Sensitizers for Photodegradation Reactions - ACS Publications

A 6000W xenon arc Atlas Weather-Ometer (k) equipped with Corning No. 77^0 borosilicate inner and outer filters was used as a laboratory light source b...
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22 Sensitizers for Photodegradation Reactions J. D . C O O N E Y and D . M . W I L E S Division of Chemistry, National Research Council of Canada, Ottawa, Canada K 1 A 0R9

Intensive research in the past eight years on sensitizers for the photodegradation of thermoplastics has led to the new concept of photodegradable plastics or Degradable P l a s t i c s . The term Degradable Plastics is commonly used to describe plastics which undergo more rapid than normal deterioration on exposure to unfiltered (although not necessarily direct) sunlight while maintaining long-term indoor stability. Owing to the action of outdoor weathering, Degradable Plastics disintegrate into tiny fragments and it is frequently claimed that these fragments are further degraded by micro-organisms (i.e., fungi, bacteria). Realistic uses for Degradable Plastics include those applications where materials are used outdoors for a limited time only and it is not economically desirable to collect the residual materials after use. Examples are agricultural mulch, films and cordage, twine, etc. Another application is that of packaging which is stored and used indoors and discarded outdoors, in other words litter for which manual collection is not practicable. Degradable Plastics are not inherently biodegradable but, for the most part, are s l i g h t l y modified conventional resins which are intentionally made more sensitive to the erythemal (sunburn) region of the sun's spectrum. One common approach to the required modification involves the addition of a photosensitive additive, hopefully with just the right UV absorption spectrum and subsequent excited state reaction mechanisms to produce early embrittlement. The other approach involves the use of a copolymer where the conventional resin monomer is copolymer!zed with a small amount of another monomer containing 307

308

UV

L I G H T INDUCED REACTIONS

IN

POLYMERS

a f u n c t i o n a l group which absorbs UV r a d i a t i o n . I d e a l l y the l i g h t - s e n s i t i v e groups absorb s t r o n g l y i n t h a t p a r t o f the sun's spectrum which passes through the atmosphere but i s not t r a n s m i t t e d by window g l a s s , i . e . , absorb i n the r e g i o n 2 9 0 t o 3 2 0 nm. The use o f a m a s t e r b a t c h i s p o s s i b l e w i t h b o t h approaches, and i t i s c l a i m e d t h a t t h e r e are no d e l e t e r i o u s e f f e c t s on p r o c e s s i n g c h a r a c t e r i s t i c s or on f i n a l p h y s i c a l properties. T h i s paper d e s c r i b e s the e f f e c t s o f UV l i g h t , the most s i g n i f i c a n t a s p e c t of outdoor w e a t h e r i n g , on a v a r i e t y o f Degradable P l a s t i c s and a l s o d i s c u s s e s the e x t e n t of b i o d e g r a d a b i l i t y o f the photodegraded samples. Examples o f Degradable

Plastics

(a) E c o l y t e Res ins : E c o P l a s t i c s L t d . , T o r o n t o , Canada. E c o l y t e r e s i n s Π Π a r e c o p o l y m e r s c o n t a i n i n g a few per cent of v i n y l ketone comonomer and are based on the i n v e n t i o n s o f P r o f e s s o r James E. G u i l l e t o f the U n i v e r s i t y of T o r o n t o . Quantum y i e l d c o n s i d e r a t i o n s account f o r the e f f i c a c y o f h a v i n g the u n i t s d i s t r i b u t e d randomly (and s p a r s e l y ) i n the backbone of ~CRi~ polymer c h a i n s . P l a s t i c s o f both the a d d i t i o n ^ and c o n d e n s a t i o n polymer types can be made (" s u i t a b l y p h o t o s e n s i t i v e i n t h i s way a l t h o u g h R2 polystyrene (Ecolyte PS), polyethylene (Ecolyte PE) and p o l y p r o p y l e n e ( E c o l y t e PP) are the f i r s t t o be developed. C o n t r o l l e d outdoor l i f e t i m e s over a v e r y wide range of time p e r i o d s (days t o months) are p o s s i b l e w i t h apparent l o n g term s t a b i l i t y i n d o o r s . The p e r s i s t e n c e of outdoor m e c h a n i c a l i n t e g r i t y depends on the c o n c e n t r a t i o n o f the ketone comonomer and i t s s p e c i f i c s t r u c t u r e , e.g., the n a t u r e of Ri and R2. The t i m e - t o - e m b r i t t l e m e n t o f E c o l y t e PS and PE i n our Weather-Ometer exposures are l i s t e d i n T a b l e s I and I I . = 0

