases in NaOH

/ P ratio to 0.95 had the effect of decreasing the condensation rate. A similar effect

of NaOH in retarding the condensation kinetics of PF resoles was also reported by

Christiansen and Gollob [51] as they varied NaOH / P ratio from 0.45 to 0.75 and

by He and Yan [52].

2.4 The Use of DSC to Study the Cure Behaviour of PF

Resoles

DSC has been used in many studies to investigate the cure behaviour of PF resoles

[see, for example, 40, 41, 45-47, 51-56]. The nature of the DSC peaks has been a

subject of extensive investigation [10, 41, 52, 55-57]. Christiansen and Gollob [51]

used DSC to follow the cure behaviour of resoles having varying F / P ratio and

pH. The DSC analysis showed two major exothermic peaks. The first peak between

98°C and 129°C was quite sharp and attributed to the addition of formaldehyde to

phenolic rings, whereas the second peak between 139°C to 151°C was always

broader and attributed to the condensation reactions involving methylol groups.

27

This hypothesis was supported by the fact that the first exothermic peak was found

to be more intense with the increase in the free formaldehyde content.

King et al. [40] used DSC to study the effects of F / P ratio and type of catalyst

(NaOH and triethylamine) on the cure characteristics of resoles. They found that

the DSC curves had two peaks at 155°C and 185°C and that the relative

significance of the peaks depended largely on the type of catalyst used and not very

much on F / P ratio. They attributed the lower peak to the formation of methylene

linkages and the higher to the formation of ether linkages. Holopainen et al [45]

also reported two exotherms at temperatures higher than about 150°C for PF

resoles with varying F / P molar ratios. These exotherms overlapped at lower F / P

ratios (1.90 - 2.00), but became well-separated at higher F / P ratios (2.15 - 2.30).

As the F / P ratio increased, the peak temperature of the first exotherm changed

only slightly, whereas the peak temperature of the second exotherm increased

significantly. The first exotherm was attributed to the formation of ether and

methylene linkages, and the second to further reactions of the resin, for example,

the condensation of ether linkages to methylene linkages eliminating formaldehyde.

It was also suggested in this study that the increase of F/P ratio resulted in increases

of methylol concentration and in amounts of methylene and ether linkages in the

rigid resin, which made the condensation of ether linkages more difficult as

evidenced by the shift of the second exotherm to higher temperatures.

A single DSC exothermic peak for certain resoles was also observed in other

studies [46, 47, 53]. He et al. [53] observed a single DSC peak at about 150°C

during the curing of both low and high viscosity resoles. This peak was attributed

to the condensation reactions to form ether and methylene linkages, since

comparative experiments in the same study suggested that the addition reactions

were almost complete prior to the curing process. The authors also noted that

addition reactions due to formaldehyde released during the cure could occur

simultaneously with condensation reactions. Park et al. [46] also observed a single

exothermic DSC peak for all samples with varying F / P and NaOH / P molar

ratios, but attributed this to the lower molecular weight of the resoles used in their

study.

28

In addition to the focus on the nature of the DSC peaks, a number of studies have

also analysed the DSC curves to obtain kinetic information [see, examples, 52, 55-

57]. The analyses commonly involved fitting the DSC data to a hypothetical

reaction model of f(α). Following this model-fitting, the Arrhenius parameters such

as reaction order (n), activation energy (Ea), pre-exponential factor (Z), reaction

rate (k) were determined by the form of f(α) assumed [41, 55-57]. However, there

are discrepant results and interpretations. For instance, Lee et al. [54] investigated

the change in Ea of thermal reactions and cure behaviour of the resole as a function

of the F / P molar ratio ranging from 1.3 to 2.5. They observed a single exothermic

DSC peak for all samples and assumed that the reactions had nth-order kinetics and

used the Kissinger method [58] to calculate Ea from the plotting of –ln(βTp2) versus

1/Tp, where β is the heating rate and Tp is the peak exotherm temperature. They

found that Ea was 17.6 kJ mol-1 for F / P molar ratio of 1.3 and decreased to 15.2 kJ

mol-1 for molar ratio of 2.5.

In the study by Park et al. which also observed a single exothermic peak as

discussed above, the authors also assumed nth-order kinetics, but used the

Borchardt-Daniels method [59] which is based on a single heating rate run for the

calculation of Ea. In this case, Ea was found to increase from 92.4 kJ mol-1 for F / P

ratio of 1.9 to 118.7 for F / P ratio of 2.5. This is in disagreement with the

decreasing trend for Ea with increasing F / P ratio as found in Lee et al. As well, the

values for Ea in Park et al. were much higher than those in Lee et al. Park et al. also

calculated Es for varying NaOH / P ratio. They found that Ea had a value of 102.0

kJ mol-1 for NaOH / P of 0.2 and decreased to 90.5 kJ mol-1 as NaOH / P was

increased to 0.50. However, as the authors pointed out, such a decreasing trend of

Ea with increasing NaOH / P ratio is not consistent with other findings that the

reactivity of PF resoles decreases under high alkaline conditions.

These studies assumed that Ea does not change with temperature. A major problem

with this approach is that it ignores the complexity of the reactions and the

complex dependence of Ea on the degree of the cure, as has been shown in a

number of studies [see, for example, 14, 30, 55]. In particular, Kiran and Iyer [60]

studied the cure behaviour of a paper – PF resole composite using DSC in the

29

dynamic heating mode up to 250°C. They observed that Ea beyond 30 %

conversion was about one-half of its value observed at the lower conversion range.

The decrease in the “apparent” Ea was attributed to diffusion limitation that became

significant beyond 30 % conversion. The authors suggested that the kinetics of the

cure could be described by the homogeneous first-order model below 30 %

conversion and by the Jander 3-dimensional diffusion model beyond 30 %

conversion. Vazquez et al. [61] used the model-free isoconversional method [62] to

analyse the cure behaviour of a commercial PF resole up to about 200°C. They

found that the apparent Ea did not remain constant, but decreased with the extent of

the cure. Such decrease in Ea was attributed to the complications caused by

diffusion limitation mechanism. Similarly, He et al. [53] analysed the DSC data

obtained from the cure of PF resoles up to 250°C using isoconversional analysis

method. They found a decrease in Ea as the cure proceeded and concluded that the

cure of PF resoles changed from a kinetic to a diffusion regime because of gelation,

vitrification and cross-linking in the system.

2.5 Concluding Remarks

It has been established by the use of various chemical, chromatographic and

spectroscopic techniques, that when phenol reacts with formaldehyde in basic

medium the reactions occur through different stages:

(a) Addition reactions to form five monomer compounds: 2-MMP and 4-MMP are

formed which may further react with formaldehyde to form 2,4-DMP and 2,6-

DMP. The reactions may proceed further to form 2,4,6-TMP;

(b) Resole formation: some condensation of the methylol phenols to form

methylene and ether linkages. Products of this type, soluble and fusible and

containing alcohols, are the pre-polymer compounds (resoles), which are used

commercially;

(c) Cure reactions: if the process proceeds further at higher temperatures, as in

practical applications, more complex condensations and rearrangements of the pre-

30

polymer intermediates occur, leading to highly condensed infusible network

structures.

Despite the overall support in the published literature for the above reaction

sequence in a general sense, the mechanisms governing the cross-linking process

encompassing the entire PF reaction cycle remain relatively unclear. This is largely

due to the heterogeneous nature of the phenomena involved and by the lack of

simple or distinctive analytical tools. Extensive efforts have gone into elucidating

the reaction pathways during the PF reaction cycle by simplifying the system and

starting with the first addition products. However, these efforts generally concerned

themselves with studies of the mechanisms and kinetics of the reactions occurring

during individual stages of the PF reaction cycle. With the advent of thermal

analytical techniques, various efforts have been made to study the thermochemical

characteristics of the entire PF reaction cycle that involve the addition of

formaldehyde to phenol, the condensation reactions of the monomers to form the

resole and the subsequent cure reactions. However, the complexity of the system,

which involves many consecutive or integrated processes, has precluded a

complete understanding of the reactions that occur during the entire process. With

the exception of a few studies, there is very limited published information on the

use of monomer compounds to elucidate the kinetics and mechanisms of the

reactions during the entire PF cycle.

The present study aims to fill this gap and to investigate the thermochemical

characteristics of the individual monomers as a function of catalyst concentration

using DSC in the temperature range up to 250°C. Within a single experiment, this

temperature range incorporates the initial lower temperature cross-linking reactions

of the monomers to form the pre-polymer compounds, through to the cure reactions

that lead to fully solid network structures occurring at higher temperatures. This

regime captures the kinetics throughout the entire cure region and was identified to

be the least well understood, and which might provide complementary information

for further understanding the cure mechanism of PF resoles. The study focuses on

the use of isoconversional analysis of the DSC data to evaluate the dependence of

activation energy on the degree of reaction conversion.

31

Another issue is that despite the extensive past efforts of using DSC to study the

cure properties of PF resoles, they were often limited to the investigation of the

nature of the DSC curves. Where kinetic analysis was carried out to obtain relevant

kinetic information, it was often mistakenly assumed that the activation energy of

the thermal reactions was constant and did not change with the extent of the cure. A

few studies have addressed this issue and demonstrated the complex dependence of

the reaction kinetics on the degree of the cure, mainly due to gelation, vitrification

and cross-linking in the system. Despite these encouraging efforts, the use of DSC

to obtain insights into mechanisms of the cure of PF resoles is still limited.

Therefore, in addition to the work on the monomers, the present study also aims to

use DSC isoconversional analysis to investigate the effects of catalyst

concentration on the cure properties of PF resoles as a whole.

2.6 References

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2. R. Houwink and G. Salomon (eds.), Adhesion and Adhesives Vol. 1,

Elsevier Publishing Company, 1965.

3. A. Knop and W. Scheib, Chemistry and Application of Phenolic Resins,

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4. A.A. Marra, Technology of Wood Bonding: Principles in Practice, Van

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5. R.W. Martin, The Chemistry of Phenolic Resins, J. Wiley, New York, 1956.

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7. J.M. Garro-Galvez, and B. Riedl, “Pyrogallol-Formaldehyde Thermosetting

Adhesives”, J. Appl. Polym. Sci. 65(2), 399-408 (1997).




32

9. I. Poljangek, B. Likozar, M. Krajnc, “ Kinetics of Hydroxymethyl Phenols

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12. L.B. Manfredi, C. C. Riccardi, O. de la Osa, A. Vazquez, “Modelling of

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13. J. Monni, L. Alvila, J. Rainio, T.T. Pakkanen, “Novel Two-Stage Phenol-

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14. N. Kamo, M. Higuchi, T. Yoshimatsu, T. Yoshimatsu, Y. Ohara, M. Morita,

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of Wood Science 48(6), 491-496 (2002).

15. C.C. Riccardi, G.A. Aierbe, J.M. Echeverria, I. Mondragon, “Modelling of

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33


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27. A. Gardziella, L.A. Pilato, A. Knop, “Phenolic Resins : Chemistry,

Applications, Standardization, Safety, and Ecology”, 2nd ed., Springer-

Verlag, New York, 2000.

28. M.F. Grenier-Loustalot, G. Raffin, B. Salino and O. Païssé, “Phenolic resins

Part 6. Identifications of Volatile Organic Molecules During Thermal

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41(19), 7123-7132 (2000).

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30. N. Kamo, M. Higuchi, T. Yoshimatsu, M. Morita, “Condensation reactions

of phenolic resins IV: self-condensation of 2,4-dihydroxymethylphenol and

34

2,4,6 trihydroxymethylphenol (2)”, Journal of Wood Science 50(1), 68-76

(2004).

31. N. Kamo, J. Tanaka, M. Higuchi, T. Kondo, M. Morita, “Condensation

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32. P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, New

York, 1953.

33. S. Gan, J.K. Gillham and R.B. Prime, “A Methodology for Characterizing

Reactive Coatings: Time-Temperature-Transformation (TTT) Analysis of

the Competition Between Cure, Evaporation, and Thermal Degradation for

an Epoxy-Phenolic System,” J. Appl. Polym. Sci. 37, 803 (1989).

34. K. Hultzsch, Angew. Chem. 61, 93-96 (1949).

35. K. Hultzsch, Chemie der Phenolharze, Springer-Verlag, Berlin, 1950.

36. D.D. Werstler, “Quantitative 13C NMR Characterization of Aqueous

Formaldehyde Resins: 1. Phenol-Formaldehyde Resins Polymer 27, 750-

756 (1986).

37. S. So and A. Rudin, “Analysis of the Formation and Curing Reactions of

Resole Phenolics”, J. Appl. Polym. Sci. 41, 205-232 (1990).

38. E. Imoto and T. Kimura, J. Chem. Soc. Japan 53, 9-11 (1950).

39. A. Pizzi, in “Handbook of Adhesive Technology”, A. Pizzi, K.L. Mittal

(Ed.), Marcel Dekker, New York, 2003.



41. G. Vazquez, J.Gonzalez-Alvarez, F. Lopez-Suevos, S. Freire, G.

Antorrena, “Curing Kinetics of Tannin-Phenol-Formaldehyde Adhesives as

Determined by DSC”, Journal of Thermal Analysis and Calorimetry 70(1),

143-149 (2005).

35

42. L. Gollob, “The Correlation Between Preparation and Properties in Phenolic

Resins”, in Wood Adhesives – Chemistry and Technology Vol. 2, A. Pizzi

(ed.), Dekker, New York, 1989, p. 121.

43. J. Monni, L. Alvila, T.T. Pakkanen, “Structural and Physical Changes in

Phenol-Formaldehyde Resol Resin, as a Function of the Degree of

Condensation of the Resol Solution”, Industrial & Engineering Chemistry

Research 46(21), 6916-6924 (2007).

44. J. Bouajila, G. Raffin, H. Waton, C. Sanglar, J.O. Paisse, M-F. Grenier-

Loustalot, “Phenolic Resins - Characterizations and Kinetic Studies of

Different Resols Prepared with Different Catalysts and

Formaldehyde/Phenol Ratios”, Polymers & Polymer Composites 10, 341

(2002).

45. T. Holopainen, L. Alvila, J. Rainio and T.T. Pakkanen, “Phenol-

Formaldehyde Resol Resins Studied by 13C-NMR Spectroscopy, Gel

Permeation Chromatography, and Differential Calorimetry”, J. Appl.

Polym. Sci. 66, 1183-1193 (1997).



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Solid-State Physicochemical Study of Resoles with Variable F / P Ratios”,

Polymer 37(4), 639-650 (1996).

48. G. Astarloa-Aierbe, J. M. Echeverria, A. Vazquez, I. Mondragon,

“Influence of the Amount of Catalyst and Initial pH on the Phenolic Resol

Resin Formation”, Polymer 41, 3311 (2000).




50. R.A. Haupt and S. Waago, “The Ionic Nature of the Phenol-Formaldehyde

Condensation Reaction and Its Effect on Polymer Propreties”, in A.W.

36

Christiansen, and A.H. Conner (eds.), Proc. of Wood Adhesives 1995, pp.

220–226 June 29–30, 1995, Portland, Oregon. Forest Prod. Soc., Madison,

Wisconsin.



52. G. He, N. Yan, “Effect of Wood on the Curing Behavior of Commercial

Phenolic Resin Systems”, J. Polym. Sci. 95(2), 185-192 (2004).



Appl. Polym. Sci. 87, 433-440 (2003).

54. Y-K Lee, D-J Kim, H-J Kim, T-S Hwang, M. Rafailovich and J. Sokolov,

“Activation Energy and Curing Behaviour of Resol- and Novolac-Type

Phenolic Resins by Differential Scanning Calorimetry and

Thermogravimetric Analysis”, J. Appl. Polym. Sci. 89, 2589-2596 (2003).

55. Byung-Dae Park, Xiang-Ming Wang, “Thermokinetic Behaviour of

Powdered Phenol-Formaldehyde (PPF) Resin”, Thermochimica Acta 433

(1-2), 88-92 (2005).

56. M.V. Alonso, M. Oliet, J.M. Pérez, F. Rodríguez, J. Echeverría,

“Determination of Curing Kinetic Parameters of Lignin–Phenol–

Formaldehyde Resol Resins by Several Dynamic Differential Scanning

Calorimetry Methods”, Thermochimica Acta 419(1-2), 161-167 (2004).

57. M.V. Alonso, M. Oliet, J. Garcia F. Rodríguez, J. Echeverría,

“Transformation of Dynamic DSC Results into Isothermal Data for the

Curing Kinetics Study of the Resol Resin”, Journal of Thermal Analysis

and Calorimetry 86(3), 797-802 (2006).

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Analytical Chemistry 29, 1702 (1957).

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Analysis to the Study of Reaction Kinetics”, J. Am. Chem. Soc. 79, 41

(1957).

37

60. E. Kiran and R. Iyer, “Cure Behaviour of Paper-Phenolic Composite

System: Kinetic Modeling”, J. Appl. Polym. Sci. 51, 353-364 (1994).

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38

Chapter 3

Methodology and Experimental Details

3.1 Methodology

As reviewed in Chapter 2, whilst there is agreement among researchers on the

general features of the PF reaction cycle, the reaction pathways governing the

entire cycle remain relatively unclear, primarily because of the complexity caused

by the large number of reactions occurring simultaneously. The use of methylol

phenol monomers to simplify the system has been advocated as a legitimate

approach to study relevant mechanistic aspects and can be very useful in providing

empirical parameters for modelling and controlling the PF reaction cycle. However,

most studies utilising this approach have been commonly concerned with the

mechanisms and kinetics of the reactions occurring during individual stages, rather

than the entire PF reaction cycle. The above considerations provided the impetus

for adopting a particular methodology, the essential features of which are described

in the following sections.

3.1.1 System parameters

3.1.1.1 Methyl phenol monomers

Given the complexity of the system, the strategy adopted in the present study

involves simplifying this system by starting with the first formed addition products

in the phenol-formaldehyde reaction and evaluating the reactions of each

independently. The five initial monomers are shown in Figure 3.1.

3.1.1.2 Reaction conditions

It is commonly accepted that the reaction pathway is dependent on the initial

formulation parameters, namely, the ratio of reactants (P / F), type and amount of

catalyst and the temperature. As this study is concerned with the condensation and

39

cure reactions, the P / F ratio is not an issue, but instead the effects of the catalyst

and temperature are the focus. The pressure of the reaction chamber is also an

important consideration.

OH

CH2OH

OH

CH2OH

OH

CH2OHHOCH2

OH

CH2OH

CH2OH

OH

CH2OH

CH2OHHOCH2

2-MMP 4-MMP 2,6-DMP2,4-DMP 2,4,6-TMP

Figure 3.1: The five initial intermediate monomers.

Catalyst: NaOH is selected as the catalyst, because it is most commonly used in the

wood adhesives industry. As discussed in chapter 2, the presence of NaOH has a

significant influence kinetically and mechanistically, both during the initial

addition reactions to form the five monomer compounds and during the subsequent

stages to form the resole and consequently the fully cured resin. Whilst there have

been studies that examined the effects of NaOH by monitoring the kinetics of the

reactions of the monomer compounds in aqueous solutions towards itself, towards

phenol and towards formaldehyde, these studies were commonly carried out under

constant conditions of pH and temperature with a varying F / P ratio. The present

study aims to investigate the condensation and curing reactions of individual

monomers in different NaOH regimes by systematically changing the molar ratio

of NaOH / monomer between zero and one. Molar ratio, instead of pH, is chosen as

the variable, since the experiments are carried out in a melt rather than in aqueous

solution. This range of NaOH is consistent with the common practice in the

production of commercial PF resole used in the wood adhesives industry.

Temperature: The condensation and curing reactions in the PF resole system are

effected by the application of heat within the temperature range of about (60 –

80)°C (333 – 353K) to 250°C (523K). At about 80°C (353K) to 100°C (373K), the

condensation reactions of the monomers occur to form the pre-polymer

compounds. The resin reaches the fully cured state with further heating to 250°C

(523K). In the manufacture of wood composites, the curing temperature varies

40

between 120°C (393K) to 250°C (523K), depending on the type of wood product.

In the present study, the monomers were heated up to 250°C (523K).

Pressure: In commercial operations, resoles are cured under heat and pressure.

Another consideration is that condensation reactions occurring under ambient

atmospheric conditions may lead to emission of volatile reaction products, usually

water and formaldehyde, within the temperature range studied. The emission of the

volatiles may obscure the results arising from the cross-linking reactions.

Therefore, experiments were conducted under a sealed cell operation to simulate

the commercial curing conditions and had the additional advantage of being able to

retain reactive volatiles. This was achieved by using hermetically sealed stainless

steel capsules that withstand the internal pressures (maximum up to 24

atmospheres).

3.1.1.3 Additional experimental parameters

Experiments were carried out in a melt, rather than in aqueous solution to further

simplify the system by excluding water and focusing purely on the effects of NaOH

on the cross-linking reactions. For optimum instrument resolution, the contact

surface between sample holder and sample should be maximized during heating the

sample. This was achieved by the use of compact fine granules in good contact

with the capsule bottom.

3.1.2 Thermal analysis by DSC

3.1.2.1 General

The most common and conventional analytical techniques employed for the

investigation of curing reactions include chemical analysis, quantitative paper

chromatography (PC), high-performance liquid chromatography (HPLC), gel

permeation chromatography (GPC), ultra violet (UV) spectroscopy, infra-red (IR)

spectroscopy and nuclear magnetic resonance (NMR). These methods are based on

monitoring the changes in concentration of reactive groups consumed or produced

in the course of reaction, and are powerful tools to characterize the chemical

composition of resoles. However, as the cross-linking advances particularly past

41

the gel point, the resin becomes completely insoluble in solvent, and as a

consequence, the whole PF reaction cycle is difficult to study by chemical and

spectroscopic means.

Although techniques such as the solid-state 13C-NMR spectroscopy with cross-

polarization and magic angle spinning (CP/MAS) can be used to study the solid-

state polymers, only a few methods can be employed to follow the whole PF

reaction cycle from beginning to end. In recent years, intensive use of thermal

techniques has been found to offer valuable insights into the intricate processes of

the formation, the curing characteristics and the thermal degradation of

thermosetting resins. Thermal analysis covers a group of techniques such as

thermomechanical analysis (TMA), thermogravimetry (TG), differential thermal

analysis (DTA), dinamomechanical analysis (DMA) and DSC. Of these, the

method mostly used to study the kinetics of cure reactions is the thermal analysis

by DSC. The following sections present a brief overview of the principle of the

DSC technique and key aspects of DSC data analysis. Detailed information

regarding these issues can be found in a number of references [1-3].

3.1.2.2 Principle of DSC

Whenever a material undergoes a change in physical state, such as melting or

transition from one crystalline form to another, or whenever it reacts chemically,

heat is either absorbed or liberated. Many such processes can be initiated simply by

raising the temperature of the material. DSC measures this heat flow into a material

(endothermic) or out of a material (exothermic). A commonality of all

thermosetting systems is the evolution of energy accompanying the cure, expressed

as heat per mol of reacting groups (kJ mol-1).

DSC is designed to determine the enthalpies of these reactions by measuring the

differential heat flow required to maintain a sample of the material and an inert

reference at the same temperature. There are two types of DSC instruments

currently used: “heat flux” and “power compensated” instruments. Although they

are fundamentally different in design, the data produced are comparable. For the

42

remainder of this work, the technique referred to as DSC is specifically that of the

power compensated method.

The power-compensated DSC consists of two separate microfurnaces for the

sample and reference and each contain one temperature sensor and a heater (Figure

3.2). Both furnaces are positioned in an aluminium block of constant temperature.

The same heating power is supplied to both furnaces according to a preset

temperature-time program. In the case of ideal thermal symmetry, the temperatures

of both furnaces are the same. When a sample reaction occurs (exothermic or

endothermic), the symmetry is disturbed due to a temperature difference between

the furnaces. This temperature difference of the measuring system can be

electronically compensated either by increasing or decreasing an additional heating

power. The compensating heating power is proportional to the measured

temperature difference between the furnaces. Thus the temperature of the sample

holder is always kept the same as that of the reference holder by continuous and

automatic adjustment of the heater power. A signal proportional to the difference

between the heat input to the sample and that to the reference is recorded as

reaction heat flow rate dH/dt as a result of calibration determined by measuring the

enthalpy of fusion of pure indium metal (99.9 % purity). The resulting enthalpy

change is then plotted against the temperature ramp as shown in Figure 3.3.

Figure 3.2: Power compensated DSC.

A peak in the measured curve occurs when the steady state is disturbed by

thermally activated heat production or consumption in the sample. A peak begins at

Ti (first deviation from the baseline), rises / falls to the peak maximum / minimum

Pt sensors

S R

Individualheaters

43

Tp and joins the base line again at Tf. The base line is that part of the measured

curve: (i) where no sample transition takes place as in region 1; and (ii) which in

the range of a peak is constructed in such a way that it connects the measured curve

before and behind the peak as if no reaction had taken place i.e. as if no peak had

developed as in region 3.

Figure 3.3: DSC dynamic scan peak.

3.1.2.3 Analysis of DSC experimental data

A major characteristic of PF reactions is that both the addition and condensation

reactions are strongly exothermic events accompanied by the liberation of heat. The

rate of the exothermic heat accompanying these chemical events may be monitored

as a function of temperature or time by DSC. The result is a profile of the rate of

enthalpy change when, as a function of temperature, the sample is heated at a

known linear rate. When the enthalpy is as a function of time, the calorimeter is

held at a constant temperature. Whilst isothermal mode permits the determination

of reaction kinetics at a particular temperature, non-isothermal measurements can

provide kinetic information over a large temperature range and do not have the

problem of having to heat the sample to the isothermal hold temperature during

which cure reaction may take place. In this work, PF reactions were monitored

under non-isothermal conditions.

