The thermal degradation of polychloroprene-II study of the products of degradation

European Polymer Jota,"nal, 1971, Vol. 7, pp. 593-602. Pergamon Press. Printed in England.

THE THERMAL D E G R A D A T I O N OF POLYCHLOROPRENE--II STUDY OF THE PRODUCTS

OF DEGRADATION

D. L. GARDNER* a n d I. C. MCNEILL Chemistry Department, University of Glasgow, Glasgow, W.2, Scotland (Received 23 June 1970)

Abstract--Studies have been made of the gaseous and liquid products and the involatile residue from the degradation of polychloroprene. Programmed heating was used; below 400 ° in addition to hydrogen chloride, small amounts of ethylene and a trace of chloroprene were detected in the volatile products. Above 400 °, methane became a significant product ; smaller amounts of hydrogen, ethylene and propylene were present. The liquid products were not fully characterized: the complex mixture contained products, which might be chloroprene dimers, and less volatile components containing aromatic structures. The involatile residue of partial degradation showed similarities to that obtained in PVC degradation, but differed in giving a clear indication of methyl groups. Conjugation was found to be much less extensive than in PVC after dehydrochlorination; triene structures predominated, and there was little contribution from structures with 12 or more double bonds in conjugation.

INTRODUCTION I r Is well established that h y d r o g e n chloride is the m a j o r p r o d u c t o f d e g r a d a t i o n o f p o l y c h l o r o p r e n e . In the previous p a p e r o f this series, m the stabilities o f various samples to h y d r o g e n chloride loss were c o m p a r e d , a n d the kinetics o f d e h y d r o c h l o r i n a t i o n were studied. Some c o m p a r i s o n s with the d e g r a d a t i o n o f P V C were also m a d e ; in particular, it was n o t e d that d e h y d r o c h l o r i n a t i o n occurs less easily in p o l y c h l o r o p r e n e , the loss o f chlorine as HC1 a m o u n t i n g only to 90 per cent o f the total available (comp a r e d with over 95 per cent for PVC), a n d the yields o f liquid (tar) p r o d u c t s a n d the involatile residue o f d e g r a d a t i o n being greater for p o l y c h t o r o p r e n e t h a n for PVC. P o l y c h l o r o p r e n e also loses p r o d u c t s which are n o t c o n d e n s a b l e at - - 1 9 6 °, in a reaction which overlaps in t e m p e r a t u r e range with d e h y d r o c h l o r i n a t i o n , u n d e r p r o g r a m m e d heating conditions. T h e p u r p o s e of the present w o r k was to investigate in detail the gaseous a n d liquid p r o d u c t s o f d e g r a d a t i o n , a n d also to study the d e v e l o p m e n t o f c o n j u g a t i o n in the residue from d e h y d r o c h l o r i n a t i o n o f p o l y c h l o r o p r e n e .

EXPERIMENTAL Analysis of gaseous products

For infra-red analysis, 200 mg samples were degraded in a closed system, initially evacuated, connected to a 10 cm i.r. gas cell. For GLC studies, the experimental arrangement of Fig. I was employed. The polymer sample was placed in tube Z and products from programmed degradation in vacuo at 5°/rain were collected in Y at -- 196°. After degradation, stopcock X was closed and the temperature of Y was raised to -- I i0 ° (at which temperature hydrogen chloride is involatile) and the apparatus section PQRS was evacuated. * Present address: I.C.I. Ltd., Mond Division, Runcorn, Cheshire, England. 593

594

D. L, GARDNER. and I. C. McNEILL

Pyrolysis section

X

R

Chromatograph S

0 Column

to )ump

[

ITI

II Argon in

I

Argon out FIG. I. Apparatus arrangement for degradations with gas chromatographic investigation of products. [Key: P,Q,X, stopcocks; Z, degradation tube; Y, cold finger; R,S, two-way stopcocks; F, fiowmeter; T, thyristor]. Stopcock X was opened and the sample expanded into the chromatograph inlet system up to stopcock S, and stopcock P was closed. The sample contained in section RS was then swept into the chromatograph. Further samples were similarly obtained by expansion from the pyrolysis side of the apparatus into the evacuated section PQRS. A 16 ft silica gel column at room temperature with an argon carrier gas flow rate of 24 mi,;min was used. Under these conditions, retention times were 6 rain for hydrogen and 9 rain for methane. The system was calibrated by comparing peak areas in the chromatogram with known amounts of hydrogen and methane introduced into the pyrolysis side of the apparatus.

