thermal oxidative degradation kinetics of flame-retarded polypropylene with intumescent...

Polymer Degradation and Stability 92 (2007) 280e291www.elsevier.com/locate/polydegstab

Thermal oxidative degradation kinetics of flame-retarded polypropylenewith intumescent flame-retardant master batches in situ

prepared in twin-screw extruder

Yinghong Chen, Qi Wang*

The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University,

24, Southern Section 1, Yihuan Road, Chengdu 610065, Sichuan, PR China

Received 16 September 2006; received in revised form 6 November 2006; accepted 16 November 2006

Available online 27 December 2006

Abstract

The thermal oxidative degradation kinetics of pure PP and the flame-retarded (FR) PP materials with intumescent flame-retardant (IFR) mas-ter batches in situ prepared in twin-screw extruder were investigated using Kissinger method, FlynneWalleOzawa method and CoatseRedfernmethod. The results showed that the activation energy order of PP and FR PP samples with different blowing agent/char former ratios obtainedby Kissinger method agrees well with that obtained by CoatseRedfern one, which well illustrates the relationship between the composition ofIFRs and their flame-retardancy, i.e. FR material with richer carbonization agent has higher activation energy for thermal oxidative degradation,hence leading to a better flame-retardancy. For FlynneWalleOzawa method, due to its adoption of Doyle approximation, the obtained activationenergy and its order of samples are very different from those of both Kissinger and CoatseRedfern methods. Criado method was finally used todetermine the degradation reaction mechanism of various samples.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Polypropylene; Intumescent flame-retardant; Thermal oxidative degradation; Kinetics

1. Introduction

In the recent years the interest in flame-retardants (FRs)used in polyolefin (especially polypropylene, i.e. PP) hasbeen focused on the intumescent flame-retardants (IFRs)[1e9] due to their halogen-free, low toxicity, absence ofdioxins and low smoke occurring in fire accidents. IFRs aregenerally composed of three components, i.e. carbonizationagent, acid catalyst and blowing agent. The involved intumes-cent mechanism of IFRs acts through the following process,i.e. the charred layers are firstly produced by the dehydrationof carbonization agent under the catalytic effect of acid cata-lyst and simultaneously expanded by the inert gases releasedfrom the blowing agent upon heating. The formed non-

* Corresponding author. Tel.: þ86 28 85405133; fax: þ86 28 85402465.

E-mail address: [email protected] (Q. Wang).

0141-3910/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.11.004

flammable and multi-cellular charred layer almost instantlystart to provide an efficient shielding and insulation for the un-derlying polymeric matrix against the direct contact with theflame and oxygen and also against the heat transfer. The un-derlying matrix is then effectively protected. There are manystudies on the applications of the intumescent flame-retardantin polypropylene [1,2,5e9]. The concerned IFRs generally in-clude the following systems, i.e. ammonium polyphosphateþmelamineþ pentaerythritol [3,6,7,10,11], ammonium poly-phosphateþ triazine compound [12,13], melamineþintrinsical flame-retarded polymer [14e16], macromolecularchar former contained [2,5,17,18] system, mixture of mela-mine phosphate with pentaerythritol [19,20] or their reactionproduct [8,9], single molecular IFR [21], and so on.

From those contributions above mentioned, it is known thatthe flame-retardancy of materials not only depends on theirthermal stability but also on their decomposition rate, char-forming rate and char yield [8,12]. For example, our previous

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281Y. Chen, Q. Wang / Polymer Degradation and Stability 92 (2007) 280e291

studies [9] showed that below 400 �C PP has better thermalstability than the other flame-retarded materials but poorerflame-retardancy. The reason for this was attributed to theviolent decomposition of PP in the temperature range400e450 �C and almost no char yield. As a consequence,the thermal decomposition details of the materials influencetheir flame-retardancy to a considerable degree and deservea deep investigation. The thermal decomposition behavior ofa material can be measured by thermogravimetric analysis(TGA). According to the TGA data, the kinetic parametersof thermal decomposition of material such as apparentactivation energy (Ea), pre-exponential factor (A), apparent re-action order (n) and rate constant (k) can be calculated usingvarious kinetic models such as Friedman [22], Kissinger[23], CoatseRedfern [24], FlynneWalleOzawa [25,26] andHorowitzeMetzger [27] methods. Consequently, the thermaldecomposition of the flame-retarded polymeric material canbe quantitatively described to reveal the flame-retardingmechanism from the viewpoint of kinetics. There are a lotof works involved in the investigation of the thermal orthermo-oxidative degradation kinetics of the flame-retardedpolymeric material [28e32]. Lesnikovich et al. investigatedthe kinetics of oxidative degradation of flame-retardant poly-esters [poly(ethylene terephthalate)] [28] and also the effectof hexabromocyclododecane (HBCD) as a fire-retardant addi-tive on PP thermal degradation [29]. Wang et al. [30e32]systematically investigated the thermal oxidative degradationbehavior of poly(ethylene terephthalate) (PET), the flame-retardant copolyester containing phosphorous linked pendentgroups and its blend with montmorillonite nanocomposite atdifferent heating rates in air using Kissinger and FlynneWalleOzawa methods. In particular, there are also a fewstudies on the thermal or thermo-oxidative degradation kinet-ics of the flame-retarded polypropylene with intumescentflame-retardants (IFRs) [5,17,33e35], e.g. Neininger et al.[33] utilized a mathematical model i.e. simplified kineticscheme to predict the global kinetics of the thermal degrada-tion of IFRs added in textile-reinforced composite. They in-vestigated the influence of the increased heating rates on thechar yield and also the relationship between the Arrhenius pa-rameters of volatile-forming reaction and those of char-form-ing reaction. Almeras et al. [5,17] modeled the thermal andthermo-oxidative degradations of intumescent PP/APP/PA6/EVA19 composites using invariant kinetic parameters (IKP)method and distinguished the two degradation steps of aboveintumescent formulations. Siat et al. [34,35] investigated thethermal and thermo-oxidative degradations of PA6, PA6/APP, PA6/EVA8/APP materials also using IKP method. Theirstudies showed that APP decreases the stability of polymermatrix and provides a protection through ablative processand EVA, however, plays a part in reduction of fuel flowrate contributing to the fire retardancy of the blends.

