Radical degradation of polyvinyl chloride

Radical degradation of polyvinyl chloride

European Polymer Journal, 1972, Vol. 8, pp. 1231-1236. Pergamon Press. Printed in England. RADICAL D E G R A D A T I O N OF POLYVINYL CHLORIDE A. H. ...

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European Polymer Journal, 1972, Vol. 8, pp. 1231-1236. Pergamon Press. Printed in England.

RADICAL D E G R A D A T I O N OF POLYVINYL CHLORIDE A. H. K. YOUSUFZAI, M. M. ZAFAR and SHABIH-UL-HASAN Pakistan Council of Scientific and Industrial Research Laboratories, Karachi-39, Pakistan (Received 12 April 1972)

Abstract--A free radical process has recently been shown by Bamford (12) to occur in the thermal degradation of polyvinyl chloride. The existence of CI radical has been established in the present work. A study has been made of the thermal degradation of PVC at 204, 217 and 232° in the presence of chloromercury-acetaidehyde (CMA). CMA decomposes alone thus: CIHgCH2CHO --, CIH/g + CH2CHO. Hg2CI, is the only product of the radical C1Hg but, when PVC is degraded in the presence of CMA, HgCI2 is obtained as a product of CI and CIHg radicals. Inhibition of dehydrochlorination in the presence of CMA is thus explained.

INTRODUCTION THE THERMALdegradation o f polyvinyl chloride has been the subject of wide interest but, its mechanism is not yet fully understood. The various hypotheses put forward can be broadly placed into three groups: (a) Numerous workers have considered the dehydrochlorination o f polyvinyl chloride to be zipper-like elimination el-a) o f hydrogen chloride; dehydrochlorination proceeds along the polymer chain resulting in the formation o f a conjugated polyene chain. The initial reaction is believed to be the elimination of a chlorine atom adjacent to a carbon--carbon double bond. (~) (b) A free radical mechanism has also been suggested by some authors, although the evidence have seen somewhat indirect. (s-8) A possible reaction scheme was proposed where initially rupture takes place at a comparatively weak C--C1 bond: ~,CH2CHCI CH2CHCICH=CH~

-* ~,~CH2CHCICH2CHCH=CH--w + CI"

(I)

CI" q- ~,~CH2CHCICH2CHC1CH= C H ~ --~ H C I + ,~CH2CHCICI-I 2 C C I C H = C H ~

(2)

CI" +~,,CH2CHCICH2CHCI-I=CH~,-, ~ H C I + ~ C H 2 C H C H = C H C H = C H ~ ,

(3)

J CI (c) The possibility o f a four-centre transition state type o f ionic mechanism cannot be ruled out, since it has been suggested that the elimination (9'1°) of hydrogen bromide from bromoethanes follows an ionic mechanism. Although each hypothesis has been based on some experimental evidence, the overall picture is still confused as the possibility o f both ionic and radical mechanisms operating simultaneously, particularly in solution, has also been postulated. (11) However, Bamford et aL c12) have recently obtained definite evidence of active free radicals participating in the degradation o f PVC, by degrading PVC in the presence o f toluene a-t and toluene a-c t~4) at 180 °. It was observed that both materials were incorporated into the polymer chain and the dehydrochlorination was found to be directly related to the extent o f their incorporation. PhaP (triphenyl phosphine) 1231

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A. H. K. Y O U S U F Z A I , M. M. Z A F A R and S H A B I H - U L - H A S A N

markedly catalysed the dehydrochlorination at 120° and a graft copolymer of methyl methacrylate and PVC was obtained in the presence of Ph3P. In the present work, a study has been made of the degradation of PVC in the presence of CMA. Since CMA degrades to give a free radical of the type CIHg at about the temperature at which PVC degrades, the products of PVC and CMA degradation might give important clues about the nature of PVC degradation.

RESULTS AND DISCUSSION The rate of dehydrochlodnation in the thermal degradation of polyvinylchloride PVC is markedly depressed by the presence of CMA. There is a remarkable similarity of the role of CMA in the pyrolysis of PVC at 204, 217 and 232 °. The degradation curves (Figs. 1-3) show that the dehydrochlorination is very much retarded in the

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FIG. 1. A = PVC; B = PVC with 2 per cent C M A ; C = PVC with 10 per cent CMA. Temperature 204 °.

