Photodegradation and Photostabilization of Coatings - American

14 Photodegradation of P o l y v i n y l C h l o r i d e

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 21, 2016 | http://pubs.acs.org Publication Date: April 8, 1981 | doi: 10.1021/bk-1981-0151.ch014

A Survey of Recent Studies W. H. STARNES, JR. Bell Laboratories, Murray Hill, NJ 07974 Although poly(vinyl chloride) (PVC) is one of the most im portant commercial polymers, its outdoor use has been restricted by its photochemical instability. The reasons for this insta bility are incompletely understood, but some progress has been made recently on this problem, and the present paper attempts to summarize the current status of fundamental knowledge in this field. This survey is not intended to be comprehensive; it is concerned primarily with work published since the early 1970's and with basic chemical principles rather than technological de velopments. The photodegradation of PVC has been discussed in other recent reviews (1,2,3,4). General Considerations Photolysis of PVC in the presence of oxygen causes oxidation of the polymer. However, under most (perhaps all) conditions, in both the presence and absence of oxygen, the photodegradation is complicated by scissions of carbon-chlorine bonds. Such scis sions may lead to the formation of conjugated polyene sequences via sequential dehydrochlorination (Equation 1). The polyenes hv -(CH-CHC1) > -(CH=CH) - + nHCl (1) z n. ii — and the initial oxidation products may undergo photooxidation, in turn, or they may be destroyed by photochemical processes of the nonoxidative variety. Thermal reactions can occur also when temperatures are sufficiently high. Thus the photodegradation of PVC produces extremely complex chemical systems whose compo sitions are difficult to determine and whose behavior is hard to predict. It is, therefore, hardly surprising to find that most of the basic problems in this field have not been solved. Recent fundamental studies in this area have been concerned with aspects such as the nature of the initiating chromophores, the chemistry of the initiation steps, the extent to which the 0097-6156/81/0151-0197$05.00/0 © 1981 American Chemical Society Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

198

PHOTODEGRADATION

AND

PHOTOSTABILIZATION

OF

COATINGS


chromophores and other r e a c t i v e groups are formed during polymeri z a t i o n and subsequent p r o c e s s i n g , the products and r e a c t i o n mechanism of the o v e r a l l degradation process, and, of course, the e f f e c t s of experimental v a r i a b l e s (wavelength and i n t e n s i t y of the i n c i d e n t l i g h t , oxygen pressure, temperature, time, sample p u r i t y , and sample t h i c k n e s s ) . Some of these i n v e s t i g a t i o n s have been c a r r i e d out w i t h l i g h t a t wavelengths below the t e r r e s t r i a l s o l a r range ( i . e . , a t wavelengths < ^290 nm) (5) , and i t should be kept i n mind that r e s u l t s obtained under circumstances such as these may not be s t r i c t l y a p p l i c a b l e to n a t u r a l weathering s i t u a t i o n s . Initiation The o r d i n a r y monomer u n i t s of PVC are not expected to absorb any t e r r e s t r i a l s o l a r r a d i a t i o n (1,_2,3.,4) . Thus, under the u s u a l ambient c o n d i t i o n s , photodegradation of the polymer must be i n i t i a t e d by chromophoric i m p u r i t i e s . These i m p u r i t i e s may simp l y be s t r u c t u r a l d e f e c t s i n the PVC i t s e l f , or they may be extraneous substances that have been i n c o r p o r a t e d i n t o the polymer. S e v e r a l of these p o t e n t i a l p h o t o s e n s i t i z e r s are discussed i n the following sections. Carbonyl Groups. Such s t r u c t u r e s could be introduced by a i r o x i d a t i o n during p o l y m e r i z a t i o n or subsequent p r o c e s s i n g of the polymer. There i s , i n f a c t , some experimental evidence f o r t h e i r formation during p o l y m e r i z a t i o n v i a the f o l l o w i n g sequence of steps (6): (1) copolymerization of v i n y l c h l o r i d e with a d v e n t i t i o u s oxygen; (2) decomposition of the r e s u l t i n g polyperoxide to form HC1, CH2°> ; (3) copolymerization of CO with v i n y l chloride. F r e e - r a d i c a l copolymers of v i n y l c h l o r i d e w i t h carbon monoxi d e have been suggested to c o n t a i n pendent C0C1 groups (as i n T.) a

