Polymer Stabilization and Degradation - American Chemical Society

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25 Polypropylene Degradation by γ-Irradiation in Air D. J . C A R L S S O N , C. J. B. DOBBIN, J. P. T. JENSEN, and D. M . WILES Division of Chemistry, National Research Council of Canada, Ottawa, Canada K1A 0R9

The γ-irradiation induced, oxidative deterioration of polypropylene film has been investigated by infrared and electron spin resonance spectroscopy and the observed changes correlated with film embrittlement. Both deterioration during γ-irradiation and during storage at 23° or 60°C after irradiation were investigated in the presence and absence of stabilizers i n cluding piperidyl derivitives, a hindered phenol and a thioester. The origin of the post-γ-irradiation deterioration was investigated by the use of a peroxyl radical trap, 2-methyl-2-nitrosopropane and selective destruction of the hydroperoxide oxidation product with reactive gases (SF and S0 ). Storage oxidation was concluded to result primarily from the slow decomposition of hydroperoxide and possibly peroxide oxidation products rather than from trapped macro-radicals as proposed perviously. Ozone may also play a minor role in the oxidative process. 4

2

Polymer degradation upon exposure to Y-or other high energy radiation is important in diverse areas including radiation sterilization (medical equipment and non-wovens for hospital use), the use of plastics in the vicinity of nuclear reactor cores and the controlled modification of polymers for industrial purposes. The radiation degradation of polyolefins in both air and vacuum has already been extensively studied (1-7). Two stages to the degradation are apparent: chemical modification during the actual radiation exposure, then a slow, insidious post-irradiation deterioration during months or years of storage in a i r . Degradation can take the form of discolouration but is more usually shown by a progressive embrittlement. 0097-6156/85/0280-0359S06.00/0 Published 1985, American Chemical Society Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

360

POLYMER STABILIZATION AND DEGRADATION

V a r i o u s t h e o r i e s f o r the p o s t - y - i r r a d i a t i o n d e t e r i o r a t i o n have been p r o p o s e d , the most f r e q u e n t l y c i t e d b e i n g the presence of l o n g lived, m a c r o - r a d i c a l s which r e s u l t from the y - i r r a d i a t i o n ( 2 - 4 ) . These r a d i c a l s are s u g g e s t e d to be t r a p p e d i n the c r y s t a l l i n e d o mains i n p o l y o l e f i n s and to m i g r a t e s l o w l y i n t o the amorphous r e g i o n s where r e a c t i o n w i t h 0 can o c c u r t o t r i g g e r on o x i d a t i o n chain. The e x p e r i m e n t a l evidence f o r t h i s mechanism i s largely d e r i v e d from e l e c t r o n s p i n resonance ( e . s . r . ) s t u d i e s o f the free r a d i c a l s p e c i e s produced by y - i r r a d i a t i o n and t h e i r subsequent r e a c t i o n w i t h oxygen. Although extremely sensitive, e.s.r, can o f c o u r s e o n l y d e t e c t f r e e r a d i c a l s p e c i e s and when used a l o n e may g i v e a b i a s e d view o f the d e g r a d a t i o n p r o c e s s . We have reinvestigated the o x i d a t i v e d e t e r i o r a t i o n of y - i r r a d i a t e d p o l y p r o p y l e n e ( P P ) by the use o f e . s . r . t o g e t h e r w i t h sample c h a r a c t e r i z a t i o n by i n f r a r e d s p e c t r o s c o p y , iodometry and s t r e s s - s t r a i n a n a l y s i s . Our e x p e r i m e n t s have been performed a t dose r a t e s and t o t a l doses i n c l u d i n g t h o s e used i n commercial y - s t e r i l i z a t i o n equipment (~ 0 . 2 Mrad h " , 2 . 5 Mrad t o t a l d o s e ) . In t h i s p a p e r , some p r e l i m i n a r y r e s u l t s on the respective r o l e s of trapped r a d i c a l s , unstable o x i d a t i o n products and ozone i n d e g r a d a t i o n s a r e c o n s i d e r e d .