Collection and analysis of liquid products Several methods were employed to collect the volatile liquid products of polychloroprene degradation. In all cases, thin layer chromatography indicated only one major component. The yield of liquid was so small that quite large amounts of polymer (10 g) had to be degraded to an advanced extent of reaction in order to obtain a significant quantity of liquid; this led, at temperatures above 300 °, to an uncontrollable "flash" degradation unless the heating rate was reduced at 300 ~ from 5=/min to 1°,!min. A further problem was that the hydrogen chloride produced in the reaction reacted quite readily with the liquid to form a red viscous gum, and it was found necessary to collect the products in a trap at --196 ° and neutralize the acid with sodium carbonate solution. Extraction of the aqueous sodium carbonate mixture (after effervescence had ceased) with ether and subsequent evaporation gave about 400 mg of yellow-brown oil. "Cold ring" products, which collected on the water-cooled upper part of the degradation tube, consisted of a sticky green fraction at the top and a brown tar at the bottom. The green fraction was removed by dissolution with ether or toluene. The brown tar and the solid residue were both extracted with chloroform; a brown pitch was obtainable by evaporation of the solution.

Examination of the residue Degradations were carried out to a predetermined temperature, using the thermal volatilization analysis (TVA) apparatus <2) and a heating rate of 5°/rain. The solid residue was examined by both infra-red and ultraviolet spectroscopy. For the former, samples were examined in the form of KBr discs (when the original polymer samples were used in small lump form) or as films on 9 mm sodium chloride discs. In the latter case, polymer samples for degradation were cast from toluene solution on to the NaCI discs before degradation, which was carried out with the polymer surface uppermost. There was an additional temperature differential of about 20 ° across the salt disc. For the u.v. spectroscopic studies, polymer samples were cast initially on the base of a silica degradation tube similar to the normal TVA degradation tube <-') but of reduced length to permit the tube to be

The Thermal Degradation of Polychloroprene--H

595

inserted directly into the beam of the spectrometer. A similar but empty tube was used in the reference beam. Twenty-milli~am films were used to study the initial part of the degradation and 2 mg firms for later stages. De~-adations were carried out to various extents in the TVA apparatus, and the u.v. spectrum of the pelymer was recorded before and after each de~adation. RESULTS A N D D I S C U S S I O N

Gaseous products of degradation Infra-red spectroscopic analyses of the gases evol~ ed from polych!oroprene indicated the presence of a number of substances (Table 1). For degradation temperatures below 400 ° under the p r o ~ a m m e d heating conditions, hydrogen chloride was by far the major component. A small amount of ethylene was also found, as well as a trace of monomer. When the products from further heating of the same sample were collected separately and analysed, further small amounts of gases were obtained, of which methane and ethylene were the main components. The quantity of ethylene was g e a t e r than for the lower temperature degradation product. The above studies were made using 200 mg samples. When much larger samples were used (up to 10 g) traces of carbon monoxide and carbon dioxide were also detected, indicating the breakdown of oxygenated structures in the poiymer. TABLE I. PRODUCTS IDENTIFIED BY INFRA-RED ANALYSIS IN T",VO FRACTIONS COLLECTED IN DIFFERENT TEMPERATURE RANGES IN THE PROGRAMMED DEGRADATION OF A POLYCHLOROPRENE SAMPLE 170---400° Hydrogen chloride Ethylene Chloroprene (trace)

400-500 ° Hydrogen chloride (trace) Methane Ethylene Propylene (trace)

