University of Groningen

New aspects of the suspension polymerization of vinyl chloride in relation to the low thermalstability of poly(vinyl chloride)Pauwels, Kim Francesca Daniëla

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1

CHAPTER 1

General introduction

Abstract

In a historical overview, the discovery of poly(vinyl chloride) (PVC) and its subsequent

development over the twentieth century, are briefly described.

The main problem around PVC is its low thermal stability caused by the presence of defects in

the molecular structure formed during the polymerization, resulting in the formation of polyene

structures in the chain due to elimination of HCl if exposed to high temperatures and UV light.

Both the formation of different types of defects and routes of dehydrochlorination are discussed in

detail.

At the end of this chapter the aim of this doctoral research and the outline of this thesis are

presented.

CHAPTER 1

2

1.1 The origin of PVC and its subsequent development

The polymerization of vinyl chloride monomer (VCM) is known since 1872. Baumann 1 was the first who produced poly(vinyl chloride) (PVC) by accident. He exposed VCM

to sunlight and obtained a white solid material that could be heated up to 130 °C

without decomposition. However, when exposed to higher temperatures the material

started to melt and simultaneously a considerable amount of acid vapor was

produced, finally resulting in a black-brown material. With some exceptions it was not

until the early twentieth century that more scientific research dealing with the

development of PVC took place.

In the late twenties 2,3 vinyl chloride copolymers were introduced. Also, the possibility

of plasticizing PVC, by using esters such as tritolyl phosphate and dibutyl phthalate,

resulting in more flexible material was discovered at that time. The introduction of

emulsion and suspension polymerization systems for PVC resins both around 1933

have been substantial advancements, just like the development of the so-called 'easy

processing' suspension resins, which were able to absorb plasticizers without

gelation. In the early thirties PVC had already been introduced in small quantities in

different types of products both in the USA and Germany.

Large-scale production, however, began in Germany in 1937. After 1939, especially

during the 2nd World War, the production started to gain commercial significance on a

worldwide basis. Due to deficiency of essential conventional materials and the

demand for certain properties, which were not possessed by any of the available

materials at that time, this war encouraged the development of a lot of plastics.

Growth of the production of PVC has been so rapid after 1939 that most countries

with any degree of industrialization produce some vinyl resin. The production and use

of rigid PVC increased dramatically in the early sixties, due to large improvements in

both heat stabilizing agents and processing equipment 4.

The low production costs and the great versatility of vinyl chloride polymers are the

two major reasons for their large share on the plastic market. The polymer can be

converted into many different products exhibiting an extremely wide range of

properties both physical and chemical by using modifying agents, such as

General introduction

3

plasticizers, fillers and stabilizers 5. The products can range from a flexible garden

hose to a rigid drainpipe, from flexible sheets for raincoats to rigid sheets for packing,

from soft toys to upholstery. Eventually, PVC compositions have succeeded in

displacing materials such as rubber, metals, wood, leather, textiles, conventional

paints and coatings, ceramics, glass, etc.

1.2 Current status

Alongside polyethylene (PE), polypropylene (PP) and polystyrene (PS), PVC is one of

the most important commercial thermoplastics in the world 4,6.

Nowadays, PVC is manufactured by three different processes: suspension, bulk or

mass and emulsion polymerization. The suspension process, however, embraces

80% of all commercial productions of PVC 7. Polymerization of VCM occurs according

to a free radical addition process, which includes initiation, propagation, chain transfer

to monomer and bimolecular termination steps 8 (Scheme 1.1).

Scheme 1.1 VCM polymerization (initiation, propagation, chain transfer and termination)

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4

PVC has many desirable characteristics that have allowed it to achieve its present

status. Despite of the enormous technical and economical importance, PVC also

possesses many problems 9-11. PVC is almost certainly the least naturally stable

polymer in commercial use.