2

1

Experimental T h i n P P f i l m ( 3 0 urn, e s s e n t i a l l y u n o r i e n t e d , H e r c u l e s P r o f a x r e s i n , 55% c r y s t a l l i n e by d i f f e r e n t i a l scanning c a l o r i m e t r y , DSC) were f r e e d from p r o c e s s i n g a d d i t i v e s by acetone e x t r a c t i o n . F i l m was y - i r r a d i a t e d in a C o A . E . C . L . Gamma c e l l (0.14 or 1 . 3 5 Mrad.h" dose r a t e ) . F i l m s were then a n a l y z e d i m m e d i a t e l y a f t e r i r r a d i a t i o n o r a f t e r a s t o r a g e p e r i o d e i t h e r a t room temperature o r a t 6 0 ° C i n a f o r c e d a i r oven. F o r comparison purposes some e x t r a c t e d PP f i l m was p h o t o - o x i d i z e d i n a xenon a r c WeatherOmeter (Atlas 6 5 0 0 watt, Pyrex i n n e r and o u t e r f i l t e r s ) . A sample of commercial atactic polypropylene (Montecatini r e s i n ) was p u r i f i e d by e x t r a c t i o n w i t h hot t o l u e n e f o l l o w e d by p r e c i p i t a t i o n from the t o l u e n e s o l u t i o n w i t h methanol t o remove i s o t a c t i c PP c o n t a m i n a t i o n . The p u r i f i e d m a t e r i a l showed no d e t e c t a b l e c r y s t a l l i n e m e l t i n g endotherm by DSC (< 0 . 5 % c r y s t a l l i n e c o n t e n t ) . However, IR i n d i c a t e d the p r e s e n c e of some s t e r e o b l o c k material. The a t a c t i c PP was c a s t i n t o t h i n f i l m s (~ 5 0 jjm) on g l a s s from heptane s o l u t i o n , vacuum d r i e d and p e e l e d from the g l a s s w i t h a sharp b l a d e . The e . s . r . s p e c t r a o f small r o l l s o f f i l m (~ 3 0 mg) were r e c o r ded on a V a r i a n E4 s p e c t r o m e t e r . F o r f i l m sample y - i r r a d i a t e d a c t u a l l y i n the e . s . r . t u b e , c o l o u r c e n t r e s i n the g l a s s were c a r e f u l l y removed by a n n e a l i n g t h e tube end b e f o r e s l i d i n g the f i l m i n t o t h i s zone and i n s e r t i o n o f the tube i n t o t h e s p e c t r o m e t e r . Measurements were performed a t a nominal power l e v e l o f 0 . 5 mW and m o d u l a t i o n a m p l i t u d e o f 5 g a u s s . In s e p a r a t e e x p e r i m e n t s s i g n a l i n t e n s i t y d i d not show s a t u r a t i o n problems f o r e i t h e r m a c r o - a l k y l o r p e r o x y l r a d i c a l s a t t h i s power l e v e l on o u r s p e c t r o m e t e r ; d e v i a 6 0

1

1/2 t i o n s from (power) dependence o n l y o c c u r r e d a t above ~ 0 . 8 mW f o r the m a c r o - a l k y l r a d i c a l s , w i t h no s a t u r a t i o n a p p a r e n t a t < 2 0 mW f o r peroxyl r a d i c a l s .

Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25. CARLSSON ET AL.

Polypropylene Degradation by y-Irradiation

361

IR s p e c t r a were r e c o r d e d w i t h a P e r k i n Elmer 1500 F o u r i e r T r a n s f o r m IR s p e c t r o p h o t o m e t e r w i t h a TGS d e t e c t o r . T y p i c a l l y 200 scans were averaged f o r each sample t o improve t h e s i g n a l - t o - n o i s e r a t i o and a l l o w s p e c t r a l s u b t r a c t i o n a t high s e n s i t i v i t i e s . Spect r a l s u b t r a c t i o n was performed u s i n g t h e 973 c n r PP band (which i s l a r g e l y i n s e n s i t i v e t o v a r i a t i o n s i n h e l i c a l c o n t e n t ) as the r e f e rence band t o be s u p p r e s s e d t o z e r o . Iodometric a n a l y s i s o f o x i d i zed f i l m s were performed as d e s c r i b e d p r e v i o u s l y ( 8 ) . Stress-strain measurements were performed on 4 mm f i l m s t r i p s on an I n s t r o n Model 1123 a t 500 % m i n - i .


1

P r i o r t o i r r a d i a t i o n , some e x t r a c t e d f i l m s were immersed i n hexane s o l u t i o n s of s t a b i l i z e r s f o r 15 h . A f t e r removal, r i n s i n g and d r y i n g t h e f i l m , t h e absorbed s t a b i l i z e r l e v e l s were q u a n t i f i e d by IR s p e c t r o s c o p y u s i n g t h e 1738 c m - e s t e r band i n a l l c a s e s . The stabilizers employed included 1,2,2,6,6-pentamethyl-4-piperidyl octadecanoate (StNCH ), 2,2,6,6-tetramethyl-4-pi peri dyl-N-oxyl (StNO-), octadecyl 3 - ( 3 , 5 - d i - t e r t . - b u t y l - 4 - h y d r o x y p h e n y l )-propionate (A0X) and d i - o c t a d e c y l 3 , 3 ' - t h i o d i p r o p i o n a t e (DSTDP). Subsequent t o i r r a d i a t i o n , some f i l m s were s u b j e c t e d t o t r e a t m e n t by o v e r n i g h t immersion i n a s o l u t i o n o f a s p i n t r a p ( 2 - m e t h y l - 2 - n i t r o s o p r o p a n e , 0.024 m o l - L " i n hexane) o r i n an atmosphere o f a r e a c t i v e gas (SF^ or S 0 ) to s e l e c t i v e l y d e r i v i t i z e the polymeric products ( 8 ) . 1