The investigation of polychloroprene by differential condensation TVA, ~3~ reported in Part I, m indicated the evolution at higher temperatures of gaseous products noncondensable in liquid nitrogen in the evacuated system. The presence of methane was established by i.r. spectroscopy. G L C of the products indicated, in addition to methane, the presence of hydrogen; this technique was used to estimate the quantities of these products evolved. The apparatus of Fig. 1 was employed, using 200-400 mg samples of polychIoroprene MC 30. Results are presented in Fig. 2. Both hydrogen and methane are given off in small amounts below 400 ° , at which temperature the rate of evolution of methane starts to increase and continues to do so up to 510 ° . Although the rate of hydrogen evolution increases slightly after 450 ° , the total amount of hydrogen produced is much less than the amount of methane. The quantity of methane liberated during the pyrolysis is much larger than in the case of PVC. Both Stromberg and co-workers (a~ and Gilbert and Kipling (s~ have examined the products of the secondary decomposition of PVC by heating the polymer until the elimination reaction was virtually complete and then degrading the residue at a higher temperature. Stromberg, working at 400 °, found that the yields of hydrogen and methane from heating dehydrochlorinated PVC for 30 rain were less than 0 . 1 % . Gilbert and Kipling heated PVC in stages of 100: between 400 and 900 °, keeping the

596

D.L. GARDNER and I. C. McNEtLL

Mefhane I

10

8

g. o

e

"D

.~_ >"

4 @

400

450

5;,3

Temperature, °C FIG. 2. Cumulative yields of hydrogen and methane from polychloroprene heated at 5°/rain.

sample at each temperature until volatilization had almost ceased. Their results have been recalculated on a molar basis in Table 2, and show much higher gas yields at 400" than reported by Stromberg. TABLE 2. GAS YIELDS FROM P V C HEATED IN STAGES OF 100 ~ (CALCL'LATED FROM RESULTS OF GILBERT AND KIPLING (s))

Temperature 400 500 600 700 800 900

Cumulative yield [mole ~/oof (CH=CH).] Hydrogen Methane 0'6 3" 2 7"5 12"2 14"7 16"3

2'2 4' 3 4'8 4"9

For a direct comparison with the programmed heating conditions of the present work, and to help resolve the conflicting data for PVC described above, 300 mg samples o f P V C (Breon 113) were degraded at 5°/rain to 437 ° and 510°; the gases were analysed by G L C in the same way as for polychloroprene. The results are shown in Table 3, along with the equivalent figures for polychloroprene reduced to the same C2 monomer units as in PVC.

597

T h e The..~mal D e g r a d a t i o n of P o l y c h l o r o p r e n e - - I 1 TABLE 3. GAS YIELDS FROM P V C AND POLYGHLOROP,KF-NEHEATED AT U ' m i n

Maximum temperature

Polymer PVC PC Pvc PC

Cumulative yield [mole }; of (CH~---CH),] Hydrogen Methane

437 437 5[0 510

0.7 0- 3 1.1 t. 1

0.4 0.8 2.8 5.8

It is notable that significant amounts of hydrogen and methane are obtained from programmed PVC degradation to 437 °, suggesting that Stromberg underestimated these products. The results of Table 3 indicate that the methane yield from polychloroprene is about twice that from PVC. In comparison with Gilbert and Kipling's PVC data, it appears that continued heating at 400 ° causes the evolution of a considerable amount of methane whereas programmed heating to 437 ° produces very little. A related observation is that the PVC residue in Gilbert and Kipling's experiments showed an infrared absorption band for the methyl g o u p , whereas no such peak was detected for PVC in the present investigation. The total amount of methane liberated by PVC when heated in stages up to 900 ° is less than the amount produced from polycNoroprene when subjected to programmed heating up to only 510°. This confirms the evidence that polychloroprene yields much more methane than PVC on carbonization of the residue from dehydrochlorination.

Liquid degradationproducts About 80 mg of a light yellow fruity smelling oil were recovered from the main band of a thin layer chromatography plate of the products from the degradation of 10 g of polymer. The oil was investigated by GLC, by i.r. and P M R spectroscopy, and by mass spectrometry, and some chemical modifications were also attempted, but positive identification was unsuccessful. GLC and mass spectrometry indicated a complex mixture of products. Some of the signals in the mass spectrum could only reasonably be explained if oxygen-containing molecules were present, and since the composition changed if the products were stored, it appeared that some of these were susceptible to oxidation. Oxygen-containing products could also result from peroxide structures likely to be present in small amount in polychloroprene, and which may be responsible for the early stages of degradation of the polymer. (') The mass spectrum suggested that the molecular weight of these liquid products corresponded approximately to that of dimers of chloroprene. Infra-red and PMR spectra indicated the presence of structures of the type CHz~C--CR1R, C1

and possibly

R~CH2--C~CHR2 C1

Chloroprene is known to form a range of dimeric products; (6-~1) although positive

