Thermal degradation of vinyl chloride-vinyl acetate copolymers — I. Bulk degradation studies by thermal volatilization analysis

European Polymer Journal, 1970, Vol. 6, pp. 679-686. Pergamon Press. Printed in England.

T H E R M A L D E G R A D A T I O N OF VINYL C H L O R I D E - V I N Y L ACETATE C O P O L Y M E R S - - I * B U L K D E G R A D A T I O N STUDIES BY T H E R M A L VOLATILIZATION ANALYSIS N. GRASSIE, I. F. M c L ~ E N and I. C. McNEILL Chemistry Department, University of Glasgow, Glasgow W.2, Scotland

(Received 28 July 1969) Abstract--The rates of production of volatile material from PVA, PVC and vinyl acetate-~inyl chloride copolymers, covering the entire composition range, have been compared using thermal volatilization analysis. It is found that, at each extreme of the composition range, incorporation of the co-monomer unit results in a copolymer less stable than the homopolymer. Minimum stability occurs for compositions of approximately 40-50 per cent VA. The proportions of acetic acid and hydrogen chloride produced from the copolymers appear to remain constant during degradation, indicating that neither is evolved preferentially, once reaction has begun. INTRODUCTION THE DEGRADATIONS of poly(vinyl chloride) (PVC) and poly(vinyl acetate) (PVA) provide an interesting contrast. In the former case, much work has been carried out yet the mechanism is still not established conclusively. The subject has been discussed recently by Geddes, (I~ Bamford and Fenton, C2) and McNeill and Neil33' ~) For the latter, however, almost the only work is that performed more than 15 years ago by Grassie/5> in which the main features observed in the degradation were satisfactorily explained. Servotte and Desreux (6> have confirmed some of Grassie's results, but some points of difference were also reported. Some aspects of PVA breakdown are considered in recent work by Gardner and McNeill. (7. s> Both PVC and PVA break down at fairly low temperatures (the former being the less stable) by initial splitting out of hydrogen chloride and acetic acid, respectively; the reaction proceeds systematically along the chain to give a highly conjugated residue ~hich will break down further by scission of the polymer backbone at higher temperatures. In PVA the acid-producing reaction has been considered to follow a molecular mechanism, (5) whereas there is strong support for the view that the corresponding reaction in PVC follows a radical chain mechanism. Copolymers of vinyl chloride (VC) and vinyl acetate (VA) have achieved commercial importance and knowledge of their mode of degradation is therefore of some value. It is also of considerable interest, however, to compare their d e ~ a d a t i o n behaviour with that of the homopolymers. Questions to which such a study might provide some answers include the following. Are the copolymers intermediate in stability between PVC and PVA or is there a maximum or minimum of stability at any particular composition ? Does degradation proceed preferentially at one or other type of chain unit, or does the reaction pass along the chain through both types of umt? Can the mechanism be established as molecular, or radical chain ? Do the copolymers discolour in the same way as the homopolymers ? *Presented at the International Symposium on Macromolecular Chemistry, Budapest, 1969. 679

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N. GRASSIE, I. F. M c L A R E N and I. C. McNEILL

N o p r e v i o u s s y s t e m a t i c s t u d y o f d e g r a d a t i o n in thts system a p p e a r s to h a v e been m a d e , a l t h o u g h L e h r l e and R o b b (9) h a v e s h o w n , using the p y r o l y s i s - g . l . c , t e c h n i q u e , t h a t u n d e r the c o n d i t i o n s o f their e x p e r i m e n t s the m a i n d e g r a d a t i o n p r o d u c t s are acetic a n d h y d r o c h l o r i c acids a n d that t h e y are p r o d u c e d in p r o p o r t i o n s c o r r e s p o n d ing to the p r o p o r t i o n o f the m o n o m e r s in the c o p o l y m e r . In this i n v e s t i g a t i o n , s a m p l e s o f the t w o h o m o p o l y m e r s a n d a r a n g e o f c o p o l y m e r s c o v e r i n g the entire c o m p o s i t i o n r a n g e b e t w e e n P V A a n d P V C h a v e b e e n p r e p a r e d a n d d e g r a d a t i o n has been s t u d i e d b o t h for b u l k s a m p l e s a n d in s o l u t i o n . T h i s p a p e r r e p o r t s d a t a o n the r e l a t i o n b e t w e e n c o m p o s i t i o n a n d t h e r m a l stability o b t a i n e d f o r bulk samples by t h e r m a l v o l a t i l i z a t i o n analysis ( T V A ) a n d t h e r m o g r a v i m e t r y ( T G ) . S o l u t i o n studies, i n c l u d i n g m e a s u r e m e n t s o f relative stability, p r o d u c t c o m p o s i t i o n , a n d c o l o u r a t i o n , are r e p o r t e d in P a r t I1.

