Thermal degradation of blends of PVC with poly(ethylene adipate)

Polymer Degradation and Stability 41 (1993) 311-317

Thermal degradation of blends of PVC with poly(ethylene adipate) I. C. McNeili Polymer Research Group, Chemistry Department, University of Glasgow, Glasgow, UK, G12 8QQ

& S. Basan Chemistry Department, Cumhuriyet University, 58140 Sivas, Turkey (Received 8 December 1992; accepted 27 December 1992)

The thermal degradation of PVC-PEAD blends has been studied over the whole composition range, using thermal volatilisation analysis and thermogravimetry. In the TVA studies, degradation products have been separated and characterised and in the TG experiments both temperature programmed and isothermal conditions have been used. The blend system has been found to show very similar behaviour to the PVC-poly(tetramethylene sebacate) system previously investigated, involving some stabilisation of PVC at low to moderate PEAD compositions, but destabilisation at high PEAD contents. A similar degradation mechanism in the blends is proposed, involving interaction of chlorine radicals from the PVC with PEAD, following by cross-linking.

INTRODUCTION

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This contribution is one of a series of studies of the stability and decompositon behaviour of blends of PVC with other polymers. In recent investigations, the systems investigated have included PVC with bisphenol A polycarbonate, 1 with polydimethylsiloxane2 and with poly(tetramethylene sebacate) ( P T M S ) . 3 In the degradation of PVC-PTMS blends it was found that at certain compositions, there was some stabilisation of the PVC in the blend and that, in general, blending resulted in an increase in the residue of degradation to 500°C under nitrogen. An interpretation of these effects in terms of degradation mechanism was given. The present study seeks to compare the effect of an alteration in the aliphatic polyester structure on the degradation behaviour. Poly(ethylene adipate) (PEAD) was selected for comparison. The structures of the two polymers

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poly(tetramethylene sebacate) The experimental approach used in this investigation was similar to that adopted in the case of PVC-PTMS blends. The homopolymers and various blend compositions were examined by thermogravimetry (TG) under dynamic nitrogen and the homopolymers and a 1:1 by weight blend were also degraded by thermal volatilisation analysis (TVA) under continuous evacuation. In these experiments, programmed heating to 500°C was employed. Using the TVA approach, it was possible to separate and examine spectroscopically the various product

Polymer Degradation and Stability 0141-3910/93/$06.00 (~) 1993 Elsevier Science Publishers Ltd. 311

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I.C. McNeill, S. Basan

fractions, from gases to the involatile residue of degradation. The polymers and a range of blends were also studied by TG under isothermal conditions at 270°C.

EXPERIMENTAL The PVC sample, as used in previous blend studies, was a commercial polymer (Breon 113), purified by reprecipitation from cyclohexanone by toluene. The PEAD sample was obtained from Aldrich Chemical Company and used as supplied. The procedures for thermal volatilisation analysis, thermogravimetry and for preparation of the polyblends were the same as reported in the case of PVC-PTMS blends. 3 In the present investigation, however, in the examination of condensable volatile product fractions separated by subambient TVA, and the cold ring fraction (tar/wax) products removed from the upper part of the degradation tube, mass spectrometry was

used in addition to infrared spectroscopy for purposes of characterisation. The two-limbed degradation tube approach was again employed in the TVA studies of blends of the polymers. This provides a clear indication of any effects due to interaction when the polymers are degraded together. Two experiments were carried out: in the first, simultaneous, separate degradation of the two polymers was carried out with one polymer in each limb (the 'unmixed' situation); in the second, a blend of the two polymers was present in both limbs (the 'mixed' situation), all other variables being kept similar.

RESULTS Thermal volatilisation analysis studies The behaviour of the individual polymers and the unmixed and mixed situations referred to above are compared in Fig. 1 for PVC and PEAD as

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TEMPERATURE, °C Fig. 1. T V A curves for: ( A ) P V C ; (B) P V C a n d P E A D u n m i x e d ; (C) P V C a n d P E A D m i x e d ; ( D ) P E A D . P o w d e r samples (50 rag); h e a t i n g rate 10°/rain.