During processing, storage and utilization, PVC degrades as it is exposed to high

temperatures, high mechanical stresses or ultraviolet light, all in the presence of

oxygen. Degradation of the polymer occurs by successive elimination of hydrogen

chloride (HCl), which is called dehydrochlorination, yielding long polyenes (Scheme

1.2), which are consequently causing discoloration, deterioration of the mechanical

properties and a lowering of the chemical resistance.

Scheme 1.2 Dehydrochlorination

Therefore, PVC requires stabilization for practically any technical application.

Stabilization mainly proceeds by the addition of compounds, which contain transition

metals like lead, tin and zinc. Scandinavian authorities, however, were the first who

issued a prohibition for the use of these heavy metals because of environmental

consideration 12. Gradually more and more countries will prohibit the use of this type

of stabilizing agents. The PVC processing industry is suffering from this decree, as no

equivalent low-priced alternatives are available.

As there is also no equivalent alternative available for PVC itself, it is of great

importance to investigate the lack of thermal stability of PVC in detail. If the problems

concerning the polymerization were better understood, it could eventually be possible

to produce a thermally more stable polymer.

General introduction

5

1.3 The causes for the low thermal stability of PVC

The ideal structure of PVC is a linear structure formed by head to tail addition of

monomer molecules to the growing polymer chain (Scheme 1.1) 13.

Thermogravimetric analysis on low molecular model compounds such as 2,4,6-

trichloroheptane, 2-chloropropane and 2,4-dichloropentane, corresponding to the

regular head-to-tail structure of PVC containing secondary chlorines only, shows that

these model compounds are stable up to at least 200-300 °C. Commercially available

PVC, on the other hand, would already degrade around 120 °C, if it were not

stabilized before processing 9,14-17.

It is known that the quality, or thermal stability, of PVC decreases when monomer

conversion increases. VCM is polymerized in a batchwise process, which means that

the monomer supply gets more and more exhausted with increasing monomer

conversion. As a consequence side-reactions by the macroradicals will increasingly

occur, resulting in the formation of a lot of different types of structural irregularities.

Some of these defects are shown to have a dramatic influence on the thermal stability 5,10,18.

The most occurring structural defects in PVC are a wide range of branches, which are

formed by various routes. Some of them seem to affect the thermal stability while

others are completely harmless. The frequently occurring branches and the most

important types of branches concerning the thermal stability of PVC are described

below.

Besides head-to-head units within the polymer backbone, head-to-head emplacement

of VCM to a growing polymer chain can also result in other types of irregularities

within the chain. Chloromethyl (MB) 19-29 and 1,2-dichloroethyl branches (EB) 18,19,29-31 result from one or two successive 1,2-Cl shifts respectively, followed by

regular chain growth as is shown in Scheme 3.1. β-Scission of a Cl radical can also

occur after these Cl shifts resulting in a polymer chain bearing a chloroallylic end

group. The Cl radical is able to reinitiate new chain growth, resulting in a polymer

CHAPTER 1

6

chain which carries a dichloroethyl endgroup 32.

Scheme 1.3 Chemical consequences of head-to-head addition during the polymerization of

VCM32

The MB and EB structures are expected to have minor, if any, influence on the

initiation of dehydrochlorination of PVC 33,34, which is also the case for the two types

of endgroups (Scheme 1.3) 35,36.

This insignificant influence of MB and EB is probably due to the absence of tertiary

chlorine at the branchpoint carbons, as it has been proven that an increase in the

amount of tertiary chlorine in the polymer chain increases the thermal degradation 37-

41. Other types of branching do contain tertiary chlorine at the branchpoint carbon

such as the 2,4-dichloro-n-butyl branch and various types of long chain branching.

The 2,4-dichloro-n-butyl branch (BB) 23,42-44 is formed via a 1,5-backbiting

mechanism (Scheme 1.4) 45-47. The growing macroradical abstracts a hydrogen atom

from the CH2 group at the fifth position in the chain leaving a radical at that point, after

which propagation continues from there and a polymer chain bearing a butyl branch is

General introduction

7

created.

