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A studv of the aroducts of PVC thermal

Polymer Degradation and Stability 49 (1995) 181-191 0 1995 Elsevier Science Limited

Ian C. McNeil& Livia Memetea & William J. Cole Polymer Research, Chemistry Department, University of Glasgow, Glasgow G12 8QQ, UK

(Received 3 January 1995; accepted 3 February 1995)

PVC thermal degradation in vacuum up to 500C has been followed by recording the relative rate of volatile product formation by thermal volatilisa- tion analysis while monitoring by mass spectrometry the formation of the main products: HCl, aromatic and aliphatic hydrocarbons, and non-condensable gases (CH,, HZ). The material balance after pyrolysis has been evaluated. The liquid fraction collected during pyrolysis was analysed by GC-MS and its composition determined by the integration of the ion current under the peaks due to different compounds. After HCl (53% of the PVC sample), the tar is the major fraction (24%). The liquid fraction (of which 80% is benzene) accounts for 7% of the original polymer. The other fractions are the char (9.5%) and gas fraction (6.6%). 10% of the Cl remained trapped in the polymer until higher degradation temperatures giving rise then to the chlorinated compounds which account for 1.75% of the liquid fraction and 0.14% of the polymer.

PVC shows two stages of degradation: during the first stage, between 200 and 360C mainly HCl and benzene and very little alkyl aromatic or condensed ring aromatic hydrocarbons are formed. It was evaluated that 15% of the polyene generates benzene, the main part accumulating in the polymer and being active in intermolecular and intramolecular condensation reactions by which cyclohexene and cyclohexadiene rings embedded in an aliphatic matrix are formed. Alkyl aromatic and condensed ring aromatic hydrocarbons are formed in the second stage of degradation, between 360 and 500C when very little HCl and benzene are formed. In this stage the polymeric network formed by polyene condensation breaks down in the process of aromatisation of the above C, rings. The mechanism of benzene formation at different temperatures was considered.

1 INTRODUCTION

The present investigation is a study of PVC pyrolysis which has relevance to waste disposal by incineration and the environmental problem it poses. The work is part of a larger project which aims at: (i) identification and quantitative determination of the pyrolysis products of PVC as a reference material and of its formulations with additives such as plasticisers, stabilisers and flame-retardants; (ii) better understanding of the pyrolysis mechanism; and (iii) intervention into the pyrolysis mechanism in order to minimize toxic product formation, offering thereby a less polluting solution to disposal of PVC scrap by incineration.

The present paper, the second in the series, is concerned with the separation, identification and quantitative determination of the pyrolysis products of PVC.

2 EXPERIMENTAL

The PVC used was a typical industrial, suspension grade sample from European Vinyls Corporation with molecular weight (A4,) 40,000 as determined by GPC. The pyrolysis was conducted under continuous evacuation (initial vacuum 10v5 mm Hg) at a heating rate of lOC/min up to SWC, when a hold time of 20 min was allowed. PVC samples (powder) of

182 I. C. McNeil1 et al.

20-200 mg, spread as a thin layer at the bottom of a silica cell, were subjected to pyrolysis. The pyrolysis products were collected, separated and analysed according to the procedures of thermal volatilisation analysis (TVA).. An account of this technique has been presented in the first paper of the series.

The products evolved through pyrolysis were collected in fractions: the cold ring or tar fraction, the liquid fraction, the residue or char, and the gas fraction. The tar contains condensed ring aromatic hydrocarbons (MW 128-250) and scission fragments of the network formed through the crosslinking of polyene. The tar has been qualitatively analysed in the earlier study. The liquid fraction is made up of compounds which condense at room temperature. The gas fraction consists of HCl together with noncon- densable gases (CH,, H,) and C,-C, hydrocar- bons. The fractions (apart from the gases) were weighed for material balance after pyrolysis. The gases were determined by difference. HCl was determined in a separate experiment in which it was retained on a short column containing CaO mixed with glass wool which allowed for the free passage of the other gases.