3

1

2

Results Immediately A f t e r y - E x p o s u r e . Exposure o f u n s t a b i l i z e d PP f i l m t o Z-3 Mrad. of y - i r r a d i a t i o n i n a i r r e s u l t s i n o x i d a t i o n which i s r e a d i l y observed i n the IR ( F i g u r e 1) a t ~ 3400 (-0H s p e c i e s ) , ~ 1715 ( c a r b o x y l s p e c i e s ) and - 1200 c m . The o r i g i n o f the l a t t e r a b s o r p t i o n i s n o t w e l l e s t a b l i s h e d , but may be i n d i c a t i v e o f C - 0 - C and C - 0 - 0 l i n k a g e s . From i o d o m e t r i c d e t e r m i n a t i o n s , t h e h y d r o p e r o x i d e (-00H) groups dominated a t t h e lower dose r a t e , whereas a l c o hol -OH was much more i m p o r t a n t a t t h e h i g h e r dose r a t e . D i s t i n c t i n c r e a s e s i n bonds s e n s i t i v e t o t h e h e l i c a l conformer o f the PP backbone a r e a l s o v i s i b l e ( f o r example a t 997 and 841 cm ). T h e s e changes have p r e v i o u s l y been o b s e r v e d i n p h o t o o x i d i z e d samples and c o r r e l a t e w i t h the b a c k b o n e - s c i s s i o n - i n d u c e d r e s t r u c t u r i n g o f the polymer. y - I r r a d i a t i o n o f PP f i l m a l s o caused a sharp drop i n t h e e l o n g a t i o n a t break of the f i l m s ( T a b l e 1 ) . T e n s i l e s t r e n g t h was l i t t l e e f f e c t e d . F o r t h e same t o t a l d o s e , both the o x i d a t i o n p r o d u c t q u a n t i t i e s and r e l a t i v e p r o p o r t i o n s as well as the e l o n g a t i o n a t break were markedly dependent on dose r a t e (Table 1 ) . In the presence of the t e r t i a r y h i n d e r e d amine ( S t N C H ) , the h i n d e r e d N - o x y l (StNO*) and t h e phenol ( A 0 X ) , the e x t e n t o f o x i d a t i o n and e m b r i t t l e m e n t were s h a r p l y r e d u c e d , w i t h A0X b e i n g most e f f e c t i v e ( T a b l e 1).. The t h i o e s t e r (DSTDP) had l i t t l e e f f e c t on t h e 3400 c m " , -OH a b s o r p t i o n b u t appeared t o a c c e l e r a t e e m b r i t t l e m e n t . F u r t h e r study o f t h i s a d d i t i v e was abandoned. The y - i r r a d i a t i o n o f an u n s t a b i l i z e d PP sample wrapped i n a t h i n , c a r e f u l l y e x t r a c t e d f i l m o f n a t u r a l r u b b e r showed a lower o v e r a l l o x i d a t i o n l e v e l by IR than a PP f i l m i r r a d i a t e d a l o n e ( T a b l e 1 ) . However, when w o r k i n g a t the h i g h e r dose r a t e (and so much s h o r t e r i r r a d i a t i o n t i m e s ) , this e f f e c t was b a r e l y d e t e c t a b l e . - 1

- 1

3

1


polymer stabilization a n d degradation

362


0.06-

-0.021 4000

.

I (i i 3000 1500 WAVENUMBER (cm )

—i 1000

-1

Figure 1 D i f f e r e n c e IR S p e c t r a o f PP F i l m s F i l m s a l l y - i r r a d i a t e d t o Z . 8 M r a d . a t 0.14 M r a d . h " ; times t o the p e r i o d o f s t o r a g e a t 23°C f o l l o w i n g t h e i r r a d i a t i o n . 1

TABLE 1. Additive

Immediate E f f e c t s Exposure

o f y - a n d UV r a d i a t i o n

3400 c m " 2 Mrad. 2 Mrad. as above 44 h Xe 2 Mrad. 2 Mrad. 2 Mrad. 2 Mrad.

— — —

StNCrL StNO? AOX DSTDP

d

e

e

e

fast slow + NR UV fast fast fast fast

on PP F i l m E o

IR Absorbance

3

C

0.015 0.025 0.018 0.015 0.005 0.004 0.003 0.015

1

refer

1715 c m "

b

L

1

0.007 0.010 0.012 0.006 0.006 0.010 0.006 N.D.

E

irr

1.5 5. N.D. 1.5 1.1 1.1 1.1 6.

a) " S l o w " r e f e r s t o 0.14 M r a d - h - i and " f a s t " t o 1.35 M r a d - h - i . b) E / E = i n i t i a l elongation at break/elongation after i r r a d i a tion. E = 1000%. c) PP f i l m i n i n t i m a t e c o n t a c t w i t h a n a t u r a l rubber f i l m . d) 9 x 10*3 m o l . k g - i . e) 2 x I O " mol-kg" . Q

1

r r

Q

2

1


25.

CARLSSON ET AL.


363

The e . s . r . spectrum of the - i r r a d i a t e d f i l m showed the p r e sence o f peroxy r a d i c a l s , as i n d i c a t e d by t h e w e l l known a s y m m e t r i c singlet i n Figure 2 (3,5). M a c r o - a l k y l r a d i c a l s were not d e t e c t e d i n t h i n f i l m s ; the complex m u l t i p l e t r e s u l t i n g from PP m a c r o - a l k y l r a d i c a l s formed by y - i r r a d i a t i o n under vacuum i s o v e r l a y e d i n F i g u r e 2 f o r comparison p u r p o s e s . F o r t h i c k e r f i l m samples [> 300 urn) i r r a d i a t e d a t t h e h i g h e r dose r a t e ( 1 . 3 5 Mrad h " ) i n a i r , a weak macro-alkyl s i g n a l was v i s i b l e under the dominant p e r o x y l signal immediately a f t e r i r r a d i a t i o n , but c o n v e r t e d q u i t e q u i c k l y ( u n d e t e c t a b l e a f t e r - 10 m i n u t e s a t 6 0 ° C o r - 2 hours a t 2 3 ° C ) t o p e r o x y l radicals. P h o t o - o x i d a t i o n o f t h e a d d i t i v e - f r e e PP f i l m gave s i m i l a r o x i dation product y i e l d s and e m b r i t t l e m e n t as exposure t o the 1.35 Mrad.h-i y - c e l l (Table 1 ) . Y