Scheme 1.4 1.5-backbiting mechanism generating a 2,4-dichloro-n-butyl branch

Long chain branching (LCB) results from hydrogen abstraction, from a

chloromethylene or a methylene unit of a polymer chain, by a growing macroradical 41,43 or possibly a chlorine atom (Scheme 1.5) 41,48. The newly formed macroradical

will propagate further, generating long chain branches. About 66% of all LCB formed

do contain tertiary chlorine at the branchpoint carbon, meaning that hydrogen

abstraction from the chloromethylene unit occurs more often.

After hydrogen abstraction from the methylene unit also an internal allylic moiety (IA)

can be formed after unimolecular β-scission of a Cl radical from the chain. This route

is accepted by Hjertberg and Sörvik 41,48, who assigned the Cl radical to be important

in both chain transfer to monomer and chain transfer to polymer, of which the latter

becomes especially important at monomer starved conditions. Starnes and

Wojciechowski 32, however, doubt about the presence of kinetically free Cl radicals

and suggest the Cl radical to be immediately attached to a monomer molecule,

initiating new chain growth, without the chance of inducing dehydrochlorination by H

abstraction from another polymer chain.

CHAPTER 1

8

Scheme 1.5 Formation of long chain branching after hydrogen abstraction from chloromethylene

and methylene by macroradicals or chlorine atoms

Both BB and LCB are mainly formed when the monomer supply is almost exhausted.

Due to monomer starvation the growing macroradicals start reacting with themselves

or with neighboring polymer chains as described. Hjertberg en Sörvik 41,44 examined

the subsaturation polymerization of VCM, which was used as a model for the

polymerization conditions at high monomer conversion in the conventional

polymerization process of VCM, assuming that the polymers with reduced stability are

formed when the monomer supply is almost exhausted 44,49. They indeed confirmed

the increase in the presence of tertiary chlorine in this so-called U-PVC. In industry

the polymerization of VCM is terminated at a monomer conversion of approximately

85 to 90%, at which the conditions are similar to those examined by Hjertberg en

Sörvik. Therefore, it is very likely that BB and LCB present in commercial PVC are

formed to a large extent during this last stage of polymerization.

Diethyl branches (DEB) 30,50-52 seem also to be present in PVC fractions produced at

very high VCM conversions and therefore when monomer supply is almost

exhausted. This type of branching is believed to contribute significantly to the thermal

degradation of the polymer, due to the presence of two tertiary chlorines at the

branchpoint carbons (Scheme 1.6).

General introduction

9

Scheme 1.6 Diethyl branches

Internal unsaturation, especially internal allylic chlorines (IA in Scheme 1.6), proved

to be one of the main structural defects influencing the thermal stability 33,41,43. Just

like the tertiary chlorines the amount of internal allylic chlorines increases dramatically

when monomer supply becomes exhausted 48. When the amount of allylic and tertiary

chlorines in commercial PVC as well as their reactivity towards thermal degradation is

taken into account, it seems that tertiary chlorine has a somewhat higher reactivity

towards dehydrochlorination.

The effect of head-to-head units 9,53 (Scheme 1.3) in PVC on the thermal stability is

still not conclusively proven 28,54,55, but it is known that the amount of these units is

very small, presumably 0-0.2 per 1000 monomeric units 28,35,56,57. So if these

structures have any influence on the thermal stability, it will be minor compared to

other structural irregularities 50,58.

The presence of oxygenated structures in the polymer chain is rather questionable.

Many groups have examined the presence and influence of these structures and their

conclusions are often contradictory 59-65. The most discussed oxygenated structure is

a cis-α,β-unsaturated ketone (Scheme 1.7). Different explanations for the effect of

this structure on the dehydrochlorination have been proposed 66-71. However, these

structures have not been detected yet, using NMR 30,48,72, which is not so strange

considering that the ketoallylic structure is present only in the range of 0.1 per 1000

monomeric units 64. Taking these low concentrations into account, their influence on

the thermal stability is likely to be insignificant. However, oxygen can be of influence

CHAPTER 1

10

on degradation during processing of the polymer at high temperatures in a normal

oxygen-containing atmosphere 9,73-80, as will be discussed later on in this chapter.

