Thermal dehydrochlorination and stabilisation of poly(vinylchoride) in solution: Part II—Effects of HCl readdition reaction

Polymer Degradation and Stability 21 (1988) 165-180

Thermal Dehydrochlorination and Stabilisation of Poly(vinylchoride) in Solution: Part lluEffects of HCI Readdition Reaction

Tran van Hoang & A. Guyot CNRS, Laboratoire des Materiaux Organiques, BP.24-69390 Lyon-Vernaison,France (Received 8 October 1987; accepted 21 October 1987)

ABSTRACT The role of HCl during the thermal degradation of PVC has been investigated. It was shown that HCI catalyses the degradation as well as itself adding to the conjugated sequences in the degraded PVC. There is competition between these two reactions. At the beginning of the dehydrochlorination, the first reaction is dominant, while readdition becomes more important as double bonds are formed. Metal or organometallic chlorides may significantly influence these effects. The reverse reaction is catalysed by organotin chlorides of high Lewis acidity. These retardation effects are a pure consequence of the partial and random reversible thermal decomposition of PVC; they do not result from the configurational rearrangement by the readdition reaction nor by the internal isomerisation assisted by tin compounds.

INTRODUCTION In our previous studies of PVC degradation using chlorohexene as a model for the allylic structure in PVC chains, we have shown that the thermal degradation of this c o m p o u n d is first accelerated by the evolved HCI and then comes to an equilibrium due to the readdition of HC1 to the newly formed conjugated double bonds. 1 In another model study, we have also 165 Polymer Degradation and StabiliO' 0141-3910/88/$03'50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

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demonstrated that this reverse reaction of HC1 is catalysed by organotin chlorides. 2 The first action of HC1 as a catalyst in PVC dehydrochlorination has been discussed by many investigators; a- 19 the catalytic action of HBr on PVBr degradation was also reported. 2° The mechanisms of the reaction were thoroughly discussed. 4'26-4° Concerning the second action of HC1, readdition to the polyene which has developed in the PVC chain, Oleinick et al. 21 observed this phenomenon in 1968, then Razuvaev et aL iv proposed a radical process for readdition in PVC black. On the contrary, Guyot and his colleagues, 22 in 1971, proposed an ionic mechanism which they have checked later in 1976--however, not by direct study on the polymer, but on model small molecules. 2 Furthermore, Tfidos and his colleagues 13 presented some arguments in favour of readdition in 1973, and in the same period Owen and Williams 24 were able to show evidence for the action of HC1 on predegraded PVC under irradiation. These literature data demonstrate that the effect of HC1 on PVC degradation has gradually been confirmed. But if the catalytic action is recognized almost unanimously, the readdition of HC1 is still a controversial subject and remains difficult to justify experimentally. The aim of this present work is to study the effects of HC1 on PVC degradation more extensively, especially the catalytic effect and the readdition reaction.

EXPERIMENTAL

Materials The PVC, specially polymerized for the study of the IUPAC Working Party, 47 is as free as possible from impurity (PVC XII, MW = ,-~94 500). The 1,2,4-trichlorobenzene and the carboxylates of calcium and zinc are products from Merck (Art. 821152). The fl-carotene is from Eastman Organic Chemicals, while all the organotin compounds were supplied by Ciba-Geigy. The gas carrier is argon of highest purity (Nerta) from Air Liquid.

Apparatus Infra-red studies were carried out using a Perkin-Elmer 297. The titration of HC1 was carried out by techniques described previously. 25 The tacticity of PVC was studied by 13C-NMR with a Brucker WP 80 apparatus.

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RESULTS AND DISCUSSION The shape of PVC degradation curves in 1,2,4-trichlorobenzene (TCB) solution depends on the temperature. At temperatures lower than 174°C, HC1 is evolved linearly with time. In the range 180-186°C, the kinetic curves show a sigmoidal shape and, finally, above 194°C they exhibit a downwards concavity (Figs 1, 2 and 3). These facts could be explained by the influence of HC1. At low temperature, the amount of HC1 generated, which is not high enough, is rapidly swept away from the reactor so there is practically no effect of HC1 on the degradation process. At higher temperature (184°C), the amount of

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Degradation curves of PVC (in mg) in 40cc trichlorobenzene (TCB) solution. At 164°C: 400 (O); 800 (0). At 174°C: 400 (~); 600 ([~); 800 (A).

HC1 generated is greater so the amount of HC1 remaining in the solution is sufficient to catalyse the degradation. However, this effect reaches a maximum point I (Fig. 2), the inflexion point, and the dehydrochlorination (DHC) rate begins to decrease, probably due to the competitive reverse reaction of HC1 addition to the double bonds. At even higher temperature, because of the abundance of HC1 and double bonds in the reactor, the reverse reaction takes place even at the beginning of the degradation process, so all the curves exhibit downwards concavity (Fig. 3). The autocatalytic effect of HC1 was then checked in two special kinds of experiment.

