Oxidative Degradation of Polymers in Presence of Ozone

Oxidative Degradation of Polymers in Presence of Ozone HAROLD C. BEACHELL and SPERO P. NEMPHOS

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0021.ch026

Chemistry Department,

University of Delaware,

Newark, Del.

The action of ozone-oxygen mixtures (2% ozone) on polyethylene in the temperature range from 2 5 ° to 1 0 9 ° C. a n d on polystyrene in the range from 5 5 ° to 1 5 4 ° C. has been studied. The products a n d the rates of the reaction were followed by the changes in the infrared spectra of the polymers during ozonization. The main products a p pear to be aldehydes a n d ketones in the case of polyethylene, while an ozonide or peroxidic complex, stable at intermediate temperatures, forms during polystyrene ozonization. The activation energy is of the order of 9 kcal. for polyethylene; for polystyrene it is considerably less. Rates of ozonization a n d oxidation of deuterated polystyrenes indicate that tertiary carbons are the seat of oxidation reaction.

T h e a c t i o n of ozone as a v i g o r o u s o x i d i z i n g agent of elastomers a n d o t h e r p o l y m e r s is w e l l k n o w n (1). T h i s r e p o r t p r e s e n t s t h e results of a s t u d y i n w h i c h t h e a c t i o n of ozone o n s u c h p o l y m e r s as p o l y e t h y l e n e a n d p o l y s t y r e n e w a s e x a m i n e d . Infrared s p e c t r a were u s e d as a t o o l f o r p r o d u c t a n a l y s i s a n d r a t e m e a s u r e m e n t s . Poly e t h y l e n e s h o w e d a m u c h h i g h e r t e n d e n c y t o ozone o x i d a t i o n t h a n p o l y s t y r e n e . T h e t e m p e r a t u r e range s t u d i e d v a r i e d f r o m 25° t o 109° C , a n d t h e o x i d a t i o n w a s m e a s u r e d i n t e r m s of m i n u t e s . I n t h e case of p o l y s t y r e n e t h e t e m p e r a t u r e range f r o m 55° t o 154° C . p r o d u c e d e q u i v a l e n t results b u t o n l y w h e n t h e o z o n i z a t i o n w a s m e a s u r e d i n t e r m s of h o u r s . S t y r e n e p o l y m e r s w i t h d e u t e r i u m a t o m s s u b s t i t u t i n g h y d r o g e n s o n t h e c h a i n s were s y n t h e s i z e d , a n d t h e i r o z o n i z a t i o n rates w e r e s t u d i e d i n a n a t t e m p t t o e l u c i d a t e t h e m e c h a n i s m of ozone a t t a c k o n t h e p o l y m e r . Experimental A p p a r a t u s . A W e l s b a c h o z o n a t o r , M o d e l T - 2 5 , p r o d u c e d a n o x i d i z i n g m i x t u r e of ozone (2%) a n d o x y g e n ( 9 8 % ) . T h e r a t e of flow w a s k e p t c o n s t a n t a t 0.025 c u b i c f o o t p e r m i n u t e . T h e p o l y m e r films, effective t h i c k n e s s 0.002 t o 0.006 i n c h , were u s e d free o r d i r e c t l y o n t h e r o c k salt p l a t e s o n w h i c h t h e y were cast. T h e r e a c t i o n vessel c o n t a i n i n g t h e f i l m s w a s k e p t i n a c o n s t a n t t e m p e r a t u r e b a t h . T h e ozone m i x t u r e w a s p r e h e a t e d t o a p p r o x i m a t e l y t h e t e m p e r a t u r e of t h e b a t h b y p a s s i n g t h r o u g h a s p i r a l t u b e i m m e r s e d i n t h e same b a t h p r i o r t o e n t e r i n g t h e r e a c t i o n vessel. P o l y e t h y l e n e . T h e p o l y e t h y l e n e w a s A l a t h o n N o . 1, w i t h a m o l e c u l a r w e i g h t b e t w e e n 18,000 a n d 19,000 ( D u P o n t ) . I t w a s d i s s o l v e d i n h o t c a r b o n t e t r a c h l o r i d e , a n d 168