However, according to those studies mentioned above, theinfluence of the compositions of IFRs (blowing agent/char for-mer ratio, etc.) on the kinetic parameters of thermal or thermo-oxidative decomposition of the flame-retarded PP material isnot reported, that is, the relationship between the thermal

kinetic parameters of the flame-retarded material withdifferent compositions of IFRs and the corresponding flame-retardancy is still not clear. Based on our previouscontributions [8,9], in this paper, we studied the kinetics ofthe thermo-oxidative degradation of the flame-retarded PP ma-terials with IFR master batches with different blowing agent/char former ratios prepared through twin-screw reactiveextrusion [8] using different kinetic models includingKissinger, FlynneWalleOzawa and CoatseRedfern methods.The kinetic parameters obtained by the above different modelswere then compared and meanwhile were connected with theflame-retardancy of the related materials. Kissinger methodand FlynneWalleOzawa method were used because theyare mostly used in the literatures and can conveniently obtainthe relative activation energy without prior knowledge of thereaction mechanism and the reaction order. In addition,CoatseRedfern method was used because it can give usmore information about the degradation process such asactivation energy (Ea), pre-exponential factor (A) and reactionorder (n). We also determined the thermo-oxidative decompo-sition reaction mechanism of the corresponding flame-retardedmaterials using Criado method.

1.1. Kinetic methods

In these methods, the involved usual symbols are summa-rized as follows:

Ea: apparent activation energies (kJ/mol�1)A: pre-exponential factor (min�1)n: apparent reaction orderR: gas constant (8.3136 J mol�1 K�1)T: absolute temperature (K)a: conversion degree or fractional weight losst: reaction time (s)b: heating rate in thermogravimetric analysis (K min�1)k: rate constant associated with the temperature

The basic assumption of the thermal kinetics is that the Ar-rhenius equation representative of the reaction rate depen-dence of the temperature can be applied in the thermalanalysis reaction, i.e., for a simple reaction Asolid / BsolidþCgas, its reaction rate da/dt can be written as

da

dt¼ k f ðaÞ ð1Þ

where a¼ (W0�Wt)/(W0�WN) (W0, Wt and WN are the ini-tial, actual and final weights of the sample, respectively, in theTG test) and the function f(a) depends on the specific decom-position reaction mechanism.

The Arrhenius equation has the following formula:

k ¼ Ae�Ea=RT ð2Þ

The combination of Eqs. (1) with (2) gives the followingrelationship:

282 Y. Chen, Q. Wang / Polymer Degradation and Stability 92 (2007) 280e291

da

dt¼ Af ðaÞe�Ea=RT ð3Þ

If the constant heating rate of TG analysis process is set asb¼ dT/dt, the conversion degree a can be expressed as thefunction of the temperature. However, the temperature is de-pendent on the heating time. Therefore

da

dt¼ da

dT

dT

dt¼ b

da

dTð4Þ

The combination of Eqs. (3) and (4) gives the followingexpression:

da

dT¼ A

be�Ea=RTf ðaÞ ð5Þ

Integration of the both sides of above equation and rear-rangement give

gðaÞ ¼Za

0

da

f ðaÞ ¼A

b

ZT0

e�Ea=RT dT ð6Þ

where g(a) is the integral function of conversion degree a. Asfar as the polymer is concerned, its degradation process, gen-erally, obeys the sigmoidal or deceleration functions. For thedifferent solid reaction mechanism, g(a) has different expres-sions (Table 1) [36]. These expressions can be used to predict

Table 1

The expressions of g(a) for the most frequently used reaction mechanism of

solid state processes [36]

Mechanism g(a) Solid state process description

Sigmoidal functions

A2 [�ln(1� a)]1/2 Nucleation and growth:

Avrami Eq. (1)


Avrami Eq. (2)


Avrami Eq. (3)

Deceleration functions

R2 [1� (1� a)1/2] Phase boundary

controlled reaction: contraction

area

R3 [1� (1� a)1/3] Phase boundary

controlled reaction: contraction

volume

D1 a2 One-D diffusion

D2 (1� a)ln(1� a)þ a Two-D diffusion

D3 [1� (1� a)1/3]2 Three-D diffusion:

Jander equation

D4 (1� 2/3a)� (1� a)2/3 Three-D diffusion:

GinstlingeBrounshtein equation

F1 �ln(1� a) Random nucleation

having one nucleus

on individual particle

F2 1/(1� a) Random nucleation

having two nucleus


F3 1/(1� a)2 Random nucleation

having two nucleus


the solid reaction mechanism reflected by the dynamic TGcurves.

Based on the differential Eq. (5) and the integral Eq. (6),different approximate expressions (Table 1) were proposedto fit the TG data. These approximate expressions are the basisof study on the thermal kinetics of the flame-retarded materialsin this paper.