598

D.L. GARDNER and I. C. McNEILL

identification was not achieved, it may be that dimers of structures I and II were present : Ct

Ct

LQ.~/j. r

The green cold ring fraction had an i.r. spectrum similar to that of the oil. Its molecular weight, by vapour pressure osmometry, was about 340. Examination of the pitch fraction by infrared spectroscopy gave results similar to those reported for liquid products by Harms C1'~ and Hummell. (~3) The presence of aromatic structures of various types was suggested, with l:2-disubstitution predominating. Unsaturated groups and methyl groups were also indicated, and a carb o w l absorption band at 1690 cm -1 (5.920/~m) showed that some of the products in this fraction were oxidized. The spectrum showed similarities with that of the residue at an advanced state of degradation (see below) and also with that of the tar obtained by pyrolysis of P V C . (1~)

c

250

300

Wavelength, rn~

FIG. 3. Ultraviolet spectra of 20 mg films of polychloroprene. Continuous lines: MC-30, (a) undegraded; and degraded at 5°/rnin to various temperatures, (b) 185 ~, (c) 210', (d) 233 ~, (e) 278 °, (f) 295 °. D o t t e d lines: PC-A degraded to (c') 233 °, and (f') 295'.

The The,."malDe~adation of Polychloroprene--II

599

Solid residue from partial degradation For the infra-red spectroscopic studies, polychloroprene samples were heated at 5°/min to various temperatures and the spectra of the residues were compared with each other and with that of the undegraded polymer. U n d e g a d e d films sometimes showed a small carbonyl peak at 1720 cm -I (5.815 ffm) due to oxidized structures; this band disappeared when the polymer was heated to 300°. Few other changes could be detected for tNs temperature except the disappearance of the small peak at 925 cm - t (10.810 ffm) due to 1:2 units. Upon heating to 344 ° , the C = C peak at 1660 cm - t (6.030 fire) decreases and absorption corresponding to conjugated C ~ C appears. A small peak appears at 960 cm -1 (10-420 ~m) due to trans-disubstituted double bonds. Weak absorption originally present at 826 cm - t (12. 100 ~m) due to the original polychloroprene double bond structure decreases because of dehydrochlorination. On further heating to 374 °, a band due to C--CH3 structures appears at 1375 c m - t (7.275 txm) and the presence of 1 : 2 or 1 : 3 disubstituted aromatic structures is now indicated. Upon heating to 405 ° and higher, aromatic absorption increases and the 960 cm - t (10.420 ~m) C ~ C band disappears. Behaviour similar to that described above was observed in progressive degradation of PVC to 405 °, except that the methyl absorption band was missing and the 960 c m - t band was more intense. It appears, however, that the pattern of aromatic substitution is simpler in the case of PVC than for polychloroprene. The explanation for the production of methyl groups in the residue in potychloroprene degradation probably lies in a tendency for the interunit bond in polychloroprene to break. For a partially degraded sample, some process such as the following might occur: c[

!

CHz '

ct

f

CHz

.CH z CL i

l CHz CH.,,

The u.v. spectra of degraded films of polychloroprene MC30 are shown in Fig. 3. They show how a distinct four-peak pattern arises early in the degradation but is obscured later by a featureless absorption which extends from low wavelengths to 450 mff (450 nm). The spectrum with the highest intensity of absorption in Fig. 3 was recorded from a film heated to 295 °, i.e. just past stage II in the programmed degradation (see Fig. 3 of part I), which seems to be the point at which the details of the spectrum become indistinct. Isothermal heating at 2L0° furnishes a set of spectra similar to those in Fig. 3. Only 10 min are required to produce a spectrum like that from programmed heating to 278°; a 295 ° type spectrum results after 100 rain. Films of PC-A were also examined and two typical spectra are included in Fig. 3. There is no change in the positions of absorption but the intensity is greatly reduced,