(b) Sty-Grade: B i o - D e g r a d a b l e P l a s t i c s , I n c . , B o i s e Idaho ( 2 ) . The approach i n t h i s case i s the i n c o r p o r a t i o n of a p h o t o s e n s i t i v e a d d i t i v e with c o n v e n t i o n a l r e s i n , e.g., benzophenone b l e n d e d w i t h polystyrene. The r a t e o f d e g r a d a t i o n outdoors can be v a r i e d by changing the amount o f a d d i t i v e used, a l t h o u g h t h e r e i s e x p e c t e d to be an upper l i m i t beyond which s e n s i t i v i t y t o a r t i f i c i a l l i g h t becomes a problem. Benzophenone has been g i v e n FDA a p p r o v a l f o r c e r t a i n p a c k a g i n g end-uses . Time-to-embrittlement s

22.

cooNEY

Sensitizers for Photodegradation Reactions

A N D WILES

TABLE I E m b r i t t l e m e n t Times

o f Degradable

Polystyrenes

Time t o E m b r i t t l e m e n t Xe Arc (hr )

Samples Ecolyte Ecolyte Ecolyte Ecolyte

"Fast" "Fast" "Slow" "Slow"

foam, 2β00μ foam, " foam, 2000μ foam, "

Ecolyte Ecolyte

"Fast" "Slow"

film film

275 T0 2^00 600° b b

23 55

Outdoors (months ) 1.0

-

>6.5

-

-

6

Sty-Grade r e s i n cone, f i l m Sty-Grade r e s i n cone. let-down 5:1 f i l m Der W i e n e r s c h i t z e l c o f f e e cup l i d , a Sty-Grade p r o d u c t , r e s i n cone, let-down 50:1

22

-

112

C o n t r o l , S t y r o n 666

U95

-

a

film

Foams are e x p e c t e d t o be slower than f i l m s owing t o t h i c k n e s s and o p a c i t y . The UV-degradable f o r m u l ­ a t i o n s l i s t e d here are not n e c e s s a r i l y those which are t o be used i n commercial p r o d u c t s . In a d d i t i o n , comparison o f e m b r i t t l e m e n t times i s not i n t e n d e d t o r e l a t e t o the r e l a t i v e m e r i t s o f the v a r i o u s mater­ ials .

^Irradiated °Turned

35 hr on each

over every 2k h r .

side.

309

#3

#2

LDPE, E c o P l a s t i c s L t d . ,

LDPE, E c o P l a s t i c s L t d . ,

#1 L D P E , E c o P l a s t i c s L t d . ,

C o n t r o l L D P E , 12μ

C o n t r o l L D P E , E c o P l a s t i c s L t d . , 6k\i

6ΐμ

Ecolyte

63μ

Ecolyte

53μ

Ecolyte

B i o - D e g r a d a b l e P l a s t i c s I n c . , LDPE, k2]A E N D E - p l a s t LDPE w i t h FAM-1, A k e r l u n d a n d R a u s i n g , 58μ

1820

2075

1^65

1180

>18

>h.5

2.5

>k.5

1550

820

U.o

6.5 .11

Outdoors (months)

750

2210

Ecoten

Xe A r c (hr) 1175

L D P E , 51μ

Polyolefin

Time t o E m b r i t t l e m e n t

Times o f D e g r a d a b l e

C o n t r o l LDPE, Dupont, l l O f l

Samples

Embrittlement

TABLE I I

kQO

315

230

85

375

215

k90

koo

Xe A r c (hr)

T i m e t o 1/10

Films o f %E

22.