EXPERIMENTAL

Materials Vinyl acetate (Hopkin & Williams) was distilled under nitrogen, the fraction boiling at 70--72 ~ being collected. This procedure removed both inhibitor and impurities. Vinyl chloride (I.C.I.) was used from the cylinder. Azobisisobutyronitrile (Eastman Kodak) recrystallized from ethanol, was used as initiator.

Filling of dilatometers The initiator (sufficient to give a final concentration of between 0" 0075 and 0-075 per cent in the mixture) was added to the dilatometer as a solution in benzene; the benzene was subsequently removed using a suction pump. The lower initiator concentrations were used for low VA copolymers and PVC. Vinyl acetate was degassed four times on a high vacuum line, distilled into a graduated reservoir and then into the dilatometer. Vinyl chloride was admitted to the vacuum line via a needle valve, condensed in a reservoir at -- 190°, degassed five times, distilled into a graduated reservoir and then into the dilatometer. These distillations were carried out from reservoirs at --65 ° at which temperature vinyl chloride is liquid.

Polymerizations Polymerizations were carried out at 60 ° in a thermostat to approximately 10 per cent conversion. Precautions had to be taken against the possibility of explosion, especially for mixtures containing around 95 per cent vinyl chloride.

Isolation and purification of the copolymers Two methods were used, depending on the composition of the copolymer. (a) Copolymers ofhigh VC content were dissolved in cyclohexanone and precipitated by methanol. This process was repeated three times and the copolymer was finally dried in a vacuum o~en for several days at approximately 45 °. (b) Copolymers with more than about 40 per cent VA were precipitated from acetone solution in a suitable water-methanol mixture, the composition of which had to be varied with copolymer composition. The copolymer was then "freeze dried" using benzene as solvent.

Copolymer compositions Copolymer compositions were determined using reactivity ratios obtained by the application of nuclear magnetic resonance spectroscopy, as already reported. (1°~ The full range of samples used in the present study comprised copolymers with 1, 9, 25, 40, 47, 75, 91, 95 and 99 mole per cent VA respectively, as well as the two homopolymers.

Thermal volatilization analysis The apparatus used was similar to that of McNeill/t~' lz~ Samples of appronimately 25 mg were examined as powders, or in freeze-dried form ( > 4 0 per cent VA). Heating rates of 5 ° and 10-~/min were used. The homopolymers and the copolymer with 47 per cent VA were also studied at 10~/min as films (25 mg) deposited from cyclohexanone on the flat base (area 10 cm') of the glass tube. The peak shape and temperature of the rate maximum for PVC differed slightly for film and powder, as might

Thermal Degradation of Vinyl Chloride-Vinyl Acetate Copolymers

681

be expected since sample thickness is important in PVC degradation, {t~ but th~s effect was less signtficant than the differences with composition. Temperatures quoted are sample temperatures.