600

D.L. GARDNER and I. C. McNEILL

thus demonstrating that the distinctive features in the u.v. spectrum are linked to the dehydrochlorination occurring at an early stage in the reaction. This polymer does not show the stage II reaction so that the dehydrochlorination producing these spectra has resulted from stage III reaction, and the only effect of the absence of stage II reaction, as far as the u.v. spectra are concerned, is to delay the development of absorption, Extension of these studies by heating 2 mg films to various temperatures up to 418 ° revealed nothing further. The more extensive conjugation in these films resulted again in broad featureless absorption, extending to about 550 m/~, but the maximum absorption occurs below 300 m/z, and the intensity beyond 450 mix is very low. Interpretation of the u.v. spectra is difficult in the absence of sufficient data on the u.v. spectral properties of chlorotrienes. The result of dehydrochlorination at a 1:4 unit is probably either a 1-chloro or a 2-chloro-l,6-dialkylhexatriene, (A) or (B), while a 2-chloro-l,4-dialkylhexatriene (C) may be formed from a 1:2 unit: ct

(A)

I

Ct

Ct

Ct

ct

f

C!, i

s"

(g)

(C)

CL

It has been found by Gardner and McNeill (~s) that the !,6-dialkylhexatriene chromophore (D)" ('D)

formed in degraded copolymers of vinyl acetate, gives peaks at 261,272 and 283 m/~ (nm). The wavelengths of the absorptions in degraded polychloroprene are 16 mix (nm) higher and there is an additional fine structure peak at 265 mt* (nm). Chlorine substitution has been shown to result in a bathochromic shift of about 10 mt* (nm) (1° so that the assignment of the u.v. peaks to structures (A), (B) or (C) is very reasonable. It is not possible, however, to determine whether one, or more than one, chromophore is present. Gardner and McNeill a 5) compared the development of conjugation in polychloroprene with that in PVC and several other polymers under similar degradation conditions. Whereas polychloroprene yields chlorotriene structures initially and ultimately gives only small amounts of polyenes over about 12 double bonds in lengh, PVC contrasts with this by giving at the first stage, very long polyenes. Absorption extends to 650 mf* (nm) even in the very early stages, and Geddes clT) has concluded that absorption above 500 m~, (rim) implies the presence of over 20 double bonds in conjugation. Thus, d e g a d e d polychloroprenes are yellow whereas degraded PVC samples are red. The contrasting u.v. spectral properties of degraded PVC and PC samples suggest that in the former case a "zipper" mechanism for HC1 loss is operative, whereas in the latter dehydrochlorination is at random in the molecule. This may be an indication

The Thermal Degradation of Polycb!oroprene--lI

601

t h a t the m e c h a n i s m in the first case is a radical c h a i n p r o c e s s w h e r e a s in the s e c o n d case it is not. F u r t h e r e v i d e n c e o n this a s p e c t c o m e s f r o m e x p e r i m e n t s o n d e g r a d a t i o n o f p o l y c h l o r o p r e n e / ' p o l y ( m e t h y l m e t h a c r y l a t e ) blends, to be r e p o r t e d s u b s e q u e n t l y in P a r t !II.

Acknowledgement--D.L.G. thanks the Science Research Council for the award of a Studentship, during the tenure of which this work was carried out. REFERENCES (I) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17)

D. L. Gardner and I. C. McNeill, EzLrop. Po(vt;t. J. 7, 569 (1971) I. C. McNeiil, Europ. Polym. J. 3, 409 (1967). 1. C. McNeill, Europ. Polym. J 6, 373 (1970). R. R. Stromberg, S. Straus and B. G. Achharm~er, J. Po!ym. Sci. 35, 355 (1959). J. B. Gilbert and J. J. Kipling, Ft~el, 41, 349 (I962). J. G. T. Brown, J. D. Rose and J. L. Simonsen. J. chem. Soc. 101 (1944) R. E. Foster and R. S. Schneider, J. Am. chem. Soc. 70, 2303 (1948). A. C. Cope and W. J. Bailey, ibid. 70, 2305 (1948). A. C. Cope and W. R. Schmitz, ibid. 72, 3053 (1950). I. N. Nazarov and A. I. Kutznetsova, Zh. Obshch. Khim. 30, 134 (1960). N. C. Billingham, P. A. Leeming, R. S. Lehrle and J. C. Robb, ?~2lture, 213, 494 (1967). D. L. Harms, Anatyt. Chem. 25, 1140 (1953). D. Hummel, Kautsch,~k Gum:hi, 11, 185 (1958). J. B. Gilbert and J. J. Kipling, Fuel, 42, 5 (1963). D. L. Gardner and I. C. McNeill, J. Thermal Analysis, 1, 389 (1969) K. Bowden, E. A. Braude and E. R. H. Jones, J. chem. Soc. 948 (1946). W. C. Geddes, Europ. Polym. J. 3, 747 (1967).