Thermal degradation of PVC-PEAD blends

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areas do not provide a direct comparison of the amounts of substances since the Pirani gauge response (rate axis) varies from substance to substance and is also not linear over the whole of the recorded range. The SATVA traces for the separation of condensable volatile products of degradation of PVC and PEAD, respectively, are shown in Fig. 2. Using IR and MS methods, the products in these separations were assigned as follows: PEAD (left to right) carbon dioxide, acetaldehyde and 2-ethylacrolein; PVC (left to right) hydrogen chloride, benzene, and toluene and other hydrocarbons. The SATVA traces for the separation of condensable volatile products of degradation of unmixed and mixed powders are shown in Fig. 3. These show negligible differences and the

powders. The two constituent polymers differ considerably in stability, the onset of degradation being approximately 70°C higher for P E A D under the conditions used. The unmixed powder behaviour is simply an addition of the contributions of equal weights of the different polymers, but for mixed powders a slight stabilisation in the temperature region for dehydrochlorination is observed, although the threshold temperature of degradation remains unchanged. When the condensable volatile degradation products of degradation of samples to 500°C in the TVA experiment were allowed to warm to room temperature under controlled conditions, the subambient TVA (SATVA) trace monitoring the volatilisation was recorded and products responsible for each peak were collected as separate fractions. It should be noted that peak

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product assignments at the peaks were (left to right) hydrogen chloride and carbon dioxide, benzene, and toluene plus 2-ethyl-acrolein. No acetaldehyde was detected. When the polymers were examined as films rather than as powders, the results were essentially similar to those of Figs 1 and 3. An examination of the cold ring fraction from degradation of P E A D under T V A conditions using IR spectroscopy showed that the spectrum wa~ indistinguishable from that of the original polymer, indicating that it consisted of oligomeric material of small enough molecular size to escape from the high temperature zone on to the upper part of the degradation tube. There was no indication of any end structures such as carboxyl or unsaturated groups. The cold ring fraction was

also examined by mass spectrometry using a heated probe. Molecular ions corresponding to cyclic monomer (mass equal to repeat unit), dimer and trimer were observed at a probe temperature of 240°C and there were no indications of the presence of end groups in the sample. Both of these techniques therefore indicate that the cold ring fraction products, which comprise the major proportion of volatile material released from P E A D , consist only of cyclic molecules with the repeat unit structure. The cold ring fraction from PVC degradation under TVA conditions has been examined in earlier investigations: it consists of a complex mixture of unsaturated molecules and some aromatic materials. A full characterisation of this fraction from PVC degraded under various

Thermal degradation of PVC-PEAD blends

conditions, using gas chromatography (GC),

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behaviour of blends containing 20, 40, 60 and 80% by weight of PEAD, respectively, is compared with that of each polymer alone. On the same diagram, the weight loss behaviour calculated on the basis of the proportions of the component polymers, assuming no effect of mixing, is also shown. The behaviour of the blend in each case differs from that calculated, but the pattern does not change in a systematic manner with composition. At the highest PEAD content, there is a clearly marked destabilisation in the region of dehydrochlorination, whereas for PEAD contents of less than 60%, some initial stabilisation is observed. The most striking difference between the observed and calculated behaviour, however, is in respect of the residue at 500-600°C. In all

GC-MS and high performance liquid chromatography has now been carried out and will shortly be reported. 4 It is concluded from the T V A experiments that the effect of mixing equal weights of the polymers is a slight stabilisation of the PVC component. N o significant changes in the nature of the volatile and cold ring fraction products have been observed under these conditions.

Thermogravimetry

Temperatureprogrammed experiments Blends spanning the whole composition range were examined by TG in dynamic nitrogen at 10°/min heating rate. In Fig. 4, the experimental

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Fig. 4. TG curves for: ( ) PVC; ( - - - ) PEAD; (. . . . ) calculated behaviour of PVC + PEAD blend assuming no interactions; and ( . . . . ) experimental behaviour of PVC + PEAD blend. Blend compositions: (a) 20%, (b) 40%, (c) 60% and (d) 80% by weight of PEAD.

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cases, this is larger than expected, and at 40 and 60% PEAD content, the residue is more than twice that calculated. Isothermal experiments The effect of mixing on the onset of dehydrochlorination is more clearly seen under milder conditions of isothermal degradation, the selected temperature being 270°C. The results are shown in Fig. 5. The addition of 20% PEAD to PVC appreciably retards the first 20% of weight loss from the blend; with 40% PEAD, the initial retardation is less evident and the weight loss subsequently exceeds that calculated. For 60 and 80% PEAD, there is a clearly marked destabilisation at this degradation temperature throughout the experiment. As with the temperature programmed experiments, these TG results suggest complex behaviour in the blends, with both stabilisation and destabilisation effects occurring and preventing a clear trend with composition change.