Sign

al

1 2

3

Temperature (T)

Ti Tp Tf

44

Various methods may be used in the analysis of the DSC curves to evaluate the

reaction progress. All kinetic studies start with the basic rate equation:

dα/dt = k f(α) 3.1

where k is the rate constant, α is the degree of conversion and f(α) is the function of

the degree of conversion. In general, k is dependent on temperature through an

Arrhenious-type equation. Thus, the rate equation can be written as:

dα/dt = f(α)A exp(-Eα/RT) 3.2

where A is the pre-exponential factor (s-1) and relates to the amount of collisions

that need to occur in a unit time to carry out the reaction, Eα (Jmol-1) is the

activation energy at a given degree of conversion, R is the universal gas constant

(8.314 J mol-1.K), and T is the absolute temperature (K).

The DSC exotherm is used to measure the two basic parameters of the reaction,

namely, the fraction reacted α and the reaction rate dα/dt. Figure 3.4 depicts a

sample dynamic DSC thermogram. At a sufficiently high enough temperature, the

cross-linking reaction begins to proceed and the onset of curing is observed as an

exothermic event. The onset temperature (Ti) is the temperature at which the

reaction begins to progress. The peak maximum (Tp) represents the maximum rate

of cure at the given scanning rate. The completion of the cure occurs when the DSC

response returns to linear behavior at the upper temperature range (Tf).

The basic assumptions in applying the DSC to curing reaction kinetics are:

(i) The liberation of heat accompanying the curing reaction can be measured

directly with the DSC and the rate of enthalpy change (dH/dt) with respect

to temperature is recorded directly.

(ii) Enthalpy change, ΔH, up to any temperature T is proportional to the number

of moles of reactants consumed.

45

(iii) The total area under the exotherm corresponds to the total enthalpy of

curing reaction, ΔHTotal, and the partial area up to a certain temperature T,

corresponds to the enthalpy, ΔH, up to that temperature.

(iv) The rate of enthalpy change, (dH/dt), relative to the instrumental baseline is

directly proportional to the reaction rate.

Figure 3.4: A dynamic DSC thermogram in the scanning mode depicting an exothermic reaction.

From the above assumptions, the reaction rate dα/dt at any point along the reaction

exotherm temperature axis is obtained by dividing the peak height dH/dt at

temperature T by the total peak area ΔHTotal (equation 3.5), while the fraction

reacted α is obtained by measuring the ratio of the partial area ΔH at temperature T

to the total peak area ΔHTotal (equation 3.3).

⎟⎟⎠

⎞⎜⎜⎝

⎛Δ

Δ=

TotalHH

α 3.3

and

Total

Total

HHH

ΔΔ−Δ

=− )1( α 3.4

ΔHTotal - ΔH ΔH

Ti Tf T

exo

endo

dtdH

Tp

α = ΔH ΔHTotal dα/dt = dH/dt ΔHTotal

Hea

t flo

w

dtdH

T

46

where α is the conversion or extent of reaction, ΔH is the enthalpy change up to a

certain temperature and ΔHTotal is the total enthalpy change with the basic

assumption that complete conversion of the reactants is achieved, and:

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡⎟⎠⎞

⎜⎝⎛

=

⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢

⎣

⎡

Δ

⎟⎠⎞

⎜⎝⎛

=⎟⎠⎞

⎜⎝⎛=

Adt

dH

Hdt

dH

dtdr

Total

α 3.5

where r (s-1), is the rate of reaction, dH / dt (mJ g-1 s-1) is the ordinate of a DSC

trace, and A (mJ), is the total area under the curve and corresponds to the total

enthalpy of reaction ΔHTotal (mJ g-1).

The values for ΔHTotal, ΔH and α at selected temperatures along the temperature

axis were computed by the Perkin-Elmer Pyris computer software used in

conjunction with the DSC instrument. The ordinates of a DSC trace, dH/dt, for the

corresponding α, is the height of the curve below the baseline at temperature T and

was manually calculated by taking the difference between the value of the heat

flow at the baseline and the value of the heat flow corresponding to the particular

position of the curve (Figure 3.4).

Traditionally, kinetic analysis of DSC curves involves the fitting of data to

hypothetical reaction model of f(α). Some examples of reaction models commonly

used are listed in Table 3.1. Following this model-fitting, the Arrhenius parameters

are determined by the form of f(α) assumed. Vyazovskin and Wight [4] pointed out

the major drawbacks of the model-fitting approach in non-isothermal experiments,

which are relevant in this study. In particular, since both T and α vary

simultaneously in non-isothermal experiments, the model-fitting approach

generally fails to achieve a clean separation between the temperature dependence

k(T) and the reaction model f(α). As a result, the form of f(α) may be chosen to fit

the data, but often at the cost of drastic variations in the Arrhenius parameters,

which compensate for the difference between the assumed and the true but

unknown f(α). Also, the model-fitting method usually aims at extracting a single

value for the activation energy for an overall process. For reactions with activation

energy varying with both temperature and the extent of conversion, such a single

47

value does not reflect changes in the reaction mechanism and kinetics during the

course of the reactions.

Table 3.1: Reaction models used to describe thermal decomposition in solids [4].

Reaction models f(α)

Power law 4α3/4

Power law 3α2/3

Power law 2α1/2

Power law 2/3α-1/2

One-dimensional diffusion 1/2α-1

Mampel (first-order) 1- α

Avrami-Erofeev 4(1- α)[-ln(1- α)]3/4



Three-dimensional diffusion 2(1- α)2/3[1 - (1- α)1/3]-1

Contracting sphere 3(1- α)2/3

Contracting cylinder 2(1- α)1/2

Second-order (1- α)2

These drawbacks can be avoided by the use of the model-free method, described in

a series of studies by Vyazovskin et al. [4-7], which is based on the realization that

although f(α) and Eα depend on the degree of conversion α, they are always the

same at a particular degree of conversion, independent of the heating rate used. The

use of this method allows the activation energy to be determined as a function of

the extent of conversion and/or temperature without making any assumptions about

the reaction model, thus eliminating the uncertainties involved in the model-fitting

approach.

The present study involves monitoring the chemical and physical changes of the

system from the initial stage consisting of the monomer compounds to the

completely cured state as a function of temperature. The evolution of the system

48

would thus involve a range of different chemical reactions with different kinetics.

To avoid the aforementioned drawbacks of the model-fitting approach, the model-

free method is chosen in the present study for the analysis of DSC data. Assuming

that f(α) and Eα are constant at a particular degree of conversion, we can obtain a

differential equation from equation 3.2:

d ln(dα/dt)α / dT-1 = -Eα / R 3.6

Compared to the ASTM E698 method [8], which follows only one point of

conversion and applies the derived activation energy at the DSC transition

maximum to the overall reaction process, the model-free method follows every

point of conversion, obtaining the activation energy at each point. Thus, it can

reveal the dependence of Eα on α and the complexity of the cure process. In

particular, Eα and the pre-exponential factor A can be evaluated from the following

expression [9]:

ln Φ / Ti 2 = - Eα

/ RTi + ln RA / Eα 3.7

where Φ is the heating rate and Ti is the temperature to reach a given degree of

conversion. A graph of ln Φ / Ti 2 versus 1/Ti should yield a straight line. The value

for Eα and the pre-exponential factor A can be obtained from the slope and

intercept, respectively.

3.1.2.4 “Effective” activation energy Eα obtained from the model-free

method

Activation energy Ea is often used to denote the minimum energy required for a

specific chemical reaction to occur. The rate equation 3.2 gives the quantitative

basis of the relationship between Ea and the rate at which a reaction proceeds. It

can be seen that either increasing temperature or decreasing the Ea (for example

through the use of catalysts) will result in an increase in the rate of reaction.

Because the relationship of reaction rate to Ea and temperature is exponential, a

small change in temperature or Ea generally causes a large change in reaction rate.

Nevertheless, the extent of change in reaction rate as the temperature changes is

less significant for reaction with a low Ea compared to reaction with a high Ea.

49

As mentioned above, a key difference between the model-fitting and the model-free

methods is that the former aims at extracting a single averaged value of Ea for the

overall process, whereas the latter reveals the dependence of Eα on the degree of

conversion, α. Thus, for a system where different reactions occur over different

temperature regimes, as in the present study, the model-fitting method is not

suitable because it ignores the changing nature of the reactions and variation of Ea

as the cure proceeds. Instead, the model-free method, adopted in this study, avoids

the assumption of homogeneous reaction kinetics and allows the monitoring of

different chemical reactions with different kinetics from the initial to the final cure

stages.

The model-free method has been applied successfully to a range of processes

including those involving competing, independent, consecutive and reversible

reactions [see, for example, 4-7, 10-15]. Although the shape of the dependence of

Eα on α obtained from this approach does not necessarily unequivocally identify the

reaction mechanisms, it in all instances reveals critical kinetic information and

sheds light on possible sequences of reactions that may occur in a particular

process.

An issue that may occur in the calculation of Ea using DSC is when the cure is

incomplete. This is a common problem in isothermal experiments where the DSC

scans are carried out at a fixed temperature below the glass transition temperature

Tg [16]. It has been shown that for incomplete cure, the model-free method yields

correct values of Ea provided the relative extents of cure are used instead of the

absolute values [17, 18]. The relative extent of cure, α, is determined using the total

enthalpy change for the incomplete cure reaction according to:

α = ΔHα / ΔHTotal 3.8

where ΔH is the enthalpy change up to fractional conversion α and ΔHTotal is the

total enthalpy change for the incomplete cure reaction.

50

In contrast, the absolute extent of cure, α’, is determined using the total enthalpy

change for complete cure reaction according to:

α’ = ΔHα / ΔHTotal, complete cure 3.9

where ΔHα is the enthalpy change up to fractional conversion, α, and ΔHTotal, complete

cure is the total enthalpy change for the complete cure reaction with complete

conversion of the reactants.

In practice, the relative extent of cure, α, is determined directly from DSC data

obtained for the incomplete cure system, whereas the absolute extent of cure, α’, is

obtained by carrying out an additional experiment to bring the system to a fully

cured state and determine ΔHTotal, complete cure. The relative extent of cure, α, has been

used extensively in literature to obtain correct values of Ea [see, for example, 10-

15]

In the present study, all samples were subjected to dynamic DSC scans in the range

25°C to 250°C. Further increases of the temperature up to 350°C did not give rise

to any additional exothermic peak. Re-scanning the cured samples to establish the

baselines only gave a flat line and did not result in any peak either. Therefore, the

samples must have achieved their maximum possible cross-linking reactions at the

end of the initial dynamic scans. It is noted that, as will be described in later

chapters, one of the effects of NaOH is to reduce the number of cross-links in

methylol phenol monomers with corresponding decreases in ΔHTotal following the

cure of these samples. Given that maximum cross-linking reactions have been

achieved for these samples, the reduction in the number of cross-links represents

the changes in their chemical and physical nature caused by the presence of NaOH,

rather than “incomplete” cure reactions.

The extent of cure, α, for all samples in the present study was determined by

measuring the ΔHα and the ΔHTotal obtained from the dynamic DSC scans. As such,

even if the cure of samples containing NaOH is regarded as “incomplete”, the α

values determined in this way are effectively the relative values. This would yield

correct values of Ea and enable comparisons between different samples.

51

3.2 Experimental Details

3.2.1 Materials

2-MMP (99 % purity) and 4-MMP (>98 % purity) were obtained from Aldrich

Chemical Company, Inc. and MERCK-Schuchardt, respectively. 2,4-DMP, 2,6-

DMP and 2,4,6-TMP were synthesised according to the method outlined by

Freedman [19-21]. Sodium hydroxide was obtained from BDH Chemicals Ltd.

(analytical grade).

3.2.1.1 Synthesis of 2,4-DMP

The synthesis of 2,4-DMP comprises three major steps as depicted in Figure 3.5.

Step 1 – preparation of compound II: This step involved the methylation of

compound I (4-hydroxy-isophthalic acid) to produce compound II (4-hydroxy-

isophthalic acid dimethyl ester). The reaction set up is shown in Figure 3.6. 4-

hydroxyisophthalic acid (compound I) (14.7 g, Aldrich Chemical Co) and methanol

(250 mL) were placed into a 500 mL round bottom flask having two ports. One

port was fitted with a condenser, and the other port stoppered. The flask was

warmed to dissolve most of the solid – the rest dissolved with the addition of dry

hydrogen chloride gas. The evolution of hydrogen chloride gas occurred by the

drop wise addition of sulphuric acid to a flask containing liquid hydrogen chloride.

The gas was carried to the flask via a gas tube (see Figure 2). The solution was

refluxed for 2 hours. The solution was then cooled at room temperature during

which time white needle crystals readily formed. The crystals were washed with

water until the filter liquor reached approximately pH 7, then dried over calcium

chloride in a vacuum desiccator to provide the 4-hydroxy-isophthalic dimethylester

as white crystals.

52

OHO OHO

OH O

O

CH3

OO

CH3

O

O

CH3

O

O

CH3

OH

OH OH

I II III IV

(ii) (iii)(i)

Figure 3.5: Reaction steps for the synthesis of 2,4-DMP.

4-HiPA + MeOH

empty H2SO4

HCl

H2SO4

H2O

H2O

glass gas tubes

silicon hose

heating mantel

Figure 3.6: Schematic for the synthesis of compound II.

Step 2 - preparation of compound III from compound II: This involved the

acetylation of compound II to produce 4-acetoxy-isophthalic acid dimethylester

(compound III). To a stirred solution of compound II (14.78 g) and pyridine (35

mL, AR grade) was added excess acetic anhydride (35 mL) over an ice bath. The

solution was allowed to stand at room temperature for 16 or more hours. The

mixture was poured into 500 mL ice water and stirred for an hour. The solution

was then acidified with the gradual addition of dilute 5N hydrochloric acid

monitored using pH indicator paper to pH 3-4. The white crystalline product was

separated by filtration, washed with saturated sodium carbonate solution (100 mL)

53

to remove unacetylated material, then with water and finally dried in a vacuum

desiccator to provide the crystalline 4-acetoxy-isophthalic acid dimethyl ester.

Step 3 – preparation of 2,4-DMP: This involved the reduction of compound III to

produce the target compound 2,4-Dimethylol phenol (2,4-DMP). The reaction set

up is shown in Figure 3.7.

1

2

3

45 6

7

8

9

1 102

3

45 6

7

8

911

H2O

H2O

CaCl2 drying tube

LiAlH4 + THF

4-AiPAdiMe + THF

pressure equalizing dropping funnel

heating mantel

Figure 3.7: Schematic for the synthesis of 2,4-DMP.

Lithium aluminium hydride (5 g) was weighed into a two necked round bottom

flask in a sealed box flushed with nitrogen gas. One of the necks was stoppered,

and tetrahydrofuran (100 mL) was slowly and carefully added, then fitted with a

condenser attached with a calcium chloride drying tube and the flask flushed again

with nitrogen gas. The lithium aluminium hydride was dissolved in the

tetrahydrofuran by refluxing for 16 hours or longer with stirring.

4-acetoxy-isophthalic acid dimethyl ester dissolved in tetrahydrofuran (100 mL)

was poured into a pressure equalising funnel and fitted to the spare port on the

reaction flask. The solution was added dropwise to the dissolved lithium

aluminium hydride over a period of 45 minutes at about 60°C (slightly below

reflux). The dropping funnel was exchanged for a thermometer and the refluxing

54

maintained for a further 15 minutes. The temperature was then reduced to 38 -

40°C and refluxing continued for 3 hours.

To decompose excess lithium aluminium hydride, tetrahydrofuran (10 mL)

containing ethyl acetate (3 mL) was cautiously added to the reaction flask and the

mixture refluxed for a further 15 minutes at about 38°C. Tetrahydrofuran (25 mL)

containing water (2 mL) was cautiously added through the condenser to the cold

reaction flask and the mixture allowed to stand 1-2 hours or overnight. A slight

stiochiometric excess of sodium potassium tartrate (~34 % w/v) was cautiously and

slowly added over an ice bath to complex the aluminium hydroxide. The water

volume was kept small due to solubility of the 2,4-DMP. The solution was stirred

for approximately one hour. The pH was lowered to pH 6 by titrating the lithium

hydroxide by-product with a 60 % w/v tartaric acid solution. Tetrahydrofuran was

removed by rotary evaporation, followed by exhaustive mechanical extraction of

the remaining aqueous layer with ethyl acetate (4 x 150 mL). The ethyl acetate

fraction liquor was concentrated on the rotary evaporator and the residue stored at –

14°C. Aggregates of crystals formed after a few hours and were collected by

filtration.

3.2.1.2 Synthesis of 2,6-DMP

The synthesis of 2,6-DMP comprised four reaction steps as shown in Figure 3.8.

Step 1 – preparation of compound II (2-hydroxy-isophthalic acid): Compound I (2-

methoxy-isophthalic acid) was obtained from Aldrich Chemical Co. Excess

hydriodic acid (25 mL) was added to compound I (4.0 g) placed in a conical flask.

The solution, which was heated to boiling and stirred for fifteen minutes, produced

the 2-hydroxy-isophthalic acid (compound II). The solution was then cooled at

room temperature and filtered (filter glass porosity 3) while washing with a small

volume of water to remove the pale yellow of the hydriodic acid. The collected

crystals were recrystallized from boiling water, by dissolving and heating in

enough water to dissolve the crystals to a clear solution. The liquor was slowly

cooled to room temperature during which time white needles of crystals formed

55

readily. These were dried in a vacuum desiccator to provide the 2-hydroxy-

isophthalic acid (compound II) as white crystals.

OHO

OCH3

O

OH

OHO

OH

O

OH

OO

CH3

O

OH

OCH3

OH

OH

OH

OO

CH3

OCH3

O

O

O

CH3

I IVII III V

Figure 3.8: Reaction steps for the synthesis of 2,6-DMP.

Step 2 – preparation of compound III (2-hydroxy-isophthalic acid dimethyl ester):

Compound II (2.94 g) weighed into a round bottom flask and dissolved in methanol

(150 mL) was refluxed for two hours with a steady gas flow from the HCl gas

admitted just prior to reflux. Reducing the volume of solvent by vacuum

evaporation facilitated the formation of crystals. The crystals were directly

recrystallized by warming the flask and allowing the liquor to cool under

refrigeration overnight, during which time large white needles of crystals of 2-

hydroxy-isophthalic acid dimethyl ester (compound III) formed readily. These were

collected by filtration and dried under vacuum.

Step 3 – preparation of compound IV (2-acetoxy-isophthalic acid dimethyl ester):

Compound III (2.284 g) and pyridine (17.5 mL) were mixed to which an excess of

acetic anhydride (12mL) was added over an ice bath, and was then allowed to stand

at room temperature over night. The solution, after being poured into ice water

(~500 mL), formed a white precipitate and was then stirred for 90 minutes. The

solution was acidified to pH 3 with hydrochloric acid (5N) and filtered over a glass

filter (filter glass porosity 4). The crystal cake was washed with saturated solution

of sodium carbonate (~250 mL) to remove unacetylated material, then with water

(2 x 300mL) and dried in a vacuum desiccator.

56

Step 4 – preparation of compound V (2,6-DMP): Compound IV (1.876 g) was

dissolved in dry tetrahydrofuran (75 mL) and added to a pressure equalising funnel.

Lithium aluminium hydride (5.67 g) was refluxed in tetrahydrofuran (75 mL) for

two hours with constant stirring. The solution was allowed to cool slightly (below

reflux), and then the tetrahydrofuran solution containing compound IV was

admitted over approximately forty minutes and the reflux was continued for three

hours. To decompose excess lithium aluminium hydride, tetrahydrofuran (10 mL)

containing ethyl acetate (3 mL) was added, then continued to reflux for a further

fifteen minutes following which the mixture was cooled to room temperature.

Tetrahydrofuran (20 mL) containing water (0.5mL) was added through the

condenser, then potassium sodium tartrate (42.2 g dissolved in 85 mL water) was

added to complex the aluminium ions, being careful to keep water volume small.

Tetrahydrofuran was removed by vacuum evaporation. The solution was titrated to

pH6 with sulphuric acid (24.5 mL) added to water (50 mL) to neutralise the alkali

present. Ethyl acetate (150 mL) was added to the aqueous mixture in a separating

funnel, the ethyl acetate layer removed and its volume reduced by vacuum

evaporation until a final volume of 20 mL was attained. Similarly, the aqueous

portion was extracted a further three times with fresh ethyl acetate (150 mL x 3).

The four flasks containing the final volume of approximately 20 mL were stored at

4oC overnight to form crystals. The crystals of 2,6-dimethylol phenol were

collected by filtration and dried in a vacuum desiccator.

3.2.1.3 Synthesis of TMP

The synthesis of TMP was carried out in a two-step process: (i) the synthesis of

lithium-trimethylol phenol; (ii) the synthesis of trimethylol phenol. The synthesis

step was only carried out to the end of the first stage until the compound was ready

to be used due to the instability of the neutral TMP.

Step 1 – preparation of lithium-trimethylol phenol: Phenol (94 g) and lithium

hydroxide monohydrate (42 g) were mixed in water (100 mL). Formaldehyde (39g

as a 36 % solution) was added. The exotherm which resulted raised the temperature

of the solution assisting it to dissolve the reagents. However, care had to be taken

not to allow the temperature to rise above 50°C. After the exotherm ceased, the

57

mixture was allowed to stand at room temperature overnight. The mixture was

poured into isopropanol (800 mL), stirred for thirty minutes during which time fine

white granular precipitate formed. This precipitate was collected on a scintered

glass filter (filter glass porosity No. 2), washed with acetone (400 mL) and dried in

a vacuum desiccator.

Step 2 – preparation of trimethylol phenol:

Preparation of the ion-exchange column: A glass column (2.5 cm ID, length 22

cm, filter porosity No.2 at the base of the column) was filled with Amberlite resin

IR-120 (H) (AR, Merck, 50 g) to attain approximately a depth of fifteen cm. The

resin was washed with water. This was followed by rinsing the column with

sodium hydroxide (1 N, 120 mL) to exhaust the resin. The column was regenerated

with hydrochloric acid (0.5 N, 120 mL) and then washed with water until neutral.

Preparation of 2,4,6-TMP: Li-TMP (5 g) was dissolved in water (200 mL) and

carefully and slowly applied to the prepared ion-exchange column making sure the

column did not run dry. The major portion of the sample was collected in the first

100 mL of eluent. After the 200 mL solution passed through, the column continued

to be washed with water until ferric chloride (60 % w/w solution) monitoring of

the eluent for the presence of phenolic OH, only showed a faint blue colour. The

collected eluents were freeze-dried.

3.2.2 Characterisation of 2,4-DMP, 2,6-DMP and TMP

1H- and 13C-NMR spectra were recorded with a Bruker AM-100 spectrometer

equipped with an Aspect 3000 computer operating at frequencies of 300 MHz and

100 MHz respectively. The monomer compounds, 2,4-DMP, 2,6-DMP and 2,4,6-

TMP, were dissolved in deuterated methanol-d4 (MEOD). All chemical shift data

are reported as δ values, in parts per million (ppm), downfield from

tetramethylsilane (TMS) as internal standard (δ 0.0).

Figures 3.9-3.11 depicts the 1H-NMR spectra and Figures 3.12-3.14 the 13C-NMR

spectra for the monomers. Table 3.2 summarises the 1H-NMR results and reports

the chemical shifts (ppm), coupling pattern (s: singlet, d: doublet, t: triplet), and

58

coupling constants (J-values given in Hz). Table 3.3 provides the 13C-NMR

chemical shifts. NMR spectral data of these monomers were in agreement with data

reported in literature [22, 23].

OH

HH

H

H

H5

4

1

2

3

CH2OH

Table 3.2: 1H-NMR chemical shifts.

Compound

2,4-DMP

ppm (multiplicity,

J-values)

2,6-DMP

ppm (multiplicity,

J-values)

TMP

ppm (multiplicity,

J-values)

p- CH2 - OH 4.64 (s) - 4.49 (d, 5.58)

o- CH2 - OH 4.48 (s) 4.71 (s) 4.63 (d, 4.90)

Ar - H1 - - -

Ar - H2 7.25 (s) 7.13 (d, 7.50) 7.03 (s)

Ar - H3 - 6.80 (d, 7.50) -

Ar - H4 7.09 (d, 2.11) 7.13 (d, 7.50) 7.03 (s)

Ar - H5 6.74 (d, 8.03) - -

59

OH

54

12

3CH2OH

6

Table 3.3: 13C-NMR chemical shifts.

Compound 2,4-DMP

ppm

2,6-DMP

ppm

TMP

ppm

o- CH2 - OH 61.05 62.09 64.17

o- CH2 - OH - 62.09 64.07

p- CH2 - OH 65.09 - 66.46

C1 155.50 155.03 161.03

C2 127.98 127.43 129.80

C3 128.65 128.08 128.64

C4 133.20 120.32 132.48

C5 128.65 128.08 128.64

C6 115.79 127.43 129.80

60

Figure 3.9: 1H-NMR spectra of 2,4-DMP

Figure 3.10: 1H-NMR spectra of 2,6-DMP

61

7.03

49

4.67

06

4.43

84

(ppm)4.04.24.44.64.85.05.25.45.65.86.06.26.46.66.87.07.27.4

Figure 3.11: 1H-NMR spectra of 2,4,6-TMP

Figure 3.12: 13C-NMR spectra of 2,4-DMP

62

Figure 3.13: 13C-NMR spectra of 2,6-DMP

161.

0300

130.

6802

130.

0398

129.

5595

128.

6447

66.4

586

64.1

715

(ppm)60708090100110120130140150160

Figure 3.14: 13C-NMR spectra of 2,4,6-TMP

63

3.2.3 DSC runs

Aqueous solutions (0.040M) of 2,MMP, 4-MMP, 2,4-DMP, 2,6-DMP and TMP,

together with solutions of (0.006, 0.012, 0.018, 0.024, 0.030, 0.040) M aqueous

sodium hydroxide were prepared and mixed at room temperature to form molar

ratios of NaOH : monomer ranging from 0.15 to 1.0. These mixtures were freeze-

dried and stored at – 5°C (268K) prior to DSC runs. The DSC runs were carried out

using a Perkin Elmer DSC (Pyris-1) under a constant purge of nitrogen gas (20

cc.min-1). Temperature calibration was performed by determining the enthalpy

change of pure indium metal (99.9 % purity). The samples were weighed directly

into the stainless steel pans specifically designed for DSC use and sealed (the mass

of the samples ranged from 4 to 9 mg). These sealed pans can withstand pressures

up to 24 atm and hence, reactive volatiles were able to be contained.