COONEY AND WILES

Sensitizers for Photodegradation Reactions

data f o r samples o f t h i s Tables I and I I .

311

t y p e o f p r o d u c t a r e shown i n

(c) E c o t e n . The a p p r o a c h u s e d b y P r o f e s s o r G e r a l d S c o t t ( U n i v e r s i t y o f A s t o n , B i r m i n g h a m , U.K.) i n v o l v e s t h e u s e o f m u l t i p l e a d d i t i v e s such as f e r r i c d i b u t y l d i t h i o c a r b a m a t e [Fe ( I I I ) DBC] o r f e r r i c stéarate, p o s s i b l y w i t h a c o b a l t , c o p p e r , chromium, manganese o r cerium s a l t . This type o f system which p r o v i d e s s t a b i l i t y d u r i n g t h e p r o c e s s i n g a n d i n i t i a l UV exposure p e r i o d s , has been used i n p o l y o l e f i n s and p o l y s t y r e n e , f o r example. A f t e r an i n d u c t i o n p e r i o d , F e ( l l l ) DBC o x i d i z e s a n d d e c o m p o s e s g i v i n g p r o d u c t s that c a t a l y z e the photodegradation o f the polymer. I t has b e e n n o t e d (3.) t h a t , a l t h o u g h i r o n d i a l k y l d i t h i o c a r b a m a t e s r e p r e s e n t d e l a y e d a c t i o n UV a c t i v a t o r a d d i t i v e s , s u l f u r - c o n t a i n i n g ligands are not e s s e n t i a l . Advantages c l a i m e d i n a d d i t i o n t o t h e i n d u c t i o n p e r i o d are t h e s m a l l q u a n t i t y o f a d d i t i v e r e q u i r e d [0.01 t o 0.05$ F e ( l l l ) D B C ] a n d t h e c o n t i n u i n g o x i d a t i v e degradation i n t h e dark f o l l o w i n g i n i t i a l photoo x i d a t i o n ( d u r i n g which t h e a d d i t i v e i s changed from s t a b i l i z e r t o p r o - d e g r a d a n t ) . I t appears t o be n e c e s s a r y , however, t o s e l e c t an a d d i t i v e l e v e l which i s i n a f a i r l y n a r r o w r a n g e f o r minimum e m b r i t t l e m e n t time. The p e r f o r m a n c e o f a n E c o t e n l o w - d e n s i t y p o l y e t h y l e n e sample i n our Weather-Ometer exposure i s shown i n T a b l e I I . The p r o d u c t s d e s c r i b e d a b o v e w e r e s e l e c t e d i n o r d e r t o i l l u s t r a t e some o f t h e p r i n c i p l e s a s s o c i a t e d w i t h t h e enhancement o f s u n l i g h t s e n s i t i v i t y . There are, i n a d d i t i o n , o t h e r e x a m p l e s (2) o f t h e c o p o l y m e r a p p r o a c h , e.g., e t h y l e n e / c a r b o n monoxide, and t h e a d d i t i v e approach. (d) E x p e r i m e n t a l M o d i f i c a t i o n o f C o n v e n t i o n a l Plastic Films. We h a v e e x a m i n e d n u m e r o u s m o d i f i c a t i o n procedures p o t e n t i a l l y capable o f reducing the embrittlement time o f commercial p l a s t i c f i l m s (mainly the polyolefins). P a r t i a l thermal o x i d a t i o n , corona d i s c h a r g e i n d i f f e r e n t e n v i r o n m e n t s , ozone t r e a t m e n t a n d γ-irradiation w e r e some o f t h e p r o c e d u r e s t h a t p r o v e d t o be i m p r a c t i c a l . T h e m a i n e f f o r t t h e n was t o f i n d a n a d d i t i v e t h a t w o u l d make l o w - d e n s i t y p o l y ­ ethylene a photodegradable p l a s t i c . A fewo f the a d d i t i v e s employed were t i t a n i u m d i o x i d e , benzophenone, ^-nitrobenzophenone, U-hydroxybenzophenone, **-( 3 - h y d r o x y e t h o x y ) b e n z o p h e n o n e , a n t h r a q u i n o n e , anthrone, benzanthrone, xanthone, deoxybenzoin,

