Ultraviolet Light Induced Reactions in Polymers

15 Photodegradation of Polyvinyl chloride

Downloaded by CORNELL UNIV on December 17, 2014 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch015

ERYL D. OWEN Chemistry Department, University College, Cardiff, Wales

1. Introduction The mechanisms of the thermal and photochemical degradation of poly(vinyl chloride) (PVC) continue to be active areas of research in polymer chemistry mainly because its high chemical resistance, comparatively low cost and wide variety of application make PVC one of the most widely used thermoplastic materials. The wide variety of forms which the material can take includes pastes, lattices, solutions, films, boards and moulded and extruded pieces and depends to a very large extent on the good electrical and mechanical properties of the polymer. In spite of these advantages the even wider application of the material has been restricted by its low thermal and photochemical stability. Thermal instability is a problem since processing of the polymer is carried out at about 200 C and the photochemical instability places a limit on the extent of the outdoor applications which can be developed. Both thermal and photochemical processes take the form of a dehydrochlorination reaction which leads to discolouration as well as extensive changes in the internal structure of the polymer which has an unfavourable effect on the desirable electrical and mechanical properties. It has become apparent that considerable similarity exists between the two degradation processes and that it is neither easy nor desirable to make a vigorous distinction between the two. Information gained from experiments on thermal degradation are often directly relevant to the analogous photochemical process. The reaction involved may be written:isopropyl CI but i n this case nineteen hydrocarbon products were detected i n addition to various organic chlorides. The alkyl aryl ketone sensitised photolysis (313 nm; ot hexane solutions of t-butyl CI reported by Harrimanl2 also gave HC1 as a major product together with 2-methyl propene. Quantum yields ranged from 0.19 when benzophenone was the sensitiser to 0.313 for EtC=0 and the involvement of the t r i p l e t state was confirmed by the quenching of the reaction by 2,5-dimethyl-hexa2,4 diene. Comparison of the range of E values (280 - 330 kJ mole"l) of the sensitisers with the dissociation energy of the C-Cl bond (327 kJ mole~l) led these authors to suggest that some charge transfer interaction was involved since there was ample evidence for such a process occuring between t r i p l e t ketone acceptors and amineiS or sulphideM. donors. In contrast,Golub— has concluded that the photolysis of 1,4 DCB sensitised by a variety of aliphatic ketone sensitisers i s a singlet state reaction. There i s ample evidence that this i s also the case when aromatic hydrocarbons act as sensitisers i n a variety of processes. Finally Whittle has shown that hexafluoroacetone i s able to sensitise the elimination of HC1 from a variety of alkyl chlorides. I t appears that the t r i p l e t or singlet state may be involved and there i s a strong possibility that the actual elimination process occurs from a vibrationally excited ground state. Clearly then different groups of workers favour the singlet, t r i p l e t or vibrationally excited ground state as the precursor for the elimination process. There i s l i t t l e evidence to c l a r i f y the extent to which catalyst end groups arising from i n i t i a t i o n or termination of the polymer T

i.e. O R

R-

+

CH =CHC1

> R-CH^CHCl

2

-^A^CHCl

+

R-

VV

>~ CHC1R

In Ultraviolet Light Induced Reactions in Polymers; Labana, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


15.

OWEN

Photodegradation of Polyvinyl chloride.