The sealed samples were heated in the DSC using an empty sample pan as

reference. Thermograms were recorded at scan speeds of (5, 7, 10, 15, 20) °C min-1

in the range 25°C (298K) to 250°C (523K). The scan was started at a relatively low

temperature compared with the curing reaction temperature so that the pans

containing the samples as well as the reference one are in equilibrium well before

the start of the reaction. Thus the total enthalpy of reaction is more accurately

acquired. Baselines were established by re-scanning the cured sample and

subtracting from the sample scan using relevant Pyris computer software. DSC

runs were made in triplicate and the maximum error in the data was ± 0.5 %.

It is noted that the purpose of the freeze-drying of the samples was to remove the

solvents, particularly water. Since the monomers are polar, there could have been

some residual water in the freeze-dried samples. Nevertheless, with the use of the

sealed pans, the results are free from potential interference due to residual water, if

any, as evidenced in the absence of the water peak in all DSC thermograms.

For optimum peak sharpness and resolution, the contact surface between pan and

sample should be maximized. This was achieved by the use of compact fine

granules in good contact with the capsule bottom.

64

3.3 References

1. G.W. Ehrenstein, G. Riedel and P. Trawiel, Thermal Analysis of Plastics:

Theory and Practice, Hanser, Munich, 2004.

2. G.W.H. Hohne, W.F. Hemminger, H.-J. Flammersheim, Differential

Scanning Calorimetry, Springer, Berlin, 2003.

3. E.L. Charsley and S.B. Warrington (eds), Thermal Analysis: Techniques

and Applications, Royal Society of Chemistry, Cambridge, 1992.

4. S. Vyazovskin and C. Wight, “Model-free and Model-fitting Approaches to

Kinetic Analysis of Isothermal and Nonisothermal Data”, Thermochimica

Acta 340-341, 53-68 (1999).

5. S. Vyazovkin, “Thermal Analysis”, Anal. Chem. 76, 3299-3312 (2004).

6. S. Vyazovkin, “On the Phenomenon of Variable Activation Energy for

Condensed Phase reactions”, New J. Chem. 24, 913-917 (2000).

7. S. Vyazovkin, “Kinetic Concepts of Thermally Stimulated Reactions on

Solids: A View From a Historical Perspective”, Int. Rev. Phys. Chem. 19,

45-60 (2000).

8. Standard Test Method for Arrhenius Kinetic Constants for Thermally

Unstable Materials, ANSI/ASTM E698 – 79, ASTM, Philadelphia, 1979.

9. H.E. Kissinger, “Reaction Kinetics in Differential Thermal Analysis”,

Analytical Chemistry 29, 1702 (1957).






Appl. Polym. Sci. 87, 433-440 (2003).

12. N. Sbirrazzuoli, S. Vyazovkin, A. Mititelu, C.l Sladic and L. Vincent, “A

Study of Epoxy-Amine Cure Kinetics by Combining Isoconversional

65

Analysis with Temperature Modulated DSC and Dynamic Rheometry”,

Macromol. Chem. Phys. 204, 1815–1821 (2003).

13. S. Vyazovkin and N. Sbirrazzuoli, “Isoconversional Kinetic Analysis of

Thermally Stimulated Processes in Polymers”, Macromol. Rapid Commun.

27, 1515-1532 (2006).

14. A. L. Daniel-da-Silva, J. C. M. Bordado and J. M. Martin-Martinez, “Use of

Isoconversional Methods to Analyze the Cure Kinetics of Isocyanate-Ended

Quasi-Prepolymers with Water”, J. Appl. Polym. Sci. 104, 1049–1057

(2007).

15. F. X. Perrin, T. M. H. Nguyen and J. L. Vernet, “Kinetic Analysis of

Isothermal and Nonisothermal Epoxy-Amine Cures by Model-Free

Isoconversional Methods”, Macromol. Chem. Phys. 208, 718–729 (2007).

16. J. M. Salla and X. Ramis, “Comparative Analysis of the Cure Kinetics of an

Unsaturated Polymer Resin Using Different Procedures”, Polym. Eng. Sci.

36, 835-851 (1996).

17. S. Vyazovkin and N. Sbirrazzuoli, “Kinetic Analysis of Isothermal Cures

Performed Below the Limiting Glass Transition Temperature”, Macromol.

Rapid Commun. 21, 85–90 (2000).

18. N. Sbirrazzuoli and S. Vyazovkin, “Learning About Epoxy Cure

Mechanisms From isoconversional Analysis of DSC Data”, Thermochimica

Acta 388, 289-298 (2002)

19. J. H. Freeman, “Synthesis of the Polymethylols of Phenols”, Journal of the

American Chemical Society 74 (24), 6257 (1952).

20. G. R. Sprengling and J. H. Freeman, “The Reaction of Phenol with

Formaldehyde”, Journal of the American Chemical Society 72 (5), 1984

(1950).

21. J. H. Freeman, “Kinetics of the Formation of Hydroxydiphenylmethanes

From Tri-methylolphenol In Alkali”, American Chemical Society, Division

of Organic Coatings and Plastics Chemistry, 27(1), 84 (1967).

66

22. M. F. Grenier-Loustalot, S. Larroque and P. Grenier, “Phenolic Resins:1.

Mechanism and Kinetics of Phenol and of The First Polycondensates

Towards Formaldehyde in Solution”, Polymer 35 (14), 3046 (1994).

23. Y. Yazaki, P. J. Collins, M. J. Reilly, S. D. Terrill and T. Nikpour, “Fast-

Curing Phenol-Formaldehyde (PF) Resins: Part1. Molecular Weight

Distribution of PF Resins”, Holzforschung 48, 41 (1994).

67

Chapter 4

Cure Properties of Mono-Methylol Phenols

4.1 Introduction

This chapter discusses the effects of NaOH on the cure properties of 2-MMP and 4-

MMP. Since the DSC runs were carried out at different scan rates, a general

discussion on the effects of scan rate on DSC thermograms will be given with

particular emphasis on the peak exotherm temperature Tp, the fractional conversion

αp at Tp and the total enthalpy of reaction ΔHT.

The main body of the chapter begins with a discussion on the effects of various

levels of NaOH on the DSC curves of the monomers and other parameters

including Tp, αp and ΔHT. The discussion then focuses on the effects of NaOH

content on the evolution of apparent activation energy Ea during the cure. The

discussion relies in part on the established knowledge of self-condensation

reactions of 2-MMP and 4-MMP, and forms the basis for the proposals of possible

mechanisms which operate during the cure.

4.2 Effects of Scan Rate on DSC Thermograms

DSC thermograms at varying scan rates for 2-MMP and 4-MMP, with or without

the presence of NaOH, display certain common features. An example is shown in

Figures 4.1 and 4.2 that show the DSC thermograms at scan rates of (5, 10, 15 and

20) °C min-1 for 2-MMP and 4-MMP, respectively, in the absence of NaOH.

4.2.1 Peak temperature Tp

It can be seen in Figure 4.1 that the peak exotherm temperature Tp shifted to higher

values along the temperature axis with increasing heating rate. Although not

presented here, similar shifts to higher temperature as the heating rate was

increased were also observed for 2-MMP and 4-MMP compounds for the entire

68

range of NaOH : MMP molar ratios used in this study. Such effect of scan rate on

Tp is a well established phenomenon, essentially due to the fact that individual

reactions have not had time to reach completion before the rapidly rising

temperature reaches the initiation temperature of adjacent higher temperature

reactions [1,2].

By way of examples, Tables 4.1 and 4.2 list the changes in Tp values with changes

in scan rate for 2-MMP and 4-MMP, respectively, with NaOH : MMP molar ratios

of 0 and 0.45. For the sake of brevity, Tp values for other molar ratios are not

presented. In the case of NaOH : 2-MMP molar ratio of 0, Tp was 148°C at scan

rate of 5 °C min-1 and shifted significantly to higher temperatures with increasing

scan rate and reached a value of 167°C at 20 °C min-1 scan rate. Similarly, for

NaOH : 4-MMP ratio of 0, Tp was 130°C when the scan rate was held at 5 °C min-1

and steadily increased to 147°C at 20 °C min-1 heating rate. Similar changes of Tp

with the increase in scan rate can also be seen in Tables 4.1 and 4.2 for 2-MMP and

4 MMP in the presence of NaOH.

2-MMP

Temperature (οC)

100 120 140 160 180 200 220 240 260 280 300

Hea

t Flo

w E

ndo

Up

(mW

)

0

5

10

15

20

25

30

35

40

45

5 oCmin-1

10 oCmin-1

15 oCmin-1

20 oCmin-1

Figure 4.1: Dynamic traces for 2-MMP at varying scan rates in the absence of NaOH.

69

4-MMP

Temperature (οC)

120 140 160 180 200 220 240 260 280 300

Hea

t Flo

w E

ndo

Up

(mW

)

5

10

15

20

25

30

35

40

45

50

5 oCmin-1

10 oCmin-1

15 oCmin-1

20 oCmin-1

Figure 4.2: Dynamic traces for 4-MMP at varying scan rates in the absence of NaOH.

4.2.2 Fractional conversion αp at Tp

As discussed in 3.1.2, the fractional conversion αp up to the peak temperature of the

exotherm was calculated according to the equation:

T

pp H

HΔ

Δ=α

where ΔHp is the area up to the peak temperature Tp, and ΔHT is the to total peak

area swept out by the exotherm.

Prime [3] has observed that, for thermoset cure, the extent of reaction at the peak

exotherm αp does not change with scan rate. This has also been observed in this

study, as evidenced by the approximately constant values of the extent of the cure

reactions αp at the peak temperature for all the samples studied. Some examples are

given in Tables 4.1 and 4.2 for 2-MMP and 4-MMP.

70

Table 4.1: Effects of scan rate on: the peak temperature of cure (Tp); total enthalpy of reactions (ΔHT); and the extent of the cure reactions at the peak of the exotherm (αp) for NaOH : 2-MMP molar ratios of 0.0 and 0.45. Maximum error from the triplicate DSC runs was 0.5 %.

NaOH : 2-MMP ratio = 0.00 NaOH : 2-MMP = 0.45 Heating rate

(°C min-1) Tp (°C) ΔHT (J g-1) αp Tp (°C) ΔHT (J g-1) αp

5

7

10

15

20

148.30

153.10

157.57

163.21

167.30

547.78

542.50

535.97

530.47

533.58

0.678

0.690

0.679

0.665

0.664

150.89

153.69

156.82

161.85

165.78

468.60

469.18

465.06

469.98

463.04

0.521

0.507

0.515

0.500

0.488

Table 4.2: Effects of scan rate on: the peak temperature of cure (Tp); total enthalpy of reactions (ΔHT); and the extent of the cure reactions at the peak of the exotherm (αp) for NaOH : 4-MMP molar ratios of 0.0 and 0.45. Maximum error from the triplicate DSC runs was 0.5 %.

NaOH : 4-MMP = 0.00 NaOH : 4-MMP = 0.45 Heating rate

(°C min-1) Tp °C ΔHT (J g-1) αp Tp (°C) ΔHT (J g-1) αp

5

7

10

15

20

198.36

204.51

212.80

220.65

227.88

389.55

386.86

387.31

385.66

395.76

0.618

0.616

0.594

0.582

0.610

129.51

133.83

137.89

143.89

147.51

438.14

443.18

436.03

446.73

436.37

0.512

0.488

0.491

0.497

0.496

4.2.3 Enthalpy of reactions ΔHT

The total enthalpy change ΔHT during the cure reactions is directly proportional to

the total area swept out by the exotherm. It can be seen from Figure 4.1 that the

peak height, hence the peak area, of the exotherm increases with increasing heating

rate. Therefore, comparison between the peak areas will be possible only when all

the exotherms are converted to the same scanning rate or to a time domain.

Consequently, the total enthalpy of reactions, ΔHT, was computed by integration of

71

the enthalpy change, dH / dt, with respect to time. This calculation was performed

using the DSC Pyris software.

In this study, no significant change in ΔHT for all 2-MMP and 4-MMP samples was

observed as the scan rate was varied. Examples are given in Tables 4.1 and 4.2

where values of ΔHT are similar for two different NaOH : MMP molar ratios listed

in Tables 4.1 and 4.2 at the five scan rates.

4.3 Effects of NaOH on DSC Thermograms

There were significant differences in DSC thermograms of the compounds as

NaOH concentration was varied. Thermograms obtained at 10°C min-1 scan rate for

2-MMP and 4-MMP in the presence of varying amounts of NaOH are shown in

Figures 4.3 and 4.4, respectively.

4.3.1 Peak temperature Tp

Generally, there was a significant effect of NaOH on 2-MMP and 4-MMP in terms

of the peak cure temperature Tp as can be seen in Figure 4.3. In the uncatalyzed

condition, both compounds had one characteristic peak exotherm at approximately

211°C. As the NaOH : MMP molar ratio was increased to 0.15, the position of the

peaks shifted significantly to lower temperatures, approximately to 162°C for 2-

MMP and to 142°C for the 4-MMP and became steady at these values for the entire

higher range of NaOH concentrations.

The decrease of the peak temperature in the presence of NaOH may be a

consequence of the diffusivity of the peak, and does not necessarily imply that the

cure of 2-MMP and 4-MMP is faster. This interpretation is supported by higher

activation energies for both 2-MMP and 4-MMP in the presence of NaOH as will

be shown in 4.4. It is relevant to note that in a study of the effect of lignin on the

cure properties of phenolic resins using DSC, Barry et al. [3] reported that whilst

the DSC peak temperature decreased in the presence of lignin, the activation

energies of lignin-containing resins were higher than those of pure resins.

72

2-MMP

Temperature (οC)

60 80 100 120 140 160 180 200 220 240 260 280

Hea

t Flo

w E

ndo

Up

(mW

)

0

10

20

30

40

50

60

70

80

90

100

110

120

No NaOH0.15 NaOH0.30 NaOH0.45 NaOH0.60 NaOH1.00 NaOH

Figure 4.3: Dynamic traces of 2-MMP in the presence of varying NaOH : 2-MMP molar ratios at 10 °C min-1 scanning rate.

73

4-MMP

Temperature (οC)

20 40 60 80 100 120 140 160 180 200 220 240 260 280 300

Hea

t Flo

w E

ndo

Up

(mW

)

5

10

15

20

25

30

35

40

45

50

55

60


Figure 4.4: Dynamic traces of 4-MMP in the presence of varying NaOH : 4-MMP molar ratios at 10 °C min-1 scanning rate.

4.3.2 Fractional conversion αp at Tp

Figure 4.5 shows the fractional conversion αp as a function of NaOH : MMP molar

ratio. It can be seen that αp decreased from 0.68 in the uncatalyzed state to 0.60 at

molar ratio of 0.15 and remained steady around 0.57 with further increase of

NaOH. In the case of 4-MMP, αp steadily decreased from 0.62 in the uncatalyzed

74

state to a value of approximately 0.50 at molar ratio of 0.30. Further increase of

NaOH concentration did not seem to have any additional effect.

NaOH : MMP molar ratio

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Frac

tiona

l con

vers

ion

( αp)

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.802-MMP4-MMP

2-MMP and 4-MMP

Figure 4.5: Fractional conversion αp as a function of NaOH : MMP molar ratio for 2-MMP and 4-MMP. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.


Figure 4.6 shows ΔHT as a function of NaOH : MMP molar ratio. It can be seen

that in the uncatalyzed condition, ΔHT for 2-MMP was 538 J g-1 and remained

steady at this value until the molar ratio reached 0.30. Thereafter, ΔHT steadily

decreased to about 425 J g-1 at molar ratios of 0.60 or higher. For 4-MMP, ΔHT

steadily increased from 390 J g-1 in the uncatalyzed state to approximately 450 J g-1

at molar ratio of 0.30. Subsequently, it steadily returned to the uncatalysed value of

390 J g-1 at molar ratio of 1.

75

It is clear that the extent of variation of ΔHT as a function of NaOH content for 4-

MMP was not as significant as for 2-MMP. A decrease in ΔHT, as in the case of 2-

MMP, may be an indication of a decrease in the extent of cross-linking. Indeed, as

will be discussed in 4.4.1, the activation energies of cross-linking reactions for 2-

MMP increased in the presence of NaOH. Therefore, it appears that the decrease in

ΔHT at molar ratios higher than 0.30 was due to lower amounts of crosslinks

formed. This explanation is consistent with the observation that although the

“cured” 2-MMP samples with high NaOH content had a solid and glassy

appearance, they disintegrated into a gel-like substance when immersed in acetone

and/or methanol. The disintegration of these samples suggests that stable

methylene linkages, which is characteristic of a cured state, were not fully formed.

NaOH : MMP molar ratio

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Ent

halp

y of

rea

ctio

n ( Δ

HT, J

g-1

)

350

400

450

500

550

6002-MMP4-MMP

2-MMP and 4-MMP

Figure 4.6: ΔHT as a function of NaOH : MMP molar ratio for 2-MMP and 4-MMP. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

76

4.4 Effects of NaOH on the Evolution of Activation Energy Ea

As described in Chapter 3, activation energy Ea for increasing extent of conversion

α was calculated according to the equation:

ln (Φ/Tα 2) = - Eα

/ RTα + ln (RA / Eα) 3.7

where Φ is the heating rate (°C min-1); Tα is the temperature to reach a given degree

of conversion, α; R is the universal gas constant (J mol-1 K-1); and A is the

frequency factor (s-1).

A graph of ln(Φ/Tα 2) versus 1/Tα should yield a straight line. The value for Eα can

be obtained from the slope of the linear graph.

For each NaOH : MMP molar ratio, thermograms were recorded at scanning rates

(5, 10, 15 and 20) oC min-1 in the range 25oC up to 250oC. Figures 4.1 and 4.2

depict that the values of the temperature at which the peak of the exotherm occurs

(Tp) increases with increasing scan rate. Similarly, the temperature (Tα) at which a

particular conversion (α) is reached shifts to higher temperature as the heating rate

is increased. The DSC Pyris version 3.52 software was used to obtain the values of

Tα for increasing values of α for each scan rate. Taking NaOH : 2-MMP molar ratio

0.45 as an example, Table 4.3 depicts the values of Tα at the four scan rates. From

these values the linear regression analysis function of SigmaPlot version 7.1 (from

SPSS Inc.) was used to generate linear graphs of ln(Φ/Tα 2) vs. 1/Tα at a set

confidence level of 95 %. Figure 4.7 shows the linear regression graphs between α

= 0.05 and α = 0.95 and the corresponding square of the correlation coefficient (r2)

values. The equations of the linear graphs were then generated by the SigmaPlot

software, from which the values of Eα at different α could be manually calculated.

Maximum error in Ea values obtained from the triplicate DSC runs was 0.5 %.

77

Table 4.3: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2-MMP molar ratio 0.45) and the corresponding values for the dependent and independent variables for equation 3.7.

Conversion α

Scan rate Φ (°C min-1)

Tα (°C) Tα (K) (1/Tα) x 10-3 (K) Ln(Φ/ Tα2)

0.05 5 118.669 391.869 2.551873203 13.55129311

10 126.56 399.76 2.501500901 14.28431383 15 133.038 406.238 2.461611174 14.72192859 20 137.173 410.373 2.43680749 15.02986528

0.10 5 125.511 398.711 2.508082295 13.5859116 10 133.639 406.839 2.457974776 14.31942015 15 139.855 413.055 2.420985099 14.75521171 20 144.149 417.349 2.396076186 15.06357788

0.20 5 133.837 407.037 2.456779113 13.62724609 10 142.161 415.361 2.407544281 14.36088114 15 148.37 421.57 2.3720853 14.79602188 20 152.824 426.024 2.347285599 15.10472364

0.30 5 139.64 412.84 2.422245906 13.65555813 10 148.081 421.281 2.373712558 14.38918523 15 154.103 427.303 2.340259722 14.82303693 20 158.344 431.544 2.317260812 15.13047122

0.40 5 144.348 417.548 2.394934235 13.67823693 10 152.588 425.788 2.348586621 14.41046823 15 158.328 431.528 2.317346731 14.842715 20 162.385 435.585 2.295763169 15.14911218

0.45 5 146.977 420.177 0.002379949 13.69079001 10 154.428 427.628 0.002338481 14.41909241 15 161.12 434.32 0.00230245 14.85561338 20 164.186 437.386 0.00228631 15.15736447

0.50 5 148.278 421.478 2.372603078 13.69697308 10 156.21 429.41 2.328776694 14.42740944 15 161.871 435.071 2.298475421 14.85906867 20 165.6 438.8 2.278942571 15.16381973

0.60 5 151.628 424.828 2.353893811 13.71280667 10 159.641 432.841 2.310317183 14.443326 15 165.464 438.664 2.279649116 14.87551769 20 169.663 442.863 2.258034652 15.18225321

0.70 5 155.19 428.39 2.334321529 13.7295059 10 163.411 436.611 2.290368314 14.46067037 15 169.303 442.503 2.259871684 14.89294469 20 173.611 446.811 2.238082769 15.20000365

0.80 5 159.415 432.615 2.311524103 13.74913429 10 167.891 441.091 2.267105881 14.4810875

78

15 173.855 447.055 2.236861236 14.91341346 20 178.266 451.466 2.215006224 15.2207324

0.90 5 165.233 438.433 2.28085021 13.77585193 10 174.012 447.212 2.236075955 14.5086506 15 179.992 453.192 2.206570284 14.94068195 20 184.584 457.784 2.184436328 15.24852719

0.95 5 169.753 442.953 2.25757586 13.79636525 10 178.739 451.939 2.212687996 14.52967952 15 184.699 457.899 2.183887713 14.96134747 20 189.46 462.66 2.16141443 15.26971716

2-MMP

1/Tα x 10-3 (K)

2.10 2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 2.75

ln( φ

/ T

α2 )

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2

15.4α = 0.05; r2 = 0.996α = 0.10; r2 = 0.997α = 0.20; r2 = 0.997α = 0.30; r2 = 0.998α = 0.40; r2 = 0.999α = 0.50; r2 = 0.998α = 0.60; r2 = 0.998α = 0.70; r2 = 0.999α = 0.80; r2 = 0.998α = 0.90; r2 = 0.999α = 0.95; r2 = 0.999

Figure 4.7: Graph of ln(Φ/Tα

2) vs. 1/Tα between α = 0.05 and α = 0.95 and the corresponding square of the correlation coefficient (r2) values for 2-MMP sample with NaOH : 2-MMP molar ratio of 0.45.

4.4.1 2-MMP

Figure 4.8 shows the effects of NaOH content on the evolution of apparent

activation energy Ea for 2-MMP as a function of the degree of conversion. It can be

seen that for most samples, there was a decrease in Ea in the initial cure stages,

79

especially when fractional conversion was about 0.10 or less. Such behaviour of Ea

may be explained by the diffusion effect on the cure kinetics at low conversions,

leading to a decrease in Ea, as proposed by Vyazovkin and Sbirrazzuoli [5]. This

phenomenon will not be investigated in the present work. Instead, the following

discussion focuses on the evolution of Ea when conversion was higher than 0.10.

2-MMP

Conversion (α)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

Ene

rgy

(Ea,

kJ

mol

-1)

80

90

100

110

120

130

140


Figure 4.8: Effects of NaOH on the evolution of apparent activation energy Ea for 2-MMP as a function of the degree of conversion. Error bars are not included in the graph because the maximum error in Ea and α values from the triplicate DSC runs was only 0.5 %.

As can be seen in Figure 4.8, for the uncatalyzed sample, there was a relatively

rapid rise of Ea from 89 kJ mol-1 to about 102 kJ mol-1 as conversion reached 0.25.

As the cure proceeded, Ea increased less rapidly and reached a value of about 108

kJ mol-1 at the end of the cure. Figure 4.8 also shows that whilst the values of Ea for

samples with NaOH were higher than the uncatalyzed sample, there were certain

differences amongst these samples. In particular, for the sample with molar ratio of

0.15, Ea increased steadily from (110 to 119) kJ mol-1 during the course of the cure

process. Further increase of NaOH content also resulted in an ascending

80

dependence of Ea on conversion until the later phase of the cure process where Ea

seemed to remain steady at about 130 kJ mol-1. There was no clear trend with

respect to the fractional conversion at which the steady Ea was reached amongst

these high NaOH content samples.

It has been shown that the shape of the dependence of Ea on conversion degree is

determined by the ratio of the partial contribution of individual reactions to the

overall reaction process [6]. Previous studies [7-8] have shown that the self-

condensation reaction of 2-MMP essentially involves the reaction of a methylol

group with a free para hydrogen on the ring of the coupling 2-MMP to form an

(o,p) methylene linkage. Yeddanapalli and Francis [9] have shown that a lesser

amount of (o,o) methyene linkage is also formed with a higher Ea (89.6 kJ mol-1)

than the (o,p) linkage (78.3 kJ mol-1). Whilst there are discrepancies in the absolute

Ea values compared to other studies [see, for instance, 10-11], the higher Ea for the

(o,o) linkage reaction is consistent with the low reactivity of ortho position of 2-

MMP. A schematic representation of the relevant condensation reactions is

presented in Figure 4.9.

OH

CH2OH

OH

CH2OH

+

OH

CH2

CH2OHOH

+ H2O

OH

CH2

CH2OH

+ H2OOH

(o,p methylene linkage)

(o,o methylene linkage)

Figure 4.9: Condensation reactions of 2-MMP.