312

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

a-bromo-£-phenylacetophenone, ( 2 - b e n z o y l v i n y l ) f e r r o ­ cene, f e r r o c e n e c a r b o x y a l d e h y d e , and 1 , 1 - d i b e n z o y l f e r rocene. The d a t a are summarized i n T a b l e s I I I , IV and V. 1

Experimental Outdoor exposures were conducted i n the summers of 1972-7 * at a U5 angle f a c i n g s o u t h , l a t i t u d e h$ Ν i n Ottawa, Canada. A 6000W xenon a r c A t l a s Weather-Ometer (k) equipped w i t h Corning No. 77^0 b o r o s i l i c a t e i n n e r and o u t e r f i l t e r s was used as a l a b o r a t o r y l i g h t source because i t s e m i s s i o n spectrum i s s i m i l a r t o t h a t o f s u n l i g h t , as i l l u s t r a t e d i n F i g . 1. The WeatherOmeter was o p e r a t e d w i t h the lamp on c o n t i n u o u s l y and the exposure chamber was m a i n t a i n e d at a b l a c k p a n e l temperature o f lU5±5°F and a r e l a t i v e h u m i d i t y o f 1

30±5%.

Polymer e m b r i t t l e m e n t was determined by l80° f o l d and/or p e n c i l puncture t e s t s . E l o n g a t i o n i n the t r a n s v e r s e d i r e c t i o n o f the LDPE samples was measured on an I n s t r o n TT-C t e n s i l e t e s t e r at 70°F and 65% r e l a t i v e humidity. The time t o break was 25-50 s e c . f o r 2 mm s t r i p s u t i l i z i n g a gauge l e n g t h o f 0.5" and jaw-type clamps. The l a b o r a t o r y treatments l i s t e d i n T a b l e s I I I , IV and V were performed by s o a k i n g the f i l m s i n s o l u t i o n s c o n t a i n i n g l.Og o f p o t e n t i a l p h o t o s e n s i t i z e r i n 250 ml o f methylene c h l o r i d e or e t h a n o l f o r 3 days at 22°C. The T 1 O 2 t r e a t m e n t s , however, i n v o l v e d a T i C l i t ( g ) i n N2 treatment f o l l o w e d by a H2O soak f o r conversion to T1O2 . Pis cussion Degradable P o l y s t y r e n e s . The E c o l y t e " F a s t " p o l y s t y r e n e foam has a h i g h e r c o n c e n t r a t i o n o f photo­ s e n s i t i v e v i n y l ketone comonomer than the E c o l y t e "Slow" foam and as a r e s u l t has a s h o r t e r i r r a d i a t i o n time t o e m b r i t t l e m e n t as l i s t e d i n T a b l e I . The " F a s t " foam was b r i t t l e a f t e r one month outdoor ex­ posure or 275 h r xenon a r c i r r a d i a t i o n . The outdoor exposure was f a c i l i t a t e d by wind and r a i n e r o s i o n . T u r n i n g t h e sample over a f t e r 35 hr xenon i r r a d i a t i o n lowered the e m b r i t t l e m e n t time t o a remarkably s h o r t p e r i o d o f 70 hr c o n s i d e r i n g the t h i c k n e s s and o p a c i t y of the foam. S i m i l a r l y the "Slow" foam had i t s xenon e m b r i t t l e m e n t time reduced from 2k00 hr t o o n l y 600 hr by t u r n i n g the foam over a f t e r each 2k h r i r r a d i ­ a t i o n p e r i o d . To o b t a i n a b e t t e r i d e a o f the r a t e of