Thermogrartmetry The homopolymers and the 47 per cent VA copolymer v,ere examined in a dynamic nitrogen atmosphere on the DuPont thermobalance using 5 nag powder or freeze-dried samples and a heating rate of 20~/min. The boat-type sample holder was of platinum, approximately ½in. long and agin. deep. RESULTS AND DISCUSSION Complete T V A curves (film samples) are shown in Fig. l(a) for PVC, PVA and the 47 per cent VA copolymer. The corresponding thermogravimetric data are given in

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Fig. l(b). It is clear that all three materials break down in two main stages. In the first stage, which comprises acid loss, the copolymer is considerably less stable than PVC, which in turn is less stable than PVA. The second stage occurs at similar temperatures (in the region of 450 °) for all the samples. Slight differences in the temperatures o f the rate maxima for corresponding processes in the T G and T V A curves are due to the different heating rates (20 ° and 10 ° per minute respectively). Since the three materials, after acid elimination, break down at the same temperature, a reasonable conclusion is that the end product of acid elimination in the case of the copolymer, as well as for both homopolymers, is a polyconjugated chain. The entire range of copolymers and the two homopolymers were examined by T V A as powder samples at two heating rates, 10 ~ and 5~/min. All the curves showed the two-stage degradation behaviour already illustrated in Fig. l(a), the temperature of the second rate m a x i m u m always being in the same range. Substantial differences in the shape and position of the first peak were found, however. Figure 2 shows the shapes and relative positions of the first peaks for a heating rate of 10~/min. To clarify the diagram, the data have been divided into two groups corresponding to

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polymers with less than and more than 50 per cent VA, respectively. Thus Fig. 2(a) illustrates the effect on stability resulting from incorporating increasing amounts of VA into the PVC chain, and Fig. 2(b) shows the corresponding effect of VC in the PVA chain. Less than 10 per cent of the other monomer produces a marked decrease in stability compared to the homopolymer, especially in the case of PVA, and apart from a minor irregularity at 95 per cent VA, there is a progressive decrease in stability as the amount of co-monomer is increased. To illustrate more clearly the initial stages of breakdown, the degradation was studied at the slower heating rate of 5°/min. By reproducing only the part of the TVA curve up to the first peak it is possible (Fig. 3) to get a clear picture of relative stability for the entire composition range. The stability increases from left to rigJat, so that the 47 per cent VA copolymer is the least stable material and PVA the most stable. The smooth change in stability with composition is very well demonstrated in Fig. 4 in which the Pirani response at a selected temperature in the programmed degradation, using the data of Fig. 3, is plotted against composition. As has been discussed previously, (12) the Pirani response is a measure of rate of volatilization. The relation between rate and Pirani response, (13~ however, is only linear up to pressures of the order of 10 .2 Torr (approximately 2 mV output on the commercial Pirani gauge employed) and thereafter becomes non-linear. Also, it is found that, for similar sample sizes, the Pirani response at the acetic acid elimination stage in PVA is considerably greater than that for dehydrochlorination in PVC. Thus the ordinate in Fig. 4 must be interpreted with caution. In particular, it is not valid to compare in

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detail relative stabilities of materials at opposite ends of the composition range. Nevertheless, the smooth trend for a decrease in stability towards the centre of the composition range is well established, and the only real uncertainty on the diagram is the relative stability of the 9 and 75 per cent VA samples. The minimum of stability must lie in the region of 45 per cent VA. Information regarding the composition of the degradation products during a temperature-programmed run was obtained using a modified version of the TVA apparatus in which a U-tube was inserted immediately before the Pirani gauge attachment point. The upper curve in Fig. 5 shows the trace obtained for the 47 per

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The results of these bulk degradation experiments suggest that in this system the acid-elimination process is facilitated by increasing heterogeneity of the chain, that it probably proceeds systematically along the chain as in the homopolymers, and that ultimately all the samples leave a similar polyconjugated residue. In order to substantiate these conclusions and provide a basis on which a mechanism may be postulated, more information is required regarding the proportions of acids produced, and on the development of conjugation. These aspects were more conveniently studied in solution and the results are reported in Part II. Acknowledgements--Thanks are due to the Science Research Council for the award of a studentshlp (to I. F. McLaren) and to the Distillers Company Limited for a grant in support of this work. Data on the film samples ~ere obtained by Mr. I. M. Duncan.