1.2. Kissinger method (differential method) [23]

Generally, Kissinger method was used to calculate the acti-vation energy of the solid reaction, which was obtained fromthe relationship between the logarithm of b and the reciprocalof the absolute temperature at the maximum reaction rate (in-flexion point).

The Kissinger method adopts the following equation:

ln

�b

T2max

�¼�

lnAR

Ea

þ ln�nð1� amaxÞn�1�� Ea

RTmax

ð7Þ

where Tmax corresponds to the temperature at the inflexionpoint of TG curve and amax is the conversion degree at the in-flexion point.

Plot of ln(b/Tmax2 ) against �1/Tmax produces a fitted straight

line. According to the slope of this straight line (Ea/R) the ap-parent activation energy Ea can be calculated, i.e. if slopeB¼ Ea/R, then Ea¼ BR. The advantage of the Kissinger modelis that the apparent activation energy can be obtained withoutthe knowledge of any thermal degradation reaction mechanismin advance.

1.3. FlynneWalleOzawa method (integration method)[25,26]

Eq. (6) is integrated using Doyle approximation [37].The logarithm of the integrated results leads to the FlynneWalleOzawa equation:

log b¼�0:457Ea

RTþ�

log

�AEa

gðaÞR

� 2:315

�ð8Þ

At a given conversion degree a, plot of log b against �1/Tmakes a fitted straight line with a slope 0.457Ea/R. Obviously,the apparent activation energy Ea at the given a value can becalculated from the value of 0.457Ea/R. The calculated Ea ofthis method is also independent of the thermal decompositionreaction mechanism of the flame-retarded material.

1.4. CoatseRedfern method [24,38]

CoatseRedfern method utilizes the following relations(Eq. (6)):

Za0

da

f ðaÞ ¼Za

0

da

ð1� aÞn ¼ gðaÞ ¼ A

b

ZT0

e�Ea=RT dT ð9Þ

where, f(a)¼ (1� a)n.


Let x¼ Ea/RT and then Eq. (9) becomes

gðaÞ ¼ AEa

bR

ZNx

e�x

x2dx ¼ AEa

bRPðxÞ

¼ AEa

bR

�e�x

x2

�1� 2!

xþ 3!

x2� 4!

x3þ/

��

yAEa

bR

�e�x

x2

�1� 2

x

��

¼ AEa

bR

(e�Ea=RT

ðEa=RTÞ2�

1� 2

ðEa=RTÞ

�)

¼ ART2

bEa

�1� 2RT

Ea

�e�Ea=RT ð10Þ

where, PðxÞ ¼RNx

ðe�x=x2Þ dx:

According to Eq. (9), g(a) can be written as different ex-pressions at different n values:when n¼ 1,

gðaÞ ¼ �lnð1� aÞ ð11aÞ

when n s 1,

gðaÞ ¼ 1

n� 1

�ð1� aÞ1�n�1

�ð11bÞ

The combination of Eq. (11) with Eq. (10) followed by therearrangement obtains

n¼ 1 ln

�� lnð1� aÞ

T2

�¼ ln

�AR

bEa

�1� 2RT

Ea

�� Ea

RTð12aÞ

ns1 ln

"1� ð1�aÞ1�n

T21� n

�#¼ ln

�AR

bEa

�1� 2RT

Ea

�� Ea

RT

ð12bÞGenerally, the logarithmic term on the right part of the

above equations is regarded as constant. Then, the determina-tion of the reaction order n can be finished by linear fitting ofthe left part of above equations’ dependence of �1/T. Then value at the best correlation coefficient (R) obtained is thereal reaction order, and the apparent activation energy Ea

and pre-exponential factor A can be hereby calculated.

1.5. Criado method [39]

The kinetic parameters (Ea, n and A) of the thermal decom-position reaction have been obtained by the non-isothermalanalytical method. The obeyed degradation reaction mecha-nism can be therefore determined using Criado method[39]. Criado et al. [39] proposed a method which can accu-rately determine the reaction mechanism in the solid reactionprocess.

Firstly, Criado et al. defined a type of Z(a) function

ZðaÞ ¼

dadt

�b

pðxÞT ð13Þ

where x¼ Ea/RT and p(x) is an approximate expressionobtained by integration against temperature, which cannot beexpressed by simple analysis formulas. Paterson [40] proposeda reasonable relationship between p(x) and P(x):

pðxÞ ¼ xexPðxÞ ð14ÞSenum and Yang [41] and Flynn [42] proposed an approx-

imate biquadratic rational expression of P(x):

PðxÞ ¼ e�x

x

x3þ 18x2þ 86xþ 96

x4þ 20x3þ 120x2 þ 240xþ 120ð15Þ

when x> 20, Eq. (15) can give error less than 10�5%, which isthe basis for analyzing the TG data in this paper.

Combining Eqs. (4), (13) and (14), the following relation-ship should be obtained:

ZðaÞ ¼ f ðaÞgðaÞ ð16ÞThe combination of Eqs. (5) with (16) results in the follow-

ing equation:

ZðaÞ ¼ b

AgðaÞda

dTeEa=RT ð17Þ

From Eqs. (4), (13) and (14), the following relationship canalso be obtained:

ZðaÞ ¼ da

dTx expðxÞPðxÞT ¼ da

dT

Ea

RTeEa=RTPðxÞT

¼ da

dT

Ea

ReEa=RTPðxÞ ð18Þ

where, x¼ Ea/RT, P(x) has the expression shown as Eq. (15),and da/dT can be calculated from the related DTG curve.