In the present case, the rise in Ea as a function of conversion may indicate a kinetic

process involving contributions of both (o,o) and (o,p) linkage reactions. It may be

that at low conversions, the partial contribution of the (o,o) linkage reaction with

higher Ea was low compared to that of the (o,p) linkage reaction. As the cure

81

proceeded, the contribution of the (o,o) linkage reaction increased and that of the

(o,p) linkage reaction decreased. This would result in an increasing dependence of

Ea on conversion, which has been shown to be characteristic of processes consisting

of parallel competing reactions [6,12].

It has been proposed that 2-MMP would be transformed to a sodium ring complex

following the addition of NaOH according to the following scheme [13]:

Na+

O OO

HNa

+

CH2OH CH2

:

Figure 4.10: The sodium ring complex.

Following the formation of the sodium ring complex, the methylol group is blocked

and its capacity to form methylene linkage is diminished. As well, the inclusion of

Na+ in the complex reduces the carbanion negative charge, which is the main

driving force for the condensation reactions. Thus, as the NaOH : 2-MMP molar

ratio increases, a higher proportion of methylol group are blocked and a higher

proportion of carbanion ions have less negative charge. As a consequence, higher

activation energy is required for the condensation reactions. This is reflected in the

results obtained for all samples with different levels of NaOH as shown in Figure

4.8. For the sample with molar ratio of 0.15, the increasing dependence of Ea on

fractional conversion is similar to that of the uncatalyzed sample. However, for

samples with higher NaOH content, Ea became steady at around 130 kJ mol-1 at

higher conversions, suggesting that the (o,p) linkage reaction was essentially

complete and that the (o,o) linkage reaction with higher activation energy was the

main reaction during this later phase. The lower ΔHT for these samples compared

to those with low NaOH content, as discussed in 4.3.3, is possibly due to the

sodium ring complex mechanism, which has the effect of making it more difficult

to form methylene linkage, especially at high NaOH content.

82

4.4.2 4-MMP


activation energy Ea for 4-MMP as a function of fractional conversion. Similar to

the case of 2-MMP, there was a decrease in Ea in the initial cure stages for most 4-

MMP samples, especially when fractional conversion was about 0.10 or less, due to

the diffusion effect. The following discussion focuses on the evolution of Ea when

conversion was higher than 0.10.

4-MMP

Conversion (α)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

Ene

rgy

(Ea,

kJ

mol

-1)

90

100

110

120

130

140

150No NaOH0.15 NaOH0.30 NaOH0.45 NaOH0.60 NaOH1.00 NaOH

Figure 4.11: Effects of NaOH on the evolution of apparent activation energy Ea for 4-MMP as a function of the degree of conversion. Error bars are not included in the graph because the maximum error in Ea and α values from the triplicate DSC runs was only 0.5 %.

As seen in Figure 4.11, for the uncatalyzed sample, there was a gradual decrease of

Ea from about 110 kJ mol-1 in the early stage to about 98 kJ mol-1 as conversion

reached 0.95. For the sample with molar ratio of 0.15, the conversion dependence

of Ea was similar to that for the uncatalyzed sample, although the rate of decrease

seemed to be greater. Further increase of NaOH content had the effect of increasing

83

Ea values for all samples, but these values remained relatively steady throughout

the cure process, rather than showing a decreasing trend. Samples with molar ratios

between 0.30 and 0.60 had Ea ranging from about (115 to 120) kJ mol-1, whereas

the sample with molar ratio of 1.00 had higher Ea in the vicinity of 128 kJ mol-1.

Similar to the case of 2-MMP, the identification of individual reactions and their

partial contributions to the overall reaction process is important in understanding

the dependence of the apparent activation energy on the extent of conversion.

Previous studies [8-9] have shown that the self-condensation of 4-MMP essentially

involves the formation of methylene linkage according to the following schemes:

CH2OH

CH2

OH

OH

OH

CH2OH

+ H2O

CH2 OH + H2O + CH2OHO

(o,p methylene linkage)

(p,p methlyne linkage)CH2OH

OH

+

Figure 4.12: Self-condensation of 4-MMP.

Both (p,p) and (o,p) linkage reactions occur at similar rates. Yeddanapalli and

Francis [9] obtained an activation energy of 72 kJ mol-1 for these reactions. The

CH2O released from the (p,p) linkage reaction also reacts with 4-MMP to form 2,4-

DMP according to the scheme shown in Figure 4.12. Eapen and Yeddanapalli [15]

reported a value of 60 kJ mol-1 for the addition reaction of CH2O to 4-MMP.

CH2OH

OH

CH2OH

+

OH

CH2OH

CH2O

Figure 4.13: Addition reaction of CH2O to 4-MMP.

84

The above discussion suggests that the cure process of 4-MMP consisted of

consecutive reactions. The process started with the self-condensation reactions with

higher activation energy, followed by the addition reaction of the product CH2O to

4-MMP with lower activation energy. The descending shape of the dependence of

Ea on conversion extent for the uncatalyzed 4-MMP suggests that the self-

condensation gradually decreased its partial contribution to the overall process as

the cure proceeded, whereas the addition reaction made a growing contribution and

became the main reaction towards the end of process. The activation energies of

the self-condensation and addition reactions can be estimated as the maximum and

minimum values of the effective activation energy (110 and 98) kJ mol-1,

respectively.

The results also suggest that the effect of NaOH on the cure property of 4-MMP

varied, depending on the amount present. At low level, NaOH did not seem to have

significant effect on Ea as well as its conversion dependence, as shown by the

results for the sample with molar ratio of 0.15. However, as the level of NaOH

increased, higher energy was required for the reactions to proceed. As discussed in

4.4.1 for the case of 2-MMP, the association between Na+ and the phenate oxygen

decreases the electron density on the phenolic aromatic ring, and therefore reduces

the carbanion negative charge, which is the main force driving the self-

condensation. A similar mechanism also operates in the case of 4-MMP. As a

consequence, as NaOH content was increased, higher proportion of carbanion ions

had less negative charge and higher activation energy was required for the self-

condensation reactions to occur. Despite this, the adverse effects of NaOH on the

self-condensation of 4-MMP should be less compared to 2-MMP, since the sodium

ring complex which blocked the ortho methylol group was not formed in the case

of 4-MMP. The comparison of the effects of NaOH on 2-MMP and 4-MMP will be

further explored in chapter 7.

It is interesting that all samples with molar ratio of 0.30 or above had “flat”

conversion dependences of Ea, suggesting that the self-condensation and addition

reactions had similar activation energies when NaOH content was high. It seems

that in these cases, the reduction of the negative charge on carbanion ions due to

the association with Na+ raised not only the energy required for the self-

85

condensation, but also the energy required for the addition reaction between CH2O

and 4-MMP.

4.5 Summary

In this chapter, the effects of scan rate on the peak exotherm temperature, Tp, the

fractional conversion, αp, at Tp and the total enthalpy of reaction, ΔHT, of 2-MMP

and 4-MMP were investigated. It has been shown that Tp shifted to higher values

with increasing heating rate, whereas αp and ΔHT did not change significantly with

scan rate. These properties are consistent with general behaviour of thermoset

materials.

The main focus of the chapter discussed the effect of various levels of NaOH on

the DSC curves of the monomers and on the evolution of apparent activation

energy, Ea, during the cure, as well as on ΔHT. Based on the shape of the

dependence of Ea on conversion degree, α, for 2-MMP, it was proposed that at low

conversions, the partial contribution of the (o,o) linkage reaction with higher Ea was

low compared to that of the (o,p) linkage reaction. As the cure proceeded, the

contribution of the (o,o) linkage reaction increased and that of the (o,p) linkage

reaction decreased. It was also suggested that the addition of NaOH resulted in the

formation of the sodium ring complex, which blocked the methylol group and

diminished its capacity to form methylene linkage. This is reflected in the

observation of higher Ea and lower ΔHT with increase in NaOH content.

The shape of the curve, Ea vs. α for 4-MMP, suggested that the cure process started

with the self-condensation reactions with higher activation energy to form (o,p) and

(p,p) methylene linkages. This was followed by the addition reaction of the product

CH2O to 4-MMP with lower activation energy. The descending shape of the

dependence of Ea on conversion extent for the uncatalyzed 4-MMP suggested that

the self-condensation gradually decreased its partial contribution as the cure

proceeded, whereas the addition reaction made a growing contribution to the

overall process and became the main reaction towards the end of the process.

86

A sodium ring complex mechanism was also suggested to operate in the case of 4-

MMP as NaOH was added with the resulting higher Ea for the condensation

reactions. Despite this, the adverse effects of NaOH on the cure of 4-MMP was less

compared to 2-MMP, since the sodium ring complex which blocked the ortho

methylol group was not formed in the former. Further comparison of the effects of

NaOH on the cure properties of 2-MMP and 4-MMP, as well as of DMP and TMP,

will be presented in chapter 7.

4.6 References

1. S.S.J. Warne and P. Bayliss, “The Differential Thermal Analysis of

Cerussite”, Amer. Mineral. 47, 1011 (1962).

2. R.C. MacKenzie and B.D. Mitchell, Differential Thermal Analysis, R.C.

Mackenzie (ed.), Acad. Press, London, 1970, Vol. 1, Chapter 4, p. 101.

3. A.O. Barry, W. Peng and B. Riedl, “The Effect of Lignin Content on the

Cure Properties of Phenol-Formaldehyde Resin as Determined by

Differential Scanning Calorimetry”, Holzforschung 47, 247-252 (1993).

4. J.S.M. Kazayawoko, B. Riedl, J. Poliquin, A.O. Barry and L.M. Matuana,

“A Lignin-Phenol-Formaldehyde Binder for Particle Board Part 1. Thermal

Characteristics, Holzforschung 46, 257-262 (1992).

5. S. Vyazovkin and N. Sbirrazzuoli, “Effect of Viscosity on The Kinetics of

Initial Cure Stages”, Macromol. Chem. Phys. 201, 199–203 (2000).

6. S.V. Vyazovkin, V.I. Goryachko and A.I. Lesnikovich, “An Approach to

the Solution of the Inverse Kinetic Problem in the Case of Complex

Processes. Part III. Parallel Independent Reactions”, Thermochimica Acta

197, 41-51 (1992).

7. J. Reese, Kunststoffe 45(4), 137-145 (1955).



Polymer 37(6), 955-964 (1996).

87



with Phenol”, Makromol. Chem. 55, 74-86 (1962).






12. S. Vyazovskin and A. Lesnicovich, “An Approach to the Solution of the

Inverse Kinetic Problem in the Case of Complex Processes. Part 1. Methods

Employing a Series of Thermoanalytical Curves”, Thermochimica Acta

165, 273-280 (1990).

13. A. Pizzi and A. Stephanou, “Phenol - Formaldehyde Wood Adhesives under

Very Alkaline Conditions - Part I: Behaviour and Proposed Mechanism,”

Holzforschung 48, 35-40 (1994).




15. K.C. Eapen and L.M. Yeddanapalli, “Kinetics and Mechanism of the

Alkali-Catalysed Addition of Formaldehyde to Phenol and Substituted

Phenols”, Die Makromolekulare Chemie 119, 4-16 (1968).

88

Chapter 5

Cure Properties of Di-Methylol Phenols

5.1 Introduction

This chapter discusses the effects of NaOH on the cure properties for 2,4-DMP and

2,6-DMP. The effects of scan rate on the peak exotherm temperature Tp, the

fractional conversion αp at Tp and the total enthalpy of reaction ΔHT for DMP are

similar to those for MMP. Therefore, for brevity, these aspects will not be

discussed here.

The chapter begins with a brief review of self-condensation reactions of DMP,

followed by a discussion on the effects of NaOH on the shape of DSC curves and

the enthalpy of reactions ΔHT. The chapter then focuses on the effects of NaOH

content on the evolution of apparent activation energy Ea during the cure. Possible

mechanisms that operate during the cure are proposed.

5.2 Self-Condensation Reactions of DMP

Previous studies [1,2] have shown that self-condensation reactions of 2,4-DMP

may occur according to the following schemes:

CH2OH

OH

CH2OH

CH2OH

OH

CH2OH

+ CH2

CH2

OH

OHCH2 CH2OH

OH + H2O + CH2O

(p, p methylene linkage)

OH

CH2OH

CH2OH

OH H2O + CH2O+

(o, p methylene linkage)

Figure 5.1: Condensation reactions of 2,4-DMP.

89

These studies have suggested that whilst the CH2O produced by the above

condensation reactions may react with the compounds present to form minor

compounds, the trisubstituted 2,4,6-TMP was not observed. The para methylol

group may also react with the ortho aromatic proton to yield a minor compound

according to:

CH2OH

OH

CH2OH

CH2OH

OH

CH2OH

+ CH2

OH

CH2OH

CH2OH

OH H2O+


OHCH2

Figure 5.2: Minor condensation reaction of 2,4-DMP.

In contrast to the high number of condensation possibilities in the case of 2,4-DMP,

these studies have shown that the condensation of 2,6-DMP proceeds mainly via

the following reaction:

OH

CH2OH

+ CH2 + H2O

OH

CH2OH

CH2OH

OH


OHCH2

OH

CH2OHOHCH2

OHCH2

Figure 5.3: Condensation reaction of 2,6-DMP.

A small quantity of higher-order oligomer with three aromatic rings has also been

observed. This has been attributed to the reaction between 2,6-DMP and the

aromatic carbon in non-substituted para position of the dimer.

The reactivity of 2,6-DMP is also higher than that of 2,4-DMP. This is because the

para position is more reactive than the ortho position due to the facts that the para

quinoid resonating structure is more stable than the ortho structure and that the

para position is not hindered like the ortho position which is close to the phenate

oxygen [3-5]:

90

OCH2OH

O

CH2OH

CH2OH

Para quinoid structure

HOCH2

Ortho quinoid structure

Figure 5.4: Para and ortho quinoid structures of 2,6-DMP and 2,4-DMP.

In acidic or neutral conditions, both 2,4-DMP and 2,6-DMP form dimethylene

ether linkages [6]. The mechanism involves the reaction between two methylol

groups and the release of one molecule of water according to the following scheme:

CH2OH

+ CH2 + H2OOHOHCH2

O CH2

Figure 5.5: Dimethylene ether linkage formation.

It is generally accepted that methylene and ether linkages are formed

simultaneously and that ether formation becomes increasingly important as the

methylol content in the phenolic ring increases [7]. Under strong alkaline

conditions, ether formation is essentially, if not completely, eliminated [8].

5.3 DSC Thermograms

Figures 5.6 and 5.7 show the DSC thermograms for 2,4-DMP and 2,6-DMP,

respectively, obtained at 10 °C min-1 scan rate in the presence of varying amounts

of NaOH. Generally, the DSC curves for 2,4-DMP and 2,6-DMP consisted of

either a single, two or three observable exothermic peaks, depending on the level of

NaOH in the sample. This is in contrast to the cases of 2-MMP and 4-MMP that

exhibited single DSC peak for all levels of NaOH.

91

2,4-DMP

Temperature (οC)

40 60 80 100 120 140 160 180 200 220 240 260 280

Hea

t Flo

w E

ndo

Up

(mW

)

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80

No NaOH0.15 NaOH0.30 NaOH0.45 NaOH0.60 NaOH0.75 NaOH1.00 NaOH

Figure 5.6: DSC thermograms for the self-condensation reactions of 2,4-DMP in the presence of varying NaOH : 2,4-DMP molar concentrations obtained at 10 °C min-1 scan rate.

92

2,6-DMP

Temperature (οC)

60 80 100 120 140 160 180 200 220 240 260 280

Hea

t Flo

w E

ndo

Up

(mW

)

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

80


Figure 5.7: DSC thermograms for the self-condensation reactions of 2,6-DMP in the presence of varying NaOH : 2,6-DMP molar concentrations obtained at 10 °C min-1 scan rate.

5.3.1 2,4-DMP and 2,6-DMP at molar ratios equal or less than 0.15

In the uncatalyzed condition, the compounds had a single exothermic peak at

211°C and 218°C for 2,4-DMP and 2,6-DMP, respectively. The broadness of the

peaks suggests a complex process and an overlapping of different reactions during

93

the cure. At the NaOH : DMP molar ratio of 0.15, the position of the peaks shifted

significantly to lower temperatures, approximately to 135°C for 2,4-DMP and to

145°C for the 2,6-DMP. Similar to the case of MMP, the shifting to lower

temperatures following the addition of NaOH may be a consequence of the

diffusivity of the peak, and does not necessarily imply that the cure of the

compounds was faster. There was also the appearance of a second peak at around

160°C for both compounds, which disappeared at higher NaOH levels. The first

peak is due to the condensation of the compounds to form methylene and ether

linkages [9,10]. This peak was broader for 2,4-DMP compared to 2,6-DMP,

probably due in part to the higher number of condensation possibilities for 2,4-

DMP. The second peak represents further reaction of the ether linkages, for

instance, to form methylene linkages and eliminate formaldehyde [9,10]. This

interpretation is supported by the disappearance of the second peak at higher NaOH

levels, reflecting the fact that the formation of ether linkages is very unlikely under

strong alkaline conditions. For the uncatalyzed samples, it is likely that similar

reactions involving the formation of ether linkages and their degradation also

occurred, but were not manifested in distinct signals due to the broadness of the

peak that encompassed a wide range of temperatures.

At molar ratios higher than 0.15, the DSC curves for both compounds changed

significantly, suggesting considerable changes in the effects of NaOH on various

condensation reactions of the compounds. Despite the similarities, there are certain

differences between the curves for 2,4-DMP and 2,6-DMP.

5.3.2 2,4-DMP at molar ratios higher than 0.15

For 2,4-DMP, the peak seemed to be broadened at molar ratios between 0.30 and

0.75 and a broad shoulder at about 153°C became evident at molar ratio of 0.75. At

the ratio of 1.00, there were two sharp peaks at about 143°C and 163°C. Similar to

the case of MMP, a sodium ring complex may form between the phenolic ring and

the ortho methylol group of 2,4-DMP. This would result in a decrease in the

capacity of the ortho methylol group to form methylene linkages, as well as a

reduction in the carbanion negative charge, which is the main driving force for the

condensation reactions [11]. For the condensation of 2,4-DMP, the effect of the

94

ring complex mechanism on the formation of (o,p) methylene linkages is likely to

be more severe than that of (p,p) methylene linkages. Further, at a particular level

of NaOH, only a proportion of ortho methylol groups are affected by the formation

of the ring complex. This would result in different reactivities of 2,4-DMP

molecules, depending on whether they are associated with Na+. Such variation in

the reactivity of 2,4-DMP, coupled with the different condensation possibilities,

may lead to the observed broadening of the DSC curves.

As the NaOH content increases, a higher proportion of ortho methylol groups are

blocked and a higher proportion of carbanion ions have less negative charge.

Where the NaOH content is sufficiently high, it would be more difficult for the

condensation reactions to occur. However, since most 2,4-DMP molecules are

affected to more or less the same extent in the presence of high levels of Na+, there

would be less variation in their reactivity and the corresponding DSC curve is

expected to be narrow. This is probably a reason for the sharp and distinct DSC

peaks as the molar ratio is 1.00. It is noted that the shoulder at about 153°C for the

sample with molar ratio of 0.75 seems to be the precursor for the second peak for

the sample with molar ratio of 1.00.

It may be that at molar ratio of 1.00, the cure process is affected not only by

chemical reactions, but also by the excessive amounts of Na+ and OH¯ ions that

may impose transport limitation on the reaction rates. This is probably a reason for

the relatively high onset temperature of the DSC curve for this sample. After the

initial formation of methylene linkages that gave rise to the first DSC peak, the

limitation on molecular mobility would become more severe, further delaying the

condensation reactions. Higher temperatures may help reduce the viscosity of the

medium and improve the mobility of reacting molecules [12]. Thus, the second

DSC peak observed for this sample may represent the regime where the

temperatures were sufficiently high for parts of unreacted 2,4-DMP molecules to

overcome the diffusion barrier and accelerate the reactions. The hypothesis of a

diffusion control mechanism and its effect on the extent of methylene linkage

formation is supported by the results for the apparent activation energy Ea and for

the enthalpy of reactions ΔHT, which will be discussed in 5.5 and 5.4, respectively.

95

There may be other explanations for the emergence of the second peak. One such

explanation is that the peak is due to the degradation of ether linkages. However,

this is very unlikely since the formation of ether linkages is insignificant, if not

eliminated, at high NaOH levels. There is also a possibility that the CH2O produced

by the condensation reactions may react at a later phase with the compounds

present, contributing to the second peak. It may be argued that the reactions of the

compounds with CH2O are favoured at strong alkaline conditions, hence the

emergence of the second peak only at high molar ratio of 1.00. However, the 2,6-

DMP sample with the same molar ratio also exhibited a similar second peak, but

there was no CH2O produced by its self-condensation reactions. Another possibility

is that the methylene linkages may undergo degradation reactions at higher

temperatures. However, this possibility seems to be inconsistent with the absence

of a corresponding degradation peak in other samples.

5.3.3 2,6-DMP at molar ratios higher than 0.15

In contrast to the case of 2,4-DMP, the DSC peak for 2,6-DMP was quite narrow at

molar ratios of 0.30 and 0.45. As discussed in 5.2, the condensation of 2,4-DMP

would proceed via both (p,p) and (o,p) positions, whereas those for 2,6-DMP

primarily via only (o,p) positions. As well, the reactivity of 2,6-DMP is known to

be higher than that of 2,4-DMP since the para position is more reactive than the

ortho position. The limited condensation possibilities and the higher reactivity of

2,6-DMP would give rise to narrower DSC curves compared to those for 2,4-DMP.

This is likely to be a reason for the narrowness of the curves at lower molar ratios

of 0.30 and 0.45, where the effect of NaOH was not significant.

At higher molar ratios, the peak became broadened and a broad shoulder at about

160°C emerged for the sample with molar ratio of 0.75. Similar to the case of 2,4-

DMP, the reactivity of 2,6-DMP molecules would be affected by the association

with Na+ that diminishes the capacity of the ortho methylol group to form

methylene linkages, as well as reducing the carbanion negative charge. Therefore,

the broadening of the DSC curves at higher molar ratios of 0.60 and 0.75 may be

explained in part by the variation in the reactivity of different 2,6-DMP molecules,

depending on whether they were associated with Na+. As well, although the

96

condensation reaction of 2,6-DMP to form dimers is limited to (o,p) methylene

linkages, these dimers can further react with 2,6-DMP at higher NaOH molar ratios

to form higher-order oligomers with three aromatic rings. The reaction rate to form

trimers for 2,6-DMP at higher molar ratios is much higher than that for 2,4-DMP

[3]. Such condensation possibilities for 2,6-DMP may also contribute to the

broadening of the peak at higher molar ratios.

The DSC curve for the sample with molar ratio of 1.00 was similar to that observed

for the corresponding 2,4-DMP sample, although the two peaks in this case were

sharper and more distinct at about 160°C and 180°C. The curve also had a

relatively higher onset temperature compared to others with lower NaOH content.

Similar to the case of 2,4-DMP, the condensation process in this case may be

complicated by diffusion limitation. As discussed previously, the limitation on

molecular diffusion could have the effect of delaying the condensation process of

2,6-DMP, and this effect would became more critical with an increasing extent of

cross-linking. Thus, after the initial condensation reactions that started at a

relatively high temperature as shown by the first DSC peak, there was a change in

mechanism from kinetic to diffusion control, which slowed down further reactions.

The second DSC peak may represent the regime where the temperatures were

sufficiently high for some of the unreacted molecules to overcome the diffusion

barrier and speed up the reactions. This hypothesis is supported by the results for

the apparent activation energy Ea and for the enthalpy of reactions ΔHT, which will

be discussed in 5.5 and 5.4, respectively.

5.4 Enthalpy of Reactions ΔHT

The effect of NaOH on ΔHT for both 2,4-DMP and 2,6-DMP are shown in Figure

5.8. It can be seen that in the uncatalyzed condition, ΔHT for 2,4-DMP was

approximately 500 J g-1. ΔHT steadily decreased with increase of NaOH level and

reduced to a value of 340 J g-1 at molar ratio of 1.00. For 2,6-DMP, the value of

ΔHT was about 530 J g-1 in the uncatalyzed condition and gradually decreased to

97

363 J g-1 at molar ratio of 0.45. ΔHT remained steady at this value until the molar

ratio reached 0.75, after which it decreased to 310 J g-1 at molar ratio of 1.00.

NaOH : DMP molar ratio

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Ent

halp

y of

rea

ctio

n ( Δ

HT, J

g-1

)

250

300

350

400

450

500

5502,4-DMP2,6-DMP

2,4-DMP and 2,6-DMP

Figure 5.8: ΔHT as a function of NaOH : DMP molar ratio for 2,4-DMP and 2,6-DMP. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

As discussed in the case of MMP, a decrease in ΔHT may be an indication of a

decrease in the extent of cross-linking. Indeed, as will be discussed in 5.5, the

activation energies of crosslinking reactions for both 2,4-DMP and 2,6-DMP

increased in the presence of NaOH. Therefore, it appears that the decrease in ΔHT

for both compounds as NaOH content was increased was due to lower amounts of

crosslinks being formed. This explanation is consistent with the hypothesis that at

high NaOH levels, the condensation process for both compounds was complicated

by diffusion control which had the effect of limiting molecular transport and

adversely affecting the extent of cross-linking. Samples with molar ratio of 1.00

appear to be most affected by the diffusion limitation with the lowest ΔHT.

98

Although higher temperatures facilitated the second phase of condensation

reactions, as suggested by the second DSC peak, it seems that this is not sufficient

to significantly improve the degree of cross-linking for these samples.