REFERENCES (1) (21 (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

W. C. Geddes, Europ. Polyrn. J. 3, 267, 733,747 (1967). C. H. Bamford and D. A. Fenton, Polymer 10, 63 (1969). I. C. McNeill and D. Nell, ;~,[akromolek. Chem. 117, 265 (1968). I. C. McNeill and D. Nell, Europ. Polym. J. 6, 569 (1970). N. Grassie, Trans. Faraday Soe. 48, 379 (1952), 49, 835 (1953). A. Servotte and V. Desreux, J. Polym. Sci. C22, 367 (1968). D. L. Gardner and h C. McNeill, Paper presented at the IUPAC Symposium on Macromolecular Chemistry, Budapest, 1969. D. L. Gardner and I. C. McNeill, unpublished. R. S. Lehrle and J. C. Robb. Nature, Lond. 183, 1671 (1959). N. Grassie, I. C. McNeill and I. F. McLaren, J. Polym. Sci. 33, 897 (1965). I. C. McNeill, J. Polym. Sci. A4, 2479 (1966). I. C. McNeiIl, Europ. Polyrn. J. 3, 409 (1967). I. C. McNeill and D. Neil, in ThermalAnalysis, Vol. 1, p. 353, ed. R. F. Schwenker and P. D. Garn Academic Press, New York (1969).

R~sum~--Les vitesses de formation de produits volatils 5. partir du PAV, du PCV et de copolym~res ac6tate de vinyle--chlorure de vinyle couvrant route la gamme de composition, ont 6t6 compar~,es par

686

N. GRASSIE, I. F. M c L A R E N and I. C. M c N E I L L

analyse de pyrolyse thermique. On a trouv6 que, pour chaque extrdmit~ clu domaine de composition, l'incorporanon de motifs du co-monomere donne naissance 5. un copolym~re moins stable que l'bomopolym~re. La stabilit~ minimale apparait pour environ 40 5. 50 pour cent de A.V. Les proporUons d'acides ac&ique et chlorhydrique formds 5. partir du copolym~re demeurent constantes au cours de la d~.gradation, ce qui indique qu'aucun ne se ddgage prdfdrentiellement une lois la r~action commencde. Sommario--Le velocits, di produzione di sostanze volatili dal PVA, PVC e dai copolimeri acetato di vinile/cloruro di vinile, in tutto i| campo di composizione, sono state paragonate usando l'analis~ di volat,lizzazione termica. Si ~ trovato che, a ciascun estremo del campo di compos~zione, l'incorporazione di units, di comonomero porta ad un copolimero meno stabile dell'omopolimero. La stab~l,t:i pi~ bassa si ha per una composizione 40-50 ~ in VA. Le preparazioni d~ acido acettco e acido cloridrico prodotte dai copolimeri sembrano rimanere costanti durante la degradazione, mdicando che nessuno dei due d sviluppato preferenzialmente una volta the la reazione d iniz~ata. Zusammenfassung---Die Geschwindigkeit der Bildung flfichtiger Stoffe aus PVA, PVC und tiber den gesamten Bereich der Zusammensetzung yon Vinylacetat-Vinylchlorid Copolymeren, wurde mit Hilfe der thermischen VerflC~chtigungsanalyse verglichen. Es wurde festgestellt, daB, bei den jeweiligen Extremen der Zusammensetzung, der Einbau der Co-Monomereinheit zu einem Copolymeren fiihrt, das weniger stabit ist als das Homopolymere. Die geringste Stabiht~it liegt bei einer Zusammensetzung yon etwa 40--50 Prozent VA. Das Verh~.Itnis der aus den Copolymeren geb~ldeten Essigs~.ure zu Chlor~asserstoff scheint fiir die Dauer des Abbaus konstant zu bleiben, ein Hinweis darauf, dab keine der beiden Sgiuren bevorzugt gebildet wird, sobald die Reaktion erst in Gang gekommen ist.