-CH CH2| C0C1 0

n

d

c o

-CH -CHC1-C0-CH -CHC12 2 0

0

r a t h e r than backbone carbonyls (7). However, other work has i n d i c a t e d that the l a t t e r type of s t r u c t u r e (£) may be c o r r e c t (8, 9). In any event, s t u d i e s w i t h a u t h e n t i c p o l y ( v i n y l c h l o r i d e co-carbon monoxide) polymers have shown that the carbonyl groups do, indeed, a c c e l e r a t e the photodehydrochlorination and photoo x i d a t i o n of these m a t e r i a l s (9,10,11). Moreover, benzophenone i s known to act as a p h o t o s e n s i t i z e r f o r the d e h y d r o c h l o r i n a t i o n of PVC (12), and the d e h y d r o c h l o r i n a t i o n of s e v e r a l simple a l k y l c h l o r i d e s has been found to be p h o t o s e n s i t i z e d by benzophenone (13,14,15) and a number of a l k y l a r y l ketones (13,14). Selfs e n s i t i z e d photodehydrochlorination has been observed w i t h 4 - c h l o r o a l k y l phenyl ketones (16,J7,18). Acetone (13,19,20,21)

Pappas and Winslow; Photodegradation and Photostabilization of Coatings ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


14.

STARNES

Photodegradation

of

Polyvinyl

199

Chloride

and s e v e r a l other a l i p h a t i c ketones (13,19) have a l s o been shown to p h o t o s e n s i t i z e the e l i m i n a t i o n of HC1 from PVC (19) and simple a l k y l c h l o r i d e s (13,19,20,21), although the s e n s i t i z a t i o n e f f i ciency of such ketones d e c l i n e s p r e c i p i t o u s l y when t h e i r a l k y l groups are l a r g e (13). Another e f f e c t i v e ketone s e n s i t i z e r f o r the photodehydrochlorination of the polymer i s hexachloroacetone (22) . S e v e r a l mechanisms have been postulated i n order to account f o r k e t o n e - s e n s i t i z e d photodehydrochlorination. Benzophenone and acetophenone have been suggested to act as s i n g l e t s e n s i t i zers v i a a c o l l i s i o n a l d e a c t i v a t i o n process (13). An a l t e r n a t i v e mechanism proposed f o r benzophenone i n v o l v e s a b s t r a c t i o n of a methylene hydrogen from PVC by the t r i p l e t ketone (Equation 2), followed by 3 s c i s s i o n of a 3

Ph C0* z

+

o

-(CH CHC1) z ii 0

• Ph C0H z I

+

o

-CHCHC1-(CH CHC1) z n-i 0

(2)

-CHCHC1-(CH CHC1) • CI' + -CH=CH-(CH CHC1) (3) z n—± z ri-i c h l o r i n e atom from the r e s u l t i n g carbon r a d i c a l (Equation 3) (12). T h i s mechanism seems c o n s i s t e n t w i t h the appearance of an absorpt i o n band at 340 nm that can be assigned to an adduct of benzophenone and r a d i c a l ^ (12), and w i t h the a b i l i t y of naphthalene (a t r i p l e t quencher) to r e t a r d both the r a t e of appearance of t h i s absorption and the r a t e of disappearance of benzophenone (23) . Arguments have been given f o r the operation of a s i m i l a r mechanism i n v o l v i n g i n t r a m o l e c u l a r hydrogen a b s t r a c t i o n i n the case of the 4 - c h l o r o a l k y l phenyl ketones (16,17,18). On the other hand, quenching s t u d i e s w i t h t e r t - b u t y l c h l o r i d e have i n d i c a t e d that the hydrogen-abstraction mechanism i s a c t u a l l y q u i t e u n l i k e l y f o r dehydrochlorinations that are p h o t o s e n s i t i z e d by benzophenone and a l k y l phenyl ketones i n an i n t e r m o l e c u l a r manner (14). An a l t e r n a t i v e scheme that can be proposed f o r such r e a c t i o n s invokes an intermediate ( t r i p l e t ketone)-substrate e x c i plex whose decomposition (Equation 4) produces a v i b r a t i o n a l l y 0