Eq. (17) was used to plot the master Z(a)ea curve of thedifferent reaction mechanisms shown in Table 1, andEq. (18) was used to plot the experimental Z(a)ea curveaccording to the TG data. The comparison of the masterZ(a)ea curve with the experimental Z(a)ea curve can easilyand accurately predict the reaction mechanism of the thermaldegradation reaction.

2. Experimental

2.1. Materials

The following materials were used as received: melaminephosphate (MP, commercial product), pentaerythritol (PER,chemically pure, supplied by Shanghai First Reagent Plant,China), polypropylene (T30s, MFI¼ 3.4 g per 10 min, as gran-ulate product supplied by Dushanzi Petrochemical Corpora-tion, Xinjiang, China).

2.2. Flame-retarded samples preparation

The IFR master batches with different blowing agent/charformer ratios, i.e MP/PER ratio, prepared by the reaction ofMP with PER using PP as carrier resin in a twin-screw


extruder reported in our previous work [8,9] were blendedwith polypropylene (PP) in the desired proportion (25 wt%IFR content in this paper) in a twin-screw extruder at190 �C. The extrudate was cut into pellets and was then injec-tion molded at 210 �C into samples for test.

2.3. Characterizations

The thermogravimetric analysis (TGA) was performed ona General V 4.1c Dupont TA2100 instrument thermal analyzerwith a heating rate of 5, 10, 15 and 25 �C/min in the temper-ature range of 25e600 �C and a dynamic air flow of 100ml/min. The amount of the sample used is 7e10 mg. Basedon the original weight loss dependence of the temperature,the first derivative data, i.e. DTG data were accordingly ob-tained (Fig. 1).

3. Results and discussion

Fig. 1 shows the TG and DTG curves of the flame-retarded PP materials with IFRs having different composi-tions, i.e. blowing agent/char former ratio (MP/PER ratio)

at different heating rates (5, 10, 15 and 25 �C/min) underair atmosphere. Almost all TG curves present a ‘‘C’’ typethermal decomposition process [36], i.e. only one thermal de-composition step in the temperature range 25e500 �C. Theunambiguous decomposition region is centered between 250and 400 �C. Relative to pure PP (Fig. 1a), the degradationof the flame-retarded PP material (Fig. 1bed) becomesmore slow. Compared with the TG curves done under nitro-gen atmosphere [8,9], the degradation regions of TG curvesof whether pure PP or IFR/PP composites shown in Fig. 1all shift to lower temperature, indicating that the presenceof oxygen decreases the thermal stability of material underheating conditions. In addition, for the flame-retarded PP ma-terial with deficiency of char former (sample with MP/PERmolar ratio 2.0, Fig. 1b), the formed charred layer experi-ences a faster thermo-oxidative degradation process in thetemperature range 350e600 �C and has a lower final charyield than the other two FR materials due to its poor thermalstability. Also, different than the char yield of other two FRmaterials, one of the sample with MP/PER molar ratio 2.0 in-creases instead with increasing the heating rate from 5 to25 �C/min. The possible reason can also be that the formed

Fig. 1. Experimental TG and DTG curves of different materials at different heating rates under air: (a) pure PP, (b) sample with MP/PER¼ 2.0, (c) sample with

MP/PER¼ 1.6 and (d) sample with MP/PER¼ 1.0.


charred layer of the sample at MP/PER molar ratio 2.0 haspoorer thermal stability than the other two samples withMP/PER molar ratios of 1.6 and 1.0 and easily degrades un-der oxygen atmosphere with a longer heating time (a lowerheating rate).

The temperature range used for data treatment of all sam-ples here is located in the main degradation region of materials,i.e. 250e400 �C. Because the TG curves of pure PP at 5 and10 �C/min are overlapped in the degradation zone, the curvesat 10e25 �C/min heating rates were only selected for use.

3.1. Kinetics analysis using Kissinger method

The inflection point temperatures of the TG curves of var-ious samples shown in Fig. 1 are included in Table 2. Accord-ing to Eq. (7), plots of ln(b/T2

max) against �1/Tmax produce thefitted straight lines as shown in Fig. 2. The obtained slopes ofvarious straight lines were used to calculate the correspondingapparent activation energy (Table 3). As can be seen, the rel-ativity of the various fitted straight lines is very good, showingthe feasibility of Kissinger method. Table 3 indicates that theapparent activation energy of thermal oxidative degradation ofPP is 74.10 kJ/mol. The apparent activation energy of theflame-retarded PP materials increases to 105.84, 117.98 and96.69 kJ/mol, respectively, after addition of PP-based IFRmaster batch with MP/PER molar ratios of 2.0, 1.6 and 1.0, re-vealing that addition of IFRs improves the thermal stability ofFR materials and hence contributes to the improvement of theflame-retardancy of material. The activation energy of the FRmaterials with different blowing agent/char former molar ra-tios decreases according to the following order: MP/PER¼ 1.6>MP/PER¼ 2.0>MP/PER¼ 1.0, which is wellcoincided with their flame-retarding property order, i.e. MP/PER¼ 1.6 (LOI value 31.5)>MP/PER¼ 2.0 (LOI value30.5)>MP/PER¼ 1.0 (LOI value 30) [8]. This indicatesthat IFR/PP FR blend with reasonable blowing agent/char for-mer (richer carbon source) has higher apparent activation en-ergy of thermal oxidative degradation, which is ascribed tothe better thermal stability of the charred layer with good qual-ity formed in the burning process. The further decompositionof the flame-retarded matrix PP is, therefore, prevented effec-tively, leading to better flame-retardancy. It can be seen thatthe relationship between the compositions of IFRs (blowingagent/char former ratio) and the flame-retardancy of the corre-sponding FR material can be well explained from the view-point of thermal kinetics.