The liquid fraction containing products up to a molecular weight of 150 was analysed by CC-MS using a Hewlett-Packard 5971 mass selective detector interfaced to a 5890 series 11 gas chromatograph and computer (Vectra QS/16s). Separations were effected with an HP1 fused silica capillary column (12.5m X 0.2mm X 0.33 pm) temperature-programmed from 50C (5 min hold) to 220C (1 min hold) at SC/min. The Grob-type injector (225C) was operated in split mode (5O:l) and the He carrier gas flow rate was 1 ml/min. Mass spectra (70 eV) were recorded in continuous scanning mode. The concentration of the components in the analysed fraction was determined by the integration of the ion current under the peaks.

In other experiments, the evolution of the volatile pyrolysis products was monitored during heating in vacuum to 500C by means of Pirani gauges using the conventional four parallel line TVA experimental arrangement. The pyrolysis products were then condensed on a liquid nitrogen trap. Another Pirani gauge placed after a liquid nitrogen trap recorded the non- condensable gases (H, and CH, in this case). The Pirani output was recorded as a function of time or temperature. For temperature measurement,

two thermocouples were used, one placed in the combustion tube just above the sample, the other in the oven, in order to assess the temperature lag between the sample and the oven, which is a common phenomenon in dynamic heating. Typically, at the beginning of the experiment the temperature lag was lo-15C, it decreased to 5-6C at 200C and then was steadily reduced to I-2C towards 500C as shown in the corres- ponding diagrams. The Pirani recording gives a measure of the relative rate of evolution of volatile products and is useful in identifying the degradation stages of the polymer, which together with the identification of the nature of the products can provide valuable information on the degradation mechanism.

A bleed to a Leda-Mass quadrupole mass spectrometer placed on line with the TVA system enabled the mass spectra of the products to be recorded in continuous scan mode (3.3 scans/min). The stored MS scans were subse- quently further analysed for the identification of the evolved products. The lines in the mass spectra produced by ions of m/e which are characteristic of certain products and which are not subject to interference from other products were identified and their intensity plotted against time (temperature) using Microsoft Excel. The intensity of the above lines is expressed as partial pressure produced in the mass spectrometer by ions of specific m/e.

3 RESULTS

3.1 Main product frictions

Table 1 presents the material balance of the products obtained through the pyrolysis of PVC in vacuum up to 500C. As can be seen, apart from HCl (53%), the cold ring products form the major fraction (24%). Of the polymer remaining

Table 1. Material balance after PVC pyrolysis

NO FrXiKCl %

I MCI (2 6 2 Cold (tar) fradm nng 24 3 3 Lquld fncttal 7.0 4 Gas fmdim (apart fim HCI) 66 5 Char 9s

A study of the products of PVC thermal degradation 183

after HCl emission, 50% degrades to tar, 20% to char, 30% to gas fraction and 30% to liquid fraction. The amount of Cl in the evolved HCl compared to the theoretical Cl content of the PVC shows that 10% of the original Cl remains in the polymer to higher temperatures than those involved in dehydrochlorination.

Table 2 presents the composition of the liquid fraction obtained through the pyrolysis of PVC, expressed as main classes of compounds. The following features can be noted.

Aromatic hydrocarbons are the main class (88%), of which the major part is benzene (80%). Aliphatic hydrocarbons account for 5.2% of theliquid fraction. These have been identified in the previous paper. Chlorinated hydrocarbons make up 1.75% of the liquid fraction and correspond to 0.14% of the original polymer.

Table 3 presents the main aromatic con- stituents of the liquid fraction obtained through the pyrolysis of PVC alone. Benzene accounts for 80% of the mixture, as already shown (and for 5.6% of the PVC). It is followed by toluene (1*8%), o&o-xylene (l-3%), ethylbenzene (1%) and 1,2 methylethylbenzene (l%), the percen- tage being relative to the liquid fraction. A surprising feature is that among the alkyl benzenes, the ortho-derivatives are formed in a higher concentration than metu- and paru- derivatives: ortho-xylene accounts for l-3%, me&- and puru-xylene for O-6%, 1,2-methylethyl benzene for l%, while 1,3- plus 1,4- derivatives make up 0.3%. This trend reflects a higher probability for the ortho-derivatives to be formed.