3

Ph CO* 2

RC1*

+ —»

1

RC1 R-

3

^ +

0

[ e x c i p l e x ] * — * Pt^CO CI- — • HC1

+

+

RCl*

i b

alkene

(4) (5)

V1D

(R = a l k y l ) e x c i t e d a l k y l c h l o r i d e that experiences d e h y d r o c h l o r i n a t i o n , e i t h e r i n a concerted manner (14) or v i a a route i n v o l v i n g the d i s p r o p o r t i o n a t i o n of f r e e - r a d i c a l intermediates (e.g., Equation 5; solvent r a d i c a l s could a l s o become involved i n t h i s p r o c e s s ) . The e x c i p l e x was o r i g i n a l l y suggested to be s t a b i l i z e d by a charge-transfer i n t e r a c t i o n i n which the a l k y l c h l o r i d e i s the


PHOTODEGRADATION

200

AND

PHOTOSTABILIZATION

OF

COATINGS

e l e c t r o n donor (14), but l a t e r workers have argued that the c h l o r i d e moiety i s the e l e c t r o n acceptor i n s t e a d (15). At any r a t e , i t would now seem that the same type of mechanism could apply to benzophenone and PVC, and that the disappearance of benzophenone and the appearance of the 340-nm a b s o r p t i o n which have been noted i n that system (12,23) might a c t u a l l y s i g n i f y the occurrence of Reaction 6 or R e a c t i o n 7 [ c f . (12) and references c i t e d t h e r e i n ] . Ph C0

+


2

Ph C0 2

+

R-

—•

Ph C0R 2

(6)

R-

(R = a polymeric carbon r a d i c a l ) The a c e t o n e - s e n s i t i z e d photodehydrochlorination of 1,4dichlorobutane i s not suppressed by t r i p l e t quenchers (20), but the f l u o r e s c e n c e of the s e n s i t i z e r i s quenched by the a l k y l c h l o r i d e (13) . These observations imply the o p e r a t i o n of a mechanism i n v o l v i n g c o l l i s i o n a l d e a c t i v a t i o n , by the s u b s t r a t e , of the acetone e x c i t e d s i n g l e t s t a t e (13,21). T h i s type of mechanism has r e c e i v e d strong support from another study i n which the f l u o r e s c e n c e of acetone and 2-butanone was found to be quenched by s e v e r a l a l k y l and b e n z y l c h l o r i d e s (24). The d e t a i l e d mechanism f o r alkanone s e n s i t i z a t i o n proposed on the b a s i s of the l a t t e r work invokes a charge-transfer ( s i n g l e t k e t o n e ) - s u b s t r a t e e x c i p l e x (24) and i s s i m i l a r to one of the mechanisms that has been suggested (15) f o r s e n s i t i z a t i o n by ketone t r i p l e t s ( c f . Equations 4 and 5). Another mechanism f o r a l k a n o n e - s e n s i t i z e d photodehydroc h l o r i n a t i o n comprises N o r r i s h type I s c i s s i o n of the ketone, followed by ground-state r e a c t i o n s of r a d i c a l s (19). However, the evidence f o r such a mechanism i s based on experiments that were c a r r i e d out i n the vapor phase (19). I n i t i a t i o n of the photodegradation of PVC by hexachloroacetone has been suggested to i n v o l v e the a b s t r a c t i o n of hydrogen from the polymer by r a d i c a l s r e s u l t i n g from the p h o t o l y s i s of the ketone's carbonc h l o r i n e bonds (22). S e v e r a l recent workers have considered the p o s s i b i l i t y that the p h o t o o x i d a t i o n of PVC i n v o l v e s o x i d a t i o n by s i n g l e t oxygen which r e s u l t s from the quenching of t r i p l e t carbonyl imp u r i t i e s (2,9,25). T h i s type of mechanism has n e i t h e r been e s t a b l i s h e d nor disproven, although, as expected, ^2 appears to be e s s e n t i a l l y u n r e a c t i v e toward undegraded PVC (26). Other r e cent observations that may be p e r t i n e n t to the c a r b o n y l - s e n s i t i z e d photodehydrochlorination of the polymer are the f a i l u r e of S-chloropropiophenone (16) and y-chlorobutyrophenone (16,17) to undergo photodehydrochlorination [the l a t t e r ketone experiences N o r r i s h type I I s c i s s i o n i n s t e a d (17)] and the occurrence of type I I s c i s s i o n (with no concomitant type I cleavage) upon i r r a d i a t i o n of 5-chloro-2-hexanone (27).