Table 2

Inflection point temperature of TG curves for different materials at different

heating rates

Heating rate

(�C/min)

Tinflection point (�C)

Pure PP MP/PER¼ 2.0 MP/PER¼ 1.6 MP/PER¼ 1.0

5 e 294.44 288.78 294.83

10 302.08 311.61 305.26 312.56

15 314.68 323.71 311.81 323.70

25 333.65 333.22 324.35 338.49

3.2. Kinetics analysis using FlynneWalleOzawa method

FlynneWalleOzawa model can also be used to calculatethe activation energy of solid-phase reaction. Its differencefrom Kissinger method is that the treatment of the TG dataat the different heating rates and at the given conversion de-gree a is required. Then, according to Eq. (8), the plot oflog b against �1/T makes a fitted straight line. The slope ofthis straight line was used to calculate the activation energyof the involved system. If the straight lines fitted at differentconversion degrees a parallel each other, this method provesto be applicable to the investigated system. Because FlynneWalleOzawa method utilizes Doyle approximation, the calcu-lated values limited in the conversion degree range of 5e20%are reliable [37]. Here, the activation energy at the conversiondegree ranging from 5% to 80% is considered, where thevalue outside the range of 5e20% is only for reference.Fig. 3 shows the fitted results of our system using FlynneWalleOzawa method. One can see that in the a range of5e35% the fitted straight lines obtained at different a valuesfor pure PP or FR PP materials are almost parallel, indicatingthe applicability of FlynneWalleOzawa method to our sys-tem. From the slopes of the fitted straight lines of various sam-ples at different conversion degrees (Fig. 3), the correspondingactivation energy was calculated. Fig. 4 shows the plots of theobtained activation energy Ea as a function of a. It can be seenthat for pure PP Ea decreases very slowly with increasing a,indicating that on one side, the slope of the fitted straightline at various conversion degree does not vary much (soFlynneWalleOzawa method is suitable for pure PP in the to-tal range of the investigated conversion degree) and more im-portantly, the thermo-oxidative degradation of pure PP obeysa simple reaction mechanism [38,43]; on the other side, thethermal stability of PP is reduced and the thermo-oxidativedegradation is accelerated, leading to the loss of its flame-re-tardancy. However, as far as the FR PP materials with differ-ent blowing agent/char former ratios are concerned, the case iscompletely different: their Ea values increase with the increaseof a, where Ea increases slowly in a range of 5e40% and,however, sharply in a range of 40e80%. So, in a rangeover 40%, FlynneWalleOzawa method is not applicable tothe FR PP materials, revealing the complex reaction mecha-nism of the thermal oxidative degradation of FR PP materials.Above results obtained are the reflection of the flame-retard-ing role of IFRs played in PP matrix: when IFR/PP blendsare heated to decompose, the involved IFRs gradually formcharred layers covering the underlying matrix to prevent thePP matrix from further decomposition, hence leading to theincrease of activation energy of thermal decomposition ofFR materials. In the late decomposition, the formation ofthe charred layers covering the underlying PP matrix tendsto be complete and has a more strongly inhibitive effect onthe decomposition of PP matrix, causing the sharp increaseof the thermal degradation activation energy of FR materialsand showing the flame-retardancy. Table 4 shows the averageactivation energy of pure PP and FR PP materials with differ-ent blowing agent/char former ratios in a range of 5e20%. As


Fig. 2. Kissinger method applied to experimental TG data at different heating rates under air: (a) pure PP, (b) sample with MP/PER¼ 2.0, (c) sample with MP/

PER¼ 1.6 and (d) sample with MP/PER¼ 1.0.

can be seen, the difference between the activation energy ofvarious samples determined by FlynneWalleOzawa methodand that determined by Kissinger method is big. In addition,the activation energy order of various samples of bothmethods is not the same, e.g. for FlynneWalleOzawamethod, the order is MP/PER¼ 1.0>MP/PER¼ 1.6> purePP>MP/PER¼ 2.0 and on the contrary, for Kissingermethod, the order is MP/PER¼ 1.6>MP/PER¼ 2.0>MP/PER¼ 1.0> pure PP. The possible reason can be related tothe Doyle approximation made in the former, because theDoyle approximation has the limitation of a range of5e20% and, however, generally the inflection point involvedin Kissinger method far exceeds this range.

3.3. Kinetics analysis using CoatseRedfern method

Kissinger method and FlynneWalleOzawa method can beused to calculate activation energy Ea without concerning thesolid-phase reaction mechanism and have the advantages ofsimplicity and shortcut. But both methods have some difficul-ties in calculating kinetic parameters such as reaction order,and pre-exponential factor, etc. Furthermore, for both methodsthe range of treatment of the TG data is narrow, e.g. Kissinger

model only considers one point (inflection) on the TG curveand FlynneWalleOzawa model is also applicable only ina range of 5e20%. However, the CoatseRedfern methodcan deal with the main degradation region of TG curve of ma-terial and only requires the TG data at just one heating rate tocalculate the related reaction order n, reaction activation en-ergy Ea and pre-exponential factor A. Here, we processed theTG data of all samples at various heating rate (5, 10, 15 and25 �C/min) and expected to find some significative results.Firstly, we supposed that a certain thermal oxidative degrada-tion reaction has a specified reaction order n and thensubstituted the supposed n value into Eq. (12). The plot ofthe left part of Eq. (12) against �1/T was fitted by computerto calculate the correlation coefficient R. Above procedure

Table 3

The activation energies (Ea) obtained using the Kissinger method for different

samples

Samples Ea (kJ/mol) Correlation coefficient (R)

Pure PP 74.10 0.9992

MP/PER¼ 2.0 105.84 0.9952

MP/PER¼ 1.6 117.98 0.9973

MP/PER¼ 1.0 96.69 0.99995


Fig. 3. Plots of log b against �1/T at various conversion values (a) in the range 5e80%: (a) pure PP, (b) sample with MP/PER¼ 2.0, (c) sample with MP/

PER¼ 1.6 and (d) sample with MP/PER¼ 1.0.

was repeated till the best R value was obtained. The calculatedreaction order at the best R value is just our required one. Sub-sequently, the activation energy and the pre-exponential factorcan be calculated from the slope and intercept of the fittedstraight line, respectively.