Scheme 1.7 Dehydrochlorination induced by α,β-unsaturated ketone structures

Many researchers thought to have established the fact that chain endgroups induce

dehydrochlorination within the polymer chain 81-84. They showed that the rate of

dehydrochlorination increased proportionally with decreasing molecular weight,

corresponding to an increasing amount of endgroups. Nevertheless, it is a bit

premature to interpret the above results as definite proof that dehydrochlorination is

controlled by reactive chain ends. Other parameters may also change inversely with

molecular weight.

In 1983 Van den Heuvel and Weber 36 performed measurements on low molecular

PVC, extracted from ordinary commercial suspension PVC, as a model for PVC itself.

They determined the type and amount of endgroups using 1H and 13C-NMR. They

found six different endgroups (Scheme 1.8) and gained better insight into the

influence of these endgroups on the thermal stability of PVC by measuring the decay

of these groups at processing temperatures.

General introduction

11

Scheme 1.8 Different types of endgroups formed during polymerization of VCM 36

They concluded that the different types of saturated and unsaturated endgroups, and

those originated from the initiator are not significantly involved in the degradation of

PVC. It should also be taken into account that the amount of the total number of

anomalous endgroups is minor compared to the other internal structural irregularities,

which are supposed to be of influence on the thermal stability of PVC.

CHAPTER 1

12

Finally, tacticity 85-88 appears to have an effect on the dehydrochlorination rate. The

isotactic triad conformation GTTG is a principal initiation site for random

dehydrochlorination 89,90 from normal monomeric units, generating well defined long

polyenes of 7-9 double bonds 91. Polymers having a higher degree of syndiotacticity

have shown to dehydrochlorinate faster, which is ascribed to a sterically facilitated

growth of all-trans polyenes 91,92.

1.4 Degradation

Despite the considerable number of studies on the mechanism of decomposition of

PVC, it is still not completely understood. Thermal decomposition 93-97 of PVC

results in an intense discoloration of the polymer, which is a result of the formation of

long conjugated polyene sequences that absorb in the visible region 13,73. It is

generally accepted that during degradation HCl molecules are eliminated in

succession along the polymer chain yielding these conjugated polyenes. The

dehydrochlorination process involves three successive steps. It starts with a relative

slow initiation of HCl loss, which is followed by a rapid zipper-like elimination of HCl

and thus the formation of polyenes, which is finally terminated. Dehydrochlorination is

initiated mainly by structural defects (e.g. allylic and tertiary chlorine) at the polymer

backbone. After the first elimination of HCl allylic chlorine has been formed, which is a

very active moiety, supporting the fast zipper-like elimination of HCl.

The main issue is the type of mechanism by which the overall dehydrochlorination

process takes place. Studies on some small aliphatic model compounds for PVC

have been inconclusive. Various schemes have been proposed, which can be

classified as involving unimolecular eliminations, ionic, free radical chain, or polaron

mechanisms.

The unimolecular mechanism is supported by many authors 66,82,97-99, especially by

Bagaloglu and Fish who suggested the six-centered concerted mechanism of

dehydrochlorination, catalyzed by HCl (Scheme 1.9) 100,101.

General introduction

13

Scheme 1.9 Unimolecular mechanism for dehydrochlorination catalyzed by HCl via a six-center

transition state

Ionic and quasi-ionic routes for dehydrochlorination have been described elaborately

over the years 14,16,102-105. In sheme 1.10 a few proposed mechanisms are shown,

such as the mechanism of dehydrochlorination involving an ion-pair, a quasi-ionic

route via a four-center transition state are shown and also the well-established

catalysis of dehydrochlorination by HCl 18.

Scheme 1.10 Ionic and quasi-ionic routes for dehydrochlorination

Both the ionic and the unimolecular routes of dehydrochlorination are highly allyl

activated.