14.

STARNES

Photodegradation

of

Polyvinyl

201

Chloride

As noted above, there i s experimental evidence to i n d i c a t e that the carbonyl groups of ( v i n y l c h l o r i d e ) - ( c a r b o n monoxide) copolymers are e f f e c t i v e s e n s i t i z e r s f o r the photodegradation of these m a t e r i a l s (9,10,11). A reasonable s e n s i t i z a t i o n mechanism can be formulated f o r t h i s system on the b a s i s of the i n f o r m a t i o n now on hand. Photodehydrochlorination of p o l y ( v i n y l chloride-co-carbon monoxide) i s not accompanied by s i g n i f i c a n t changes i n polymer molecular weight when i t i s c a r r i e d out under a i r w i t h a h i g h pressure mercury lamp (11,28). I t a l s o occurs under n i t r o g e n at wavelengths > 294 nm with no a p p r e c i a b l e a l t e r a t i o n s i n the pos i t i o n and i n t e n s i t y of the o r i g i n a l IR carbonyl absorption (9). Thus the p h o t o i n i t i a t i o n o c c u r r i n g i n t h i s system does not seem to r e q u i r e N o r r i s h s c i s s i o n s or any other permanent s t r u c t u r a l changes i n the immediate v i c i n i t y of the s e n s i t i z i n g carbonyl groups. Moreover, the i n i t i a l stage of the photodehydrochlorinat i o n i s s t r o n g l y i n h i b i t e d by ^ 2 (9,25), a r e s u l t which suggests [by analogy with other work (20)] that alkanone e x c i t e d s i n g l e t s are not i n v o l v e d i n the s e n s i t i z a t i o n process. These f i n d i n g s would seem to be i n keeping w i t h an i n i t i a t i o n mechanism (Equat i o n s 8 and 9) that i s

V

II

-C-

+

-CH CHC1- ^ 2 2

-(CH CHCl)* 2

ib

^

*

[exciplex] — *

• -CH=CH-

+

0

II

-C-

+

*

-(O^CHCl)^-

HCl

( g )

(9)