Fig. 4. Activation energies obtained by FlynneWalleOzawa method versus

conversion degree (a) for different samples.

The fitted results including reaction order, activation energyand pre-exponential factor of the involved materials at variousheating rates are summarized in Table 5. Just as an example,Fig. 5 shows the linear fitting process of pure PP and FR PPmaterials with different blowing agent/char former ratios atthe preset reaction order at heating rate of only 15 �C/min ac-cording to Eq. (12). One can see that the heating rate, however,does not influence the reaction order of one sample much. Forexample, whatever heating rate has been used, the obtained re-action orders of pure PP and FR PP materials with MP/PERmolar ratios of 2.0, 1.6 and 1.0 always become close to 1.1,2.1, 2.45 and 1.75. Obviously, the thermal oxidative degrada-tion of PP can be considered close to 1-order reaction kineticsand possesses a simple degradation reaction mechanism.

Table 4

The average activation energies of various samples obtained by FlynneWalle

Ozawa method in the range 5e20% conversion degree

Samples Ea (kJ/mol)

Pure PP 109.54

MP/PER¼ 2.0 91.51

MP/PER¼ 1.6 127.10

MP/PER¼ 1.0 143.43


Table 5

The kinetic parameters of different samples at the optimum correlation coefficient obtained using CoatseRedfern method at different heating rates

Heating rate

(b, �C/min)

Samples Reaction

order (n)

Activation

energy

(Ea, kJ/mol)

Pre-exponential

factor (A)

Correlation

coefficient (R)

5 Pure PP 1.2 104.10 4.917� 108 0.9987

MP/PER¼ 2.0 2.1 109.00 2.317� 109 0.9989

MP/PER¼ 1.6 2.3 130.48 2.432� 1011 0.9995

MP/PER¼ 1.0 1.8 107.62 7.630� 1008 0.9984

10 Pure PP 1.0 112.26 4.849� 109 0.9893

MP/PER¼ 2.0 2.2 132.76 4.297� 1011 0.9999

MP/PER¼ 1.6 2.4 152.61 2.994� 1013 0.9996

MP/PER¼ 1.0 1.8 124.92 4.302� 1010 0.9992

15 Pure PP 1.2 113.54 6.263� 109 0.9982

MP/PER¼ 2.0 2.0 136.01 6.591� 1011 0.9995

MP/PER¼ 1.6 2.5 167.68 8.959� 1014 0.9995

MP/PER¼ 1.0 1.7 130.44 1.494� 1011 0.9994

25 Pure PP 1.0 121.96 3.415� 1010 0.9987

MP/PER¼ 2.0 2.0 158.76 7.030� 1013 0.9999

MP/PER¼ 1.6 2.6 175.53 4.091� 1015 0.9984

MP/PER¼ 1.0 1.7 135.88 4.874� 1011 0.9979

Although the heating rate has a great effect on the activationenergy of each sample, i.e. the bigger heating rate, the higheractivation energy (because the sample with greater heating ratehas higher thermal stability in the whole degradation region),

it does not affect the size order of activation energy of thesesamples shown as follows: MP/PER¼ 1.6>MP/PER/PER¼ 2.0>MP/PER¼ 1.0> PP. Above analysis indicatesthat the heating rate does not influence the thermal oxidative

Fig. 5. Determination of kinetic parameters by plots of the left part in Eq. (12) against �1/T using CoatseRedfern method: (a) pure PP, (b) sample with MP/

PER¼ 2.0, (c) sample with MP/PER¼ 1.6 and (d) sample with MP/PER¼ 1.0.


degradation reaction mechanism of PP system in TG experi-ments to a considerable degree. It can also be seen that al-though there is a big discrepancy on the Ea value for varioussamples between CoatseRedfern method (at various heatingrate) and Kissinger method, both methods have obtained thesame activation energy order just mentioned above. BecauseKissinger model is independent on the thermal degradationreaction mechanism, the CoatseRedfern method utilized totreat TG data is feasible. Addition of IFRs increases the pre-exponential factor A of PP by 1e5 factor of the order (Table5), showing the more complex reaction mechanism of thermaloxidative degradation of FR PP materials. The thermal oxida-tive degradation of pure PP is simple 1-order reaction, but thereaction order of pure PP after adding IFRs with MP/PER mo-lar ratios of 2.0, 1.6 and 1.0 increases to 2.1� 0.1, 2.45� 0.15and 1.75� 0.05, respectively.

