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)
CHAPTER 1
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.
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