operable at ambient s o l a r wavelengths and i n v o l v e s e x c i p l e x f o r mation from a carbonyl t r i p l e t and a chloromethylene-containing p o r t i o n of the polymer, followed by a d i s s o c i a t i o n process that regenerates ground-state carbonyl and produces a v i b r a t i o n a l l y e x c i t e d polymer segment that e l i m i n a t e s HCl i n a stepwise or concerted manner. Other p o s s i b l e v a r i a t i o n s of the mechanism would i n v o l v e the d i r e c t occurrence of d e h y d r o c h l o r i n a t i o n from the e x c i p l e x intermediate, or d e h y d r o c h l o r i n a t i o n f o l l o w i n g d i s s o c i a t i o n of the e x c i p l e x i n t o a c a t i o n - a n i o n r a d i c a l p a i r . This scheme i s , i n f a c t , d i r e c t l y analogous to the a r y l ketone mechanism of Equations 4 and 5 (14,15). In the case of o r d i n a r y commercial PVC, the importance of carbonyl p h o t o s e n s i t i z a t i o n i s not e n t i r e l y c l e a r . I t s occurrence i s c o n s i s t e n t with the reported a b i l i t y of 02 to r e t a r d the i n i t i a l photodehydrochlorination of PVC i t s e l f (9,29). Nevertheless, t h i s evidence i s not c o n c l u s i v e , and other work, discussed i n the f o l l o w i n g s e c t i o n , has suggested that another s e n s i t i z a t i o n mechanism predominates w i t h PVC under some c o n d i t i o n s , at l e a s t . 3

Alkene Linkages. Using (250-350)-nm i r r a d i a t i o n (30,31) or the u n f i l t e r e d l i g h t from a high-pressure mercury lamp (32), B a l a n d i e r and Decker have measured quantum y i e l d s under n i t r o g e n


PHOTODEGRADATION


202

AND

PHOTOSTABILIZATION

OF

COATINGS

and oxygen atmospheres f o r the d e h y d r o c h l o r i n a t i o n , chain s c i s s i o n , and c r o s s - l i n k i n g of PVC i n s o l u t i o n (30,31) and i n f i l m s (31,32). The v a l u e s obtained^ were found to be independent of the extent of r e a c t i o n , d e s p i t e the i n c r e a s i n g a b s o r p t i o n of l i g h t by the polyene s t r u c t u r e s that were formed (30,31,32). Thus the constancy of the quantum y i e l d s was taken as evidence f o r the i n i t i a t i o n of photodegradation by unsaturated s t r u c t u r e s i n the o r i g i n a l polymer (30,31,32). Since the quantum e f f i c i e n c i e s were not a f f e c t e d s i g n i f i c a n t l y by changes i n l i g h t i n t e n s i t y or the polymer c o n c e n t r a t i o n (30), the photodegradation appears to have been p r i m a r i l y a non-chain i n t r a m o l e c u l a r process. Polyene t r i p l e t s t a t e s are u n l i k e l y to have been i n v o l v e d i n the mechanism, owing to the low t r i p l e t energies (33) and very low intersystem c r o s s i n g e f f i c i e n c i e s (34) expected f o r chromophores of t h i s g e n e r a l type. Moreover, the photodehydrochlorination showed no evidence of i n h i b i t i o n by t r i p l e t molecular oxygen (30, 31). A l l of these observations seem c o n s i s t e n t w i t h the simple process of R e a c t i o n 10 (which might i n v o l v e the d i s p r o p o r t i o n a 1

- (CH=CH)*-CHC1CH - — > ii Z 0

-(CH=CH) 11+1

+

HCl

(10)