3.4. Determination of the reaction mechanism of thermaloxidative degradation using Criado method

The kinetic parameters at 15 �C/min heating rate obtainedby CoatseRedfern method (Table 5) were substituted intoEqs. (17) and (18). The Z(a)ea master curves can be plottedusing Eq. (17) according to different reaction mechanisms

g(a) shown in Table 1. Here, the used experimental TG dataare still from TG curve at 15 �C/min heating rates. Accord-ingly, the Z(a)ea experimental curves can be plotted usingEq. (18). Fig. 6aed shows the Z(a)ea master and experimen-tal curves of pure PP and FR PP samples with MP/PER molarratios of 2.0, 1.6 and 1.0. As can be seen, the experimentalcurve of pure PP (Fig. 6a) nearly overlaps the master curveZ(F1), indicating that the thermal oxidative degradation ofpure PP belongs to F1 reaction mechanism (random nucleationhaving one nucleus on individual particle, i.e. the identicalprobability of nucleation occurring at each active site) withrate-controlling step of the nucleation process [36,44e46]. Be-cause the solid-phase reaction obeying F1 mechanism oftenhas the feature of 1-order kinetics [47], the thermal oxidativedegradation of PP is a 1-order kinetic reaction, which is in ac-cordance with the result obtained by CoatseRedfern method.After adding IFRs, for all FR PP samples, at the initial stage,the involved system has F1 reaction mechanism. With the evo-lution of the thermal oxidative degradation reaction, the in-volved system, however, gradually transits towards F2

mechanism (random nucleation having two nucleus on indi-vidual particle) and experiences A2 (nucleation and growth eAvrami Eq. (1)), A3 (nucleation and growth e Avrami Eq. (2))and A4 (nucleation and growth e Avrami Eq. (3)) mechanisms

Fig. 6. Determination of the thermo-oxidative degradation reaction mechanism by plots of Z(a) versus a using Criado model: (a) pure PP, (b) sample with MP/

PER¼ 2.0, (c) sample with MP/PER¼ 1.6 and (d) sample with MP/PER¼ 1.0.


[36,45,46] in turn. Above analysis indicates that on one side,the thermal oxidative degradation of the flame-retarded PPsystem has very complicated reaction mechanism, and onthe other side, for our PP systems (whether pure PP or FRPP materials), their oxidative degradations would experiencenucleation process of the reactants, which may be random,not followed by rapid surface growth [47,48]. In addition,the flame-retarded system with different blowing agent/charformer ratios presents different reaction mechanisms, e.g., atthe end of the degradation, the sample with MP/PER molarratio of 2.0 has F2 mechanism, the sample with MP/PER molarratio of 1.6 experiences F2 mechanism and gradually deviatesfrom this mechanism (a more complex reaction mechanism),and the sample with MP/PER molar ratio of 1.0 is far fromF2 mechanism.

4. Conclusions

The thermal oxidative degradation of pure PP and flame-retarded PP materials with different blowing agent/char formerratios was analyzed using Kissinger, FlynneWalleOzawa andCoatseRedfern methods. The corresponding kinetic parame-ters were calculated to well interpret the relationship betweenthe composition of IFRs (blowing agent/char former ratio) andthe flame-retardancy, i.e. FR material with appropriate blow-ing agent/char former ratio has higher activation energy, hencebetter flame-retardancy. Finally, Criado method was success-fully utilized to predict the reaction mechanism of thermal ox-idative degradation of pure PP and IFR/PP composites. Themain results obtained in this article were summarized asfollows.

(1) In the presence of oxygen, the thermal stability of IFR/PPblend is decreased. The thermal oxidative stability of theflame-retarded materials with deficiency of carbon sourceis poor.

(2) The Kissinger modeling shows that the linear relativity ofthe fitting obtained on pure PP and IFR/PP systems isgood. The calculated thermal oxidative degradation activa-tion energy of pure PP and FR PP materials with MP/PERmolar ratios of 2.0, 1.6 and 1.0 is 74.10, 105.84, 117.98and 96.69 kJ/mol. The order of the activation energy ofthe various samples is in good agreement with that of theirflame-retardancy, elucidating the connection of the blow-ing agent/char former ratio with the correspondingflame-retardancy.

(3) The results obtained by FlynneWalleOzawa methodshow that in the conversion degree a ranging from 5 to35%, the fitted straight lines of PP and IFR/PP materialsare nearly parallel, proving the applicability of FlynneWalleOzawa method to our systems. The obtained ther-mal oxidative degradation activation energy of pure PPand FR PP materials with MP/PER molar ratios of 2.0,1.6 and 1.0 is 109.54, 91.51, 127.10 and 143.43 kJ/mol.The Ea value and the size order are inconsistent withthat obtained by Kissinger method, which can be attrib-uted to the Doyle approximation applied by the former.

However, plots of Ea versus a can well describe theflame-retardation process of FR PP materials.

(4) The reaction order of pure PP and FR PP materials withMP/PER molar ratios of 2.0, 1.6 and 1.0 was calculatedas 1.1� 0.1, 2.1� 0.1, 2.45� 0.15 and 1.75� 0.05 byCoatseRedfern method. The order of the correspondingthermal oxidative degradation activation energy of abovematerials is in accordance with that obtained by Kissingermethod.

(5) The thermal oxidative degradation of pure PP with F1

mechanism having the feature of deceleration functionwas proved to obey the simple 1-order kinetic rule. How-ever, at the initial degradation stage, IFR/PP system showsF1 reaction mechanism. With the conduction of the ther-mal oxidative degradation, the flame-retarded systemgradually becomes close to F2 mechanism and experiencesA2, A3 and A4 mechanisms.

Acknowledgements

This work is supported by National Natural Science Foun-dation of China (20404009) and National Basic Research Pro-gram of China (2005CB623800).

References

[1] Wu Q, Qu B. Polymer Degradation and Stability 2001;74:255e61.