211

influence the photochemical s t a b i l i t y but this w i l l clearly be determined by the nature of R and i t s absorption and photo chemical characteristics. Effects of R on the thermal s t a b i l i t y are contraversial since a l l of the studies carried out so far have been plagued by the presence of traces of unreacted catalyst present not as a polymer end group but trapped within the polymer. These traces of free catalyst are undoubtedly much more effective than those present as end groups particularly since transfer reactions mean that only 20-40% of the polymer have R ends. Similar conclusions can be drawn regarding end group unsaturation since the nature of the chromophore i s such that absorptions would be i n the far ultraviolet unless the unsaturation was an extended system of conjugation. Thermal s t a b i l i t y of these systems however i s much more clearly defined and i t may be concluded that end group unsaturation contributes to the i n s t a b i l i t y of low molecular weight polymers only, while the i n s t a b i l i t y of high molecular weight fractions is more strongly influenced by some other factor, probably the presence of branch points or internal double bonds. Many of the attempts which have been made to implicate particular structural features as a cause of thermal or photochemical i n s t a b i l i t y have u t i l i s e d low molecular weight model compounds and there are several considerations which severely limit any extrapolations which can be made from such studies. F i r s t l y although small model compounds undergo unimolecular homogeneous reactions, the introduction of a surface component could cause a dramatic reduction i n the energy of activation and so any possible surface effect could be very important. Secondly many of the studies which have been carried out i n solution are complicated by factors such as solvent participation or the catalytic effect of HC1. Thirdly, as well as requiring information on the effect of particular structural features, the frequency of their occurrence i n the polymer i s also essential. Before enough information of this type i s available however much more information on the kinetics of vinyl chloride polymerisation are required, b) Chromophores. It i s possible to imagine many chromophores which could arise during processing of the polymer but the most effective i n affecting photostability are probably /C * 0, -00H and metal ions Μ^ . Ketones and hydroperoxides have low intensity absorptions i n the ultraviolet region and there is no reason why these should be much affected by attachment to a polymer chain. Some comments have already been made regarding the ketone sensitised photolysis of model compounds which occurs by transfer of energy from the ketone to the alkyl chloride. In addition, several other possible mechanisms of i n i t i a t i o n are possible. For example the ketone may decompose by the Norrish Type I mechanism with the formation of free radicals thus:+


U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

212

-C-C-CI I

N

>

N

-C-C-+-CI I

.

>

-C-+C+-CI I

Alternatively the Norrish Type II process may operate since the p o s s i b i l i t y of a Y hydrogen being available i n the polymer i s high g i l l h* - C - C - C - C — N


,M

f i -C*C

v

I I + C * C

i l " >

Y

I A further and very important p o s s i b i l i t y i s the transfer of energy from the ketone sensitiser to molecular oxygen forming one of the two low lying singlet states of oxygen ketone + 0 ( £~) * ketone + 0 or Δ ) . 3

3

3

X

2

1

2

Reactions of singlet oxygen with isolated olefins having a l l y l i c hydrogen atoms are extremely rapid and i n this sense isolated double bonds at any point i n the polymer are a source of photochemical i n s t a b i l i t y „ i

Λ Λ

·

UUn

ι -c=ç-ç- + o ( A ) I

* > -c-ç>ç

1

1

2

Yet another possible mode of i n i t i a t i o n i s hydrogen abstraction thus:3

ketone + RH

> ketone *-H + R'

so that at least four well documented mechanisms exist by which a t r i p l e t ketone sensitiser can i n i t i a t e photodegradation namely i ) decomposition with the formation of free radicals, i i ) energy transfer to the polymer, i i i ) energy transfer to molecular oxygen, iv) hydrogen abstraction with consequent formation of free radicals. Alkyl hydroperoxides which may also be formed i f oxygen is present during processing have an absorption which extends to about 320 nm which may lead to the formation of free radicals thus : R00H —

R

0

- + -OH

Metal ions act as efficient sensitisers for this process as well as forming free radicals by the process:Λχ"

>

(n

M -

1)+

( n - l ) X - • Χ*

The nature of X may effect the position of \ considerably.

m a x

very


15. O W E N

Photodegradation of Polyvinyl chloride

213


c) Impurities* Two main classes of impurities affect the photostability of PVC. The f i r s t type, traces of catalyst, have already been discussed i n section (a) while the second type, traces of solvent, are important when the polymer i s i n the form of thin films which have been cast from a solvent. Such traces are extremely d i f f i c u l t to remove completely and are undoubtedly responsible for much of the irreproducibility and disagreement which i s apparent i n the literature. One of the best solvents for PVC is tetrahydrofuran (THF) but this solvent i s particularly d i f f i c u l t to remove. Its removal i s very necessary however since i t reacts very readily with dissolved oxygen to form a photolabile peroxide whose decomposition products are very effective i n i n i t i a t i n g the photodegradation reaction. Mechanism of the Reaction It i s probable that once the i n i t i a t i o n has been accomplished, the mechanisms of the thermal and photochemical reactions have much i n common. For that reason i t i s profitable to make some comments about the mechanism of the reaction, most of which result from work on the thermal process. Until about ten years ago most workers i n the f i e l d believed that a radical mechanism operated. This view depended to a large extent on analogy with the results of the thermal decomposition of other polymers. ^5 1959 two mechanisms were put forward simultaneously by Winkler — and by Strombergi§ which differed i n the details of the initiation process but agreed that propagation occurred by a "Zipper" process which may be written In