t i o n of r a d i c a l s r e s u l t i n g from C-Cl homolysis). Other arguments i n support of such a mechanism have been presented by R e i n i s c h et a l . (35), and here i t i s a l s o of i n t e r e s t to note that the u n s e n s i t i z e d p h o t o l y s i s of l i q u i d a l l y l c h l o r i d e appears to i n v o l v e C-Cl homolysis as the primary photochemical process (36). I s alkene s e n s i t i z a t i o n important f o r the PVC polyenes that absorb a t ambient s o l a r wavelengths? The a v a i l a b l e i n f o r m a t i o n does not p r o v i d e an unambiguous answer to t h i s q u e s t i o n . Decker found that the average l e n g t h of the polyenes increased w i t h i n c r e a s i n g time of i r r a d i a t i o n and caused an enhanced a b s o r p t i o n of the i n c i d e n t l i g h t having wavelengths g r e a t e r than 366 nm (32). T h i s o b s e r v a t i o n , together w i t h the time independence of the d e h y d r o c h l o r i n a t i o n quantum e f f i c i e n c y , could be taken as e v i dence f o r p h o t o s e n s i t i z a t i o n by polyenes absorbing i n the ambient s o l a r range (32). Furthermore, a comparable d e h y d r o c h l o r i n a t i o n quantum y i e l d was obtained from the 514.5-nm l a s e r p h o t o l y s i s of a PVC f i l m that had been degraded p r e v i o u s l y by UV i r r a d i a t i o n under n i t r o g e n (32). On the other hand, Gibb and MacCallum observed that the polyenes formed from PVC upon i r r a d i a t i o n under n i t r o g e n at 240-560 nm d i d not e f f e c t i v e l y s e n s i t i z e f u r t h e r photodegradation during subsequent i r r a d i a t i o n under n i t r o g e n w i t h higher-wavelength l i g h t (37). R e d i s t r i b u t i o n of the polyene sequence lengths was observed instead (37), and i t was concluded from these f i n d i n g s that the p r i n c i p a l i n i t i a t i n g chromophores are conjugated dienes and t r i e n e s when > 240-nm i r r a d i a t i o n i s employed (37). Other workers have r e p o r t e d that the r a t e of the photodehydrochlorination of PVC i s so low at wavelengths >310 nm that long conjugated polyenes i n the polymer (n > 3 i n Equation 1)



14.

STARNES

Photodegradation

of Polyvinyl

Chloride

203

can be analyzed f o r q u a n t i t a t i v e l y v i a t h e i r s e l e c t i v e photooxid a t i o n using (310-450)-nm l i g h t (38). The reason f o r these divergent r e s u l t s i s , a t present, unc l e a r . However, i n view of the d i f f e r e n c e noted above with r e gard to i n h i b i t i o n by t r i p l e t oxygen, i t seems that the photos e n s i t i z a t i o n mechanism f o r the systems o f B a l a n d i e r and Decker (30,31,32) must have d i f f e r e d from that which operated i n the systems of Braun e t a l . (9,25). For the photodegradations performed w i t h simulated t e r r e s t r i a l i n s o l a t i o n , i t can be argued that carbonyl groups were the i n i t i a l a c t i v e s e n s i t i z e r s , and that long polyenes assumed the predominant r o l e o f s e n s i t i z a t i o n i n the l a t t e r stages of the process. F u r t h e r measurements of quantum y i e l d s a t ambient s o l a r wavelengths might help to s e t t l e t h i s p o i n t , e s p e c i a l l y i f the measurements were performed on polymers containing d i f f e r e n t amounts of polyene s t r u c t u r e s . I n t h i s connection, i t should be noted that polymers containing long polyenes are extremely s u s c e p t i b l e t o thermal dehydrochlor i n a t i o n (39,40) (apparently a t temperatures as low as 3G°C!) (40), and that t h i s process may complicate quantum y i e l d measurements i n some s i t u a t i o n s , p a r t i c u l a r l y i f a focused l a s e r beam i s used as the source of the i n c i d e n t l i g h t . Alkene s e n s i t i z a t i o n of the photodegradation of PVC seems to be supported by other observations that have been reported i n the recent l i t e r a t u r e . One of these i s the i n c r e a s e d r a t e s of nonoxidative photodegradation that have been found f o r polymers which were subjected to a p r e l i m i n a r y nonoxidative thermal treatment (41,42). Moreover, the benzophenone-photosensitized d e h y d r o c h l o r i n a t i o n of PVC has been shown to undergo an autoa c c e l e r a t i o n which can be a t t r i b u t e d t o a supplementary photos e n s i t i z a t i o n by the conjugated polyene product (12). A l s o of i n t e r e s t i n t h i s regard i s the f i n d i n g that the production of f r e e r a d i c a l s during the UV i r r a d i a t i o n of PVC under vacuum apparently can be s e n s i t i z e d by polyenes that have been created during a p r e l i m i n a r y p h o t o l y s i s (43,44). A c o r r e l a t i o n has been reported between the p h o t o o x i d i z a b i l i t y of s e v e r a l PVC samples and the number of long-chain ends i n these polymers (45). The mechanistic s i g n i f i c a n c e of t h i s r e s u l t i s r a t h e r u n c e r t a i n , as the authors (45) do not s t a t e how the number of long-chain ends was v a r i e d . Nevertheless, i t i s perhaps worth n o t i n g t h a t , i n the absence of added c h a i n - t r a n s f e r agents, a major f r a c t i o n of the long-chain ends should cons i s t of a l l y l i c c h l o r i d e groups (46,47). C a t a l y t i c hydrogenation of PVC causes a s i g n i f i c a n t r e d u c t i o n i n the r a t e of the polymer's subsequent photodegradation (32). Although t h i s r e s u l t i s c o n s i s t e n t w i t h the occurrence of alkene s e n s i t i z a t i o n (32), i t can a l s o be a t t r i b u t e d , per se, to the removal o f other p o s s i b l e s e n s i t i z e r s such as carbonyl groups and peroxide l i n k a g e s . On the b a s i s o f s p e c t r o s c o p i c observations, Verdu e t a l . (48) have argued that polyene sequences are the p r i n c i p a l