1

Post-y-Degradation. During storage of a p r e - i r r a d i a t e d f i l m , o x i dation continues steadily as can be seen from the IR changes i n F i g u r e 1. The p e r o x y l r a d i c a l p o p u l a t i o n decays monotonously to zero (Figure 2 ) . The decay o f p e r o x y l r a d i c a l s under a v a r i e t y o f s t o r a g e c o n d i t i o n s i s shown i n F i g u r e s 3 and 4. R a d i c a l s decayed somewhat f a s t e r i n a t a c t i c PP as compared to the i s o t a t i c film. F i l m immersion i n hexane caused the p e r o x y l s i g n a l t o decay quite q u i c k l y b u t immersion i n a hexane s o l u t i o n o f the r a d i c a l - s c a v e n g i n g n i t r o s o compound c a u s e d t h e f a s t e s t d e c a y a t 2 3 ° C . The r a t e o f peroxyl r a d i c a l decay showed a pronounced temperature dependence, b e i n g e x t r e m e l y r a p i d a t 6 0 ° C y e t e s s e n t i a l l y z e r o a t - 5 ° C (not shown). Some p e r o x y l r a d i c a l decay data a r e combined w i t h d a t a f o r the a c c u m u l a t i o n o f -OH s p e c i e s i n F i g u r e 4 f o r 23°C s t o r a g e . Changes i n t h e c a r b o n y l r e g i o n absorbance and i n e l o n g a t i o n a t break f o r the same samples s t o r e d a t 60°C a r e c o l l e c t e d i n F i g u r e s 5 and 6 f o r a d d i t i v e f r e e and a d d i t i v e c o n t a i n i n g samples. That the p r o t e c t i v e e f f e c t o f AOX, StNO* and StNCH continues i n the p o s t - y p e r i o d i s clearly visible, w i t h AOX the most e f f e c t i v e but a l s o seriously discoloured (yellow). These s t u d i e s were made a t 60°C t o reduce t h e p r o t r a c t e d l i f e t i m e a t room t e m p e r a t u r e f o r t h e s t a b i l i z e d s a m p l e s . As compared t o 2 3 ° C , t h e 60°C a g i n g c a u s e d about a x 10 a c c e l e r a t i o n i n t h e r a t e o f o x i d a t i v e d e t e r i o r a t i o n o f the u n s t a b i l i z e d f i l m as measured both by changes i n t h e IR and e l o n g a t i o n a t b r e a k . IR data f o r the -OH a b s o r p t i o n s a t 3400 c n r f e l l i n t o two d i s t i n c t p a t terns. In t h e a d d i t i v e - f r e e samples and w i t h DSTDP, the 3400 c m " a b s o r p t i o n i n c r e a s e d r a p i d l y and was always n u m e r i c a l l y about t w i c e the 1715 c n r a b s o r p t i o n . F o r t h e S t N C H , StNO- and AOX c o n t a i n i n g samples, t h e low i n i t i a l 3400 c m " absorption (Table 1) either s t a y e d r o u g h l y c o n s t a n t , o r even d e c r e a s e d s l i g h t l y up t o 1000 hours at 60°C. I t i s a l s o worth m e n t i o n i n g t h a t the c a r b o n y l y i e l d ( F i g u r e 5) appears to bear l i t t l e r e l a t i o n s h i p t o t h e e l o n g a t i o n a t break f o r the s t a b i l i z e d films (Figure 6 ) . A t 60°C t h e p r e - p h o t o o x i d i z e d f i l m was a l s o e x t r e m e l y u n s t a b l e as shown by t h e growth o f carbonyl species (at 1715 c m " ) and the drop i n e l o n g a t i o n a t break ( F i g u r e s 5 and 6 ) . 3

1

1

1

3

1

1

The o x i d a t i o n o f a s e r i e s o f a d d i t i v e f r e e samples i s shown i n F i g u r e 7. The p o s t - U V - i r r a d i a t e d and p o s t - y - i r r a d i a t e d , b u t n i t r o s o treated, films show very similar oxidation rates. However, y -


POLYMER STABILIZATION AND DEGRADATION


364

50 Figure 2 Films a l l Film A l l other stored in

GQUSS

E . S . R . S p e c t r a o f PP F i l m s y - i r r a d i a t e d t o 2.0 M r a d . a t 1.35 M r a d - h " . i r r a d i a t e d under vacuum. c u r v e s r e f e r t o f i l m i r r a d i a t e d i n a i r a t - 35°C a i r f o r the shown times a t 6 0 ° C . 1