Table 4 presents the constituents of the class of chlorinated hydrocarbons produced through the pyrolysis of PVC. The main chlorinated com- pounds are chlorinated alkenes and chlorinated aromatics, all regarded as hazardous to health.

Table 2. The composition of the liquid fraction (classes of compounds) collected during the pyrolysis

of the PVC sample

c- % 96

Bmrsns 19.14 Alkyl amna,c 5.22

Alkenyl .mmat,c 0 83

Cc.l&nd rmg alonlatic 2.40

T&al .-t,c 88 20 AlhO 2.47 C!yClWlkNWS 0 80 Alkmes I .52 cyckmkmm 0.40 Total aliphraic 5 19 Chlorinated hydmxbms I IS others 4.87 Total IOOGQ

Table 3. Quantitative determination of the main aromatic products in the liquid fraction

No PWdW %

1 796

/ \ 5 -o- 06

-

3.2 Thermal volatilisation analysis

The evolution of the volatile compounds constituting fractions l-4 of Table 1 was followed by TVA and the results are presented in Fig. 1, which shows the response of five Pirani gauges and two thermocouple outputs as a function of temperature. The evolution of non-condensable

by the response placed after the

gases (HZ and U-L,) is indicated in the trace for the Pirani gauge -196C trap (curve 5).

T&le 4. Chlorinated hydrocarbons pyrolysis of PVC

NO Crnpamd % NO

Cl

16 +

01 25

Cl CI

20 +,a

SO1 26

Cl

21 II

02 27

Cl

22 ?I

01 26

23 /t/ o-2

Cl 24 w 02

formed in the

%

o-15

01

02

016

02

036

184 1. C. McNeil1 et al.

500 5

01 : : 04 120 1.30 200 240 230 320 300 4w 440 480

Temperature (C)

Fig. 1. TVA diagram recorded as Pirani gauge output (relative rate) vs temperature for PVC thermal degradation. Key to traces: 1-4, volatile products (condensable) recorded with four Pirani gauges placed on parallel lines: 5, noncondensable gases (Hz, CH,); 6,7, thermocouples output.

Figure 1 shows that the volatile compounds are evolved in two degradation stages: between 200 and 360C and between 360 and 500C. These have already been identified by TVA. HCl and benzene are the major degradation products in the first stage. During the second stage, breakdown products of the crosslinked polyene are formed which were qualitatively analysed as components of the tar, liquid and gas fractions. The amounts of volatile products at the second stage are smaller, as can be seen in Fig. 1. A small amount of non-condensable gases is formed

(4 1 WE.03 P.w3m

O.OOE-03

7 WE.00

g o.ocEm

E 5.cQE-m

j 4.OOE-09

3OoE-08

2wE-05

1OOE-a

O.OOE+W

30

E

@I

0 50 loo 150 200 250 300 350 4Ou 450 5OQ

Tempemtun (C)

I OOE-08

3OOE-00

2wE-09

1 .JOEJm 82

OOOE+OO _s

0 50 KM 150 200 250 300 350 UXI 450 5al

Tsmperalum (C)

Fig. 2. Ion monitoring during PVC thermal degradation: (a) benzene (78) HCI (36-38); (b) toluene (91,92).

towards the end of the second stage and very little if any, in the first stage.