OZONE CHEMISTRY AND TECHNOLOGY Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BEACHELL A N D NEMPHOS-OXIDATIVE

169

DEGRADATION OF POLYMERS

t h e f i l m s were cast e i t h e r o n r o c k s a l t p l a t e s w h i c h were used d i r e c t l y o r o n t h e s u r face of m e r c u r y . T h e films were d r i e d o v e r n i g h t i n a v a c u u m o v e n a t 5 0 ° C . Polystyrenes. α-Deuteriostyrene w a s p r e p a r e d b y r e d u c t i o n of a c e t o p h e n o n e w i t h l i t h i u m a l u m i n u m d e u t e r i d e a n d d e h y d r a t i o n of t h e e n s u i n g a l c o h o l . β - D e u t e r i o s t y r e n e was p r e p a r e d b y t h e h y d r o l y s i s w i t h d e u t e r i u m oxide of t h e G r i g n a r d of β - b r o m o s t y r e n e (4)· β, β - D i d e u t e r i o s t y r e n e a n d α, β, β - t r i d e u t e r i o s t y r e n e were s y n t h e s i z e d b y the following scheme: —C=CK

+ Na- »

—C=CNa

— C = C N a + D,() ->

—C=CD

D2SO4

—teCD + D 0

>


2

HgS0

(S0

3

—C—CD H Ο

4

3

+ DoO -> D S 0 ) 2

4

L1AID4

—C—CD Ο

3

ι

> — CD=-CD dehydration\

2

—CH=CD

L1AIH4

2

T h e f o l l o w i n g m e t h o d w a s used t o p r e p a r e a, β - d i d e u t e r i o s t y r e n e (PBr + 3 D 0 -> 3 D B r + D 3 P O 3 ) 3

- C ^ C D

+ DBr

2

Dibenzoyl :—> peroxide + 0

—CD=CDBr

2

T h e G r i g n a r d of β - b r o m o d e u t e r i o s t y r e n e w a s t h e n p r e p a r e d a n d y i e l d e d a, β - d i d e u teriostyrene u p o n hydrolysis. T h e m o n o m e r s as w e l l as u n d e u t e r a t e d s t y r e n e were p o l y m e r i z e d i n sealed glass t u b e s , i n t h e absence of c a t a l y s t s , i n a c o n s t a n t t e m p e r a t u r e b a t h a t 80° C . T h e m e a n m o l e c u l a r w e i g h t of t h e o b t a i n e d p o l y m e r s w a s i n t h e range of 1,000,000 e x c e p t f o r t h e β - d e u t e r i o - a n d t h e a, β - d i d e u t e r i o p o l y s t y r e n e s . I n t h i s case t h e m o l e c u l a r w e i g h t was m u c h l o w e r e v e n a f t e r a p o l y m e r i z a t i o n t i m e of s e v e r a l weeks. T h e reason for t h i s i n h i b i t i o n lies p e r h a p s i n t h e presence of i m p u r i t i e s i n t h e s a m p l e s f r o m t h e G r i g n a r d o r t h e b r o m i d e . T h e p o l y m e r s were r e p r e c i p i t a t e d f r o m m e t h a n o l a n d d r i e d o v e r n i g h t i n a v a c u u m o v e n a t 60° C . T h e s t r u c t u r e of t h e p o l y m e r s w a s v e r i f i e d b y i n f r a r e d s p e c t r a a n d b y s p e c t r o g r a p h ^ m e a n s (3). T h i s m e t h o d , u s i n g w a t e r s a m p l e s c o l l e c t e d u p o n c o m b u s t i o n of e a c h p o l y m e r i n a p u r e o x y g e n a t m o s p h e r e , i n v o l v e s e s s e n t i a l l y t h e e x c i t a t i o n of h y d r o g e n a n d d e u t e r i u m a t o m s b y a m i c r o w a v e e x c i t e r . T h e a l p h a lines i n t h e B a l m e r region are resolved a n d measured b y spectrograph, a n d their intensities are compared w i t h those of s t a n d a r d samples so t h a t t h e p e r cent c o n t e n t of d e u t e r i u m m a y be determined. Results a n d