5.5 Effects of NaOH on the Evolution of Activation Energy

Ea



ln (Φ/Tα 2) = - Eα

/ RTα + ln (RA / Eα) 3.7







(5, 10, 15 and 20) oC min-1 in the range 25oC up to 250oC. The temperature (Tα) at

which a particular conversion (α) is reached shifts to higher temperatures as the

heating rate is increased. For each scan rate, the DSC Pyris version 3.52 software

was used to obtain the values of Tα for increasing values of α. Taking the sample

with NaOH : 2,4-DMP molar ratio of 0.45 as an example, Table 5.1 depicts the

values of Tα at the four scan rates. From these values the linear regression analysis

function of SigmaPlot version 7.1 (from SPSS Inc.) was used to generate linear

graphs of ln(Φ/Tα 2) vs. 1/Tα at a set confidence level of 95 %. Figure 5.9 shows the

linear regression graphs between α = 0.05 and α = 0.95 and the corresponding

square of the correlation coefficient (r2) values. The equations of the linear graphs

were then generated by the SigmaPlot software, from which the values of Eα at

different α could be manually calculated. Maximum error in Ea values obtained

from the triplicate DSC runs was 0.5 %.

99

Table 5.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2,4-DMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation 3.7.

Conversion α

Scan rate Φ (°C min-1)

Tα (°C)

Tα (K)

(1/Tα) x 10-3 (K)

Ln ((Φ/ Tα)2)

0.05 5 98.459 371.659 2.690638462 13.44539145

10 107.72 380.92 2.625223144 14.18776385 15 113.12 386.32 2.588527645 14.62138228 20 116.549 389.749 2.565753857 14.92673816

0.10 5 104.608 377.808 2.646847076 13.47821017 10 114.148 387.348 2.581657837 14.22123212 15 118.98 392.18 2.549849559 14.65149204 20 121.82 395.02 2.531517392 14.95360507

0.20 5 111.9555 385.1555 2.59635394 13.51673221 10 121.66 394.86 2.53254318 14.25964764 15 126 399.2 2.50501002 14.68697529 20 129.176 402.376 2.485237688 14.99050622

0.30 5 117.388 390.588 2.560242506 13.5447445 10 126.638 399.838 2.50101291 14.28470402 15 131.508 404.708 2.470917303 14.71438184 20 134.515 407.715 2.452693671 15.01686908

0.40 5 121.743 394.943 2.532010948 13.56692081 10 130.748 403.948 2.475566162 14.30515741 15 135.729 408.929 2.445412284 14.73513329 20 138.842 412.042 2.42693706 15.03798285

0.50 5 125.055 398.255 2.510954037 13.58362292 10 134.302 407.502 2.453975686 14.32267677 15 139.327 412.527 2.424083757 14.75265352 20 141.862 415.062 2.409278614 15.05258809

0.60 5 128.384 401.584 2.490139049 13.60027137 10 137.695 410.895 2.433711776 14.33926051 15 142.731 415.931 2.404244935 14.76908896 20 145.27 418.47 2.389657562 15.06894268

0.70 5 131.622 404.822 2.47022148 13.61633284 10 141.019 414.219 2.41418187 14.35537473 15 146.115 419.315 2.384841945 14.78529506 20 148.871 422.071 2.369269625 15.08607937

0.80 5 135.602 408.802 2.446171985 13.63589977 10 145.138 418.338 2.390411581 14.37516453

100

15 150.485 423.685 2.360244049 14.80603071 20 153.279 426.479 2.344781337 15.10685853

0.90 5 139.722 412.922 2.421764885 13.65595534 10 150.018 423.218 2.362848461 14.39835992 15 155.006 428.206 2.335324587 14.82725898 20 157.975 431.175 2.319243926 15.12876035

0.95 5 143.609 416.809 2.39918044 13.67469408 10 154.219 427.419 2.339624584 14.41811469 15 159.094 432.294 2.313240526 14.84626203 20 162.299 435.499 2.296216524 15.14871727

2,4-DMP

1/Tα x 10-3 (K)

2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85

ln ( φ

/ T

α2 )

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2


Figure 5.9: Graph of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α = 0.95 and the

corresponding square of the correlation coefficient (r2) values for 2-MMP sample with NaOH : 2,4-DMP molar ratio of 0.45.

5.5.1 2,4-DMP


activation energy Ea for 2,4-DMP as a function of the degree of conversion. Similar

101

to the case of MMP, there was a decrease in Ea in the initial cure stages for many

2,4-DMP samples, especially when fractional conversion was about 0.1 or less, due

to the diffusion effect. The following discussion focuses on the evolution of Ea

when conversion was higher than 0.10.

As can be seen in Figure 5.10, for the uncatalyzed sample, there was a steady rise

of Ea from about 90 kJ mol-1 as conversion was 0.15 to about 110 kJ mol-1 at the

end of the cure. The introduction of NaOH had the effect of increasing the Ea for all

samples, although there were clear differences amongst these samples. In

particular, for the sample with molar ratio of 0.15, Ea appeared to be steady at

about 103 kJ mol-1 up to conversion of 0.4. Thereafter, it gradually increased, and

when conversion was 0.8, it showed a rapid rise to about 140 kJ mol-1 and

remained there until the end of the cure process. For the sample with molar ratio of

0.30, the cure started off with a higher Ea of about 110 kJ mol-1, which appeared to

decrease somewhat to a value of about 102 kJ mol-1 at the end of the cure.

2,4-DMP

Conversion (α)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

ener

gy (E

a, k

J m

ol-1

)

80

90

100

110

120

130

140

150

160


Figure 5.10: Effects of NaOH on the evolution of apparent activation energy Ea for 2,4-DMP as a function of the degree of conversion. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

102

The pattern of Ea evolution changed significantly with a further increase in NaOH

content. For molar ratios between 0.45 and 0.75, Ea values increased with NaOH

content and with the degree of conversion. They started off at values between about

100 kJ mol-1 and kJ mol-1 and reached values between about 120 kJ mol-1 and 128

kJ mol-1 at the end of the cure. It is noted that, for molar ratio of 0.60, Ea had a

decreasing trend at conversions lower than 0.4 before starting to rise. This is in

contrast to the cases of molar ratios of 0.45 and 0.75 which had an increasing trend

for Ea from the beginning of the cure process. Reason for this anomalous behaviour

is not clear.

For molar ratio of 1.00, Ea rapidly rose to about 150 kJ mol-1 when conversion was

0.25. Thereafter, it gradually decreased with the increase in conversion to about

140 kJ mol-1 when the conversion reached about 0.8, at which point there was a

slight rise before rapidly decreasing again.

As discussed in the case of MMP, an increasing dependence of the activation

energy on conversion degree is characteristic of processes consisting of parallel

competing reactions. For the uncatalyzed sample, the rise in Ea with the increase in

fractional conversion may indicate a kinetic process involving contributions of

reactions to form (p,p) and (o,p) methylene linkages, as well as the dimethylene

ether linkages. It may be that at low conversions, the partial contributions of

reactions with lower Ea including the formation of ether and (p,p) methylene

linkages were predominant. As the cure proceeded, the contributions of the (o,p)

linkage reaction with higher Ea increased and that of the (p,p) linkage reaction

decreased. As discussed previously, given the wide range of temperatures

encompassed by the DSC curve, it is likely that the condensation reaction of the

ether linkages to form methylene linkages also contributed to the apparent Ea,

particularly towards the later phase of the cure.

It is expected that the activation energy of condensation reactions of 2,4-DMP

would increase with the addition of NaOH due to the formation of a sodium ring

complex between the phenolic ring and the ortho methylol group. This is generally

consistent with the observation that the higher the NaOH content, the higher the Ea

required. As discussed previously, the effect of the ring complex mechanism on the

103

formation of (o,p) methylene linkages is likely to be more severe than that of (p,p)

methylene linkages. Further, there would be a variation in the reactivity of 2,4-

DMP molecules at a particular NaOH level, depending on whether they are

associated with Na+. In this case, the increasing dependence of Ea on fractional

conversion for molar ratios of 0.45, 0.60 and 0.75 suggests: (i) decreasing

contributions of reactions with lower Ea, for instance, reactions between molecules

not affected by the ring complex to form (p,p) linkages; and (ii) increasing

contributions of reactions with higher Ea, for instance, reactions between molecules

affected by the ring complex to form (o,p) linkages. It is interesting that Ea for

samples with molar ratios of 0.15 and 0.30 did not follow this ascending trend, but

appeared relatively “flat”, suggesting that the partial contributions of individual

reactions in these cases were relatively constant throughout the whole cure process.

It is noted that the rapid rise of Ea towards the end of the cure in the case of molar

ratio of 0.15 represents the energy required for further reactions of the ether

linkages to form methylene linkages.

As suggested earlier, the condensation of 2,4-DMP is likely to be influenced by

diffusion limitation at molar ratio of 1.00. The diffusion limitation does not change

the identity of the reactions, but affects the reactions by involving a transport step

and imposing limitation on the rates of the reactions. It has been shown that

diffusion limitation causes a decrease of the apparent activation energy with

increasing extent of polymerisation [12]. Such a descending trend of Ea is evident

in this case after Ea reached the maximum value of about 150 kJ mol-1 at

conversion of 0.25. The slight rise in Ea at conversion of about 0.8 before resuming

the descending trend is consistent with the earlier suggestion that the temperature at

this point was sufficiently high to allow part of the unreacted 2,4-DMP molecules

to overcome the diffusion barrier and speed up the reactions.

5.5.2 2,6-DMP


activation energy Ea for 2,6-DMP as a function of the degree of conversion. Similar

to previous cases, there was a decrease in Ea in the initial cure stages for most 2,6-

DMP samples, especially when fractional conversion was about 0.10 or less, due to

104

the diffusion effect. The following discussion focuses on the evolution of Ea when

conversion was higher than 0.10.

As can be seen in Figure 5.11, Ea for the uncatalyzed sample was relatively

constant at about 96 kJ mol-1 throughout the entire process, although there appeared

to be a slight decrease from fractional conversion of 0.8 onwards to a value of

about 91 kJ mol-1 at the end of the cure process. For the sample with molar ratio of

0.15, there was a steady increase of Ea from about 90 kJ mol-1 at conversion of 0.10

to about 104 kJ mol-1 as conversion reached 0.7. This is followed by a slight

decrease to about 100 kJ mol-1 at conversion of about 0.9, where there was a rapid

rise to about 122 kJ mol-1 at the end of the cure.

2,6-DMP

Conversion (α)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

ener

gy (E

a, k

J m

ol-1

)

70

80

90

100

110

120

130

140

150

160

170

180

190No NaOH0.15 NaOH0.30 NaOH0.45 NaOH0.60 NaOH0.75 NaOH1.00 NaOH

Figure 5.11: Effects of NaOH on the evolution of apparent activation energy Ea for 2,6-DMP as a function of the degree of conversion. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

105

Further increases of NaOH content generally had the effect of raising Ea in all

cases. For molar ratio of 0.30, Ea started off at about 110 kJ mol-1 at the beginning

of the cure and, but instead of having an ascending trend as in the case of molar

ratio of 0.15, it gradually decreased to reach a value of about 100 kJ mol-1 at the

end of the process. Ea for molar ratio of 0.45 also started at about 110 kJ mol-1, but

remained relatively steady at this value until conversion reached 0.7, after which it

gradually decreased to about 105 kJ mol-1 at the end of the cure.

Ea for the samples with molar ratios of 0.60 and 0.75 were about 122 kJ mol-1 at

conversion of 0.10. It appeared to have a descending trend with increase in

conversion, although such a descending trend of Ea was convoluted by the rises and

falls that corresponded to the emergence of small exotherms observed in the DSC

curves of these samples. For the sample with molar ratio of 1.00, Ea had an

ascending trend and reached a value of 180 kJ mol-1 at conversion of about 0.3

before starting a deep descending trend down to 80 kJ mol-1 when conversion

reached about 0.8. Thereafter, Ea rose rapidly to about 140 kJ mol-1 at conversion

of 0.95.

In the uncatalyzed state, the condensation of 2,6-DMP is likely to proceed mainly

via the formation of ether bridges and (o,p) methylene linkages. The “flat”

conversion dependence of Ea for the uncatalyzed sample suggests that the

condensation of 2,6-DMP was governed by constant contributions of these

reactions throughout the cure. This is consistent with the commonly accepted view

that methylene and ether linkages are formed simultaneously [7]. The slight and

gradual decrease in Ea from fractional conversion of 0.8 onwards may be due to

diffusion limitation, which could have some effect as the extent of cross-linking

increased. It is noted that a similar decrease in Ea towards the end of the cure was

also observed for samples added with NaOH.

The reactivity of 2,6-DMP molecules would be affected by the association with

Na+ that diminishes their capacity to proceed with condensation reactions.

However, as discussed previously, not all 2,6-DMP molecules would be affected

by the sodium ring complex, especially at lower levels of NaOH. The ascending

trend of Ea at molar ratio of 0.15 suggests that reactions between molecules not

106

affected by Na+ (i.e., with lower Ea) decreased their contributions and reactions

between molecules affected by Na+ (i.e., with higher Ea) and became more

dominant as fractional conversion increased. Similar to 2,4-DMP, the rapid rise of

Ea at conversion of 0.9 represents the energy required for further reactions of the

ether linkages to form methylene linkages.

At higher molar ratios, the extent of association between 2,6-DMP molecules and

Na+ is expected to increase. When most 2,6-DMP molecules are associated with

Na+, their reactivity would be more uniform. Given that at high molar ratios, the

major condensation reaction of 2,6-DMP is the formation of (o,p) methylene

linkages, such uniformity in 2,6-DMP reactivity would result in constant Ea

throughout the cure. This is perhaps the reason for the “flat” conversion

dependence of Ea at higher molar ratios of 0.30 and 0.45. Some effect of diffusion

limitation for both of these samples is also apparent as suggested by the descending

trend of Ea towards the end of cure. Note the more extensive effect for molar ratio

of 0.30, the reason for which is not clear.

Whilst not apparent from the DSC curves, diffusion limitation appears to prevail

from the early phase of the cure for molar ratios of 0.60 and 0.75, as suggested by

the descending conversion dependence of Ea. It seems that the excessive amounts

of Na+ and OH¯ ions were effective in imposing transport limitation on reacting

molecules, which became increasingly important as the condensation reactions

progressed. The rises and falls that convoluted the descending trend of Ea towards

the later phase of the cure of both molar ratios is probably due to the possibility

that parts of unreacted 2,6-DMP molecules gained sufficient energy at higher

temperatures to overcome the diffusion barrier and speed up the reactions.

A further increase of NaOH to 1.00 molar ratio exacerbated the effect of diffusion

limitation, as suggested by the higher onset temperature for the DSC curve. It is

possible that the diffusion of 2,6-DMP molecules to bring about chemical reactions

in the early phase of the cure was facilitated by high temperature, leading to the

observed ascending conversion dependence of Ea during this phase. As the

fractional conversion reached 0.3 where the cross-linking was more extensive,

diffusion control became more dominant, leading to the descending trend for Ea

107

with increase in the extent of conversion. Again, the rapid rise of Ea as conversion

reached 0.8 is presumably due to those unreacted 2,6-DMP molecules that could

overcome the diffusion barrier and accelerate the reactions.

5.6 Summary

A major focus of this chapter was to investigate the effects of NaOH content on the

evolution of apparent activation energy, Ea, during the cure of 2,4-DMP and 2,6-

DMP, as well as on total enthalpy of reaction, ΔHT. For 2,4-DMP, it was proposed

that at low conversions, partial contributions of reactions with lower Ea including

the formation of ether and (p,p) methylene linkages were predominant. As the cure

proceeded, the contributions of the (o,p) linkage reaction with higher Ea increased.

It was suggested that the addition of NaOH resulted in the formation of the sodium

ring complex, which diminished the capacity of the monomer to form methylene

linkages, particularly the (o,p) linkage. This was reflected in the rise of the Ea with

increase in NaOH content. At the high NaOH molar ratio of 1.00, it was proposed

that the condensation of 2,4-DMP was influenced by diffusion limitation

mechanism in the later stage of the cure, which became increasingly important as

the condensation reactions progressed and resulted in a descending trend of Ea. The

diminished capacity of 2,4-DMP to form methylene linkages was also reflected in

lower ΔHT values with increase in NaOH content.

For 2,6-DMP, the condensation was suggested to proceed mainly via the reactions

to form ether bridges and (o,p) methylene linkages with constant contributions of

these reactions throughout the cure. The sodium ring complex mechanism was also

suggested to operate in the case of 2,6-DMP which became more dominant with

increase in NaOH content. At higher NaOH content, ether bridges did not form and

the major condensation reaction of 2,6-DMP was the formation of (o,p) methylene

linkages, as reflected in constant Ea throughout the cure. In contrast to 2,4-DMP,

the diffusion limitation mechanism appeared to operate at lower NaOH content for

2,6-DMP, as suggested by the descending conversion dependence of Ea from the

early phase of the cure for molar ratios of 0.60 or above.

108

Further comparison of the effects of NaOH on the cure properties of 2,4-DMP and

2,6-DMP, as well as of MMP and TMP, will be presented in chapter 7.

5.7 References



Polymer 37(6), 955-964 (1996).







4. M.F. Grenier-Loustalot, S. Larroque, D Grande and P. Grenier, “Phenolic

Resins: 2. Influence of Catalyst Type on Reaction Mechanisms and

Kinetics”, Polymer 37(8), 1363-1369 (1996).




6. E. Imoto and T. Kimura, J. Chem. Soc. Japan 53, 9-11 (1950).

7. R.W. Martin, The Chemistry of Phenolic Resins, J. Wiley, New York, 1956,

p. 134.


Springer-Verlag, New York, 1979, p. 44.




Polym. Sci. 66, 1183-1193 (1997).

109

10. P. Luukko, L. Alvila, T. Holopainen, J. Rainio and T.T. Pakkanen, “Effect

of Alkalinity on the Structure of Phenol-Formaldehyde Resol Resins”, J.

Appl. Polym. Sci. 82, 258-262 (2001).



J. Appl. Polym. Sci 83, 1415-1424 (2002).

12. S.V. Vyazovkin, “On the Phenomenon of Variable Activation Energy for

Condensed Phase Reactions”, New J. Chem. 24, 913-917 (2000).

13. A.O. Barry, W. Peng and B. Riedl, “The Effect of Lignin Content on the

Cure Properties of Phenol-Formaldehyde Resin as Determined by

Differential Scanning Calorimetry”, Holzforschung 47, 247-252 (1993).

14. J.S.M. Kazayawoko, B. Riedl, J. Poliquin, A.O. Barry and L.M. Matuana,

“A Lignin-Phenol-Formaldehyde Binder for Particle Board Part 1. Thermal

Characteristics”, Holzforschung 46, 257-262 (1992).


1801 (1983).



17. T.H. Goswami and M.M. Naiti, “The Characterization of Trimethylol

Phenol by Thermal Analysis”, Thermochimica Acta 197, 453-462 (1992).

110

Chapter 6

Cure Properties of Tri-Methylol Phenols

6.1 Introduction

This chapter discusses the effects of NaOH on the cure properties for TMP. The

effects of scan rate on the peak exotherm temperature Tp, the fractional conversion

αp at Tp and the total enthalpy of reactions ΔHT for TMP are similar to those for

MMP. Therefore, for brevity, these aspects will not be discussed here.

The structure of this chapter is similar to that of chapter 5. It begins with a brief

review of self-condensation reactions of TMP, followed by a discussion on the

effects of NaOH on the DSC curves and the enthalpy of reactions ΔHT. The

discussion then focuses on the effects of NaOH content on the evolution of

apparent activation energy Ea during the cure. Possible mechanisms that operate

during the cure are proposed.

6.2 Self-condensation reactions of TMP

Previous studies [1,2] have shown that self-condensation reactions of 2,4-DMP

may occur according to the following schemes:

111

OH

CH2OH

CH2 + H2O

CH2OH

CH2OH

OH

(p, p methylene linkage)

OHCH2

OHCH2

CH2OH

2 x

OHCH2

HO + CH2O

CH2 + H2O

CH2OH

CH2OHHO


OHCH2

OHCH2

HO + CH2O

Figure 6.1: Condensation reactions of TMP.

Despite the larger number of methylol groups in the ortho position, the formation

of (p,p) methylene linkage involving condensation between two methylol groups in

the para position is favoured, compared to (o,p) methylene linkage. Higher-order

oligomer with three aromatic rings may also be formed from the methylol group in

the para position of compound with (o,p) methylene linkage:

CH2

CH2

CH2OHHOOHCH2

OHCH2

HO

OH

CH2OH

OHCH2

Figure 6.2: Chemical structure of trimer following condensation reactions of TMP.

The methylol groups on TMP are more reactive towards themselves compared to

those on MMP and DMP. Jones [3] has shown that the condensation rate of TMP

reaches a maximum near pH 8.5 and decreases with increasing pH.

112

Ether linkages are formed under slightly acidic and neutral conditions. Because of

the high methylol content, ether formation is likely to be more important for TMP,

compared to MMP and DMP [4]. Quinone methides are also formed in the

presence of acid or alkaline catalyst [5]. As with MMP and DMP, the stability of

ether linkages is limited up to about 160°C, beyond which they decompose to

methylene linkages [4, 6, 7].

6.3 DSC Thermograms

Figure 6.3 shows the DSC thermograms for TMP obtained at 10 °C min-1 scan rate

in the presence of varying amounts of NaOH. It can be seen that the DSC curve for

the uncatalyzed sample was quite broad, encompassing the temperature range

between about 90°C and 160°C. A similar DSC curve was also observed for the

sample with molar ratio of 0.15. The broadness of the curves suggests a complex

process and an overlapping of different reactions during the cure. As discussed

above, the formation of ether bridges, (p,p) and (o,p) methylene linkages are major

condensation reactions of TMP. Further increases of NaOH content had significant

effect on the shape of the DSC curves. At molar ratio of 0.45, the curve became

narrower with a sharp peak at about 147°C and a smaller peak at about 137°C. As

the molar ratio was increased to 0.60, the smaller peak disappeared and the curve

had a single peak at about 137°C that was even sharper compared to that of 0.45

molar ratio. At molar ratio of 0.75, this peak was still present at about 130°C, but

there was an additional small exotherm with peak at 146°C. Although further

increase of NaOH to 1.00 molar ratio appeared to enhance the significance of the

exotherm at 146°C, the sharp peak at about 130°C did not seem to change.

The similarity between the DSC curves of the uncatalyzed sample and the sample

with molar ratio of 0.15 suggests that this level of NaOH did not have significant

influence on the cure properties of TMP. As will be seen in the analysis of the

apparent activation energy Ea in 6.5, diffusion limitation appears to be an important

mechanism in the condensation of TMP for both samples. This is in contrast to the

case of DMP, where the effect of diffusion limitation was only apparent at much

higher NaOH content. The importance of diffusion limitation for TMP samples,

113

even in the uncatalyzed condition, may be due to the higher concentration of

methylol groups in TMP that would give rise to a higher degree of molecular

branching, and higher amounts of ether and methylene linkages, hence a bulkier

and more rigid structure.

TMP

Temperature (οC)

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Hea

t Flo

w E

ndo

Up

(mW

)

0

10

20

30

40

50

60

70

80

90

100

110

120

130


Figure 6.3: DSC thermograms for the self-condensation reactions of TMP in the presence of varying NaOH : TMP molar concentrations obtained at 10 °C min-1 scan rate.

114

The narrowing of the curve with the emergence of two peaks at 0.45 molar ratio

suggests that the condensation reactions were shifted to higher temperatures at this

NaOH level. This is probably due to the association between Na+ and TMP which

could slow down the condensation reactions of TMP, particularly those involving

the ortho methylol groups. The two peaks in the curve may represent the different

reactivities of TMP molecules, depending on whether they are associated with Na+,

and the different condensation possibilities. The early peak at 137°C is probably

due to (p,p) methylene linkage formation, which is expected to be least affected by

Na+. Some (o,p) methylene linkages could also be formed from TMP molecules not

affected by Na+ at this stage. The sharp peak at about 147°C may be due mainly to

reactions between TMP molecules affected by Na+ to form (o,p) methylene

linkages. As will be seen in 6.4 and 6.5, the effect of diffusion limitation was also

prominent at 0.45 molar ratio.

At higher molar ratio of 0.60, the delaying effect of Na+ is expected to be more

severe. This may be a reason for the disappearance of the lower temperature peak.

The sharp single DSC peak at 137°C for the entire cure process suggests that the

condensation was most effective within a narrow range of temperatures. At this

NaOH level, it is possible that most TMP molecules would be affected by Na+ and

the differences in their reactivity become less significant. Although the sharp peak

still persisted at higher molar ratios, the exotherm with peak at 146°C seemed to

increase its significance with increase in NaOH from (0.75 to 1.00) molar ratio. It

may be that at these high NaOH levels, the condensation reactions that gave rise to

the sharp peak were ineffective and that the exotherm at 146°C is due to reactions

of residual TMP molecules which could overcome the diffusion barrier at higher

temperatures. These issues will be further explored in 6.4 and 6.5.

6.4 Enthalpy of Reactions ΔHT

The effect of NaOH on ΔHT for TMP is shown in Figure 6.4. It can be seen that

ΔHT for TMP was approximately 365 J g-1 in the uncatalyzed condition and slightly

increased to about 372 J g-1 at molar ratio of 0.15. Increasing the NaOH molar ratio

115

to 0.45 had the effect of increasing ΔHT to a value of 420 J g-1. Thereafter, ΔHT

increased more slowly and reached a value of 435 J g-1 at molar ratio of 1.00.

TMP

NaOH : 2,4,6-TMP

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Ent

halp

y of

rea

ctio

n ( Δ

ΗΤ,

J g

-1)

350

360

370

380

390

400

410

420

430

440

4502,4,6-TMP

Figure 6.4: ΔHT as a function of NaOH : TMP molar ratio for TMP Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

As discussed previously, an increase in ΔHT may indicate that the extent of cross-

linking is higher. All the cured samples appeared glassy, rigid and were not

affected when immersed in acetone and/or methanol, suggesting that the samples

reached a fully cured state at the end of the cure process. Therefore, it is unlikely

that the difference in the extent of cross-linking between the samples, if any, would

contribute significantly to the increase in ΔHT as NaOH content was increased.