[2] Almeras X, Le Bras M, Hornsby P, Bourbigot S, Marosi Gy, Keszei S,

et al. Polymer Degradation and Stability 2003;82:325e31.

[3] Bourbigot S, Le Bras M, Delobel R, Breant P, Tremillon J-M. Polymer

Degradation and Stability 1996;54:275e87.

[4] Liang HB, Shi W, Gong M. Polymer Degradation and Stability

2005;90:1e8.

[5] Almeras X, Dabrowski F, Le Bras M, Poutch F, Bourbigot S, Marosi G,


[6] Demir H, Arkısx E, Balkose, Ulku S. Polymer Degradation and Stability

2005;89:478e83.

[7] Anna P, Marosi Gy, Bourbigot S, Le Bras M, Delobel R. Polymer


[8] Chen YH, Liu Y, Wang Q, Yin H, Aelmans N, Kierkels R. Polymer


[9] Wang Q, Chen YH, Liu Y, Yin H, Aelmans N, Kierkels R. Polymer

International 2004;53:439e48.

[10] Chiu S-H, Wang W-K. Polymer 1998;39(10):1951e5.

[11] Chiu S-H, Wang W-K. Journal of Applied Polymer Science

1998;67:989e95.

[12] Liao KR, Liu J, Lu ZJ, Lu YS. Acta Scientiarum Naturalium Universitatis

Sunyatseni (Natural Science Edition) (China) 1998;37(2):32e4.

[13] Liu J, Liao KR, Lu ZJ. Polymer Materials Science and Engineering

(China) 1999;15(1):73e5.

[14] Yang CP, Hsiao SH. Journal of Polymer Science Part A: Polymer Chem-

istry 1990;28:871.

[15] Nagata M, Tsutsumi N, Kiyotsukuri T. Journal of Polymer Science Part

A: Polymer Chemistry 1988;26(1):235e45.

[16] Banks M, Ebdon JR, Johnson M. Polymer 1993;34:4547e56.

[17] Almeras X, Dabrowski F, Le Bras M, Delobel S, Bourbigot S, Marosi G,


[18] Bras ML, Bourbigot S, Felix E, Pouille F, Siat C, Traisnel M. Polymer

2000;41:5283e96.

[19] Lv P, Wang ZZ, Hu KL, Fan WC. Polymer Degradation and Stability

2005;90:523e34.


[20] Balabanovich AI. Thermochimica Acta 2005;435:188e96.

[21] Halpern Y, Mott DM, Niswarder RH. Industrial Engineering Chemistry

Product Research and Development 1984;23:233e8.

[22] Friedman HL. Journal of Polymer Science Part C 1964;6:183e8.

[23] Kissinger HE. Analytical Chemistry 1957;29:1702e6.

[24] Coats AW, Redfern JP. Nature 1964;201(491):68e9.

[25] Flynn JH, Wall LA. Journal of Research of the National Bureau of

Standards, Section A: Physics and Chemistry 1966;70(6):487e9.

[26] Ozawa T. Bulletin of the Chemical Society of Japan 1965;38:1881e6.

[27] Horowitz HH, Metzger G. Analytical Chemistry 1963;35(10):1464e7.

[28] Lesnikovich AI, Levchik SV, Levchik GF. Journal of Applied Polymer

Science 1986;31(6):1943e50.

[29] Vyazovkin SV, Bogdanova VV, Klimovtsova IA, Lesnikovich AI. Journal

of Applied Polymer Science 1991;42:2095e8.

[30] Wu B, Wang YZ, Wang XL, Yang KK, Jin YD, Zhao H. Polymer


[31] Zhao H, Wang YZ, Wang DY, Wu B, Chen DQ, Wang XL, et al. Polymer


[32] Wang DY, Wang YZ, Wang JS, Chen DQ, Zhou Q, Yang B, et al.

Polymer Degradation and Stability 2005;87:171e6.

[33] Neininger SM, Staggs JEJ, Horrocks AR, Hill NJ. Polymer Degradation

and Stability 2002;77:187e94.

[34] Siat C, Bourbigot S, Le Bras M. Polymer Degradation and Stability

1997;58(3):303e13.

[35] Siat C, Le Bras M, Bourbigot S. Fire and Materials 1998;22(3):

119e28.

[36] Nunez L, Fraga F, Nunez MR, Villanueva M. Polymer 2000;41:4635e41.

[37] Doyle CD. Nature 1965;207(4994):290e1.

[38] Liu ZH. Introduction of thermal analysis. Beijing: Chemical Industrial

Press; 1991.

[39] Criado JM, Malek J, Ortega A. Thermochimica Acta 1989;147:377e85.

[40] Paterson WL. Journal of Computational Physics 1971;7(1):187e90.

[41] Senum GI, Yang RT. Journal of Thermal Analysis 1977;11(3):445e9.

[42] Flynn JH. Thermochimica Acta 1997;300(1e2):83e92.

[43] May CA. Epoxy: chemistry and technology. New York: Marcel Dekker;

1988.

[44] Satava V, Skvara F. Journal of the American Ceramic Society

1969;52(11):591e5.

[45] Sestak J, Berggren G. Thermochimica Acta 1971;3(1):1e12.

[46] Ma S, Hill JO, Heng S. Journal of Thermal Analysis 1991;37:1161e77.

[47] Alshehri SM, Monshi MAS, Abd El-Salam NM, Mahfouz RM. Thermo-

chimica Acta 2000;363:61e70.

[48] Sharp JH, Bindley WG, Achar BNN. Journal of the American Ceramic

Society 1966;49(7):379e82.

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