CHCHCl^—^ / ~ N ^ CH=CH

>

^^^CH=CH^^

+ CI*

+C1 · — > ~CH=CH«CH* C H C l ^ ^ +HC1

and was supported to some extent— by earlier work on the gas phase decomposition of alkyl chlorides. At about the same time i t was found that the presence of small quantities of salts like FeCl^ or ZnClj had a strong accelerating effect on the rate of the dehydrochlonnation reaction which led some authors to believe that an ionic mechanism could be operative at least i n the absence of oxygen. Support for the idea that at least a p a r t i a l separation of charge i n the transition state was possible began to grow and gained impetus from the following observations:a) typical radical inhibitors like hydroquinone did not affect the rate of dehydrochlorination i n a nitrogen atmosphere, b) chlorine has never been detected as a reaction gfoguct, c) the catalytic effect of HC1 i n the absence of oxygen i s d i f f i c u l t to explain by means of a radical mechanism but easier to explain i n terms of a non-radical process proceeding v i a a c y c l i c , six membered transition state, d) the effect of solvent, i n particular nitrobenzene, f i r s t


214

U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

mentioned by Goto and F u j i i l 5 and later confirmed by BengoughlSjJJ as well as the relation between the rate of dehydrochlorination and the dielectric constant of the s o l v e n t is typical of non-radical processes, e) the low molecular weight model compounds which undergo thermal decomposition i n the gas phase by a homogeneous, f i r s t order, monomolecular process, probably do so v i a a polar transition state. In spite of the increasing support for an ionic mechanism and the evidence which has been advanced for the presence of carbonium ion species i n Pwff8,29 there are s t i l l many workers who support a free radical mechanism. The work of Bamford2£ is typical and involves carrying out the degradation i n labelled C H CH T or C H C ^H3 toluene at 180°C where i t is found that both Τ and are incorporated into the polymer chain. The evidence for both types of mechanism i s so convincing and the conditions so d i f f i c u l t to standardise that i t i s tempting to speculate that both mechanisms may operate given the right conditions. Indeed Guyot^l h suggested that the i n i t i a l step has an ionic mechanism and that this might i n i t i a t e a radical process which predominates when high concentrations of HC1 are present. Papko and Pudov2£ have gone one step further and suggested that radical, molecular and ionic mechanisms may make contributions which depend on the conditions. In particular they suggest that the presence of HC1 may induce an ion-molecule reaction. I t i s becoming apparent from results such as these that the presence of HC1 may be very important and i t s presence may dictate which mechanism predominates. 1


6

5

2

6

5

a s

Termination and Extent of Dehydrochlorination Various estimates have been made of the length and distribution of the polyene sequences which result from the dehydrochlorinat ion reaction. GuyotJL suggested a range of 5-12. Geddes-22 thought that 13-15 was more typical while Braun£-t believed that values as high as 25-30 were possible. More recent work*5^3P favours the shorter range ^ 12 while the more sophisticated ^ computer based spectrum matching procedure of Daniels and Rees— shows that the range 3-14 applies to the sample degraded under their carefully defined conditions but that the temperature and duration of degradation may be c r i t i c a l . The method used by most workers to analyse the distribution of polymers i s ultraviolet spectroscopy and this i s undoubtedly the main reason for the considerable disparity i n the results obtained by different investigators. Polyene sequences are very reactive and the spectra may be considerably affected by interaction with a) the solvent, b) solvent derived peroxides, c) molecular oxygen, d) HC1, or by undergoing intramolecular or intermolecular Diels-Alder type reactions. a) Russian authors— have shown that the NMe group of dimethylformamide causes a bathochromic shift of polyene 2



15.