Rather, as will be discussed in 6.5, such change in ΔHT with NaOH content is

likely due to the increase in the activation energies of cross-linking reactions for

TMP as NaOH content was increased.

116

It is noted that ΔHT for MMP, particularly 2-MMP, and DMP generally decreased

with the increase in NaOH content, whereas the reverse trend was observed for

TMP. In this work, the decrease in ΔHT with NaOH content for 2-MMP, 2,4-DMP

and 2,6-DMP is suggested to be due to higher activation energies of condensation

reactions, as well as to diffusion limitation which had the effects of limiting

molecular transport and adversely affecting the extent of cross-linking. In the case

of TMP, although activation energies and diffusion limitation were also increased

as NaOH content was increased, their impacts on the extent of cross-linking

appeared to be limited, presumably due to the higher reactivity and higher amount

of methylol groups present in TMP molecules.

6.5 Effects of NaOH on the Evolution of Activation Energy Ea



ln (Φ/Tα 2) = - Eα

/ RTα + ln (RA / Eα) 3.7







(5, 10, 15 and 20) oC min-1 in the range 25oC up to 250oC. The temperature (Tα) at




with NaOH : 2,4,6-TMP molar ratio of 0.45 as an example, Table 6.1 depicts the

values of Tα at the four scan rates. From these values the linear regression analysis

function of SigmaPlot version 7.1 (from SPSS Inc.) was used to generate linear

117

graphs of ln(Φ/Tα 2) vs. 1/Tα at a set confidence level of 95 %. Figure 5.9 shows the

linear regression graphs between α = 0.05 and α = 0.95 and the corresponding

square of the correlation coefficient (r2) values. The equations of the linear graphs

were then generated by the SigmaPlot software, from which the values of Eα at

different α could be manually calculated. Maximum error in Ea values obtained

from the triplicate DSC runs was 0.5 %.

Table 6.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH : 2,4,6-TMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation 3.7.

Conversion

α Scan rate Φ (°C min-1)

Tα (°C)

Tα (K)

(1/Tα) x 10-3 (K)

Ln (Φ/ Tα2)

0.05 5 94.8 368 2.7174 13.425604

10 102.083 375.283 2.6647 14.157946 15 106.842 380.042 2.6313 14.588614 20 110.369 383.569 2.6071 14.894771

0.10 5 103.797 376.997 2.6525 13.473912 10 110.764 383.964 2.6044 14.203683 15 114.925 388.125 2.5765 14.630705 20 118.25 391.45 2.5546 14.935448

0.20 5 115.69 388.89 2.5714 13.536031 10 122.454 395.654 2.5275 14.263665 15 126.035 399.235 2.5048 14.687151 20 128.322 401.522 2.4905 14.986257

0.30 5 124.135 397.335 2.5168 13.578997 10 130.851 404.051 2.4749 14.305667 15 134.692 407.892 2.4516 14.730055 20 137.156 410.356 2.4369 15.029782

0.4 5 128.818 402.018 2.4875 13.602432 10 136.006 409.206 2.4438 14.331022 15 139.828 413.028 2.4211 14.755081 20 142.51 415.71 2.4055 15.055708

0.50 5 132.032 405.232 2.4677 13.618357 10 140.032 413.232 2.4199 14.350603 15 144.108 417.308 2.3963 14.775699 20 147.005 420.205 2.3798 15.077218

0.60 5 135.882 409.082 2.4445 13.637269 10 143.836 417.036 2.3979 14.36893 15 148.43 421.63 2.3717 14.796307 20 151.672 424.872 2.3537 15.099308

118

0.70 5 139.193 412.393 2.4249 13.653391

10 146.639 419.839 2.3819 14.382328 15 151.504 424.704 2.3546 14.810835 20 154.573 427.773 2.3377 15.112918

0.80 5 142.008 415.208 2.4084 13.666997 10 149.142 422.342 2.3677 14.394216 15 154.295 427.495 2.3392 14.823935 20 157.507 430.707 2.3218 15.126588

0.90 5 145.348 418.548 2.3892 13.683021 10 154.144 427.344 2.34 14.417764 15 159.755 432.955 2.3097 14.849318 20 163.275 436.475 2.2911 15.153194

0.93 5 147.713 420.913 2.3758 13.69429 10 154.858 428.058 2.3361 14.421102 15 162.9 436.1 2.2931 14.863793 20 166.836 440.036 2.2725 15.169445

TMP

1/Tα x 10-3 (K)

2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90

ln( φ

/ T

α2 )

13.2

13.4

13.6

13.8

14.0

14.2

14.4

14.6

14.8

15.0

15.2


Figure 6.5: Graph of ln(Φ/Tα

2) vs. 1/Tα between α = 0.05 and α = 0.93 and the corresponding square of the correlation coefficient (r2) values for 2,4,6-TMP sample with NaOH : 2,4,6-TMP molar ratio of 0.45.

119


activation energy Ea for TMP as a function of the degree of conversion. Similar to

previous cases, the discussion focuses on the evolution of Ea when the conversion

was higher than 0.10, since the initial cure stage for most samples was greatly

influenced by the diffusion effect.

TMP

Conversion (α)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

ener

gy (E

a, kJ

mol

-1)

100

110

120

130

140

150

160

170No NaOH0.15 NaOH0.45 NaOH0.60 NaOH0.75 NaOH1.00 NaOH

Figure 6.6: Effects of NaOH on the evolution of apparent activation energy Ea for TMP as a function of the degree of conversion. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

As can be seen in Figure 6.6, Ea for the uncatalyzed sample was about 142 kJ mol-1

at conversion of 0.1 and steadily decreased to a value of 110 kJ mol-1 as conversion

reached 0.9. This is followed by a rapid decline to about 100 kJ mol-1 at the end of

the cure. Similar descending dependence of Ea on conversion was also observed for

the sample with molar ratio of 0.15. However, Ea for this sample started out at a

higher value of about 160 kJ mol-1 at conversion of 0.1, and rapidly declined to 137

120

kJ mol-1 at conversion of 0.3. Thereafter, the rate of decline was similar to that of

the uncatalyzed sample, although Ea values were slightly higher.

The dependence of Ea on conversion degree changed dramatically with further

increases of NaOH content. For molar ratio of 0.45, Ea had a value of about 125 kJ

mol-1 at 0.1 conversion and rapidly rose to the maximum value of 150 kJ mol-1 at

0.2 conversion. It appeared to remain steady at this value until conversion reached

0.4. At this point, Ea started to decline to about 133 kJ mol-1 at 0.6 conversion, and

then rose to 140 kJ mol-1 at 0.8 conversion before declining rapidly to 114 kJ mol-1

at the end of the cure.

For the sample with molar ratio of 0.60, Ea was about 127 kJ mol-1 at 0.1

conversion and steadily rose to the maximum value of 155 kJ mol-1 at 0.5

conversion. After reaching the maximum value, Ea gradually declined to about 110

kJ mol-1 at the end of the cure. It is noted that the maximum Ea for this sample was

higher and was reached at a later phase in the cure, compared to the maximum Ea

for molar ratio of 0.45.

The shape of the conversion dependence of Ea for molar ratios of 0.75 and 1.00 was

similar to that for molar ratio of 0.60. However, for these samples, the rise of Ea

seemed to be more rapid with higher maximum value and the descending

dependence also occurred at earlier stages in the cure. In particular, for molar ratio

of 0.75, Ea started to rise from the beginning of the cure and reached a value of

about 161 kJ mol-1 at conversion close to 0.4 before starting to decline. For higher

molar ratio of 1.00, the maximum Ea was 162 kJ mol-1 and this value was reached

at lower conversion of 0.3. The decline of Ea for these samples proceeded in two

stages. The first stage was relatively rapid, whereas the second stage which started

at conversions of about 0.5 – 0.6 was much slower. It is noted that the decline for

molar ratio of 1.00 was convoluted by a slight rise in Ea at conversions of about 0.7

– 0.8 which corresponds to the exotherm with peak at 146°C in the respective DSC

curve.

In the uncatalyzed state, the condensation of TMP is likely to proceed via a range

of reactions including the formation of ether bridges, (p,p) and (o,p) methylene

121

linkages. However, instead of having an ascending dependence on conversion as

in the cases of 2-MMP and 2,4-DMP (see 4.4.1 and 5.5.1, respectively), or a “flat”

dependence as in the cases of uncatalyzed 2,6-DMP (see 5.5.2), Ea for this sample

exhibited a descending trend with conversion. A descending dependence of Ea on

conversion was also observed for uncatalyzed 4-MMP, which is suggested to be

due to consecutive reactions, starting with the self-condensation reaction with

higher activation energy, followed by the addition reaction of the product CH2O to

4-MMP with lower activation energy (see 4.4.2). In the case of uncatalyzed TMP,

diffusion limitation is likely to have a major role in bringing about the observed

descending trend of Ea. Indeed, the higher concentration of methylol groups in

TMP, compared to those in MMP and DMP, is expected to give rise to higher

degree of molecular branching, as well as higher amounts of ether and methylene

linkages. Therefore, as the condensation progressed, the polymeric structure would

become bulkier and more rigid, thus limiting molecular transport and providing

steric hindrances which would shield internal methylol groups from further

reactions [8-10]. It is noted that although the condensation reactions of TMP

produce CH2O, addition reaction of CH2O to TMP is not possible.

The similarity between Ea evolution of the uncatalyzed sample and that of the

sample with molar ratio of 0.15 suggests that the addition of NaOH to this level did

not have a significant effect. However, as discussed above, there were considerable

changes in Ea evolution at molar ratios of 0.45 or higher. It is important that these

changes be considered in conjunction with the changes in the respective DSC

curves. In particular, the shape of the DSC curves at molar ratio of 0.45 or higher

suggests that the condensation reactions were shifted to higher temperatures at

these NaOH levels. This can be seen in Figure 6.7 which shows the fractional

conversion as a function of temperature for TMP samples with various NaOH

molar ratios.

122

TMP

Temperature (οC)

90 100 110 120 130 140 150 160 170

Frac

tiona

l con

vers

ion

( α)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Figure 6.7: Fractional conversion as a function of temperature for TMP samples with various NaOH molar ratios. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

Thus, for molar ratio of 0.45, the rise in Ea during the initial condensation stage up

to conversion of 0.2 corresponds to temperatures up to 122°C. At this temperature,

the conversion for the uncatalyzed sample or the sample with molar ratio of 0.15

already reached a value of about 0.4. It appears that there was a sufficient amount

of Na+ at 0.45 molar ratio to effectively slow down the kinetics of condensation

reactions, particularly those involving the ortho methylol groups. On the other

hand, delaying the reactions to higher temperatures could provide the reacting

molecules with higher energies to overcome the diffusion resistance, at least at the

early stage where the formation of cross-links was not substantial. Higher

temperatures could also lower the viscosity of the medium and further promote

molecular transport [11]. As a result, instead of having a descending dependence

on conversion as in the cases of 0.15 molar ratio or uncatalyzed samples with lower

123

onset temperatures, Ea for 0.45 molar ratio sample exhibited an ascending trend up

to conversion of 0.2 and appeared to remain steady until the conversion reached

0.4. This stage corresponds to the early peak at about 137°C in the DSC curve of

the sample. It is likely that during this stage, the more reactive TMP molecules

were those not associated with or affected by Na+ and the predominant reaction was

the formation of (p,p) methylene linkages.

As the extent of crosslink formation increases, the effect of diffusion limitation is

expected to be more severe, thus imposing considerable limitation on molecular

transport. This is probably the reason for the descending trend of Ea from

conversions of 0.4 to 0.6. The second rise in Ea from conversions of 0.6 to 0.8 may

represent the regime where the temperatures were sufficiently high for unreacted

TMP molecules to overcome the diffusion barrier. The predominant reactions

during this stage may be the formation of (o,p) methylene linkages by TMP

molecules which were affected by Na+.

Figure 6.7 also shows that the condensation reactions for 0.60 molar ratio were not

as effective as those for 0.45 molar ratio in the initial stage up to about 133°C. The

slowing down of the condensation during this stage is probably due to the enhanced

association between TMP molecules and Na+ at higher molar ratio. However, from

about 133°C onwards, the reactions became much more rapid and the conversion

reached 0.8 at about 141°C. It can be seen in Figure 6.7 that this narrow

temperature range corresponds to the rise and fall of Ea between conversions of 0.3

and 0.8. The rise is likely due to condensation reactions with increasing

contributions from those with higher activation energies and the fall due to the

increasing effect of diffusion limitation as condensation reactions progressed. As

discussed in 6.3, the fact that the reactions were most effective within a relatively

narrow range of temperatures suggests that at this NaOH level, most TMP

molecules were affected by Na+ and the differences in their reactivity were not as

significant as in the case of 0.45 molar ratio.

Further increases of NaOH to molar ratios of 0.75 and 1.00 seemed to enhance the

delaying effect of Na+, as can be seen in Figure 6.7 where conversions up to about

130°C for these samples were lower than those for 0.60 molar ratio. However,

124

condensation reactions were much more rapid from 130°C to 135°C with

conversion rising from 0.30 to 0.50 within this temperature range. It is possible that

some TMP molecules that had been shielded from reactions in the earlier phase

gained sufficient energies to react at this stage. The higher energies required for

reactions are reflected in the higher Ea for these molar ratios shown in Figure 6.6.

The rise in Ea up to conversion of 0.3 - 0.4 is attributed to the increasing

importance of reactions with higher activation energies, whereas the subsequent

descending trend up to conversion of about 0.5, to increasing effect of diffusion

limitation.

The condensation up to conversion of about 0.5 corresponds to the sharp peak in

the DSC curves of these samples, shown in Figure 6.3. It can also be seen in Figure

6.3 that in addition to the sharp peak, the DSC curves of these samples showed an

exotherm with peak at 146°C, which was not present in the case of 0.60 molar

ratio. As suggested in 6.3, it may be that the effect of NaOH was much more severe

at molar ratios of 0.75 and 1.00 such that the reactions that gave rise to the sharp

peak did not condense all TMP molecules and that the emergence of the subsequent

exotherm is due to condensation reactions of residual TMP molecules which could

overcome the diffusion barrier at higher temperatures. These reactions proceeded at

a slower rate than the preceding condensation stage, as suggested by the change in

the slope of the respective curves shown in Figure 6.7, and were affected by

increasing contribution from diffusion limitation, as suggested by the descending

trend of Ea from conversion of about 0.5 to the end of the cure shown in Figure 6.6.

6.6 Summary

This chapter focused on the effects of NaOH content on the evolution of apparent

activation energy, Ea, during the cure of TMP, as well as on total enthalpy of

reaction, ΔHT. It was proposed that in the uncatalysed state, the condensation of

TMP proceeded via a range of reactions including the formation of ether bridges,

(p,p) and (o,p) methylene linkages. These reactions appeared to be influenced

significantly by the diffusion limitation mechanism, as suggested by the observed

descending trend of Ea. It was suggested that the higher concentration of methylol

125

groups in TMP, compared to those in MMP and DMP, would give rise to higher

degree of molecular branching, as well as higher amounts of ether and methylene

linkages, thus limiting molecular transport as the cure progressed.

The sodium complex mechanism was also suggested to be operative in the case of

TMP and became quite severe for samples having NaOH molar ratios of 0.45 or

higher. This was evidenced in the shifting of the onset temperatures for

condensation reactions of these samples to higher values, consistent with the

slowing down of reaction kinetics due to the diminished capacity of TMP

molecules to form methylene linkages, particularly those involving the ortho

methylol groups. The increased severity of the sodium complex mechanism was

also reflected in higher Ea values for these samples and in their descending

dependence on conversion, particularly during later phases of the cure.

The addition of NaOH also had the effect of increasing ΔHT. Because all TMP

samples, irrespective of NaOH content, reached a fully cured state at the end of the

cure process, it was suggested that such increase in ΔHT was due to the increase in

the activation energy of cross-linking reactions for TMP as NaOH content was

increased.

Further comparison of the effects of NaOH on the cure properties of TMP with

those of MMP and DMP will be presented in chapter 7.

6.7 References



Polymer 37(6), 955-964 (1996).

2. D.J. Francis and L.M. Yeddanapalli, “Kinetics and Mechanism of the

Alkali Catalysed Condensations of Di- and Tri-Methylol Phenols by

Themselves and with Phenol”, Die Makromolekulare Chemie 125, 119-

125 (1969).

126


1801 (1983).



5. T.H. Goswami and M.M. Naiti, “The Characterization of Trimethylol

Phenol by Thermal Analysis”, Thermochimica Acta 197, 453-462 (1992).

6. R.W. Martin, The Chemistry of Phenolic Resins, J. Wiley, New York,

1956, p. 128.




Polym. Sci. 66, 1183-1193 (1997).

8. A. Kumar, U. K. Phukan, A. K. Kulshreshtha and S. K. Gupta, “Molecular

Weight Distribution in Novolac Type Phenol Formaldehyde

Polymerization”, Polymer 23, 215-221 (1982).

9. A. Kumar, A. K. Kulshreshtha and S. K. Gupta, “Modelling of Phenol-

Formaldehyde Polymerization Reaction”, Polymer 21, 317-324 (1980).

10. L. Collob, “The Correlation Between Preparation and Properties in

Phenolic Resins” in Wood Adhesives. Vol. 2, A. Pizzi (ed.), Dekker, New

York, 1989, p. 121.

11. S.V. Vyazovkin, “On the Phenomenon of Variable Activation Energy for

Condensed Phase Reactions”, New J. Chem. 24, 913-917 (2000).

127

Chapter 7

Comparison of Effects of NaOH on the Cure

Properties of Mono-, Di- and Tri-Methylol Phenols

7.1 Introduction

In previous chapters, the DSC thermograms, the conversion dependence of

apparrent activation energy Ea and the enthalpy of reactions ΔHT of individual

methylol phenols at different NaOH levels have been analysed in an effort to obtain

further insights into the cure properties of MMP, DMP and TMP. A key

mechanism that has been discussed is the strong inclusion of Na+ in the methylol

phenols that diminish the capacity of these compounds to participate in

condensation reactions. Another important mechanism that has been suggested to

affect the reactions is the limitation on molecular diffusion, especially when the

cure is carried out in the presence of excessive amounts of Na+ and OH¯ ions.

These effects have been shown not only to depend on the level of NaOH, but also

on the amount and position of methylol groups in the methylol phenols.

Differences in the DSC thermograms, in the evolution of Ea with extent of

conversion and in ΔHT between different samples have been used to assess the

extent of these effects and to gain insights into various mechanistic aspects of the

condensation process. On this basis, proposals for possible mechanisms that

operate during the cure process of individual methylol phenols at different NaOH

levels have been put forward.

This chapter aims to provide a summary of the findings and compare the cure

properties of individual methylol phenols. In previous chapters, where appropriate,

some differences in the cure properties of these compounds have been identified

and explanations have been proposed. In this chapter, the comparison will be

128

carried out in a more systematic manner in an effort to provide a consistent overall

picture of relevant mechanisms operating during the cure process.

7.2 MMP

7.2.1 2-MMP

During the cure of 2-MMP in the uncatalyzed state, the apparent activation energy

Ea rose relatively rapidly from 89 kJ mole-1 to about 102 kJ mole-1 as conversion

reached 0.25. As the cure proceeded, Ea increased less rapidly and reached a value

of about 108 kJ mole-1 at the end of the cure. It has been suggested that the rise in

Ea as a function of conversion is due to a kinetic process involving variable

contributions of both (o,o) and (o,p) linkage reactions. At low conversions, the

partial contribution of the (o,o) linkage reaction with higher Ea was low compared

to that of the (o,p) linkage reaction. As the cure proceeded, the contribution of the

(o,o) linkage reaction increased and that of the (o,p) linkage reaction decreased.

The addition of NaOH generally had the effect of raising Ea. This has been

suggested to be due to the formation of the sodium ring complex, which

presumably has the effects of blocking the ortho-methylol group and reducing the

carbanion negative charge. In particular, at 0.15 molar ratio, Ea increased steadily

during the course of the cure from 110 kJ mole-1 at the beginning and reached 119

kJ mole-1 at the end of the process. Similar to the uncatalyzed sample, the

ascending dependence of Ea on conversion suggests an increasing contribution of

the (o,o) linkage reaction, and a diminishing importance of the (o,p) linkage

reaction, as the cure progressed.

Further increases of NaOH up to 1.00 molar ratio also resulted in an ascending

dependence of Ea on extent of conversion. However, instead of persisting

throughout the entire cure process, the ascending dependence of Ea for these

samples was maintained only in the initial stage, after which Ea had a “flat”

dependence with value at about 130 kJ mole-1. It has been suggested that during

this later phase, the (o,p) linkage reaction was essentially complete and the (o,o)

linkage reaction with higher activation energy became the dominant reaction.

129

The addition of NaOH to 2-MMP also had the effect of decreasing the enthalpy of

reactions ΔHT, especially at higher NaOH content. In particular, ΔHT was 538 J g-1

in the uncatalyzed condition and remained steady around this value until the molar

ratio reached 0.30. Thereafter, ΔHT steadily decreased to about 425 J g-1 at molar

ratios of 0.60 or higher. Such decrease in ΔHT has been suggested to be due to

lower amounts of cross-links formed at higher molar ratios, as higher activation

energies were required for the cross-linking reactions.

7.2.2 4-MMP

In contrast to 2-MMP where there was an ascending dependence of Ea on

conversion, Ea of the uncatalyzed 4-MMP gradually decreased from about 110 kJ

mole-1 in the early stage to about 98 kJ mole-1 at the end of the cure. It has been

suggested that the cure process of 4-MMP consisted of consecutive reactions,

starting with the self-condensation reactions with higher activation energies to form

(p,p) and (o,p) linkages, followed by the addition reaction of the product CH2O to

4-MMP with lower activation energy. The descending dependence of Ea on

conversion is attributed to decreasing contributions of the self-condensation

reactions as the cure proceeded, and to the growing contribution of the addition

reaction towards the end of the process. It is noted that (p,p) and (o,p) linkage

reactions of 4-MMP have been found to occur at similar rates with an activation

energy of 72 kJ mole-1 [1], whereas the addition reaction of CH2O to 4-MMP had a

lower activation energy of 60 kJ mole-1 [2].

Whilst NaOH molar ratio of 0.15 did not have significant effect on Ea as well as its

conversion dependence, higher levels of NaOH content had the effect of increasing

Ea, which had values ranging from about 115 kJ mole-1 to 120 kJ mole-1 for molar

ratios between 0.30 and 0.60, and about 128 kJ mole-1 for molar ratio of 1.00. The

increase in Ea is attributed to the association between Na+ and the phenate oxygen,

which reduces the carbanion negative charge and diminishes the capacity of 4-

MMP to participate in self-condensation reactions. It has also been suggested that

the adverse effects of NaOH on 4-MMP should be less compared to 2-MMP, since

the sodium ring complex is not formed in the former case. Indeed, whilst a low

NaOH molar ratio of 0.15 did not change the cure properties of 4-MMP to any

130

significant extent, the same NaOH level had a prominent effect on Ea values of 2-

MMP. Also, although Ea values of 4-MMP at higher molar ratios were higher than

those of the uncatalyzed sample, they were generally lower compared to those of

the corresponding 2-MMP counterparts. In addition, the fact that the decrease in

ΔHT with NaOH content in the case of 2-MMP was more significant than that in

the case of 4-MMP, is consistent with a more significant effect of NaOH in

reducing the extent of cross-linking in 2-MMP.

Another difference between 2-MMP and 4-MMP at molar ratios higher than 0.15 is

that whilst Ea of the former increased with conversion in the early stage of the cure,

Ea of the latter had a “flat” dependence on conversion throughout the entire cure

process. As suggested, the ascending conversion dependence of Ea for 2-MMP

indicates a difference in the kinetics of condensation reactions with (o,o) linkage

reaction increasing its contribution and (o,p) linkage reaction becoming less

important as the cure progressed. An ascending conversion dependence of Ea was

also observed for the uncatalyzed 2-MMP. Apparently, despite having the effect of

raising Ea, the presence of NaOH did not significantly alter the relative kinetics of

the condensation reactions. In contrast, the “flat” conversion dependence of Ea for

4-MMP suggests that the self-condensation and addition reactions had similar

activation energies at higher NaOH content. It seems that a higher NaOH content

not only raised the energy required for the self-condensation reactions, but also the

energy required for the addition reaction between CH2O and 4-MMP.

7.3 DMP

7.3.1 2,4-DMP

Whilst the DSC curves for 2-MMP and 4-MMP showed a single exotherm at all

levels of NaOH, those for 2,4-DMP exhibited considerable differences with the

emergence of well-separated exotherms and/or broadening of the curves,

depending on NaOH content. This has been suggested to be due partly to the

operation of the sodium ring complex mechanism that gave rise to a variation in the

reactivity of 2,4-DMP molecules at a particular NaOH level, depending on whether

they are associated with Na+. Another factor that affected the shape of the DSC

131

curves is thought to be the range of condensation possibilities which essentially

consisted of reactions to form (p,p) and (o,p) methylene linkages, as well as ether

linkages at low NaOH levels.

A particular feature of the DSC curve for 2,4-DMP at molar ratio of 0.15 is the

emergence of two peaks at around 135°C and 160°C. It has been suggested that the

first peak (135°C) is due to the condensation reactions to form methylene and ether

linkages, whereas the second peak (160°C) represents further reaction of the ether

linkages, for instance, to form methylene linkages and eliminate formaldehyde. In

contrast, the DSC curves of 2-MMP and 4-MMP at similar molar ratio did not

show a similar distinct peak for the degradation of ether linkages. Holopainen et al.

[3] have reported that the condensation and ether degradation peaks of methylol

phenols overlap when the degree of methylol substitution is low and become

progressively separated with increasing substitution. This is likely to be the reason

for the absence of the ether degradation peak for the MMP samples and the

emergence of such a peak for the 2,4-DMP sample.