Yamashita, however, degraded the polymer and reported cyclic dimer as the main product, accompanied by cyclic monomer and cyclic 3-oxopentamethylene ester. 6 In this investigation, when PEAD was degraded, the main fraction of products was found to consist of cyclic oligomers. In this respect, the polymer behaves similarly to other aliphatic polyesters such as PTMS,3 polylacetide7,s and polyglycollide.9 Also formed are carbon dioxide, acetaldehyde and 2ethylacrolein. Comparison of the TVA and TG curves shows that these more volatile products are formed from the start of decomposition, which occurs a little below 300°C under programmed heating at 10°/min. A small proportion of non-condensable gas is evolved above 350°C, probably carbon monoxide. Whilst the oligomeric products can be formed readily by a non-radical ester interchange mechanism, the formation of the other more volatile products suggests chain homolysis. These products are easily related to the original chain structure: O

DISCUSSION

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Degradation of poly(ethylene adipate) Although the degradation of PEAD has not been studied previously in depth, some previous work on the degradation of this polymer has been reported. Liiderwald5 pyrolysed PEAD in the mass spectrometer and claimed that the results indicated cleavage at the ester group to give ketenic and hydroxyl ends. Hashimoto and

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No evidence has been found to support Liiderwald's claim that ketenic and hydroxyl ends result from cleavage at the ester group.

Thermal degradation of P V C - P E A D blends

Degradation of PVC-PEAD blends The T V A data for a blend containing equal weights of the polymers indicate some stabilisation in the temperature region of dehydrochlorination. Over a wider range of blend compositions, however, the T G results show complex behaviour, with initial stabilisation of the polymer, seen at lower P E A D contents, being replaced by m a r k e d destabilisation at higher P E A D contents. The most striking feature of the T G data is the considerable increase in residue at 500-600°C in the blends c o m p a r e d to that expected in the absence of interaction. It is also clear from the product investigation by T V A and S A T V A that the same products are formed from the blends and from the c o m p o n e n t polymers. This pattern of behaviour resembles that already reported for blends of P V C with poly(tetramethylene sebacate). The principal difference found in the case of P E A D blends is that the interaction appears to be less d e p e n d e n t on the physical state of the blend. Both powder and film blends of P V C and P E A D show interaction effects, whereas in the case of PTMS the effects appeared to be limited to films. Stabilisation of P V C in the region of dehydrochlorination in other blend systems has been explained on the basis of loss of some chlorine radical chain carriers in the radical dehydrochlorination mechanism. 1° These are assumed to abstract hydrogen atoms from the second polymer. W h e n this occurs with a polyester such as PTMS or P E A D , it appears that cross-linking results: this provides an

317

explanation for the considerable increase in involatile residue in the blends. Although loss of chlorine radicals stabilises the P.VC, it is possible that cross-linking makes evolution of hydrogen chloride from the blends more difficult: the consequent increase in autocatalysis explains the destabilisation seen at some compositions. The complexity of the behaviour observed may reflect the contributions of these two opposing effects.

ACKNOWLEDGEMENT The support of N A T O through a collaborative research grant (ref. C R G 890497) is acknowledged with thanks.

REFERENCES 1. McNeill, I. C. & Basan, S., Poly. Deg. Stab., 39 (1993), 145. 2. McNeill, I. C. & Basan, S., Poly. Deg. Stab., 39 (1993), 139. 3. McNeill, I. C. & Basan, S., Poly. Deg. Stab., 33 (1991) 263. 4. MeNeill, I. C. & Memetea, L. T., Poly. Deg. Stab. (in press). 5. Liiderwald, I., In Developments in Polymer Degradation, Vol. 2, ed. N. Grassie. Elsevier Applied Science, London, 1979, p. 77. 6. Hashimoto, S. & Yamashita, T., Doshisha Daigaku Rikogaku Kenkyu Hokoku, 17 (1976) 102. 7. McNeill, I. C. & Leiper, H. A., Poly. Deg. Stab., 11 (1985) 267. 8. McNeill, I. C. & Leiper, H. A., Poly. Deg. Stab., U (1985) 309. 9. McNeill, I. C. & Leiper, H. A., Poly. Deg. Stab., 12 (1985) 373. 10. McNeill, I. C., Grassie, N., Samson, J. N. R., Jamieson, A. & Straiton, T., J. Macromol. Sci.-Chem., A12 (1978) 503.