3.3 Evolution of individual products

3.3.1 Hydrogen chloride and benzene The evolution of the individual products with temperature was followed by plotting the intensity of the lines produced in the mass spectra by their characteristic ions vs tempera- ture. Figure 2(a) shows the evolution of benzene (m/e 78) and HCl (m/e 36, 38). It is evident that these are evolved mainly during the first degradation stage and only to a minor extent during the second stage. However, Fig. 2(a) shows that although HCl is produced at the same time as benzene (at 200C) it appears that the HCI curve (m/e 36) initially lags behind the benzene curve, which was a reproducible trend in all the present experiments. Although the delay is expressed in terms of temperature, it is believed that time and not temperature governs the phenomenon, which may possibly anomaly due to initial HCl adsorption glassware. The effect is being investigated.

be an on the further

3.3.2 Toluene and other aromatics Toluene formation as a function of temperature was followed using its ions at m/e 91 and 92 (Fig. 2(b)). While in the first degradation stage there are no other compounds contributing to the 91 line, in the second stage this is no longer valid. Compounds such as xylene, ethylbenzene, propylbenzene, butylbenzene and their isomers are the main sources of the 91 line in the second stage. However, the 92 line can be safely regarded as characteristic only of toluene, since other substances giving the same ion (especially C,-alkylbenzenes) are formed in very low concentrations. Figure 2(b) shows that a small part of the toluene is formed in the same process as benzene. The major part of the toluene is formed in a different process in the second stage. This conclusion has been also verified by the GC analysis of the liquid fractions collected separ- ately for each degradation stage. Styrene formation was followed by its molecular ion (104) and so was ethylbenzene and the isomers of xylene (106), all presented in Fig. 3(a). They are all formed in a minor amount during the first stage and almost entirely during the second stage.

A study of the products of PVC thermal degradation 185

(4 3.wE-00 3.OcE-w

2.sOE-OQ

F ~2.OE-00

j 1.sOE-09

l.WE-OQ

0 s 10 1s m 25 30 35 40 45 50 TemmmlumrC)x0.1

.m .

: ?I

I ;

0 5 lo 15 m 25 30 35 40 45 50

Tanpentun CC)xO.l

Fig. 3. Ion monitoring during PVC thermal degradation: (a) styrene (NM), C3-C5 alkyl benzene (105), ethylbenzene and

xylene isomers (106); (b) indene (115, 116).

Figure 3(a) shows also the intensity for evolution of the ion with m/e 105 characteristic of C,-C, alkyl benzenes which are present in this system. These products are also formed mainly in the

(4 ZOOE-OQ- .d

lmlE-o~- .

:. .

l.eE-OS ..

1.4E-00 8 ;

0

- 12oE-O~-

S I l.OEoo ..

1 8.OOE-10 ..

' cs 117

. -a .

e.OE-10

4.Oms10 : z.OOE-10

_*pQ?,,,

__...yy : , '4Al8

O.OOEIoo L, .: . 118

0 5 lo 15 m 25 30 35 40 45 50 Temmrmum~~IxO.1

(b) T o.ooE-10

moE-10

7oOE.10 I

0 s 10 is m 25 30 35 40 45 50 T.mpemlum~C)xO.~

Fig. 4. Ion monitoring during PVC thermal degradation: (a) indane, methylindane and C,-alkenylbenzenes (117, 118);

(b) naphthalene (128).

second degradation stage. The same observation is valid for indene (ions 115, 116, Fig. 3(b)) and also for indane and methyl indanes (structures 15 and 16, Table 3) and C,-alkenylbenzenes (structures 7 and 8, Table 3) having contributions to the ions 117 and 118, the formation of which during degradation is presented in Fig. 4(a). Naphthalene (128, Fig. 4(b)) is formed only during the second degradation stage.

In the case of indene, indane (and other compounds) and naphthalene, the curves are very noisy due to the low amount in which these compounds are formed. In Figs 3(b), 4(a) and 4(b), which show their formation as a function of temperature, the experimental data are displayed in two curves representing the upper and lower levels within which the experimental results are scattered. Nevertheless, the trend in the intensity variation is clear.