Another distinguishing feature of the DSC curves for 2,4-DMP is that at molar

ratio of 1.00, there were two distinct peaks at about 143°C and 163°C. The

narrowness of the peaks have been suggested to be due in part to less variation in

the reactivity of 2,4-DMP molecules since most would be affected to more or less

the same extent in the presence of high level of Na+. Another factor that is thought

to be operative at this molar ratio is the excessive amounts of Na+ and OH¯ ions that

may impose limitation on molecular diffusion. It has been suggested that after the

initial formation of methylene linkages at the relatively high onset temperature that

gave rise to the first DSC peak (143°C), the limitation on molecular mobility could

become more severe, making it more difficult for further condensation to proceed.

The second DSC peak (163°C) is thought to be due to the unreacted 2,4-DMP

molecules that could overcome the diffusion barrier and accelerate the reactions at

higher temperatures.

In terms of the apparent activation energy Ea, for the uncatalyzed sample, there was

a steady rise of Ea from about 90 kJ mole-1 in the early stage to about 110 kJ mole-1

at the end of the cure. Such ascending dependence of Ea on conversion has been

132

suggested to be due to a predominance of reactions to form ether and (p,p)

methylene linkages with lower activation energies at lower conversions, and an

increasing contribution of the (o,p) linkage reaction with higher activation energy

as the cure proceeded. As expected, the addition of NaOH had the effect of raising

the activation energies of condensation reactions due to the formation of the

sodium ring complex. In particular, Ea appeared to increase with increases in

NaOH content and reached a value as high as 140 kJ mole-1 at 0.75 molar ratio.

Generally, the ascending conversion dependence of Ea was still observed for

different NaOH molar ratios. This has been attributed to: (i) decreasing

contributions of reactions with lower Ea, for instance, reactions between molecules

not affected by the sodium ring complex to form (p,p) linkages; and (ii) increasing

contributions of reactions with higher Ea, for instance, reactions between molecules

affected by the ring complex to form (o,p) linkages.

The conversion dependence of Ea at molar ratio of 1.00 was distinctly different

from that at lower molar ratios in that there was an initial rise of Ea to about 150 kJ

mole-1 at 0.25 conversion, followed by a steady decrease to about 140 kJ mole-1

when the conversion reached about 0.80. Afterwards, there was a slight rise before

rapidly decreasing again. Consistent with the interpretation of the respective DSC

curve, the descending trend between 0.25 and 0.80 conversions has been attributed

to the diffusion limitation that caused a decrease of the apparent activation energy

with increasing extent of polymerisation. As well, the slight rise after 0.80

conversion has been suggested to be due to unreacted 2,4-DMP molecules which

could overcome the diffusion barrier and speed up the reactions at higher

temperatures.

Whilst the results suggest that diffusion limitation was important for 2,4-DMP at

1.00 molar ratio, there was no evidence that a similar mechanism operated in the

cases of 2-MMP and 4-MMP. This may be due in part to the bulkier nature of 2,4-

DMP molecules that could exacerbate the limitation on molecular mobility caused

by excessive amounts of Na+ and OH¯ ions, thus raising the onset temperatures for

condensation reactions as observed. Also, as the condensation reactions proceeded,

the higher concentration of methylol groups in 2,4-DMP would give rise to higher

133

degree of molecular branching and higher amounts of ether and methylene

linkages, thus rendering a more rigid polymer structure with less effective

molecular diffusion, compared to those formed by 2-MMP and 4-MMP molecules.

The change in enthalpy ΔHT with NaOH content is another parameter that provides

further insights into the cure properties of 2,4-DMP. In particular, ΔHT for 2,4-

DMP decreased steadily with increase in NaOH content from about 500 J g-1 in the

uncatalyzed state to about 340 J g-1 when NaOH molar ratio reached 1.00. This has

been attributed to the lower amounts of cross-links formed at higher molar ratios. It

is noted that within a similar range of NaOH content, the decrease in ΔHT in the

case of 2,4-DMP was more significant than those in the cases of 2-MMP and 4-

MMP. This is consistent with the diffusion limitation mechanism which had the

effect of diminishing the extent of cross-linking in 2,4-DMP.

7.3.2 2,6-DMP

A main difference between 2,4-DMP and 2,6-DMP was that the shape of the DSC

curves for the latter was narrower and did not appear to change significantly at

molar ratios of 0.45 or less, apart from the appearance of the distinct second peak at

0.15 molar ratio. This second peak, as in the case of 2,4-DMP, has been attributed

to further reactions of the ether linkages to form methylene. The narrowness of the

curves has been attributed in part to the limited condensation possibilities of 2,6-

DMP where reactions proceeded primarily via only (o,p) positions, compared to

those for 2,4-DMP where reactions occurred via both (p,p) and (o,p) positions. As

well, the higher reactivity of 2,6-DMP, compared to that of 2,4-DMP, is another

factor that has been suggested to contribute to the narrow shape of the curves. The

apparent stability of the DSC curves of 2,6-DMP at these molar ratios has been

attributed to a less severe effect of NaOH on the reactivity of 2,6-DMP, given that

one ortho methylol group is still available for reactions, whilst the other is

restrained by the association with Na+.

At higher molar ratios, the effects of NaOH were more significant and the shape of

the DSC curves became broadened at molar ratios of 0.60 and 0.75, before

emerging as two distinct sharp peaks at molar ratio of 1.00. Similar to the case of

134

2,4-DMP, the broadening of the curves has been explained in part by the variation

in the reactivity of different 2,6-DMP molecules, depending on whether they were

associated with Na+. Likewise, a molecular diffusion control mechanism which

slows down the condensation reactions until higher temperatures has also been

used to explain the emergence of the two sharp peaks at 1.00 molar ratio. It is noted

that the first and second peaks for 2,6-DMP at 1.00 molar ratio were more distinct

and well-separated, compared to those for 2,4-DMP. This is likely to be due to the

greater effect of diffusion limitation for 2,6-DMP at higher molar ratios, as

apparent from the following comparison of the conversion dependence of Ea of the

two compounds.

In the uncatalyzed state, the conversion dependence of Ea was “flat” at about 96 kJ

mole-1. This has been suggested to be due to constant contributions of reactions to

form ether bridges and (o,p) methylene linkages throughout the cure. At molar

ratios between 0.15 and 0.45, Ea of 2,6-DMP generally had either an ascending or a

“flat” dependence on conversion. Given that the major condensation reaction of

2,6-DMP is the formation of (o,p) linkages, such behaviour of Ea has been

attributed to the variation in reactivity of 2,6-DMP molecules due to the extent of

their association with Na+ at a particular NaOH level. Some effects of diffusion

limitation were also apparent for 2,6-DMP at 0.30 and 0.45 molar ratios,

particularly towards the end of cure.

As mentioned above, data on the conversion dependence of Ea suggest that

diffusion limitation mechanism had greater effects on 2,6-DMP than on 2,4-DMP,

especially at higher molar ratios. In particular, diffusion limitation appeared to

prevail from the early phase of the cure for 2,6-DMP at molar ratios of 0.60 and

0.75, as suggested by the descending conversion dependence of Ea for these

samples. This is in contrast to the predominance of chemical reactions for 2,4-DMP

at similar molar ratios, as suggested by the ascending trend of Ea for these samples.

The greater effects of diffusion limitation mechanism for 2,6-DMP can also be seen

in the case of 1.00 molar ratio where Ea for this sample initially rose to a high value

of 180 kJ mole-1 at 0.3 conversion, then sharply decreased down to about 80 kJ

mole-1 at 0.8 conversion before rapidly increasing to about 140 kJ mole-1 towards

135

the end of the cure. Note that Ea for 2,4-DMP at 1.00 molar ratio also had a similar

pattern of evolution with conversion, but to a much lesser extent. Reasons for the

difference in the effects of diffusion limitation on the two compounds are not clear.

The decrease in the enthalpy of reactions ΔHT with NaOH content for 2,6-DMP

appeared to be more significant than that for 2,4-DMP. In particular, ΔHT

decreased from about 530 J g-1 in the uncatalyzed condition to a value of 310 J g-1

at molar ratio of 1.00. The greater change in ΔHT for 2,6-DMP is consistent with

the greater effects of diffusion limitation mechanism which further diminished the

extent of cross-linking in the structure of this compound.

7.4 TMP

As discussed previously, diffusion limitation was absent in the cure of 2-MMP and

4-MMP, but was important in the condensation of 2,4-DMP and 2,6-DMP at higher

levels of NaOH. For TMP, this mechanism played a major role even in the

uncatalyzed condition, as suggested by the steady descending dependence of Ea on

conversion for the uncatalyzed TMP from about 142 kJ mole-1 at the beginning to

about 100 kJ mole-1 at the end of the cure. This has been attributed to the higher

concentration of methylol groups in TMP, compared to those in MMP and DMP,

that would give rise to higher degree of molecular branching, and higher amounts

of ether and methylene linkages. The formation of ether bridges, (p,p) and (o,p)

methylene linkages are thought to be major condensation reactions of TMP.

The effects of diffusion limitation did not appear to change at molar ratio of 0.15,

but became increasingly severe at molar ratios of 0.45 or higher, as evidenced in

the considerable changes in the DSC curves and in the conversion dependence of

Ea of these samples. In particular, at 0.45 molar ratio, the onset temperatures for

condensation reactions increased significantly and the DSC curve became narrower

with the emergence of a smaller peak at about 137°C and a sharp peak at about

147°C. The smaller peak represents the first condensation stage during which Ea

increased to about 150 kJ mole-1 at 0.2 conversion and remained steady at this

value until the conversion reached 0.4. The sharp peak represents the second

condensation stage where there was a descending trend of Ea to about 133 kJ mole-1

136

at 0.6 conversion, followed by a rapid rise to kJ mole-1 at 0.8 conversion before

declining rapidly to 114 kJ mole-1 at the end of the cure.

The shape of the DSC curve and the complex evolution of Ea at 0.45 molar ratio

have been attributed to the association between Na+ and TMP which further

exacerbated the diffusion limitation imposed by TMP itself. In particular, it has

been suggested that during the first condensation stage, TMP molecules not

associated with Na+ were more reactive and would preferentially react to form (p,p)

methylene linkages. As the condensation proceeded, the effect of diffusion

limitation became more severe, leading to the descending trend of Ea from 0.4 to

0.6 conversions. The evolution of Ea from 0.6 conversion towards the end of the

cure has been explained on the basis that further increases in temperature would

allow some unreacted TMP molecules to overcome the diffusion barrier and

accelerate the condensation, before being restrained again by the diffusion control

mechanism. The predominant reactions during this stage are thought to be the

formation of (o,p) methylene linkages by TMP molecules which were affected by

Na+.

Further increases of NaOH content resulted in further changes in the DSC curves

and in the conversion dependence of Ea, which are consistent with increasingly

enhanced association between Na+ and TMP, and a more severe effect of diffusion

limitation mechanism. In particular, the condensation reactions at 0.60 molar ratio

were further shifted to higher temperatures compared to those at 0.45 molar ratio

and the DSC curve exhibited a sharp single peak at 137°C, instead of two peaks as

in the case of 0.45 molar ratio. Correspondingly, Ea showed an initial rise to the

maximum value of 155 kJ mole-1 at 0.5 conversion and a subsequent descending

trend to about 110 kJ mole-1 towards the end of the cure. It has been suggested that

the shifting of the condensation reactions to higher temperatures is due to enhanced

association between Na+ and TMP, and that the emergence of a single sharp peak is

due partly to a more uniform reactivity of TMP molecules and an increased effect

of diffusion limitation at this molar ratio.

Increases of NaOH to molar ratios of 0.75 and 1.00 further shifted the condensation

to higher temperatures, which is consistent with a more severe effect of TMP – Na+

137

complex in delaying the reactions. The sharp peak still persisted, but an exotherm

with peak at 146°C started to emerge with increased significance at 1.00 molar

ratio. The shape of the conversion dependence of Ea for these samples was similar

to that for 0.60 molar ratio. However, the rise of Ea seemed to be more rapid with

higher maximum value and the descending dependence also occurred at earlier

stages in the cure. It has been suggested that at these high molar ratios, the effect of

NaOH was much more severe so that the initial condensation that gave rise to the

sharp peak was not efficient, and that the emergence of the subsequent exotherm is

due to reactions of residual TMP molecules which could overcome the diffusion

barrier at higher temperatures.

As discussed previously, the enthalpy of reactions ΔHT for MMP and DMP

decreased with increases in NaOH content. This has been attributed to the lowering

of the extent of cross-linking in these compounds due to higher activation energies

required and the diffusion limitation mechanism at higher molar ratios. However,

for TMP, an increasing dependence of ΔHT on NaOH content was observed, which

started at about 365 J g-1 for the uncatalyzed sample and reached 435 J g-1 as NaOH

was increased to 1.00 molar ratio. This is the case despite the increase in activation

energies of condensation reactions and the enhanced effect of diffusion limitation

at higher molar ratios for TMP. The higher reactivity and higher amount of

methylol groups present in TMP molecules have been suggested to be contributing

to the more efficient cross-linking reactions in TMP samples.

7.5 Summary

This chapter summarised and compared the effects of NaOH on the cure properties

of individual methylol phenols. A key mechanism that was suggested to operate

during the cure of the monomers in the presence of NaOH is the formation of the

sodium ring complex that diminishes the capacity of the monomers to participate in

condensation reactions, particularly those involving ortho-methylol groups. At a

particular NaOH level, the monomer molecules may have a range of reactivity,

depending on whether they are associated with Na+. Such variation in the reactivity

and the different condensation possibilities of the monomers are critical factors

governing the cure behaviour of the monomers. An effect of the sodium ring

138

complex that was commonly observed for all monomers, was the raising of the

apparent activation energy, Ea. For TMP, an additional effect was the shifting of

the onset temperatures for condensation reactions to higher values as NaOH

content is increased.

Another important mechanism that was suggested to operate during the cure was

the limitation on molecular diffusion that had the effect of slowing down the


mechanism was more pronounced with increases in the amount of the methylol

groups in the monomers, which would give rise to higher degree of molecular

branching and higher amounts of methylene linkages, thus limiting molecular

transport as the cure progressed. Increasing NaOH content in the monomers also

had the effect of exacerbating the effect of this mechanism. As shown by the shape

of the dependence of Ea on the degree of conversion, the diffusion limitation

mechanism is absent in the cases of 2-MMP and 4-MMP, and most pronounced in

the case of TMP. For 2,4-DMP and 2,6-DMP, the presence of this mechanism was

only apparent in samples with higher NaOH content.

Differences in the effects of these mechanisms between monomer samples with

different NaOH content, together with the established chemistry of condensation

reactions, were used as a basis to explain the shape of the DSC curves, the

dependence of Ea on the degree of conversion and the heat of reactions ΔHT. The

partial contributions of condensation reactions to form para-para and ortho-para

linkages, as well as ortho-ortho linkages on rare occasions, at different stages of

the cure, were also proposed for individual monomers at different NaOH levels.

7.6 References



with Phenol”, Makromol. Chem. 55, 74-86 (1962).




139




Polym. Sci. 66, 1183-1193 (1997).

140

Chapter 8

Cure Properties of PF Resoles

8.1 Introduction

As discussed in chapters 1 and 2, many DSC studies of PF resoles often mistakenly

assumed that the activation energy of the thermal reaction is constant and did not

change with the extent of the cure. This approach ignores the complexity of the

reactions and the complex dependence of Ea on the degree of the cure. As reviewed

in Chapter 2, the limited number of studies which investigated the variation of Ea

during the cure, suggested a decrease in Ea as the cure proceeded and concluded

that the cure of PF resoles changed from a kinetic to a diffusion regime due to

cross-linking in the system. Nevertheless, these studies did not investigate the

effects of NaOH on the cure kinetics of the resoles.

This chapter uses the model-free kinetic analysis of DSC data to investigate the

variation of activation energy, Ea, during the cure of resoles having different NaOH

/ P molar ratios, and from here, to obtain further insights into the cure mechanisms.

Gel permeation chromatography (GPC) and gel time techniques are also used to

provide complementary information. An issue that will be addressed in this chapter

is whether the retardation effect on the cure kinetics of the resoles caused by the

sodium complex ring mechanism, is confined to 2-MMP or also applies to other

methylol phenols present in the resoles. This chapter will also investigate the

effects of NaOH content and methylol substitution on the diffusion limitation

mechanism that operates in the later stage of the cure.

The chapter contains an experimental section that describes the synthesis of the

resoles and the experimental techniques employed including GPC and gel time

techniques. The DSC methods are similar to those used for the monomers and are

not presented here. It will be shown that the outcomes of both studies of the

monomers and the resoles are complementary to each other and provide a

141

consistent overall picture of relevant mechanisms operating during the cure

process.

8.2 Experimental

8.2.1 Resole synthesis

Three PF resole formulations were synthesised with F / P molar ratio of 2.0, while

the NaOH / P molar ratios were 0.30, 0.50 and 0.70. The materials used were 100%

phenol, 46 % NaOH, 54 % formaldehyde and water. The reactions were conducted

in a laboratory glass reactor equipped with a thermometer, stirrer and a reflux

condenser. The temperature was controlled with an electric heating mantle and an

ice water cooling bath.

The initial charge consisted of all the phenol and half of the NaOH content. After

heating the components to 60°C, half of the formaldehyde was slowly added over

thirty minutes and the reaction continued at this temperature for a further thirty

minutes. The reaction was then allowed to heat to 80°C and held at that

temperature for thirty minutes. The remaining sodium hydroxide and water were

charged, then cooled to 60°C. The reaction was maintained at this temperature for

30 minutes, during which time the remaining formaldehyde was slowly added. The

reaction temperature was then increased to 75°C and maintained at this temperature

until the final viscosity reached approximately 300 mPa.s. The reaction was

stopped by cooling rapidly to room temperature. In order to monitor the reactions,

samples were periodically taken from the reaction mixture and their viscosity

determined at 25°C using a Mettler Rheomat RM 180 rotational viscometer.

The solids content of the resoles were 42 % to 46 % and the final free

formaldehyde contents were less than 0.1 %.

8.2.2 GPC

The GPC procedure to determine the molecular weight distribution of the PF

resoles was outlined in Yazaki et al. [1]. Essentially, the GPC of the resoles was

performed using a Waters Associates instrument comprising Waters 710B injection

142

system, Waters 590 programmable HPLC pump and Waters Lambda-max 481 UV

detector fitted with a series of Shodex columns having dimensions 300mm length

by 8mm ID and packed with styrene-divinylbenzene copolymers. The column

series consisted of KF 800P (guard column, 10 mm by 4.6 mm ID), KF 803, KF

802.5 and KF 802. HPLC grade tetrahydrofuran (THF) was used as the mobile

phase. A sample (5mg) of PF resin was dissolved in THF (5 mL) and toluene (1

µL) was added as an internal standard. An aliquot (50 µL) of the sample solution

was injected, eluted with the THF at a flow rate of 1 mL/min and UV absorption

was detected at 280 nm. Polystyrene standards (nominal molecular weights

100000, 9000, 2000 and 580) were used for calibration. The resulting

chromatograms were analysed for weight-average molecular weight (Mw) and

number-average molecular weight (Mn). All GPC data collection and manipulation

was performed on a 4100 computing integrator (Spectra-Physics, San Jose, CA)

and Millenium Software (Waters Corporation, Milford, MA) respectively.

8.2.3 Gel time

Gelation is defined as the point at which a resole ceases to be a viscous liquid and

becomes a soft, elastic, rubbery solid. The procedure to determine the gel time for

the resoles essentially consisted of filling a test tube of 1.25 cm diameter with a

sample of the resoles to approximately half of its capacity. The test tube was then

placed into a constant-temperature bath at 100°C with an inserted stirring glass rod

of 4mm, which was used as a probe. The glass rod was removed frequently for

observation. The gel time was the time taken for the sample to reach a specified gel

strength, taken as a point at which truly viscous flow was no longer observed.

Duplicate determinations were carried out [2].

8.2.4 DSC experiments

DSC runs and kinetic analysis of DSC data were carried out in accordance with the

methods and procedures described in chapter 3.

143

8.3 Results and Discussion

8.3.1 GPC

The molecular weight measurements and the calculated polydispersity (Mw / Mn) of

the PF resoles as functions of NaOH / P molar ratio are shown in Figure 8.1. It can

be seen that Mw increased steadily with increase of NaOH from about 1700 at

molar ratio of 0.30 to about 5500 at molar ratio of 0.70. Likewise, the

polydispersity of the resoles increased progressively from about 2.0 at 0.30 molar

ratio to about 5.0 at 0.70 molar ratio.

PF resoles

NaOH / P molar ratio

0.3 0.4 0.5 0.6 0.7

Wei

ght-

aver

age

mol

ecul

ar w

eigh

t (M

w)

1000

2000

3000

4000

5000

6000

Poly

disp

ersi

ty (M

w /

Mn)

1

2

3

4

5

6

Molecular weightPolydispersity

Figure 8.1: The weight-average molecular weight (Mw) and the polydispersity (Mw/Mn) of PF resoles as functions of NaOH / P molar ratio.

The increasing trend of Mw with increase of NaOH / P molar ratio is in agreement

with the results reported in previous studies [3-5]. Gollob [3] explained that since

the addition of NaOH has the effect of thinning the resoles, the methylol phenols in

the resole with higher NaOH content must react to a greater extent to reach a

144

constant viscosity end point. Polydispersity gives a measure of the range of

molecular size in the resole. The higher polydispersity for resoles with higher

NaOH content may reflect the larger range of reaction products with different

molecular weights arising from the greater extent of condensation as NaOH content

was increased.

8.3.2 Gel time

Figure 8.2 shows that the gel time of the PF resoles increased as NaOH / P ratio

was increased, indicating that the resole reactivity decreased at higher NaOH

content. This is in agreement with previous gel time studies, which found a similar

decreasing trend of the reactivity of PF resoles as NaOH / P molar ratio was

increased within the range similar to that used in the present study [4-7]. It is noted

that Haupt and Waago [5] found that for PF resoles having NaOH / P molar ratio

less than about 0.30, the reactivity of the resoles increased with increases in the

ratio.

PF Resoles


0.3 0.4 0.5 0.6 0.7

Gel

tim

e (s

)

400

600

800

1000

1200

1400

1600

1800

Figure 8.2: The gel time of PF resoles as a function of NaOH / P molar ratio.

145

Since gel time is measured by the cessation of viscous flow, a major critique of the

use of gel time to study reactivity is that the interpretation of the results is

complicated by factors that influence the viscosity of the resoles [8]. In the present

study, the use of resoles having similar viscosity would eliminate these

complications. The consistency between the gel time results and the DSC data, as

well as further discussion on the effects of NaOH on resole reactivity, will be

presented in the following sections.

8.3.3 DSC curves

Figure 8.3 shows the DSC thermograms of the PF resoles having different NaOH /

P molar ratios obtained at 10 °C min-1 scan rate. It can be seen that for the resole

with a NaOH / P molar ratio of 0.30, the DSC curve was quite sharp and had a peak

at about 139°C. Further increases of the NaOH / P ratio to 0.50 and 0.70 had the

effect of broadening the curve and shifted the peak temperatures to higher values of

about 147°C and 152°C, respectively.

It is noted that all DSC thermograms of the resoles obtained in this study showed a

single exothermic peak. The presence of a single peak is consistent with a number

of studies [see, for example, 4, 9, 10], but not in agreement with some others that

showed two exothermic peaks [see, for example, 6, 11-13]. Park et al. [4] attributed

the single peak in their study to the lower molecular weight of the resoles used,

which ranged from 486 to 701. However, this does not explain the presence of a

single peak for the resoles in this study, which had much higher molecular weight

(1700 – 5500). It is likely that the single peak in the present study is due to

condensation reactions involving the methylol groups to form mostly methylene

linkages [6]. The absence of a sharp “addition” peak between 98°C and 130°C [6,

13] is likely due to the insignificant level of residual formaldehyde in the resoles to

give rise to addition reactions with phenolic rings. Likewise, the absence of a peak

representing the degradation of ether linkages at a temperature higher than that of

the condensation peak [12, 13] is probably because the formation of ether linkages

was insignificant under the alkaline conditions used.

146

PF Resoles

Temperature (οC)

40 60 80 100 120 140 160 180 200 220

Hea

t Flo

w E

ndo

Up

(mW

)

0

10

20

30

40

50

60

NaOH / P = 0.70NaOH / P = 0.50NaOH / P = 0.30

Figure 8.3: DSC thermograms of the PF resoles having different NaOH / P molar ratios obtained at 10 °C min-1 scan rate.

Figure 8.4 shows the fractional conversion of the cure reactions of the resoles as a

function of temperature. It can be seen that for the resole with NaOH / P = 0.30, the

cure kinetics was relatively slow at temperatures below 132°C where the

conversion degrees were less than about 0.3. Thereafter, the reactions accelerated

significantly and reached a conversion of 0.9 at about 145°C, before slowing down

147

towards the end of the cure process. The fast reaction kinetics within a relatively

narrow temperature range reflects the sharp exotherm in the DSC curve of the

resole.

Increasing the NaOH / P molar ratio had the effect of delaying the cure reactions to

higher temperatures, most notably when the conversions were above 0.3. Indeed, it

can be seen in Figure 8.4 that higher temperatures were required for the resoles

with 0.50 and 0.70 NaOH / P molar ratios to reach the same degree of conversion

as that of the resole with 0.30 molar ratio. For instance, whilst the 0.30 molar ratio

resole reached 0.9 conversion at 145°C, the 0.50 and 0.70 molar ratio resoles

reached the same conversion degree at 161°C and 165°C, respectively. The slow

reaction kinetics for these resoles over a broad range of temperatures reflects the

broad exotherm in the DSC curves of the respective resoles.

PF Resoles

Temperature (οC)

100 110 120 130 140 150 160 170 180

Con

vers

ion

( α)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0


Figure 8.4: Fractional conversion of the cure reactions of the resoles as a function of temperature.