initial stages of the degradation; then, after a considerable time, a regular pattern with the diminishing of the steepness of rate curve is obtained. It has also been observed that the retardation of the dehydrochlorination is very much dependent on CMA concentration; after a certain critical concentration, it becomes minimum and almost independent of any further increase of CMA concentration. The activation energies obtained from the Arrhenius plots of the rates of dehydrochlorination of PVC, with 2 and 10 per cent CMA are 28, 26 and 34 kcal/mole respectively, (evaluated from the steep portions of the curves; Fig. 4). The residues of the mixed degradation of PVC and CMA on analysis gave mercuric chloride (HgCI2) as an additional product. The formation of HgC12 is only possible if there were C1 and HgC1 radicals present in the system. The presence of the C1

Radical Degradation o f Polyvinyl Chloride

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1234

A.H.K. YOUSUFZAI, M. M. ZAFAR and SHABIH-UL-HASAN

radical in the degradation of PVC have recently been established by Bamford according to the following mechanism. ~wCH2CHCI~ --~ ~,~CH2CH~W+ Cl"

etal.cl2) (1)

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(2)

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FIG. 4. A = PVC; B = 2 per cent C M A ; C ---- 10 per cent C M A .

An additional reaction in this scheme will be: CI" + "HgC1 ~ HgC]2

(5)

Although there is a possibility of formation of HgCI2 by abstraction of chlorine from PVC: PVC + "HgC1 -. HgCI2 + PVC

(6)

Nevertheless, such a reaction would not explain the observed strong inhibition of dehydrochlorination of PVC in the presence of CMA (Figs. I-3). The pyrolysis of CMA in an atmosphere of HCI at 217 ° did not give a trace of HgCI2 in the pyrolysate. The pyrolysis of CMA (13) at 204, 217 and 234 ° in an atmosphere of nitrogen and initiated by a free radical initiator (benzoyl peroxide) and retarded by a free radical scavenger (hydroquinone). It was concluded that the

Radical Degradation of Polyvinyl Chloride

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d e g r a d a t i o n o f C M A t a k e s p l a c e by a free r a d i c a l m e c h a n i s m . T h e r u p t u r e takes place at C - - H g b o n d to give: C 1 H g C H 2 C H O ~ CIH~ + (~H2CHO M e r c u r o u s chloride, a c e t a l d e h y d e a n d ketene were identified b y g a s - c h r o m a t o g r a p h y a n d i.r. s p e c t r o s c o p y as p r o d u c t s o f d e g r a d a t i o n . A d e t a i l e d k i n e t i c study ~a~) o f C M A pyrolysis h a s been m a d e a n d is being pubfished separately. T h e r e t a r d a t i o n o f the d e h y d r o c h l o r i n a t i o n o f P V C with C M A can be e x p l a i n e d a c c o r d i n g to the a b o v e m e c h a n i s m [Eqn. (5)]. I n the m i x e d d e g r a d a t i o n , there is a c o m p e t i t i o n between the CI a n d HgC1 r a d i c a l p r o d u c e d f r o m C M A . A t low C M A c o n c e n t r a t i o n , less HgC1 r a d i c a l s are available for scavenging C! r a d i c a l s f r o m P V C hence there is less i n h i b i t i o n o f d e h y d r o c h l o f i n a t i o n . A f t e r a critical c o n c e n t r a t i o n o f C M A , further increase in C M A c o n c e n t r a t i o n does n o t greatly alter the rate o f d e h y d r o c h l o r i n a t i o n . This m a y be due to the fact t h a t m u t u a l t e r m i n a t i o n o f the r a d i c a l H g C I b e c o m e s m o r e p r o b a b l e , whereas CI radical a t t a c k i n g the d e g r a d i n g P V C r a d i c a l r e m a i n s the a l m o s t the same. EXPERIMENTAL Materials and methods 1. Polyvinyl chloride. PVC was prepared using azo-bis-isobutyronitrile as initiator. The reaction

time was 35 hr at 40 °. The polymer was dissolved in cyciohexanone, precipitated in methanol and dried in a vacuum oven at 50°. l~lv for the polymer was 75,800. 2. Chloromercur.vaeetaldehyde (CMA). CMA was prepared by the method of Nesmeyanov. " ~ Vinyl acetate (86 g) was added to mercuric acetate (320 g) in 1" 5 1. water, the precipitate of mercury salt filtered off and 75 g potassium chloride added to the filtrate; the crystalline chloromercuryacetaldehyde (78 per cent) formed was washed with distilled water (m.p. 131-132°; lit. 129-130). 3. Degradation: apparatus and procedure. Degradation was carried out in apparatus similar to that used by Bengough et al." x~ with slight modification. PVC powder was weighed in a small fusion tube and introduced in the degradation tube, with a flow of nitrogen gas. The pyrolysis tube was immersed in constant temperature vapour baths made from benzyl alcohol (b.p. 204 °) m-nitrotoluene (b.p. 232 °) naphthalene (b.p. 217°). The evolved HC1 gas was measured as described by Bengough et al. ~11~ A fixed weight of 0. 238 g of PVC was used for every run and a constant and steady flow of nitrogen was maintained. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)