3.3.3 Aliphatic hydrocarbons Figures 5(a) and 5(b) show aliphatic hydrocarbons through

the evolution of their lines at m/e

67 (cyclopentene), 55-57 (C,,-C,, alkenes) and 57 (C,,,C,, alkanes). As can be seen, they all have small peaks situated towards the higher

1 .la5oo I\ _,_ l o.OOE*w : _ ,_--,

0 SO 100 150 200 2x) 3aO 350 4Oa 450 SaJ

50 100150 200 250 MO 350 400 *50 500

T-W)

Fig. 5. Ion monitoring during PVC thermal degradation: (a) cyclopentene (67); (b) C,0-C,3 alkenes (55-57) and C,,-C,,

alkanes (57).

186 I. C. McNeil1 et al.

5mE-10-~ 2

,j>

0ooE.M).

0 so 1w 150 xl0 25n c?m 350 400 450 !m Tempermum CC)

Fig. 6. Ion monitoring during PVC thermal degradation: methane (16) and H, (2).

temperatures of the first degradation stage, between 300 and 37oC, which develop into important peaks in the second stage.

3.3.4 Non-condensable gases Figure 6 illustrates the evolution of HZ (m/e 2) and CH, (m/e 16) with temperature. The amount of H, might be underestimated due to the low sensitivity of the quadrupole mass spectrometer for m/e under 10 amu. However, the general tendency of H, evolution can be observed. H, and CH, are formed only in minor amount, if any, in the first degradation stage. They are evolved in detectable amounts in the second stage, at temperatures above 400C.

4 DISCUSSION

4.1 Aspects of PVC degradation from previous work

It is known that the thermal degradation of PVC begins at a relatively low temperature, soon after T. (8OoC).4 The initiation of the dehydrochlorina- tion (DHC) was correlated with the labile sites-defects-of the chemical structure: the allylic chlorine atom of the internal unsaturation, the tertiary chlorine atom of the butyl branches and the head-to-head units formed in the course of synthesis. The head-to-head structures have an increased tendency of splitting Cl, but do not propagate the DHC more readily than PVC, as shown by studies on model compounds prepared by chlorinating cis-l,4-polybutadiene.s The con- centration of the above defects was evaluated as Oel-O-2/1000, 0*5-l/1000, and under 0.2/1000 vinyl chloride (VC) units, respectively.h Although the rate of degradation has been clearly correlated with the concentration of allylic and tertiary Cl, there is no definite evidence on the

nature of the initiation act at low temperatures (SO-120C).

At the same time, it is possibile that physical defects contribute to DHC. A certain confor- mation of syndiotactic segments of chains in which the Cl atoms are tram relative to a double bond makes the DHC very easy, while the presence of a Cl atom gauche to the unsaturation favours the termination of polyene growth. Syndiotactic chains of up to 13 VC units are present in commercial suspension PVC samples, as part of very small imperfect crystallites dispersed in the polymer matrix. Overall, the conformation responsible has a concentration of the same order of magnitude as the defects of the chemical structure and it is likewise dispersed.

At high temperatures, the initiation of DHC takes place in the whole mass of the polymer by the random scission of the secondary chlorine atoms. At intermediate temperatures, chain ends can have an important contribution in DHC, if proved to have labile structure. It is largely accepted in the literature that chain ends contribute to the formation of mononuclear hydrocarbons, as reviewed below.

Benzene formation begins at low temperatures (soon after T,) as soon as polyenes of suitable length are formed through DHC. Polyenes are very active in crosslinking and molecular enlargement has been observed at a very low degree of DHC (0.5% at lSOC). Several mechanisms for benzene formation in the early stages of thermal degradation have been advanced: the cyclisation of a triene radical situated at a chain end and formed through DHC followed by the scission of the macro- molecular chain, the thermal condensation of a triene within the chain, or the reaction of a triene in the triplet state easily achievable by thermal excitation. The last hypothesis is still very appealing though it did not gain experimen- tal support with the passing of the years.