R~sum~---On a/~tudi6 les produits gazeux et liquides ainsi que le r6sidu non volatil de la d~gradation du polychloropr~ne. On utilise un chauffage progamm6; en-dessous de 400 ° de petites quantit~s d'~thyl~ne et des traces de chloropr~ne sont d6tect/~es dans les produits volatils en plus du gaz chlorhydrique. Au-dessus de 400:,le m6thane devient un produit important; on trouve de petites quantit6s d'hydrog~ne, d'6thyl~ne et de propyl~ne. Les produits liquides n'ont pas 6t6 enti6rement caract6ris6s: le m6lange complexe renferme des produits qui peuvent 6tre des dim6res duchloropr~ne,et des composants moins volatits renfermant des structures aromatiques. Le r6sidu non votatiI de la d6gradation partielle pr6sente des analogies avec celui obtenu par d~gadation du PCV mais en diff~re par la presence nette de ~oupements m~thyles. On a trouv~ que la conjugaison est beaucoup moins importante que dans le PCV apres d~,hydrochloration; les structures tri6nes pr6dominent et on observe la contribution de structures contenant 12 doubles liaisons ou plus en conjugaison.

Sommario---Si sono studiati i prodotti liquidi e gassosi e i residui non volatili derivati da!la degradazione del policloroprene. Si ~ impiegato il metodo di riscaldamento pro~ammato. Sotto i 400% nei prodotti volatili si rivelarono, oltre al cloruro d'idrogeno, piccoli quantitativi di etilene e tracce di cloroprene. Sopra i 400 °, il metano divent6 un prodotto notevole; erano pure presenti piccoli quantitativi di idrogeno, etilene e propilene. Non si individuarono completamente i prodotti liquidi: la complessa miscela conteneva prodotti che potevano essere dimeri di cloroprene, e componenti meno volatili contenenti strutture aromatiche. I residui non volatili della degradazione parziale presentavano analogie a quelli ottenuti nella degradazione del PVC, per6 differivano nel dare una chiara indicazione dei gruppi metilici. Si trov6 ehe la coniugazione dopo deidroclorurazione era molto meno ampia che in PVC; erano predominanti strutture trieniche, mentre non erano numerose quelle con 12 o pifi doppi legami coniugati.

602

D . L . G A R D N E R and I. C. McNEILL

Zusammenfassung--Beim Abbau yon Polychloropren wurden die gasfOrm,igen und flfissigen Produkte sowie der nichtfli.ichtige Rtickstand untersucht. Es wurde mit programmiertem Erhkzen gearbeitet; unter 400 ° wurden in den fltichtigea Produkten neben Chtorwasserstoff geringe Mengen .3Lthylen und Spuren yon Chloropren nachgewiesen. I]'ber 400: wurde Methan eine wesentliche Komponente; geringere Mengen an Wasserstoff, .~thylen und Propylen waren anwesend. Die fl~ssigen Produkte wurden nicht vollst~indig charakterisiert; die komplexe Mischung enthielt Produkte, die Chloropren Dimere sein kiSnnen, und weniger flOchtige Komponenten, die aromatische Strukturen enthalten. Der bei partiellem Abbau gefunden~ nicht fli~chtige RiJckstand zeigte ~hnlichkeit mit dem. der beim Abbau yon PVC gebildet wird, er unterschied sich aber durch einen eindeutigen Hinweis auf Anwesenheit yon Methylgruppen. Es wurde festgestellt, dab Konjugation wesentlich geringer ist als bei PVC nach Chlorwasserstoffabspaltung; es/.i berwogen Trienstrukturen, Strukturen mit 12 oder mehr Doppelbindung in Konjugation bildeten nur einen geringea Anteil.