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Degradation curves of PVC (in mg) in 40 cc TCB solution at 184°C: 200 ( + ); 400 ((3); 600 (*); 800 (@).

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Degradation curves of PVC (in mg) in 40 cc TCB solution. At 194°C: 200 (a); 400 (b); 600 (c). At 204°C: 200 (d); 400 (f).

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First, a TCB solution is bubbled with argon until a very small amount of PVC is added and is degraded in the reactor. The amount of evolved HC1 dissolved in the solution is consequently very small and will serve as catalyst; a large amount of PVC is then added to the solution which is again bubbled with the argon stream. The total amount of HC1 liberated during the pretreatment time is less than 5% of the initial amount of HC1 generated by the degradation of the large amount of added PVC. Figure 4 shows that the degradation rate is very sensitive to HC1 and the inflexion points of the catalysed curves appear sooner than in the non-catalysed curve.

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Fig. 4. HCI evolution from degradation of 400mg PVC at 184°C in 40cc TCB solution containing a small quantity of predegraded PVC: (O), in absence of pretreated PVC;( x ), in pretreated PVC (10mg) solution for 65 min; (O), in pretreated PVC (20mg) solution for 67 min; (#), in pretreated PVC (20mg) solution for 143 min.

In the second set of experiments, the reactor was closed for various times, allowing the accumulation of HC1; when the reactor is opened and HC1 allowed to be removed by the gas stream, an increase of the DHC rate, as well as a shift of the inflexion point to lower time are observed (Fig. 5). The true catalyst for DHC is believed to be a charge transfer complex between HCI and the polyene sequences resulting from the DHC, like the one formed from HC1 and 2-4 hexadiene in dichloroethane solution. The latter has been shown previously to be able to catalyse strongly the readdition reaction. 2 But in degraded PVC, the polyene sequences are generally

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longer than two and in this case fl-Carotene may be considered to be a better model than a simple diene. Previously, we have reported some UV and N M R evidence for the readdition of HC1 to fl-Carotene. In NMR, for example, new proton resonances appear around 4.8 ppm attributed to the formation of an allylic chlorine structure. 2 Figure 6 shows the appearance of C-C1 stretching bands at 620, 650 and 705 c m - 1 in the IR for, probably, various conformers of the product of that reaction which was carried out in acetone solution (35%) at 35°C. The colour of the solution changes from deep red to dark green with increasing time. On addition of fl-carotene to the solution of PVC, a net decrease of the D H C rate is observed, the effect being more pronounced at higher temperatures (Figs 7A, 7B, 7C). The behaviour is complex. At 184°C, for example, a very small amount of fl-carotene can suppress the autocatalytic shape of the D H C curve (Fig. 7B); for higher amounts of fl-carotene, the D H C rate becomes practically independent of the amount. At higher temperature (Figure 7C), a surprising maximum effect is observed for intermediate amount of tiC. Readdition of HC1 to the double bonds of tiC via a charge transfer complex HCI-flC 1 is an obvious explanation of the decrease of the rate of evolution of HC1.

Thermal dehydrochlorination and stabilisation of P VC

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The same decrease of D H C rate is observed in experiments carried out in the presence of predegraded PVC. In these cases, a first degradation step is carried out with a certain a m o u n t of PVC and, during this time, HC1 is removed. Then a second a m o u n t of PVC is introduced and the quanity of HC1 evolved after that time which is taken as zero time, is recorded. Such experiments are illustrated in Figs 8 and 9. In Fig. 8, a comparison is shown between the effect o f tic and o f an equivalent a m o u n t of PVC predegraded at different times at 204°C. If this time is long enough, despite the fact that the a m o u n t o f PVC is doubled, the whole D H C rate is also reduced. Contrarily, for the experiments illustrated in Fig. 4, the a m o u n t o f predegraded PVC is very low and then the rate is enhanced. This was because, after the predegradation, there are not enough double bonds to allow for a rapid readdition o f HC1; the same is true with a

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Thermal dehydrochlorination and stabilisation of P VC

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174

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large a m o u n t o f PVC but predegraded for a short time (curve b in Fig. 8). In general, a very large retardation effect is not observed, due to the fact that the predegraded PVC continues to generate HC1. If the degradation is carried out in the presence of an HC1 acceptor, such as calcium stearate, readdition is not allowed. Typical data are presented in Fig. 10. The observed evolution o f HC1 is, o f course, very low and very slow (curve a), because most o f the HC1 reacts with the calcium stearate to give calcium chloride. The actual D H C , which can be estimated from the