200

HOURS

400 DECAY

then

80C

Figure 3 P e r o x y l R a d i c a l Decay i n PP F i l m A l l f i l m s y - i r r a d i a t e d i n a i r a t 0.14 M r a d - h " t o 2.8 Mrad. d o s e . S p e c t r a measured a t p e a k - t o - p e a k maximum f o r 0.030 g s a m p l e s . I s o t a c t i c PP f i l m , s p e c t r o m e t e r g a i n 2 x 1 0 : • F i l m stored at 23°C i n a i r . • F i l m s t o r e d a t 60°C i n a i r . A F i l m immersed i n hexane immediately a f t e r i r r a d i a t i o n . • F i l m immersed i n 2-methyl-2-nitrosopropane/hexane s o l u t i o n immediately a f t e r i r r a d i a t i o n . A t a c t i c PP f i l m , s p e c t r o m e t e r g a i n 5 x I O : O Film stored in a i r at 60°C. 1

2

3



25. CARLSSON ET AL.


365

Figure 4

Comparison o f P e r o x y l Decay and O x i d a t i o n Growth After y-Irradiation F i l m i r r a d i a t e d i n a i r a t 0.14 M r a d - h " to 2.8 M r a d . t o t a l d o s e . 1

Figure 5 Film Oxidation After y - I r r a d i a t i o n F i l m s y - i r r a d i a t e d i n a i r t o 2.0 M r a d . dose a t 1.35 M r a d - h " , then aged i n a i r a t 6 0 ° C . • No a d d i t i v e . A AOX (2 x 10"2 mol-kg-i). • StNCH (9 x 1 0 - 3 m o L k g - i ) , • StNO- (2 x 10-2 mol-kg-i). F i l m p r e - p h o t o - o x i d i z e d i n a i r (Xe a r c , 44 h ) , then aged a t 60°C:- O 1

3


366

P O L Y M E R STABILIZATION


T

1

1

1

Figure 6 Film Embrittlement A f t e r Samples and a e t a n s as i n F i g u r e 5.

A N D

DEGRADATION

r

y-Irradiation

800

HOURS AT

23°C

Figure 7 F i l m O x i d a t i o n A f t e r I r r a d i a t i o n Exposures y -1 r r a d i a t e d H l m s M r a d . , a t 0.14 M r a d - h - M : — C T R a d i c a l s d e s t r o y e d by a b r i e f immersion o f the f i l m i n 2 - m e t h y l 2 - n i t r o s o p r o p a n e s o l u t i o n . A H y d r o p e r o x i d e s d e s t r o y e d by a b r i e f (15 h) exposure o f the f i l m t o gaseous S 0 . P h o t o - o x i d i z e d f i l m (95 h, Xe a r c exposure i n a i r ) : # 2


25. CARLSSON ET AL.


i r r a d i a t e d f i l m which had been exposed t o e i t h e r gaseous S 0 (not shown) immediately a f t e r i r r a d i a t i o n showed n e g l i g i b l e oxidation.

367

2

or SF further 4

Discussion Polymer (RH) o x i d a t i o n , i r r e s p e c t i v e of the p r e c i s e i n i t i a t i o n p r o c e s s , can be e x p r e s s e d l a r g e l y i n terms of the c l a s s i c a l , thermal o x i d a t i o n scheme ( r e a c t i o n s 1-3) (9). In the case of PP, t e r t . R. + 0 R 0 - + RH 2 R0 -

R0 ROOH + R. some n o n - r a d i c a l

2


(1) (2) (3)

2

9

o

products

peroxyl radicals are dominant and their termination is quite complete (reacton 4) {]). Only p e r o x i d e formation is truely a t e r m i n a t i o n s t e p , whereas 3 - s c i s s i o n t o form a ketone i s e x p e c t e d t o CH 1

2 ~CH -C~ 9

2

1

CH

q

CH

0

C

3

+CH -C~ + 0 9

2

1

+ ~CH -C

9

9

2

2

1

CH -C-0-0-C-CH, I l CH CH Q

0

0

PP CH

C H

scission

0

^CH -C~ + 2

/

~CH -C~

~CH

2

CH^ 3

c

+

* *

CH

9

0

OH

CH, I C ~ I H (4)

be an i m p o r t a n t backbone s c i s s i o n p r o c e s s . In a y - i n i t i a t e d , t h e r mal o x i d a t i o n , f r e e r a d i c a l s w i l l be g e n e r a t e d t h r o u g h o u t the sample c r o s s - s e c t i o n a f t e r a c o m p l e x , r a p i d cascade of r e a c t i o n s i n v o l v i n g i o n s and e x c i t e d s t a t e s ( r e a c t i o n 5 ) . Even a t the h i g h e r degrees of o x i d a t i o n (~ 0.2 m o l ^ k g ) y - i n t e r a c t i o n w i t h o x i d a t i o n p r o d u c t s i s much l e s s l i k e l y than C-H bond s c i s s i o n based on the sheer c o n c e n - 1

RH

—wv*R. ^ R . 1

+ H« + R.