22. COONEY AND WILES

Sensitizers for Photodegradation Reactions

313

TABLE I I I Embrittlement

times

o f Low-Density Polyethylene

Hours t o Embrittlement

Treatments LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE

Films

, ΙΙΟμ, c o n t r o l + Ti0 (2.0%) , lOOu, c o n t r o l + 1,1 -dibenzoylferrocene (1.3%) + b e n z o p h e n o n e (10%) , 110y, c o n t r o l + ferric dibutyldithiocarbamate + 2-methylanthraquinone + 1,3,5-triacetylbenzene + 9-fluorenone + flavone + Uchromanone + dibenzanthrone + carbazole + benzophenone 2

1

1520 330 1350 288

660 1225 825 920 920 1030 1030 1030 1030 1030 1030

TABLE I V Embrittlement

Times o f H i g h - D e n s i t y

Polyethylene

Treatments HDPE, 2 3 y , c o n t r o l HDPE + T i 0 (2.3%) HDPE + a n t h r a q u i n o n e HDPE + a n t h r o n e HDPE + b e n z a n t h r o n e HDPE + 2 - m e t h y l a n t h r a q u i n o n e HDPE + 1,3 , 5 - t r i a c e t y l b e n z e n e HDPE + a - b r o m o - p _ - p h e n y l a c e t o p h e n o n e HDPE + 9 - f l u o r e n o n e - U - c a r b o x y l i c a c i d HDPE + f l a v o n e 2