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Fig. 10. Degradation at 184°C of 400mg PVC in 40cc TCB solution (+), and in the presence of 25 mg calcium stearate: (O), degradation estimated by HC1 gas evolution; (~), degradation estimated from the sum of HCI gas and CaCI2 formed in the reactor. The curve (O) resulting from the subtraction of the two curves (~) and (+), measures the quantity of HCI readded to the double bonds in PVC. coulometric titration of total C1- ions from HCI gas evolved and from samples taken from the solution (curve b), is higher than in the absence o f the calcium compound. The difference (curve c) m a y be taken as a reasonable estimate o f the readdition because CaCI 2 can be considered as neutral in both degradation and readdition reactions. O f course, if calcium is replaced by Zinc stearate or acetate, the apparent and actual D H C rates are much higher (Fig. 11). With the alkyltin trichlorides, though they do not consume HC1, the D H C rates are slowed d o w n significantly. The best explanation of the difference is that the alkyltin chlorides are good catalysts for the readdition of HC1. This

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Fig. 11. Degradation of 400mg PVC in 40cc TCB at 184°C in the absence ((3) and the presence of 4 mg zinc stearate (A), 1.2 mg of zinc acetate (+), 5 #1 of butyltin trichloride (~) and 8 #1 of octyltin trichloride (@).

is clearly shown in Fig. 12 which illustrates the effect of addition of various amounts of Butyl-di and tri-chloride. These compounds should be effective for both D H C and HC1 readdition. In both cases, the catalytic activity seems to be related to the Lewis acidity of the organotin chlorides. It may be surprising that ZnCI 2, which is a strong Lewis acid, seems mostly to catalyse the DHC. A possible reason for this is the ability of ZnC12 to destroy the double bonds through a crosslinking reaction. 41 Such crosslinking is not observed with organotin chlorides. The ability of tin compounds to catalyse both D H C and its reverse reaction may explain why their net effect may be more or less oriented towards the stabilisation or degradation as shown by contradictory results reported in the literature. The data in Fig. 12 tend to show a maximum stabilising effect for a certain a m o u n t of the trichloride; the nature of the alkyl group may also be important. The octyl group does favour catalysis towards D H C a little more strongly than the Butyl group. The net result may also depend on the amount of tin compound. A final series of experiments (Fig. 13) shows that there is an antisynergistic effect when both tiC and alkyltin chloride are present. If the a m o u n t of additives is large enough, the D H C may even be accelerated. The situation is

176

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Thermal dehydroehlorination and stabilisation ~[ P VC

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then complex because various kinds of complex may exist: polymer polyenes with either HC1 or tin chlorides, tiC with HC1 or tin compounds and possibly a ternary complex. All of these may affect both reactions, D H C and readdition. Another possibility is that as a result of the competition of degraded PVC and tiC for the capture of HC1, the tiC in the presence of tin chloride becomes more reactive towards HCI due to the/~C-tin chloride complex; but contrarily to the addition of HC1 to degraded PVC, the grafting of HC1 on tiC in most cases forms an allylic structure which is more sensitive to decomposition in the presence of Lewis acid compounds. Finally, two basic questions may be posed to try to explain the ability of organotin chlorides to stabilize PVC. Firstly, does readdition lead to the same structure as the initial PVC or is there a possibility of a change in tacticity? Actually, tacticity is also a factor which influences the stability of PVC itself as suggested by several authors.42-45 So it might be suggested that a more stable PVC could result from rearrangement of the polymer configuration. Table 1 shows the results of a 13C-NMR study of the tacticity of PVC before and after thermal degradation in TCB at 184°C in the presence of trioctyltin monochloride. TABLE 1

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The change in configuration is not very significant. Secondly, does the retardation effect find its origin in the isomerisation of the labile chlorines in PVC into more stable chlorides rather than to readdition, as suggested by Wirth and Andreas. 46 It is quite difficult to distinguish between the isomerisation and readdition reaction because each gives rise to the same retardation phenomenon. In order to clarify this situation, experiments have been carried out to compare the behaviour of calcium stearate (CaSt) and tetraoctyltin (TOT). Both compounds react with HC1, the first giving calcium chloride which is known to be neutral towards both DHC and readdition. The second c o m p o u n d is transformed into a mixture of octyltin chlorides, which may act as isomerisation catalysts. The results shown in Fig. 14 indicate that TOT is less efficient than CaSt in decreasing the evolution of HC1. Thus the

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conclusion is that either the expected isomerisation does not take place or, if it does, this effect has no consequence on the polymer stability. Our general conclusion is that readdition is an important feature of the thermal degradation process of PVC and may explain the apparent stabilizing effect of organotin chlorides although the mechanism of that readdition is not fully understood.

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Thermal dehydrochlorination and stabil&ation of PVC 8. 9. 10. ! 1. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

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45. Behnisch, J., Zimmerman, H. & Andrers, H., Poly. Deg. andStab., 13, 113 (1985). 46. (a) Wirth, H. O. & Andreas, H., Pure andAppl. Chem., 49, 627 (1977); (b) Wirth, H. O., Muller, H. A. & Wehner, W., 2nd International Conference of Degradation and Stabilisation Dubrovnik, Oct. 4--6 (1978). 47. Guyot, A., Pure & Applied Chem., 57, 833 (1985).