(5a) (5b)

t r a t i o n of C-H s i t e s . We have f o l l o w e d the i n c r e a s e of -OH and )C=0 s p e c i e s by IR f o r f i l m s i r r a d i a t e d p r o g r e s s i v e l y up to 5 Mrad t o t a l dose. The b u i l d u p o f both p r o d u c t s was a c c u r a t e l y l i n e a r w i t h dose up t o t h i s p o i n t , c o n s i s t e n t w i t h the complete i n s e n s i t i v i t y of the o x i d a t i o n p r o d u c t s t o the y - i r r a d i a t i o n p r o c e s s . Thus i n c o n t r a s t t o p h o t o - i n i t i a t e d o r t h e r m a l l y i n i t i a t e d o x i d a t i o n i n which h y d r o p e r o x i d e s c i s s i o n must o c c u r to generate the first r a d i c a l s , a n d , through the i n t e r m e d i a c y of the m a c r o - a l k o x y l radic a l s , some c h a i n s c i s s i o n , c h a i n s c i s s i o n d u r i n g y - i r r a d i a t i o n o n l y o c c u r s as the r e s u l t of the i r r a d i a t i o n i t s e l f ( r e a c t i o n 5b) and the termination process (reaction 4). The c o m p l e x i t y of the c a r b o n y l envelope ( F i g u r e 1) indicates t h a t r e a c t i o n s o t h e r than 3 - s c i s s i o n of t e r t . - a l k o x y l r a d i c a l s c o n t r i b u t e to t h i s a b s o r p t i o n . O t h e r sources i n c l u d e t e r m i n a t i o n of


P O L Y M E R STABILIZATION A N D

368

DEGRADATION

primary o r secondary p e r o x y l r a d i c a l s and f u r t h e r o x i d a t i o n of u n s t a b l e p r o d u c t s such as a l d e h y d e s . The s m a l l e r e f f e c t on e l o n g a t i o n a t break o f the 2.5 Mrad. dose a t the f a s t e r dose r a t e ( T a b l e 1) i s not e x p e c t e d from r e a c t i o n 1 to 5. The h i g h e r dose r a t e s h o u l d g e n e r a t e more r a d i c a l s , l e a d to a h i g h e r r a t e of peroxyl selft e r m i n a t i o n and so generate more c h a i n s c i s s i o n and s c i s s i o n p r o ducts (carbonyl s p e c i e s ) but l e s s o x i d a t i o n p r o d u c t ( - 0 0 H ) . The d i s c r e p a n c i e s w i t h t h i s scheme may r e s u l t from o t h e r t e r m i n a t i o n r e a c t i o n s (such as R* + R or R* + R0 * e t c . ) becoming i m p o r t a n t a t t h e h i g h e r dose r a t e as a r e s u l t of some 0 d e p l e t i o n i n more h i g h l y o x i d i z e d domains. 2


2

The s t a b i l i z a t i o n e f f e c t d u r i n g i r r a d i a t i o n of the p i p e r i d y l compounds and the phenol most l i k e l y r e s u l t from the s c a v e n g i n g o f p e r o x y l r a d i c a l s but the phenol i s c o n v e r t e d t o y e l l o w quinone p r o ducts (10). P i p e r i d y l compounds are much l e s s e f f i c i e n t scavengers o f p e r o x y l r a d i c a l s , but can a l s o compete w i t h oxygen f o r the macroa l k y l s ( r e a c t i o n 1) ( 1 1 ) . None o f the s t a b i l i z e r systems c o m p l e t e l y s u p p r e s s e d the y - i n i t i a t e d o x i d a t i o n s , which i s not s u r p r i s i n g i n view of the h i g h number of randomly d i s p e r s e d i n i t i a t i o n p r o c e s s e s from y - i r r a d i a t i o n . The a c c e l e r a t i o n o f e m b r i t t l e m e n t by the t h i o e s t e r (DSTDP), which i s n o r m a l l y b e l i e v e d t o s t a b i l i z e v i a a h y d r o p e r o x i d e d e c o m p o s i t i o n mechanism r a t h e r than r a d i c a l s c a v e n g i n g , i s unexpected but may r e s u l t from the r a p i d o x i d a t i o n of the thio a d d i t i v e to unstable i n t e r m e d i a t e s . Horng and Klemchuk have a l s o r e p o r t e d t h a t DSTDP i s i n e f f e c t i v e a g a i n s t y - d e t e r i o r a t i o n ( 6 h The y - i r r a d i a t i o n of samples i n a i r o c c u r s i n the presence of some ozone formed by the r a d i a l y s i s of the a i r i t s e l f . (The smell o f ozone i s always d e t e c t a b l e when the Gamma c e l l i s o p e n e d . ) Ozone can a t t a c k s a t u r a t e d p o l y o l e f i n s to g e n e r a t e r a d i c a l s and i n i t i a t e o x i d a t i v e chains (12). The small but r e p r o d u c i b l e r e t a r d a t i o n of t h e o x i d a t i o n o f the PP f i l m s i n t i m a t e l y wrapped i n a n a t u r a l r u b b e r f i l m i s then c o n s i s t e n t w i t h an 0 component to the o x i d a t i o n , the u n s a t u r a t e d rubber s h i e l d i n g the PP f i l m to some e x t e n t from 0 a t t a c k by means of the r a p i d C L - u n s a t u r a t i o n r e a c t i o n . As w e l l as f r e e r a d i c a l g e n e r a t i o n , the 0 - P P r e a c t i o n may a l s o l e a d to u n s t a b l e o z o n i d e and p e r o x i d e i n t e r m e d i a t e s . The pos t - y o x i d a t i o n i s markedly r e t a r d e d by the t h r e e a d d i t i v e s ( F i g u r e s 5 and 6 ) . The p h e n o l i c a d d i t i v e i s o u t s t a n d i n g but i t s performance i s marred by the y e l l o w i n g o f the f i l m as the phenol i s c o n v e r t e d to quinone p r o d u c t s . Horng and Klemchuk have found t h a t p h e n o l i c , p h o s p h i t e and a secondary h i n d e r e d amine were e s s e n t i a l l y unchanged by a 2.5 M r a d . y - d o s e , and so a v a i l a b l e t o suppress the p o s t - y d e t e r i o r a t i o n ( 6 h The p r o t r a c t e d thermal o x i d a t i o n which f o l l o w s the end of y - i r r a d i a t i o n c o u l d stem from two s o u r c e s : 1) M a c r o - a l k y l r a d i c a l s t r a p p e d i n the c r y s t a l l i n e domains. 2) Decomposition of unstable o x i d a t i o n p r o d u c t s . The former mechanism has f r e q u e n t l y been o b s e r v e d , but u s u a l l y o n l y f o r samples exposed t o e l e c t r o n beam o r y - i r r a d i a t i o n a t 77°K under vacuum f o l l o w e d by a n n e a l i n g a t ambient, then 0 admission (2-4). Under our experimenal c o n d i t i o n s our data appear t o l a r g e l y r e f u t e the o c c u r r e n c e o f p r o c e s s 1 . The d e t e c t i o n s o l e l y o f p e r o x y l r a d i c a l s i n t h i n f i l m s and the r a p i d post-y c o n v e r s i o n o f the macro3