Films

Hours t o Embrittlement 925 ^ 385 ^20 ^20 ^50 ^50 ^75 575 650

UV

314

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

p h o t o - o x i d a t i o n , f i l m s were p r e p a r e d from t h e foams. F i l m s c a s t from C C 1 s o l u t i o n s o f t h e " F a s t " and "Slow" foams e m b r i t t l e d i n 23 a n d 55 h r x e n o n i r r a d i a t i o n , r e s p e c t i v e l y , whereas t h e c o n t r o l p o l y s t y r e n e f i l m r e q u i r e d 1*95 h r i r r a d i a t i o n t o p r o d u c e e m b r i t t l e m e n t . The p h o t o s e n s i t i v e v i n y l k e t o n e comonomer c o n c e n t r a t i o n c a n be v a r i e d t o c o n t r o l t h e l i f e t i m e o f t h e D e g r a d ­ able polystyrene t o s u i t the desired use. B i o - D e g r a d a b l e P l a s t i c s I n c . recommend t h a t t h e i r S t y - G r a d e r e s i n c o n c e n t r a t e b e l e t - d o w n 10:1 o r 20:1 f o r u s e i n f o r m u l a t i n g a D e g r a d a b l e p o l y s t y r e n e (2_) . A f i l m produced by employing t h e r e s i n c o n c e n t r a t e l e t - d o w n 5:1 e m b r i t t l e d i n 22 h r w h i l e a c o m m e r c i a l p r o d u c t b e l i e v e d t o b e l e t - d o w n 50:1 (5.) e m b r i t t l e d w i t h a 112 h r x e n o n i r r a d i a t i o n . A film of the resin concentrate embrittled i n 6 hr; therefore, v a r i a t i o n of t h e let-down r a t i o can c o n t r o l and vary t h e l i f e t i m e of t h e p o l y s t y r e n e d u r i n g outdoor u s e . D e g r a d a b l e L o w - D e n s i t y P o l y e t h y l e n e (LDPE) F i l m s . T y p i c a l xenon a r c e m b r i t t l e m e n t times f o r u n s t a b i l i z e d l o w - d e n s i t y p o l y e t h y l e n e a r e l600-2U00 h r s . The c o n ­ t r o l s chosen f o r t h i s r e s e a r c h a r e l i s t e d i n Table I I ; t h e t h i n LDPE g a r m e n t b a g e m b r i t t l e d i n 1820 h r a n d t h e E c o P l a s t i c s LDPE c o n t r o l became b r i t t l e a f t e r 2075 h r xenon i r r a d i a t i o n . The t h i n g a r m e n t b a g was n o t b r i t t l e , a s d e t e r m i n e d b y a f o l d t e s t , a f t e r 18 m o n t h s outdoor exposure. The D u p o n t " S c l a i r " LDPE c o n t r o l l i s t e d i n T a b l e I I was u n u s u a l s i n c e i t h a d s h o r t e m b r i t t l e m e n t t i m e s o f 1175 h r i n t h e W e a t h e r - O m e t e r a n d 6.5 m o n t h s o u t d o o r s . T h i s f i l m was u s e d as a y a r d s t i c k t o m e a s u r e t h e D e g r a d a b l e LDPE f i l m s s i n c e a D e g r a d a b l e P l a s t i c s h o u l d n o t l a s t more t h a n 6 m o n t h s , p e r h a p s l e s s , o u t d o o r s a n d £1200 h r i n t h e xenon a r c Weather-Ometer. E c o t e n LDPE e m b r i t t l e d i n 2210 h r s i n t h e W e a t h e r O m e t e r a n d -11 m o n t h s o u t d o o r s . T h i s sample o f Ecoten does n o t a p p e a r t o be a s a t i s f a c t o r y D e g r a d a b l e P l a s t i c because o f t h e h i g h l e v e l o f T1O2 added t o t h e f i l m . C l e a r Ecoten f i l m w i t h o u t t h e t i t a n i u m d i o x i d e pigment was t e s t e d o u t d o o r s i n I s r a e l a s a g r i c u l t u r a l m u l c h and f o u n d t o e m b r i t t l e i n l e s s t h a n f o u r m o n t h s (6). B i o - D e g r a d a b l e P l a s t i c s I n c . b r o w n LDPE f i l m was b r i t t l e i n 750 h r x e n o n i r r a d i a t i o n a n d f o u r m o n t h s outdoors. T h i s l i f e t i m e c o u l d p r e s u m a b l y be d e c r e a s e d or i n c r e a s e d by c h a n g i n g t h e m a s t e r b a t c h r a t i o . E N D E - p l a s t LDPE w i t h f o r t i f i e d " a d d i t i v e m i x t u r e o n e " ( F A M - l ) i s m a r k e t e d as a D e g r a d a b l e P l a s t i c b y A k e r l u n d a n d R a u s i n g i n Sweden. A c l e a r f i l m o f ENDEp l a s t was b r i t t l e i n 1550 h r x e n o n i r r a d i a t i o n a n d >1*.5 m o n t h s o u t d o o r s a n d a p p e a r s t o h a v e a l o n g e r H