It is considered that at higher temperatures benzene and alkyl benzenes are formed by an intramolecular process (cyclisation) which starts at the chain ends of the macromolecules, while the inner part of the macromolecules is bound into a crosslinked network. The network is formed through: (i) Diels-Alder condensation of double bonds belonging to different chains generating cyclohexene rings and within the chain generating cyclohexadiene rings, which then become aromatic: and (ii) crosslinking

A study of the products of PVC thermal degradation 187

through free radical attack on unsaturation. The extent to which either of these processes takes place is not known, but both crosslinked and conjugated structures have been identified.13 The Diels-Alder condensation, however, is considered a major process, for the use of a dienophile strongly reduces PVC crosslinking.14 The present picture of PVC as it degrades at temperatures above 250C is that of a crosslinked internal core to which loose branches are attached. The branches or chain ends produce benzene and other aromatic hydrocarbons by the cyclisation of the free ends.

The breakdown of the crosslinked network produces condensed ring aromatic hydrocarbons with or without aliphatic substituents and aliphatic hydrocarbons.

4.2 The chain ends in suspension PVC

In the framework of the above picture and since we are concerned with the degradation at all temperature levels it is worth examining the structure of chain ends. The most probable structure of the PVC macromolecule obtained by suspension has been the object of an excellent review.6

In suspension polymerisation, the major termination path is by chain transfer to the monomer. This is a complex process consisting of several reactions by which one Cl atom is expulsed and chain ends of the type a (1-chloro, 2-alkene) - Scheme 1 - are created. Chain end a is referred to as pseudoterminal unsaturation. The Cl atom reinitiates the polymerisation producing chain ends of type b (1,2- dichloroalkane). This is a very efficient process, reinitiation by Cl becoming the main way of initiation: four-five polymer molecules per initiator residue are formed. Towards very high conversions, under conditions of monomer starvation, the termination is by chain transfer to

? CICHZ - y - CH2 - ~t+--C, - C - CH2 - CHCI - CH2 - C2C, Cl Cl L Hz b L C

1

6

7 a CH2CI

Scheme 1. The structure of the suspension PVC macromolecule.

the polymer and polymer branching starts. The most frequent branches are C, (type c, Scheme 1) which shows that backbiting is the most probable transfer process. The termination at high conversions can also take place by H abstraction from any chain, hence, a number of normal (1,3-dichloroalkane) chain ends, -CH( Cl)-CH,- CH,Cl, are formed.

Let us consider the C, branches among chain ends. On average, in suspension PVC the concentration of chain ends is: 0.8-0.9 chain ends of the type b, 0.7 chain ends of type a and O-2 normal chain ends per molecule, 0.20-0.25 initiator fragments units and 0.5-l C, branches per 1000 VC units. Considering the molecular weight of the present PVC sample, 40,000, one can calculate the proportion of chain ends (Table 5). As already known6 and presented here in Table 5, structures a and b are the most frequent chain ends. Another observation is that C, branches (hence tertiary Cl atoms) are quite numerous.

Considering the structure of the PVC molecule with three chain ends as in Scheme 1 (including the C, branches), the MW of our PVC sample and the amount of benzene being formed through pyrolysis (5.6%), one can estimate that 9.5 molecules of benzene are formed per chain end.

Little is known about the tendency to DHC of chain ends of a particular structure. Table 6 shows the activation energy (E,) for HCl elimination of model compounds. Long chains ending in the structures shown in Table 6 are expected to have lower E, than the mic- romolecular compounds in the same table. As one can see, 1-chloro, 2-alkene chain ends have the highest tendency to DHC, followed by 1,2-dichloroalkane and finally by normal chain- ends. For comparison, Table 6 also includes the E, for the elimination of an allylic Cl associated with a terminal double bond (compound No. 4) which, if substituted at both ends (compound No. 5) is a model for internal unsaturation and is

Table 5. The percentage of chain ends of different structures in a typical suspension PVC sample