3

3

2


25.

CARLSSON ET AL.


369

a l k y l r a d i c a l s i m p l i e s t h a t a l l of the d e t e c t e d r a d i c a l s a r e i n 0 a c c e s s i b l e (amorphous o r d e f e c t i v e c r y s t a l l i n e ) r e g i o n s . A macroa l k y l r a d i c a l / h y d r o g e n atom p a i r o r m a c r o - a l k y l p a i r g e n e r a t e d i n the c r y s t a l l i n e domains ( r e a c t i o n 5) p r o b a b l y recombines q u i c k l y i n the r i g i d domain. A l t e r n a t i v e l y , H» may hydrogen a b s t r a c t c l o s e t o the geminate m a c r o - a l k y l r a d i c a l and r a p i d c o m b i n a t i o n of the two m a c r o - a l k y l s then takes p l a c e . F u r t h e r m o r e , the p e r o x y l r a d i c a l s i n the y - i r r a d i a t e d a t a c t i c sample ( c o m p l e t e l y n o n - c r y s t a l l i n e ) ( F i g u r e 3) decayed i n a s i m i l a r f a s h i o n to those i n the s e m i - c r y s t a l l i n e isotatic film. In both the a t a c t i c and i s o t a c t i c samples, PP0 « decay appears to f i t a good second o r d e r r e l a t i o n s h i p as e x p e c t e d from r e a c t i o n 4. However, the r e l a t i v e second o r d e r r a t e c o n s t a n t f o r p e r o x y l decay i s about a f a c t o r of 6 f a s t e r i n the a t a c t i c p o l y mer as compared t o the i s o t a c t i c f i l m . F u r t h e r m o r e , from an ongoing study of the e f f e c t of f i l m morphology on PP0 * decay we do f i n d a slower decay as m o r p h o l o g i c a l p e r f e c t i o n i n c r e a s e s . T h i s may r e s u l t from a p r o g r e s s i v e i n c r e a s e i n r e s t r i c t i o n of peroxyl m o b i l i t y w i t h i n the amorphous domains. F i n a l l y , the f a c t t h a t a low m o l e c u l a r weight a l k a n e a n d , even more d r a m a t i c a l l y , a s o l u t i o n of a r a d i c a l t r a p can a c c e l e r a t e the PP0 * decay i m p l i e s t h a t the r a d i c a l s i t e s are i n the amorphous domains. The e f f e c t of hexane appears to be analogous to the e f f e c t of a " m o b i l i z e r " (an a l k a n e o i l ) reported p r e v i o u s l y (2). From the e . s . r . o f f i l m s t r e a t e d w i t h the n i t r o s o s o l u t i o n s , t h e r e was an i n d i c a t i o n t h a t a new r a d i c a l s p e c i e s was formed, but was always weak i n comparison w i t h the r e s i d u a l p e r o x y l signal. N i t r o s o compounds are known to scavenge peroxyl r a d i c a l s , but the peroxy N - o x y l p r o d u c t i s not s t a b l e a t ambient temperature and decays through a s e r i e s of u n s t a b l e i n t e r m e d i a t e s ( r e a c t i o n 6) 2


2

2

2

(14). R0 * 2

+4-NO

•4-N-0-0-R—^—• 0.

non r a d i c a l

products (6)