22. COONEY AND WDLES

Sensitizers for Photodegradation Reactions

315

l i f e t i m e t h a n w a r r a n t e d f o r most u s e s o f D e g r a d a b l e Plastics. T h i s l i f e t i m e p r e s u m a b l y c a n be d e c r e a s e d by l o w e r i n g t h e m a s t e r b a t c h l e t - d o w n r a t i o . The E c o l y t e LDPE f i l m s a r e n u m b e r e d i n r e l a t i o n to the c o n c e n t r a t i o n o f p h o t o s e n s i t i v e v i n y l ketone comonomer p r e s e n t i n t h e p o l y m e r i e . 1>>2>3. T h e E c o l y t e #1 LDPE c o n t a i n i n g t h e m o s t v i n y l k e t o n e c o ­ monomer was b r i t t l e i n 820 h r i n t h e W e a t h e r - O m e t e r and 2.5 m o n t h s o u t d o o r s . The E c o l y t e #2 was b r i t t l e i n l l 8 0 h r a n d t h e E c o l y t e #3 r e q u i r e d lU65 h r w h i l e t h e E c o P l a s t i c s L t d . LDPE c o n t r o l r e q u i r e d 2075 h r xenon arc i r r a d i a t i o n t o reach e m b r i t t l e m e n t . This s e r i e s o f E c o l y t e s demonstrates t h e v a r i e d and con­ t r o l l e d l i f e t i m e s t h a t c a n be d e s i g n e d i n t o D e g r a d ­ able P l a s t i c s by c o n t r o l l i n g c o n c e n t r a t i o n o f t h e photosensitizer. The d i s c r e p a n c y b e t w e e n x e n o n a r c a n d o u t d o o r weathering data f o r theBio-Degradable P l a s t i c s Inc. LDPE a n d t h e E c o l y t e #1 LDPE m o s t l i k e l y o c c u r s b e ­ c a u s e t h e x e n o n a r c l a m p was o p e r a t e d w i t h a n UV e n ­ ergy s l i g h t l y below, r a t h e r than s l i g h t l y above, t h a t o f n o o n summer s u n l i g h t a s d e p i c t e d i n F i g . 1. The lower xenon a r c o u t p u t would g r e a t l y reduce t h e energy i n t h e 285-315 nm r e g i o n w h e r e t h e E c o l y t e #1 a b s o r b s r a d i a t i o n b u t w o u l d n o t have as g r e a t an e f f e c t i n t h e 315-3^0 nm r e g i o n w h e r e B i o - D e g r a d a b l e P l a s t i c s I n c . LDPE a p p e a r s t o a b s o r b r a d i a t i o n . In a second experiment t h e f u n c t i o n a l l i f e t i m e o f t h e D e g r a d a b l e LDPE P l a s t i c s was e x a m i n e d . The f u n c ­ t i o n a l l i f e t i m e was d e f i n e d a s t h e x e n o n a r c i r r a d i a ­ t i o n time r e q u i r e d t o reduce t h epercent elongation a t b r e a k b y 9/10. T h e s e v a l u e s l i s t e d i n T a b l e I I a r e d e t e r m i n e d from t h e e l o n g a t i o n as a f u n c t i o n o f i r r a d i a t i o n t i m e c u r v e s i n F i g s . 2 a n d 3. The o r d e r o f i n c r e a s i n g t i m e b y w h i c h f i l m s r e a c h 10$ o f t h e i r o r i g i n a l elongation i squite s i m i l a r t o t h eorder o f embrittlement times. T h u s , t h e f o r m e r c a n be u s e d as a convenient i n d i c a t o r o f r e l a t i v e s u s c e p t i b i l i t y t o b r i t t l e n e s s. A f e w o f t h e p h o t o s e n s i t i v e a d d i t i v e s t h a t we h a v e t r i e d a r e l i s t e d i n T a b l e I I I , I V a n d V. T a b l e I I I h a s t h e h o u r s o f x e n o n a r c i r r a d i a t i o n t o em­ brittlement of treated low-density polyethylene, Table IV contains r e s u l t s f o r t r e a t e d h i g h - d e n s i t y polyethylene while Table V contains r e s u l t s for treated polypropylene films. I t s h o u l d be n o t e d t h a t t h e s e a d d i t i v e s w e r e d e s i g n e d t o i n c r e a s e t h e number o f c h r o m o p h o r e s on a n d n e a r t h e f i l m s u r f a c e s . I n principle the "optimal" concentration of the "right" chromophores s h o u l d enhance t h e a b s o r p t i o n o f l i g h t

U V L I G H T INDUCED REACTIONS

316

IN POLYMERS

TABLE V E m b r i t t l e m e n t Times o f P o l y p r o p y l e n e F i l m s Hours t o Embrittlement

Treatments PP, 27y, c o n t r o l PP + T i 0 (5.9*) PP , acetone e x t r . PP , acetone e x t r . PP, acetone e x t r . PP , acetone e x t r . acetophenone PP , acetone e x t r . PP , acetone e x t r . f e r r o c e n e (0.2%) PP , acetone e x t r . quinone PP, acetone e x t r . benzene PP , acetone e x t r .

2k0

2

+ anthraquinone + anthrone + a-b romo-]D-phenyl+ xanthone + 1,1* d i b e n z o y l -

16 155 50 65 65 75 85

+ 2-methylanthra-