Type

b

StNCtUW %

CICHrCHCHr 40-32 Cl

a ClCHrCH=CHCH>- m-28 c Tertiary Cl at G branches 14-2s

lllfutor terminated 8-7 s NW& CICHrCHr 8-7 5

Table 6. Activation energies for HCI eliminations from model compounds [Ref. 151

Model wmpound

H,C-CHJTiKI HC-CHCHKI

Cl H,CCH=CH-CH>CI H,C-CHCH=CHz

Cl -HK-CH-CH=CH-

Cl

E. (kalhole)

55 I 54 9(-ZHCI)

52 2 48 5

A study of the products of PVC thermal degradation 189

The macroradical I formed through tertiary Cl scission (reaction 1) would usually split in position d with the formation of macroradical II and macromolecule III (reaction 2). Macroradical II has the structure of the growing chain in the polymerisation and it is not known to promote zip dehydrochlorination. Its main reaction route in the absence of monomer is likely to be H atom abstraction from another chain or from its own chain (backbiting at the fifth C atom). Hence, it is improbable that radical II has a direct role in benzene formation which has to be sought in another reaction.

The Cl atom expelled in reaction (1) will give a random attack on any macromolecule. The affinity of Cl atom for the CH, group6 would lead to H abstraction through which radical IV is formed. Radical IV can split (reaction 3) leading to radical V which has a structure more favourable to zip DHC. The electron withdraw- ing effect of Cl polarizes the bonds in radical V as shown in Scheme 2 favouring zip DHC by which radical VI results after three double bonds have been fomed (reaction 4). Radical VI has a resonance form VII. It is proposed that radical VII attacks the double bond at the sixth C atom closing a cyclohexadiene ring and forming radical VIII (reaction 6) which can then split the bond to the ring as this bond is weakened by the withdrawing effect exerted by the Cl atom on the radical. The scission is followed by aromatisation and H atom rearrangement so that no H transfer from other molecules is needed. Benzene formation has been shown to be intramolecular not only with respect to C, but also to H. Aromatic ring formation takes place after scission and does not infringe the orbital symmetry interdiction which operates with the aromatisation of substituted rings. Moreover, reaction (6) regenerates radical V which can resume the cycle producing several benzene molecules per radical chain end.

The alternative formation of toluene in reaction (6) from radical VIII would imply the scission of the bond between the C atom bearing the radical and that bearing the Cl atom. Though this is possible, it is not highly probable due to the withdrawing effect of the Cl which shortens and reinforces the bond. The repeated formation of benzene at the radical chain-end apparently prevents the formation of polyenes containing more than three double bonds in this particular place. Consequently, the formation of condensed

rings like naphthalene, indene etc., from radical chain ends is of lower probability, a fact which has the support of our experimental data since very little of these compounds are formed in the same stage as benzene. Our findings do not agree with the conclusions of a previous study in which it was conclued that condensed ring aromatic hydrocarbons and alkyl aromatic hydrocarbons are formed by the same mechanism as benzene.

The C, branch has a small, but finite probability of being eliminated by the scission of bond e. The chlorobutene isomers identified in Table 4 (Nos 19 and 20) in low concentration can be formed in this process (reactions 7 and 8). It is not clear at this stage whether the saturated homologue (chlorobutane, No. 21 in Table 4) is formed in the same process or not.

The formation of macromolecule structure III with vinylidene groups close to chain ends could explain the crosslinking observed at very low degrees of DHC (O-3%)8 through Cl atom attack on this pendant unsaturation.

The formation of benzene at higher tempera- tures through DHC of loose chain ends followed by cyclisation starts with 1-chloro, 2-alkene type chain ends (pseudoterminal un- saturation). Scheme 3 explains the formation of benzene and other aromatic hydrocarbons from chain-ends of this type.

The mechanism proposed in Scheme 3 accounts for benzene being formed through repetitive scission of the chain ends of pseudoter- minal unsaturation and for the main identified alkyl aromatics being formed at smaller yields.