204

PHOTODEGRADATION

A N DPHOTOSTABILIZATION

O F COATINGS

s e n s i t i z e r s f o r the photooxidation of ( v i n y l chloride)-Ccarbon monoxide) and ( v i n y l chloride)-oxygen copolymers a t 300-450 nm. For the case of the p o l y ( v i n y l chloride-co-carbon monoxide)s, t h i s c o n c l u s i o n i s d i f f i c u l t to r e c o n c i l e with the d i f f e r e n t i n i t i a l k i n e t i c e f f e c t s of that were found f o r the systems of Braun et a l . (9,25) as compared t o those of B a l a n d i e r and


Decker (30,_31) . Alkene l i n k a g e s that are formed during processing seem t o p l a y an important r o l e i n the photooxidation of PVC (49,50,51), and some c o n s i d e r a t i o n has been given r e c e n t l y t o the p o s s i b i l i t y of a degradation mechanism i n v o l v i n g energy t r a n s f e r from exc i t e d carbonyl groups to alkene l i n k a g e s i n the polymer (9). On the other hand, carbonyl quenching by p o l y e n y l r a d i c a l s i n PVC has been invoked i n order t o account f o r a u t o i n h i b i t i o n during photooxidative degradation (52). Some workers have mentioned the p o s s i b i l i t y of s i n g l e t oxygen formation i n PVC v i a the quenching of e x c i t e d polyenes (25,29) o r cyclohexadienes (53). Peroxides. D i r e c t o r s e n s i t i z e d p h o t o l y s i s o f an oxygenoxygen l i n k a g e would produce alkoxy r a d i c a l s that might i n i t i a t e the degradation of PVC. Indeed, the p e r o x i d i c g e l f r a c t i o n o f m i l d l y processed polymer has been found to be extremely suscept i b l e t o subsequent photooxidation (49). The hydroperoxide concentration of such a g e l can be c o r r e l a t e d w i t h the i n i t i a l photooxidation r a t e (49), but the o l e f i n i c u n s a t u r a t i o n produced c o n c u r r e n t l y during processing a l s o appears to c o n t r i b u t e to the photooxidative i n s t a b i l i t y of the polymer (50,51). Enhanced r a t e s of photodegradation have been reported a l s o f o r PVC prepared i n the presence of oxygen (9,10,48) and, a t X >320 nm, f o r PVC containing peroxide that was introduced by o z o n i z a t i o n (52). In these cases, both carbonyl groups and peroxides may have c o n t r i b u t e d t o the p h o t o a c t i v a t i o n e f f e c t [however, see (48)1. S e v e r a l carbonyl-containing peroxide a d d i t i v e s have been shown t o i n c r e a s e the i n i t i a l r a t e o f the nonoxidative photod e h y d r o c h l o r i n a t i o n of PVC (54). I n s t u d i e s w i t h polymeric ketones u n r e l a t e d s t r u c t u r a l l y to PVC, the e x c i t e d s i n g l e t and t r i p l e t s t a t e s o f the carbonyl groups i n these polymers were found t o s e n s i t i z e 0-0 homolysis a t r a t e s approaching d i f f u s i o n c o n t r o l (55). S i m i l a r r e a c t i o n s may w e l l occur i n o x i d i z e d v i n y l c h l o r i d e polymers. S o l v e n t s , Additives, and Extraneous I m p u r i t i e s . The r a t e of PVC photodegradation under a i r at wavelengths >250 nm i s s a i d t o be increased by small amounts of r e s i d u a l tetrahydrofuran (THF) or dichloroethane (56). On the other hand, r e s i d u a l THF has been reported not t o enhance the d e h y d r o c h l o r i n a t i o n of the polymer during i r r a d i a t i o n under n i t r o g e n a t X >240 nm (41). Nevertheless, under the c o n d i t i o n s of the l a t t e r study, THF was found to i n c r e a s e the r e l a t i v e concentrations of the s h o r t e r polyene products (41). T h i s e f f e c t was a t t r i b u t e d t o a f a c i l e