Of the p r o d u c t s from y - i n i t i a t e d thermal o x i d a t i o n , p e r o x i d e s , h y d r o p e r o x i d e s and o z o n i d e s are the prime c a n d i d a t e s to decompose q u i c k l y enough a t room temperature to i n i t i a t e the p o s t - y o x i d a t i o n . From F i g u r e 4, the r a t e of post-y o x i d a t i o n slows and becomes a p p r o x i m a t e l y c o n s t a n t a f t e r 5-600 h a t 2 3 ° C , which c o r r e s p o n d s t o the p o i n t a t which the peroxyl s i g n a l has e x t e n s i v e l y d e c a y e d . It i s t e m p t i n g t o a s s o c i a t e the f a s t e r , p o s t - y o x i d a t i o n stage to a c o m b i n a t i o n of the r e s i d u a l p e r o x y l r a d i c a l e f f e c t and o x i d a t i o n p r o d u c t e f f e c t but the subsequent, steady o x i d a t i o n r e g i o n s o l e l y t o o x i d a t i o n product decomposition. However, from F i g u r e 7, p o s t - y o x i d a t i o n i s stopped c o m p l e t e l y by S0 ( o r SF^) t r e a t m e n t of the f i l m . Both treatments d e s t r o y -OOH groups (8) y e t were found t o cause n e g l i g i b l e change i n the p e r o x y l s i g n a l l e v e l . This result c l e a r l y p o i n t s to the r e s i d u a l p e r o x y l l e v e l c o n t r i b u t i n g l i t t l e to the post-y o x i d a t i o n . When the p e r o x y l p o p u l a t i o n was q u i c k l y d e s t r o y e d by 2 - m e t h y l 2-nitrosopropane treatment, post-y o x i d a t i o n s t i l l occurred ( F i g u r e 7). In a d d i t i o n , f i l m which had been p r e - o x i d i z e d by x e n o n - a r c p h o t o - i n i t i a t e d o x i d a t i o n showed an i d e n t i c a l , s l o w , thermal o x i d a t i o n d u r i n g s t o r a g e , as w e l l as a r a p i d e m b r i t t l e m e n t i n the a c c e l e r a t e d aging at 60°C (Figure 6). The p h o t o - o x i d a t i o n o f PP i s w e l l 2


P O L Y M E R STABILIZATION A N D D E G R A D A T I O N

370


documented to produce hydroperoxide and lesser amounts of peroxides, but only low concentrations of peroxyl radicals. The thermal instability of peroxidic groups in several polymers has, in fact, been invoked previously as a source of oxidation initiation at moderate temperatures. Clough and Gillen suggested that thermal decomposition of -OOH sites promoted the degradation of polyethylene and polyvinyl chloride) during exposure to low dose rates of y-irradiation in a nuclear power station (15). Citovicky et al and Kishore have also invoked peroxidation of^PP by ozone or y-exposure as a source of initiation of thermal degradation (16-17). Conclusions The post-y oxidation appears to result predominantly from the slow decomposition of hydroperoxides and/or some other unstable peroxidic product. The more rapid, early rate of oxidation (Figure 4) might include a component from an extremely unstable oxidation product (ozonide, peroxide, etc.) not present after photo-oxidation and destroyed by the nitroso treatement. Work is continuing on the characterization of the products from y-initiated oxidation, but is made difficult by the problems of quantifying the peroxide species. Degradation both during y-irradiation and during post-y storage can be prevented by phenolic and piperidyl stabilizers. The latter were less effective than the phenol, but did not cause discolouration. Other hindered amines with a higher radical scavenging efficiency would be extremely valuable. Acknowledgment Issued as NRCC 23466. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Geymer, D.O., "The Radiation Chemistry of Macromolecules", Dole, M. E d . , Academic Press, New York, 1973, vol. 2, p.4. Dole, M., Adv. Rad. Chem. 1974, 4, 307. Dunn, T.S. and Williams, J . L . , J . Ind. Irrad. Technol. 1983, 1, 33. Bohm, G.G.A., J . Polym. S c i . 1967 A2, 5, 639. Tsuji, K., Adv. Polym. S c i . 1973, 12, 131. Horng, P. and Klemchuk, P, Plast. Eng. 1984, (April), 35. Decker, C. and Mayo, F.R., J . Polym. S c i . , Polym. Chem. Ed. 1973 11, 2847. Carlsson, D.J. and Wiles, D.M., Macromolecules 1969, 2, 597. Garton, A . , Carlsson, D.J. and Wiles, D.M., Dev. Polym. Photochem., 1980, 1, 93. Carlsson, D.J. and Wiles, D.M., J . Macromol. S c i . , Rev. Macromol. Chem., 1976. C14, 155. Wiles, D.M., Tovborg Jensen, J . P . and Carlsson, D . J . , Pure Appl. Chem., 1983, 55, 1651. Krisyuk, B . E . , Popov, A.A. Griva, A.P. and Denisov, E . T . , Dokladi Akad. Nank S.S.S.R. 1983, 269, 400. Vieth, W. and Wuerth, W.F., J . Appl. Polym. S c i . , 1969, 13, 685. Chatgilialoglu, C . , Howard, J . A . and Ingold, K.U., J . Amer. Chem. Soc., 1982, 47, 4361. Clough, R.L. and Gillen, K.T., J . Polym. S c i . , Polym. Chem. E d . , 1981, 19, 2041. Klemchuk; Polymer Stabilization and Degradation ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

25. CARLSSON ET AL.

Polypropylene Degradation by y-irradiation

16. Citovicky, P., Mikulasova, D. and Chrastova, V., Euro. Polym. J . 1976, 12, 627. 17. Kishore, K, J . Macromol. S c i . , Chem., 1983, A19, 937.


RECEIVED October 26, 1984


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Polymer Stabilization and Degradation - American Chemical Society

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