In Scheme 3, the 1-chloro, 2-alkene chain end is polarised as shown, which makes the dissociation of Cl atom easier (reaction 9). Radical XI is formed in which the bonds are polarised by the electron withdrawing Cl atoms and the tendency to conjugation between the double bond and the radical. This will render DHC easier by which radical XII (reaction 10) is formed. Radical XII has an alternative resonance form XIII which would be expected to be somewhat more stable. Both radicals, XII and XIII, can explain the formation of aromatic hydrocarbons. This process will be illustrated here with radical XIII which can split in positions a, b and c. Though route a is still most probable due to the maximum gain in free energy for benzene formation, routes b (leading to toluene) and c (leading to styrene, ethylbenzene and o&o-xylene) have increased probability com-

I. C. McNeil1 et al.

- CH - CH* - CH - ;H* - CHtCH2-CH = CH - &I2 + a I I Cl Cl I Cl

XI - 3HCI

-..-~y2- i - - CH CH CH-CH-CH.CH_-CH*

XII 1 -~-c*-cH-cH=CH-cH.CH-cH=CH~

XIII

c ba

__t

!I % $ t dH CH=CH-CH.CH-CHCCH*

a

(-: cCH - a$- CH = CH - iH2 + C&

Cl 1 tic,

XI

b - y - CH2 + CH3- C&

c,

Scheme 3. Aromatic hydrocarbon formation from pseudo- unsaturated chain ends.

pared to the formation of the same compounds from macroradical VIII in Scheme 2. Scheme 3 is also consistent with benzene being generated repeatedly through the regeneration of radical XI (reaction 12).

The formation of me&- and para- isomers of xylene cannot be explained as arising from radicals formed at marginal, loose branches of the structures in Scheme 1. Their formation can only be explained by the scission of the crosslinked network containing benzenoid str- uctures formed by Diels-Alder condensation.

5 CONCLUSIONS

PVC thermal degradation in vacuum up to 500C has been studied by following the relative rate of volatile product evolution in thermal volatilisa- tion analysis experiments and by monitoring the formation of the main products by mass spectrometry. The material balance after pyroly- sis combined with the GC-MS analysis of the liquid fraction shows that HCl is the major fraction (53% of the polymer), followed by tar (24%), char (9.5%), liquid fraction (7%) and gas fraction (6.6%). 10% of the Cl atoms remain in the polymer after HCl evolution ceases. The quantitative evaluation of the products in the liquid fraction shows that benzene accounts for 80% of the liquid fraction and for 5.6% of the

polymer. The chlorinated hydrocarbons (mainly chlorinated alkenes and chlorinated aromatics) account for 1.75% of the liquid fraction and for 0.14% of the polymer.

PVC shows two degradation stages. During the first one, between 200 and 360C HCl and benzene are evolved but very little alkyl aromatic hydrocarbons (toluene, xylene isomers, ethylben- zene, etc.) or condensed ring aromatics (naph- thalene, indene, indane, etc.). The molar ratio of benzene and HCl shows that only 15% of the double bonds produced through dehydroch- lorination generate benzene. Most of the double bonds accumulate in the polymer and are active in crosslinking through intramolecular Diels- Alder condensation and through free radical attack. As a result, a crosslinked network is created which contains cyclohexene and cyclohe- xadiene rings embedded in an aliphatic matrix. In the second degradation stage (360-5ooC), the aromatisation of the above rings takes place with much scission. As already suggested in the literature, the aromatisation of the substituted rings cannot occur if it necessitates orbital overlapping of the substituted C atoms, due to orbital symmetry interdictions. Hence scission is a prerequisite for aromatisation. Scission gener- ates an important amount of alkyl aromatic and condensed ring aromatic hydrocarbons, as well as C,-C,, aliphatic hydrocarbons and a small amount of hydrogen.

The mechanism of benzene formation thro- ughout the temperature range has been con- sidered beginning with scission at the tertiary Cl (the most abundant labile site) and continuing with the formation of benzene through the cyclisation of chain ends, of which the pseudoter- minal unsaturation is the most labile structure to dehydrochlorination.

ACKNOWLEDGEMENT

Support from SERC for the work reported is acknowledged with thanks.

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