CHAPTER 1

14

Also the radical mechanism for dehydrochlorination has been discussed 73,81,84,98,106,107. As the attack of a chlorine radical on the PVC chain is supported by

many of them, this mechanism is depicted in Scheme 1.11 18. The chlorine radical

abstracts a methylene hydrogen atom, forming HCl. The new macroradical formed

will dissociate a chlorine radical adjacent to this radical and a new double bond is

formed and the newly released chlorine radical will attack the neighboring methylene

group immediately. This cycle will repeat many times yielding a polyene sequence in

the chain.

Scheme 1.11 Radical mechanism of dehydrochlorination

Polaron (ion-radical) mechanisms for dehydrochlorination have also been proposed 108-112. In Scheme 1.12 a mechanism, which involves an allylic cation radical, is

shown. According to Tran et al. 112 who reviewed the ion-radical mechanism of

thermal dehydrochlorination not only polarons can propagate dehydrochlorination, but

also solitons and bi-polarons.

Scheme 1.12 Polaron mechanism of dehydrochlorination

General introduction

15

This route of dehydrochlorination can not be the only one, as the polarons or solitons

originate from a sequence of at least three conjugated double bonds. However if

these are formed they enhance the dehydrochlorination process of PVC.

The type of medium, in which the degradation of the polymer takes place, seems to

be of influence on the type of mechanism of dehydrochlorination. Both the ionic and

the polaron mechanism are supported in polar solvents 93,110,113,114. This type of

medium accelerates the dehydrochlorination considerably, due to the ionization of the

C-Cl bonds and the creation of polarons from solitons.

As evidence for the validity of the radical mechanism, the presence of free radicals

was shown by electron spin resonance spectroscopy (ESR) 37,115,116. Another

argument in support of the radical mechanism of degradation was the acceleration of

degradation after adding some free radical initiators 37,81,115. However, other authors

did mention that addition of some radicals did not accelerate the degradation and

adding radical traps did not retard the degradation 37,84,98. Furthermore, a few

scientists have mentioned that the observed ESR signal did not belong to free

radicals, but presumably to a type of radical, which is associated with an unpaired

electron of a π-system. Later, this signal was more specifically related to the presence

of solitons and polarons 109,116.

The initiation of dehydrochlorination and the formation of longer polyene sequences,

both catalyzed by HCl, are established facts. However, just like the doubts about the

type of mechanism of dehydrochlorination also the type of catalysis is still not clear.

As well ionic 96,117, radical 95, polaronic 112 and molecular 93,94,97 routes of HCl

catalyzed dehydrochlorination are suggested.

Bacaloglu and Fisch 97 even mentioned the likelihood of the occurrence of HCl

catalyzed initiation of dehydrochlorination from an ordinary monomer unit, via a

concerted six-center transition state (Scheme 1.13).

CHAPTER 1

16

Scheme 1.13 Initiation of dehydrochlorination from an ordinary monomer unit catalyzed by HCl

Photodegradation 73,84,98,118-120 is considered to occur according to a radical

mechanism. Initiation of degradation occurs by excitation of the polymer by irradiation

of the material with UV light eliminating a chlorine atom, which can initiate

dehydrochlorination at other polymer chains. The extent of degradation depends

primarily on the presence of photosensitive chromophores in the polymer chain as

irregular structures and impurities, e.g. hydroperoxides, carbonyl groups, unsaturation

and metal salts, often present in processed polymeric materials 121-123. Whatever the

nature of the chromophores initially absorbing UV light in the original material,

polyene structures, which rapidly accumulate in photolyzed PVC, become the

predominant absorbing chromophores due to their large extinction coefficients.

In the presence of oxygen both thermal- and photodegradation are enhanced.

There are various interpretations of the mechanism of the thermo-and photo-

oxidative degradation of PVC. It is, however, almost generally accepted that in the

presence of oxygen, radical chain reactions play an important role 9,73,124. The

dehydrochlorination in oxygen is much faster than in nitrogen 81,115,120,125. This is

probably due to the superposition of oxygen-initiated radical processes on the

dehydrochlorination reactions 9, which is supported by the observation that the high

rate of HCl elimination in thermo-oxidation is reduced by antioxidants 119. Also the

degradation of PVC containing peroxide groups formed due to the presence of

oxygen during polymerization 126 was found to proceed faster compared with the

degradation of PVC, prepared in an inert atmosphere.