148

The progressive retardation of the reaction kinetics with increases in the NaOH / P

ratio observed in the DSC data is in agreement with the gel time results presented

in 8.3.2. As discussed previously, the retardation effect of NaOH has been reported

in a number of studies, but most of these studies were based on the gel time

technique. Although DSC technique has been employed in a number of PF resole

studies, its use to study the effect of NaOH on the cure kinetics is limited. The DSC

data in the present study provide an independent confirmation of the retardation

effect of NaOH on the cure kinetics of PF resoles.

It has been suggested that a key mechanism for the progressive retardation of the

cure kinetics of PF resoles at increasingly higher NaOH content is the formation of

the sodium ring complex that blocks the ortho-methylol group and reduces the

carbanion negative charge, which is the main force driving the PF condensation

reactions under alkaline conditions [7]. However, there is uncertainty as to whether

only 2-MMP moieties in PF resoles are involved in the retardation effect. The

results of individual monomers in the present study strongly suggest that the

retardation effect of NaOH was not confined to 2-MMP, but also applied to other

methylol phenols present in the resoles. In addition, resoles with higher NaOH / P

molar ratios would have higher proportions of DMP and TMP, and should be more

reactive towards condensation reactions, given that the degree of substitution

increases with increasing NaOH content and that the reactivity of methylol phenols

increases with increasing substitution [5, 14, 16, 17]. The opposite trend observed

for the reactivity of the resoles as NaOH / P molar ratio was increased provides

additional confirmation that methylol phenols other than 2-MMP were also

affected by NaOH.


Figure 8.5 shows ΔHT for the cure reactions of the resoles as a function of NaOH /

P molar ratio. It can be seen that ΔHT had a value of 254 J g-1 at molar ratio of 0.30

and steadily decreased to about 200 J g-1 at molar ratio of 0.70. A decrease in ΔHT

may be an indication of a decrease in the extent of cross-linking. Alternatively, it

may suggest that less energy is required for the cross-linking. As will be discussed

149

in 8.3.5, the activation energies of condensation reactions for the resoles increased

with increasing NaOH / P ratio. Therefore, it appears that the decrease in ΔHT was

due to lower amounts of cross-links formed during the cure process.

PF Resoles


0.3 0.4 0.5 0.6 0.7

Ent

hapl

y of

rea

ctio

n ( Δ

HT, J

g-1

)

190

200

210

220

230

240

250

260

Figure 8.5: ΔHT as a function of NaOH / P molar ratio for PF resoles. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

It is noted that all the cured resoles having different NaOH content had a solid and

glassy appearance and were not affected when immersed in acetone and/or

methanol, suggesting that the resoles reached a fully cured state at the end of the

cure process. As discussed in 8.3.1, prior to the cure process, the condensation

reactions of methylol phenols in resole having higher NaOH content would have

proceeded to a greater extent to reach a constant viscosity end point. It may be that

the prior condensation during the making of the resole would reduce the amounts

of methylol groups available for further condensation during the cure process,

leading to lower ΔHT as shown by the DSC data.

150

It is also interesting that the values of ΔHT for the resoles ranged from about 200 J

g-1 to 250 J g-1, whereas higher values between about 300 J g-1 to 550 J g-1 were

obtained for the monomers, depending on the type of monomers and the level of

NaOH content. It is likely that the prior condensation of the methylol groups during

the making of the resoles could be a factor contributing to the lower ΔHT values for

the resoles.

8.3.5 Effects of NaOH/ P molar ratio on the evolution of activation energy Ea

As described in Chapter 3, activation energy (Ea) for increasing extent of

conversion α was calculated according to the equation:

ln (Φ/Ti 2) = - Eα

/ RTi + ln (RA / Eα) 3.7






For each NaOH : P molar ratios, thermograms were recorded at scanning rates (2.5,

5, 10 and 20) oC min-1 in the range 25oC up to 250oC. The temperature (Tα) at




with NaOH / P molar ratio 0.30 as an example, Table 8.1 depicts the values of Tα at

the four scan rates. From these values the linear regression analysis function of

SigmaPlot version 7.1 (from SPSS Inc.) was used to generate linear graphs of

ln(Φ/Tα 2) vs. 1/Tα at a set confidence level of 95 %. Figure 8.6 shows the linear

regression graphs between α = 0.05 and α = 0.95 and the corresponding square of

the correlation coefficient (r2) values. The equations of the linear graphs were then

generated by the SigmaPlot software, from which the values of Eα at different α

151

could be manually calculated. Maximum error in Ea values obtained from the

triplicate DSC runs was 0.5 %.

Table 8.1: The DSC Pyris computer software generated values of Tα at four scan rates (for NaOH / P molar ratio of 0.30) and the corresponding values for the dependent and independent variables for equation 3.7.

Conversion α

Scan rate Φ

(°C min-1)

Tα (°C)

Tα (K)

(1/Tα) x 10-3 (K)

Ln (Φ/ Tα2)

0.02 2.5 82.878 356.078 2.8083734 12.66659 5 96.412 369.612 2.7055399 13.434346 10 102.706 375.906 2.6602395 14.161263 20 116.069 389.269 2.5689176 14.924274

0.05 2.5 89.911 363.111 2.7539788 12.705708 5 102.216 375.416 2.6637117 13.465507 10 111.447 384.647 2.5997863 14.207237 20 124.483 397.683 2.5145656 14.967043

0.1 2.5 97.12 370.32 2.7003672 12.745026 5 110.137 383.337 2.6086707 13.507267 10 119.778 392.978 2.5446717 14.250092 20 132.793 405.993 2.4630967 15.008404

0.15 2.5 102.661 375.861 2.660558 12.77473 5 115.562 388.762 2.5722679 13.535373 10 125.364 398.564 2.5090073 14.278321 20 139.257 412.457 2.4244952 15.039996

0.2 2.5 106.824 380.024 2.6314128 12.79676 5 119.414 392.614 2.5470309 13.555092 10 130.085 403.285 2.479636 14.301872 20 143.113 416.313 2.4020389 15.058607

0.25 2.5 110.585 383.785 2.6056255 12.816456 5 123.287 396.487 2.5221508 13.574724 10 133.286 406.486 2.4601093 14.317684 20 146.886 420.086 2.380465 15.076651

0.3 2.5 114.187 387.387 2.5813979 12.835139 5 126.306 399.506 2.5030913 13.589895 10 136.419 409.619 2.441293 14.33304 20 149.806 423.006 2.3640327 15.090505

0.35 2.5 117.899 391.099 2.5568974 12.854212 5 129.037 402.237 2.4860965 13.603521 10 139.265 412.465 2.4244481 14.346888 20 152.431 425.631 2.3494529 15.102878

152

0.4 2.5 121.249 394.449 2.535182 12.87127

5 131.518 404.718 2.4708563 13.615819 10 141.956 415.156 2.4087331 14.359894 20 154.335 427.535 2.3389898 15.111805

0.45 2.5 124.206 397.406 2.5163183 12.886208 5 133.864 407.064 2.4566162 13.627379 10 143.698 416.898 2.3986683 14.368268 20 156.005 429.205 2.329889 15.119602

0.5 2.5 126.559 399.759 2.5015072 12.898014 5 136.605 409.805 2.440185 13.640801 10 145.453 418.653 2.388613 14.37667 20 157.401 430.601 2.3223355 15.126096

0.55 2.5 129.173 402.373 2.4852562 12.91105 5 138.973 412.173 2.4261657 13.652324 10 148.103 421.303 2.3735886 14.38929 20 159.256 432.456 2.312374 15.134693

0.6 2.5 131.826 405.026 2.4689773 12.924193 5 141.415 414.615 2.4118761 13.664139 10 150.088 423.288 2.3624577 14.398691 20 161.637 434.837 2.2997123 15.145675

0.65 2.5 133.761 406.961 2.4572379 12.933725 5 143.632 416.832 2.3990481 13.674804 10 152.054 425.254 2.3515358 14.407958 20 163.592 436.792 2.2894192 15.154646

0.7 2.5 135.704 408.904 2.4455618 12.943252 5 145.227 418.427 2.3899031 13.682443 10 154.138 427.338 2.340068 14.417736 20 165.587 438.787 2.2790101 15.16376

0.75 2.5 137.995 411.195 2.4319362 12.954426 5 147.929 421.129 2.3745693 13.695316 10 156.392 429.592 2.3277901 14.428257 20 168.503 441.703 2.2639647 15.177008

0.8 2.5 139.856 413.056 2.4209792 12.963457 5 150.408 423.608 2.3606731 13.707055 10 159.915 433.115 2.3088556 14.444592 20 170.795 443.995 2.2522776 15.187359

0.85 2.5 141.581 414.781 2.4109108 12.971792 5 151.994 425.194 2.3518676 13.714529 10 161.676 434.876 2.2995061 14.452707 20 173.348 446.548 2.2394009 15.198826

0.88 2.5 143.053 416.253 2.4023851 12.978877 5 153.924 427.124 2.3412405 13.723587 10 163.087 436.287 2.2920692 14.459186 20 175.791 448.991 2.2272161 15.209738

153

0.9 2.5 145.653 418.853 2.3874725 12.991331 5 155.751 428.951 2.3312686 13.732123 10 165.585 438.785 2.2790205 14.470604 20 179.055 452.255 2.2111419 15.224225

0.95 2.5 148.954 422.154 2.3688038 13.007031 5 159.221 432.421 2.3125611 13.748237 10 169.545 442.745 2.2586365 14.488573 20 184.111 457.311 2.1866957 15.24646

0.98 2.5 152.233 425.433 2.3505464 13.022506 5 164.492 437.692 2.2847116 13.772469 10 175.022 448.222 2.2310373 14.513162 20 188.981 462.181 2.1636545 15.267645

Resole

1/Tα x 10-3 (K)

2.15 2.20 2.25 2.30 2.35 2.40 2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95

ln( φ

/ T

α2 )

12.5

13.0

13.5

14.0

14.5

15.0


Figure 8.6: Graphs of ln(Φ/Tα 2) vs. 1/Tα between α = 0.05 and α = 0.95 and the

corresponding square of the correlation coefficient (r2) values for a resole sample having NaOH / P molar ratio 0.50.

154

The cure of a PF resole is greatly influenced by the types of monomers present and

the reaction pathways to form methylene linkages (as well as ether linkages under

acidic or neutral conditions). As discussed in chapter 2, the formation of the

methylene linkages in the resole usually involves the reaction of a methylol group

either with another methylol group, or with a proton on the aromatic ring (Schemes

V and VI, respectively, Figure 2.2). Due to the high reactivity of para-methylol

groups in the condensation reactions, methylene linkages formed are mainly in the

form of (o,p) and (p,p) linkages with the (o,o) linkages rarely forming [18-20].

Various studies using DSC have also indicated that the cure of PF resoles is

complicated by the diffusion limitation mechanism, particularly towards the later

phase of the cure, as shown by a corresponding decrease in the apparent activation

energy Ea [21-23].

This section presents the results on the dependence of apparent activation energy Ea

on the degree of conversion for the resoles having different NaOH / P molar ratios.

This information provides insights into the relative contributions of (o,p) and (p,p)

methylene linkage formation throughout the cure process, as well as the role of the

diffusion limitation mechanism, as a function of NaOH / P molar ratio.

Figure 8.7 shows the evolution of apparent activation energy Ea for the resoles with

different NaOH / P molar ratios as a function of the degree of conversion. Similar

to the cases of monomers, the initial cure stage of the resoles could be affected by

the diffusion effect and will not be investigated in the present work. Instead, the

following discussion focuses on the evolution of Ea when conversion was higher

than 0.1.

155

PF Resoles

Conversion (α)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Act

ivat

ion

ener

gy (E

a, kJ

mol

-1)

50

60

70

80

90

100

110

120


Figure 8.7: Effects of NaOH / P on the evolution of apparent activation energy Ea for the resoles as a function of the degree of conversion. Maximum error from the triplicate DSC runs was 0.5 %. Error bars are not included in the graph since they were close to the font size of the data points.

As can be seen in Figure 8.7, for the resole with NaOH / P = 0.30, there was a

steady rise of Ea from about 60 kJ mole-1 at conversion of 0.1 to 109 kJ mole-1 as

the conversion reached about 0.6. Thereafter, Ea gradually decreased as the cure

progressed and had a value of about 105 kJ mole-1 at the end of the cure. Although

a similar pattern of dependence of Ea on conversion was also observed for the

resoles with higher molar ratios, there were also clear differences. In particular, for

the resole with NaOH / P = 0.50, Ea started off at a higher value of about 80 kJ

mole-1 and progressively increased to the maximum value of 111 kJ mole-1 at

conversion of 0.7, before declining steadily towards the end of the cure. The

decline of Ea was more significant compared to the 0.30 molar ratio resole and a

value of 101 kJ mole-1 was reached at the end of the cure. Increasing the NaOH / P

molar ratio to 0.70 had the effect of further increasing Ea to about 90 kJ mole-1 at

156

the beginning of the cure. A similar increasing dependence of Ea was also observed

up to about 0.7 conversion, where Ea reached the maximum value of about 115 kJ

mole-1. The subsequent decline of Ea for this resole was even more extensive with a

lower Ea of 93 kJ mole-1 obtained at the end of the cure.

As discussed in previous chapters, the shape of the dependence of Ea on conversion

degree is determined by the ratio of the partial contributions of individual reactions

to the overall reaction process. Given that the most important condensation

reactions during the cure of the resoles involve the formation of (p,p) and (o,p)

methylene linkages and that the para-position is more reactive than the ortho-

position, it is suggested that the partial contributions of the (p,p) linkage reactions

were dominant at low conversions and that the (o,p) linkage reactions became more

dominant as the cure proceeded. This would result in an increasing dependence of

Ea on conversion observed in the earlier stage of the cure of the resoles. It is noted

that the higher values of Ea for resoles having higher NaOH / P ratio in this stage

are consistent with the retardation effect of NaOH caused by the sodium ring

complex mechanism.

The descending dependence of Ea for all resoles when conversions were higher

than 0.6 – 0.7 is likely due to the effect of the diffusion limitation mechanism

which became increasingly important as the cure proceeded. The differences in the

extent to which Ea declined between different resoles provide insights into factors

that may exacerbate the effect of this mechanism. The DSC data showed that the

decline of Ea was more extensive for resoles with higher NaOH / P ratios,

suggesting a more important role of the diffusion limitation mechanism for these

resoles. A possible reason for this behavior is related to the types of addition

products formed. As shown in the study of the monomers, the effect of the

diffusion limitation mechanism is most extensive in the case of TMP and becomes

less important for DMP, whereas there is no evidence that this mechanism operates

in the case of MMP. Given that the degree of methylol substitution increases with

increasing NaOH content, it is possible that the resoles with higher NaOH / P ratios

would have higher proportions of DMP and TMP, and therefore would be more

affected by the diffusion limitation mechanism. An excessive amount of NaOH is

157

another factor that may limit molecular diffusion, as shown in the study of the

monomers. Therefore, the higher amount of NaOH in resoles with higher NaOH / P

ratios could also exacerbate the severity of the diffusion limitation mechanism.

8.4 Summary

In this chapter, the thermochemical properties of PF resoles having different NaOH

/ P molar ratios were investigated. This is the first time the model-free kinetic

analysis of DSC data has been applied to investigate the variation of activation

energy, Ea, during the cure of PF resoles having different NaOH / P molar ratios.

GPC and gel time techniques have also been used to provide complementary

information. The results obtained from both resole and monomer studies provided

insights into the operation of the sodium complex ring mechanism, as well as the

effects of NaOH content and methylol substitution on the diffusion limitation

mechanism.

It was found that the weight-average molecular weight Mw and the polydispersity

of the resoles increased with increasing NaOH / P ratio. This was suggested to be

due to different extents of condensation in the resoles to reach a constant viscosity

end point. The gel time measurements and DSC data showed that the cure kinetics

of the resoles decreased as NaOH content was increased, which is consistent with

the retardation effect caused by the sodium ring complex mechanism. On the basis

of the study of the monomers and GPC data, it was argued that the operation of this

mechanism was not confined to 2-MMP, but also applied to other methylol phenols

present in the resoles.

The DSC data also showed that the activation energies of the cure reactions

increased with increasing NaOH / P ratio. From the data on the dependence of Ea

on the extent of conversion, it was suggested that the cure of the resoles proceeded

through two stages. The first stage is characterised by an ascending trend of Ea up

to conversion of 0.6 – 0.7. It was proposed that during this stage, the partial

contributions of reactions to form the (p,p) linkages were dominant at low

conversions and that the (o,p) linkage reactions became more significant as the cure

proceeded. The second stage is characterised by a descending trend of Ea to the end

158

of the cure, which suggested an increasing contribution of the diffusion limitation

mechanism. The effect of this mechanism was more extensive for the resoles

having higher NaOH / P ratio. This was attributed to a higher degree of methylol

substitution and a higher amount of NaOH present in these resoles, both of which

were shown in the study of monomers to have the effect of exacerbating the

severity of the diffusion limitation mechanism.

The above findings have practical implications in the development of PF resole

adhesive systems capable of curing faster at lower temperatures. As described in

Chapter 2, it is commonly thought that a higher degree of methylolation would lead

to more rigid structures with more three dimensional cross-linking and that the

addition of NaOH as a catalyst would speed up the cure process. The findings of

the current research show that increasing the degree of methylolation and the

amount of NaOH would increase the contributions of the diffusion limitation

mechanism and the retardation effect, which in turn would slow down the

condensation reactions. Clearly, for PF resole formulations with a particular F / P

molar ratio, there is an optimal level of NaOH / P molar ratio where the cross-

linking reactions are encouraged and the diffusion mechanism is minimised. The

present results indicate that for a system with a F / P molar ratio of 2, which is

commonly used in the industry, a NaOH / P ratio of 3 is sufficient to produce

resoles with a fully cross-linked network. Higher NaOH / P ratios would slow

down the cure reactions due to increasing importance of both the sodium ring

complex and the diffusion limitation mechanisms.

8.5 References

1. Y. Yazaki, P. J. Collins, M. J. Reilly, S. D. Terrill, T. Nikpour, “Fast-

Curing Phenol-Formaldehyde (PF) Resins: Part 1. Molecular Weight

Distribution of PF Resins”, Holzforschung 48, 42-48 (1994).




159






J. Appl. Polym. Sci 83, 1415-1424 (2002).

5. R.A. Haupt and S. Waago, “The Ionic Nature of the Phenol-Formaldehyde

Condensation Reaction and Its Effect on Polymer Propreties”, Wood

Adhesives, 220-226 (1995).






8. R.A. Haupt and T. Sellers, Jr., “Characterizations of Phenol-Formaldehyde

Resol Resin”, Ind. Eng. Chem. Res. 33, 693-697 (1994).


Solid-State Physicochemical Study of Resoles with Variable F / P Ratios”,

Polymer 37(4), 639-650 (1996).



Appl. Polym. Sci. 87, 433-440 (2003).






Polym. Sci. 66, 1183-1193 (1997).

160

13. P. Luukko, L. Alvila, T. Holopainen, J. Rainio and T.T. Pakkanen, “Effect

of Alkalinity on the Structure of Phenol-Formaldehyde Resol Resins”, J.

Appl. Polym. Sci. 82, 258-262 (2001).


Springer-Verlag, New York, 1979, p. 41-43.






Polymer 37(6), 955-964 (1996).




18. D. D. Werstler, “Quantitative 13C NMR Characterization of Aqueous

Formaldehyde Resins: 1. Phenol-Formaldehyde Resins Polymer 27, 750-

756 (1986).

19. S. So and A. Rudin, “Analysis of the Formation and Curing Reactions of

Resole Phenolics”, J. Appl. Polym. Sci. 41, 205-232 (1990).

20. M.G. Kim, Y. Wu and L.W. Amos, “Polymer Structure of Cured Alkaline

Phenol-Formaldehyde Resol Resins with Respect to Resin Synthesis Mole

Ratio and Oxidative Side Reactions”, J. Polym. Sci. Part A: Polym. Chem.

35, 3275-3285 (1997).

21. E. Kiran and R. Iyer, “Cure Behaviour of Paper-Phenolic Composite

System: Kinetic Modeling”, J. Appl. Polym. Sci. 51, 353-364 (1994).




161



Appl. Polym. Sci. 87, 433-440 (2003).

162

Chapter 9

Conclusions and Future Work

In the present study, the thermochemical properties of individual PF monomers as a

function of NaOH in the temperature range up to 250°C were investigated using

DSC. DSC experiments were conducted in the dynamic mode and the kinetic

analysis of the DSC data was carried out using the model-free method. A particular

focus of the study was on the changes in the shape of the DSC curves, the

dependence of apparent activation energy Ea on the degree of conversion and the

enthalpy of reactions ΔHT as the NaOH content in the monomers was varied. These

changes, together with the established chemistry of condensation reactions, were

used to elucidate relevant reaction mechanisms during the cure.

A key mechanism that was suggested to operate in the presence of NaOH is the

formation of the sodium ring complex that diminishes the capacity of the

monomers to participate in condensation reactions, particularly those involving

ortho-methylol groups. At a particular NaOH level, the monomer molecules may

have a range of reactivity, depending on whether they are associated with Na+.

Such variation in the reactivity and the different condensation possibilities of the

monomers are critical factors governing the cure behaviour.

Another important mechanism that was suggested to operate during the cure is the

limitation on molecular diffusion that had the effect of slowing down the


mechanism was more pronounced with increases in the amount of the methylol

groups in the monomers and in the levels of NaOH. The increase in the extent of

cross-linking is another factor that exacerbates the significance of this mechanism

as the cure proceeds.

Whilst the effects of these mechanisms are consistently manifested in various DSC

parameters, the results regarding the dependence of Ea on the degree of conversion

for the monomers at different NaOH levels are particularly useful in providing

163

insights into possible pathways that condensation reactions may proceed. In

particular, the partial contributions of reactions to form (p,p) and (o,p) linkages, as

well as (o,o) linkages in rare occasions, at different stages of the cure were

proposed for each monomer at different NaOH levels.

Apart from the focus on PF monomers, this study also investigated the effects of

NaOH / P ratio on the cure properties of PF resoles as a whole. A particular

emphasis of the study was on the use the model-free kinetic analysis of DSC data

to investigate the variation of activation energy Ea during the cure of the resoles,

and from here, to obtain further insights into the cure mechanisms. The outcomes

of both the studies of the monomers and the resoles are complementary to each

other and provide a consistent overall picture of relevant mechanisms operating

during the cure process. In particular, the sodium ring complex mechanism that had

the retardation effect on the cure kinetics of the resoles was demonstrated

independently by both gel time measurements and DSC data. It was suggested that

the operation of this mechanism is not confined to 2-MMP, but also applies to other

methylol phenols present in the resoles.

On the basis of the data on the dependence of Ea on the extent of conversion, it was

suggested that the cure of the resoles proceeded through two stages. The first stage

is characterised by an ascending trend of Ea up to conversion of 0.6 – 0.7, followed

by the second stage which exhibited a descending trend of Ea to the end of the cure

process. It was proposed that the partial contribution of reactions to form the (p,p)

linkages are dominant at low conversions and that the contribution of the (o,p)

linkage reactions become more significant as the cure proceeds. The descending

trend of Ea was attributed to the increasing importance of the diffusion limitation

mechanism in the second stage of the cure. The effect of this mechanism was more

extensive for the resoles having higher NaOH / P ratio. This was attributed to a

higher degree of methylol substitution and a higher amount of NaOH present in

these resoles, both of which were shown in the study of monomers to have the

effect of exacerbating the severity of the diffusion limitation mechanism.

The findings in the present study have practical implications in the development of

PF resole adhesive systems capable of curing faster at lower temperatures. Indeed,

164

it is commonly thought that higher degree of methylolation would lead to more

rigid structures with more three dimensional cross-linking and that the addition of

NaOH as a catalyst would speed up the cure process. The present research shows

that increasing the degree of methylolation and the amount of NaOH would

increase the contributions of the diffusion limitation mechanism and the retardation

effect, which in turn would slow down the condensation reactions. Clearly, for PF

resole formulations with a particular F / P molar ratio, there is an optimal level of

NaOH / P molar ratio where the cross-linking reactions are encouraged and the

diffusion mechanism is minimized. The present results indicate that for a system

with a F / P molar ratio of 2, which is commonly used in the industry, a NaOH / P

ratio of 3 is sufficient to produce resoles with fully cross-linked network. Higher

NaOH / P ratio would slow down the cure reactions due to increasing importance

of both the sodium ring complex and the diffusion limitation mechanisms.

The present study proposes possible pathways that condensation reactions may

proceed, as well as possible contributions of reactions to form (p,p) and (o,p)

linkages, during the cure of different monomers and PF resoles. Further work is

required to confirm these possibilities. The work would involve analysing the

chemical structures of the products at different stages of the cure using

complementary techniques such as NMR and FTIR. The confirmation or otherwise

of these proposed mechanisms using these techniques is necessary to improve the

understanding of the cure mechanism of PF resoles.

Another area that requires further research is the effects of F / P molar ratio on the

cure properties of PF resoles. As reviewed in Chapter 2, the effects of F / P molar

ratio have been investigated in various studies. However, these studies commonly

ignore the complexity of the reactions and assume that Ea does not change with

temperature. As has been shown throughout the present study, the model-free DSC

method avoids the assumption of homogeneous reaction kinetics and allows the

monitoring of different chemical reactions with different kinetics via the

dependence of Ea on the degree of the cure. Therefore, the issue of the effects of F /

P molar ratio should be revisited using the model-free method to analyse the DSC

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data, together with NMR and FTIR to provide complementary structural

information at different stages of the cure, as suggested above.

These additional data will add to the knowledge obtained in the present study and

aid in the development of PF resole systems capable of bonding under a wide range

of gluing conditions with the ability to cure faster at lower temperatures.

thermochemical properties of methylol phenol monomers and

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