14.

STARNES

Photodegradation

of

Polyvinyl

205

Chloride

r o t a t i o n around the C-C bonds of the longer polyene sequences, owing to the a b i l i t y of THF to f u n c t i o n as a p l a s t i c i z e r (41). Tetrahydrofuran has been reported to e x h i b i t an absorption maximum at 280 nm (52,56), but s e v e r a l workers have shown that t h i s band i s not produced by the p u r i f i e d solvent (30,41,57). Oxidation products from THF have been invoked i n order to account f o r the appearance of the 280-nm band i n PVC f i l m s that are s o l v e n t - c a s t from THF i n a i r (J57, 58). However, i n some reported cases (56,59), t h i s band was undoubtedly produced, at l e a s t i n p a r t , by a p h e n o l i c a n t i o x i d a n t (2.6-di-tert-butvl-£-cresol) (_59) i n the s o l v e n t . Since c e r t a i n j>-alkylphenols have now been shown to be powerful p h o t o s e n s i t i z e r s f o r the d e h y d r o c h l o r i n a t i o n of PVC (60), i t i s c l e a r that a n t i o x i d a n t p h o t o s e n s i t i z a t i o n might w e l l have been r e s p o n s i b l e f o r some of the e f f e c t s a t t r i buted p r e v i o u s l y (56) to THF alone. On the other hand, enhanced r a t e s of photodegradation under a i r have a l s o been observed f o r PVC f i l m s cast from p u r i f i e d THF (57), a r e s u l t which has been a s c r i b e d to r a d i c a l formation during the photooxidation of r e s i d u a l solvent (57,61). Rabek et a l . (61) have shown that t h i s photooxidation produces a-HOO-THF, a-HO-THF, and y-butyrolactone, and they have found that the hydroperoxide product i s an e f f e c t i v e s e n s i t i z e r f o r the photodehydrochlorination of PVC at A = 254 nm (61). Studies w i t h model compounds have demonstrated that photodehydrochlorination i s s e n s i t i z e d by £-cresol t r i p l e t s v i a a charge-transfer e x c i p l e x intermediate i n which the a l k y l c h l o r i d e i s the e l e c t r o n acceptor (15). The d e t a i l e d mechanism suggested f o r t h i s process (15) i s o u t l i n e d i n Equations 11 and 12. o

o

* ArOH

+

RC1

RCl" — •

Cl"

Photodegradation and Photostabilization of Coatings - American

Recommend Documents