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215

absorptions while Daniels and ReeslZ have had to assume shifts i n order to obtain agreement between their experimental and synthetic spectra. b) Many workers suspected that the behaviour of PVC films which had been cast from solution depended on the nature of the solvent used. Some authors^ attributed the improved resistance to discoloration of films prepared from THF to the fact that oxygen containing impurities i n the solvent retarded the formation of poly exes. In a very careful study however, Daniels^ has shown that of the various products of both dark and photo chemical reactions of oxygen with tetrahydrofuran only the oL -hydroperoxytetrahydrofuran reacts with the polyene sequences. This reaction takes the form of an addition to the terminal double bond of the polyene sequence. Films cast from THF and containing trace amounts of this solvent w i l l therefore be subject to variable behaviour unless oxygen i s rigorously removed. c) As well as reacting with the solvent to form unstable peroxidic products, molecular oxygen i n the gas phase or i n solution may have a very profound effect on the degradation process. The mechanisms by which i t reacts are undoubtedly very complex and have been summarised i n several good reviews of the photooxidation of polymers . > . Its accelerating effect on the degradation (measured by HC1 evolution) i s due at least i n part to the formation of)C 0 and -00H chromophores which sensitise further decomposition. It also undergoes a very rapid photochemical reaction with polyene sequences which make the measurement of the extent of degradation in the presence of oxygen by spectroscopic measurements i n the v i s i b l e and ultraviolet regions meaningless. d) The effect of HC1 on the rate and extent of the degradation reaction has long been a matter of controversy. Technologists involved with the processing of PVC have long interpreted the stabilising effect of HC1 acceptors as evidence that HC1 i s a catalyst for the degradation process. The opposite view held by Arlmani and Druesdow" however was widely accepted for many years. The definate catalytic effect found by Rieche— and TalaminiâS however has been amply confirmed by more recent work=. and i s now generally accepted. Our own work has shown z2 conclusively that a rapid photochemical addition of HC1 to the polyene sequences of degraded PVC results i n a bleaching reaction which i s the reverse of the photodehydrochlorination. The reaction, which i s wavelength dependent reaches a photo stationary state i n which a new polyene distribution i s formed which depends on the wavelength of the irradiating light and the concentration of HCl. Results for the photo addition of HC1 to the model compound diphenyloctatetraene (DPOT) indicate that the reaction i s slower i n ethanol than i n hexane and therefore do not support an ionic mechanism. The thermal dehydrochlorination reaction i n the liquid phase on the other s


U V LIGHT


216

INDUCED

REACTIONS IN

POLYMERS

hand i s undoubtedly accelerated by the presence of free HCI^AZ o-nitrophenol— or strong electrophiles such as ZnC^ or SnCl^ 4? especially i n polar solvents while non-polar solvents inhibit. In these systems, the rate of reaction depends on the dielectric constant and on the electron accepting power of the medium, results which lead to the inevitable conclusion that an ionic mechanism or at least a strongly polarised transition state is involved. Clearly the role played by HC1 i s not completely understood. Its presence may f a c i l i t a t e ionic mechanisms which may not be significant when i t s concentration i s low. Since i t s concentration may vary very considerably from one experimental arrangement to another the mechanisms involved may vary accordingly. e) It is necessary to mention one other factor which effectively terminates the dehydrochlorination reaction when the PVC i s i n the form of thin films. This i s the formation of the polyenes in a thin strongly absorbing surface layer which effectively protects the bulk of the film from further degradation^. When this layer i s separated from the bulk of the film i t i s found to be highly insoluble and therefore cross linked to some extent. Any spectrophotome t r i e analysis of degraded films therefore should take this fact into account since the polyenes are not homogeneously distributed throughout the film as i s often assumed. In order to overcome this problem the films described in our work£2 were cast from solutions using PVC which had been thermally degraded and were homogeneous so that the surface layer did not present a problem. Stabilisation of PVC At least four types of additives have been used to stabilise PVC towards photodegradation. The f i r s t type are merely strong ultraviolet absorbers which function by dissipating as thermal energy light which would otherwise be absorbed by a sensitiser and i n i t i a t e photodegradation. Some of the most effective are of the hydroxybenzophenone or 2 -hydroxybenzotriazole type which u t i l i s e internal hydrogen bond formation i n the dissipative mechanism. A second type of stabilisers are quenchers which can accept the energy of an excited sensitiser molecule and convert i t harmlessly into heat. One molecule which can do this very effectively i s oxygen but the singlet oxygen so formed may be more damaging than the original sensitiser. For this reason quenchers should be as non selective as possible i e . they should quench as large a range of excited states as possible. One very useful class of molecules i n this respect are Ni(II) chelates which quench both t r i p l e t sensitiser and singlet oxygen. I t i s interesting though that dLamagnetic square planar complexes are more effective i n stabilising polypropylene ,