Many authors suggested comparable reaction schemes, in which oxygen amplifies

dehydrochlorination due to generation of multiple radicals by means of peroxide

formation (Scheme 1.14) 73,84,98,118-120.

General introduction

17

Scheme 1.14 Influence of oxygen on dehydrochlorination reaction of PVC; the formation of

peroxy radicals

Besides the primary process of degradation, which is the elimination of HCl,

secondary reactions such as chain scission 81,82,120,125,127, crosslinking 67,73,84,97,98,120,125,128 and benzene formation 106,107,117,129-133 also occur.

In nitrogen atmosphere only crosslinking and no chain scission occurs, while in

oxygen atmosphere chain scission is definitely present but also some crosslinking

occurs. An increase in degradation temperature or an increase in illumination with UV

light increases the amount of crosslinking 78,80,134-137. An increase in the amount of

oxygen in the atmosphere, however, enhances chain scission at the cost of

crosslinking.

PVC as received from the manufacturer is converted into finished products by high

shear mixing processes such as milling, intensive mixing and extrusion. These

processes will always occur in the presence of a certain amount of oxygen. PVC

suffers from chain scission during these processing conditions of high mechanical

shear, which are high enough to set up critical stresses in the polymer 53,133. It has

been shown that radical acceptors added during processing act as stabilizers against

CHAPTER 1

18

mechanical degradation 138.

Thus mechano-chemical degradation 78,80,134-137 of PVC comprises chain scission,

crosslinking, but also initiation of radical chain dehydrochlorination reactions and thus

high-energy deformations during processing constitute another important

consideration in the prevention of degradation of PVC.

1.5 Aim and overview of this thesis

As already highlighted in the section about the current status of PVC, the polymer

suffers from a low thermal stability, which has until now been restrained satisfactorily

by the addition of heavy metal based stabilizing agents. However, because of severe

environmental legislation the use of this kind of stabilizers becomes prohibited all over

Europe. Industry suffers from this decree, as no equivalent low-priced alternatives are

available yet. Therefore, Senter set up a research program called IOP /

Environmental Technology – Heavy Metals, thereby stimulating both the development

of these alternative stabilizing agents and the improvement of the polymerization

process of VCM in order to obtain more naturally thermally stable PVC. During this

doctoral research, which participated in this research program, the polymerization

process of VCM was examined. It is necessary to understand the ins and outs of the

suspension polymerization process before any attempt can be made to produce a

more thermally stable polymer. Therefore, this doctoral research embraces a detailed

study of the suspension polymerization of VCM in a 1-l autoclave and the

investigation into the reason of the poor thermal stability of PVC.

In Chapter 2 the optimization of the suspension polymerization of VCM at 57.5 °C in

a one-liter autoclave is described in order to obtain a comparable product as is

produced in industry. For this particular research project two of these devices have

been build and optimized.

Chapter 3 reports the development of the polymerization process and the properties,

such as molecular structure, molecular weight, porosity and thermal stability, of the

resulting PVC product with increasing monomer conversion. Polymers produced up to

General introduction

19

a high monomer conversion level seem to be thermally less stable, than polymers

produced up to only low monomer conversion.

In Chapter 4 the occurrence of a heat effect during the polymerization, just after the

start of the pressure drop, is discussed. The origin of this so-called hot spot and the

final consequences on the molecular properties of PVC are of main interest.

Finally the consequences of the addition of a nonsolvent or a solvent for PVC to the

polymerization mixture are described in Chapter 5 and 6, respectively. The

polymerization, which occurs in a two-phase system, can be changed dramatically in

this way. Of interest are among others changes in molecular structure, thermal

stability and the appearance of the hot spot.

1.6 References

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