N. Grassie, Chemistry o f high polymer degradation processes, p. 294. Butterworth, London (1956). C. S. Marvel, J. H. Sample and M. F. Roy, J. Am. chem. Soc. 61, 3241 (1939). K. Tibor, P. Balint and F. Tudos, Mosy. Kern. Kapta. 23(II), 616 (1968). B. Baum and L. H. Wartman, J. Polym. Sci. 28, 537 (1958). F. J. Arlman, J. Polym. ScL 12, 543 (1954). A. A. Miller, J. phys. Chem. 63, 1755 (1959). I. Ouchi, J. Polym. Sci. A3, 2685 (1965). D. E. Winkler, J. Polym. Sci. 35, 3 (1959). A. Maccoll, and P. J. Thomas, Nature, Lond. 176, 392 (1955). S. Benson Advances in photochemistry (Edited by W. A. Noyes, G. S. Hammond and J. N. Pitts Jr., Vol. II, p. 1. Interscience, New York. W. I. Bengough and H. M. Sharpe Makromolek. Chem. 66, 31, 45 (1963). C. H. Bamford and D. F. Fenton, Polymer 10, 63 (1969). A. H. K. Yousufzai and M. M. Zafar, Pak. J. scient, ind. Res. (in press). A. H. K. Yousufzai, M. M. Zafar and A. A. Usmani, Pak. J. scient, ind. Res. (in press). Nesmeyanov and Lutsenko. Dokl. Akad. Nauk S S S R 59, 707 (1948).

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YOUSUFZAI, M. M. Z A F A R and SHABIH-UL-HASAN

R6sum6__Bamford t ~z) a r6c.cmment mis en 6videncc un proccssus radicalaire qui apparait dans la d6gradation thermique du poly(chlorure de vinyle). Dans le pr6sent travail, on a 6tabli l'existenc¢ de radicaux CI. On a 6tudi6 la d6gradation thermiquc du PVC a 204, 217 et 232 ° en pr6sence de l'ac6tald6hyde chloromercurique (CMA). Le C M A soul se d6c.omposc solon: CIHgCHzCHO -~ CIH~ -k CH2CHO. HgzCl2 est le seul produit du radical CIHg, mais q u a n d la d6gradation du PVC est effectu6¢ en pr6scncc de CMA, on obticnt HgClz comme produit des radicaux CI et CIHg. On expliquc ainsi l'inhibition de la d6hydrochlorhydratation en pr6scnce de CMA.

Sommario--Reccntemente Bamford (~2) ha mostrato che nella degradazione termica del cloruro di polivinile si ha un proccsso a radicale iibero. Nel prcscnte lavoro si 6 stabilita l'esistenza del radicale CI. Si 6 eseguito uno studio sulla degradazione termica del PVC alle temperature di 204, 217 e 232 ° in presenza di acctaldeide di cloro-mercurio (CMA), il quale, da solo, si decompone nel seguente mode: CIHgCHzCHO -~ CIHg + (~HzCHO. Hg2CI~ 6 l'unico prodotto del radicale CIHg, per6, quando il PVC si degrada in presenza di CMA, si ottiene HgCI2 come prodotto dei radicali CI e CIHg. In tal mode si spiega l'inibizione della deidroclorurazione in prescnza di CMA.

Zusammenfassung--Kfirzlich wurd¢ von Bamford (~z) gezeigt, dal3~beim-thermischen Abbau yon Polyvinylchlorid ein radikalischer Prozess stattfindet. In der vorliegcnden Arbeit wurd¢ die Existenz yon Cl Radikalen nachgewiesen. Eine Untersuchung des thermischen Abbaus yon PVC bei 204, 217 und 232 ° wurd¢ boi Gegenwart yon Chlorquecksilberacetaldehyd (CMA) durchgeffihrt. C M A allein zerf'~illt in folgender Weis¢: CIHgCH2CHO --,- ClH~ + CH2CHO. HgzCl2 ist das einzigo Produkt aus dem Radikal CIHg, wenn jedoch PVC bei Gegenwart yon C M A abgebaut wird, wird HgClz ¢rhalten als ein Produkt aus C1 und CIHg Radikalen. Die Hemmung der Chlorwasscrstoffabspaltung bci Gegenwart yon C M A wird damit erkl~t.

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