15. O W E N


217

than paramagnetic ones. One explanation which has been suggested is that energy transfers to the higher energy t r i p l e t ligand states of the paramagnetic compares are endothermic while transfers to the lower energy ligand f i e l d states of the diamagnetic complexes are exothermic though s t e r i c a l l y hindered. Essentially similar results have been obtained for polystyrene!! The efficiencies of this type of quencher are generally higher than those of the absorber type discussed above but the fact that they are coloured may be unacceptable. The third class of stabilisers are included only for the sake of completeness but the mechanism by which they operate are probably the best understood. They may be described as antioxidants and are effective because they intercept the radical chain carriers or decompose the peroxides or hydroperoxides which are potential radical i n i t i a t o r s . The fourth class which are peculiar to PVC have one thing in common namely an a b i l i t y to react wifti HC1. They include such diverse substances as inorganic lead salts, heavy metal soaps, organo-tin compounds, epoxides and many others. Although their a b i l i t y to react with HC1 may be a contributory factor their main function i s undoubtedly to react with and deactivate potential sites of i n i t i a t i o n such as branch points or areas of unsaturation.u2



218

UV LIGHT INDUCED REACTIONS IN POLYMERS

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36. Minsker K.S., and Krac E.O., Vyoskomol Soedin., (1971), A13, 1205. 37. Daniels V. and Rees N.H., J.Polym.Sci., (1974), 12, 2115. 38. Smirnov L.V. and Popov K.R., Int.Symp.Makromol.Chem., Prepr.(1969), 5, 461. 39. Daniels V. Ph.D. thesis Univ.of Wales, (1974). 40. CicchettiO.,Adv. Polymer Sci., (1970), 7, 70. 41. KarspukhinO.N.and Slobodetskaya E.M., Russ.Chem.Revs., (1973), 42. 42. Druesdow D. and Gibbs C.F., Nat.Bur.Stds Circ., (1953), 525. 43. Talamini G., Cinque G. and Palma G., Mat.Plast., (1964), 30, 317. 44. Corsato-Arnaldi Α., Palma G. and Talamini G., Mat.Plast., (1966), 32, 50. 45. Mayer Z. and Obereigner Β., Eur.Polym.J., (1973),9,435. 46. Kimura G. Yamamoto K. and Sueyoski Κ., Yuki Gosei Kagaku Kyokaishi, (1965), 23, 136. 47. Leprince P. Rev. Inst. Fr. Petroli Ann Combust. Liquides (1959), 14, 339. 48. Bailey R.J., Ph.D. thesis, Univ.of Wales (1972). 49. Owen E.D. and Williams J.I., J.Polym.Sci., (1974), 12, 1933. 50. Adamczyk A. and Wilkinson F., J.Appl.Polym.Sci., (1974), 18, 1225. 51. Harper D.J. and McKellar J.F., J.Appl.Polym.Sci., (1974), 18, 1233. 52. Anderson D.F. and McKenzie D.A., J.Polym.Sci., A-1, (1970), 8, 2905.


Ultraviolet Light Induced Reactions in Polymers

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