Discussion

Polyethylene. T h e a c t i o n of ozone o n p o l y e t h y l e n e w a s s t u d i e d i n t h e t e m p e r a t u r e range f r o m 2 5 ° t o 109° C . T h e r e a c t i o n w a s f o l l o w e d q u a l i t a t i v e l y a n d q u a n t i t a t i v e l y b y i n f r a r e d s p e c t r a . T h e p r o d u c t s a p p e a r e d t o be o f t h e same n a t u r e as those of 0 o x i d a t i o n s of p o l y e t h y l e n e (2)—i.e., t h e f o r m a t i o n of a l d e h y d i c a n d k e t o n i c g r o u p s as i n d i c a t e d b y t h e a p p e a r a n c e of a s t r o n g a b s o r p t i o n b a n d i n t h e r e g i o n of 5.9 m i c r o n s , a n d t h e existence of h y d r o x y l g r o u p s as s h o w n b y t h e 2 . 9 m i c r o n b a n d . P o l y e t h y l e n e is r e a d i l y o x i d i z e d i n t h e presence of ozone, as e v e n s h o r t p e r i o d o z o n i z a t i o n s c a r r i e d o u t a t t e m p e r a t u r e s as l o w as 2 5 ° C . y i e l d e d c o n s i d e r a b l e c o n c e n t r a t i o n s of c a r b o n y l a n d h y d r o x y l g r o u p s . P u r e 0 o x i d a t i o n s d i d n o t y i e l d comparable results u n t i l t h e reaction t e m p e r a t u r e was raised a p p r o x i m a t e l y 2

2


ADVANCES

170

IN CHEMISTRY SERIES

100° h i g h e r , t h u s m a n i f e s t i n g t h e v i g o r o u s a c c e l e r a t i n g effect of ozone as a n o x i d i z i n g agent. K i n e t i c a l l y t h e r e a c t i o n w a s f o l l o w e d b y t h e r a t e of i n c r e a s e of t h e c a r b o n y l b a n d a t 5.9 m i c r o n s . T h i s was accomplished b y applying the Beer's a n d Lambert's l a w relationship. Log ( J / / ) = 0

-edC

I t w a s a s s u m e d t h a t I/I , t h e p e r cent t r a n s m i t t a n c e , m a y b e u s e d i n t h e k i n e t i c e q u a t i o n s i n s t e a d o f C, t h e c o n c e n t r a t i o n o f c a r b o n y l , because t h e film t h i c k n e s s , d, a n d t h e e x t i n c t i o n coefficient, e, r e m a i n c o n s t a n t . P l o t s of l o g (I/Io) vs. t i m e of o z o n i z a t i o n s h o w e d t h a t t h e r e a c t i o n h a d a v e r y h i g h i n i t i a l r a t e of o x i d a t i o n w h i c h


0

2.00

T=20 C E

20

40

60

T I M E IN M I N U T E S

Figure 1.

Ozonization of polyethylene

Rate of increase of >

C=0

band (5.85 microns)

g r a d u a l l y d e c r e a s e d w i t h t i m e ( F i g u r e 1). I t w a s i n d i c a t e d t h a t t h e r e a c t i o n t a k e s p l a c e m a i n l y o n t h e s u r f a c e o f t h e p o l y m e r , w h i l e i n t h e l a t e r states w h e n m o s t of t h e a c t i v e sites o n t h e s u r f a c e h a v e b e e n d e s t r o y e d , diffusion of o x i d i z i n g gas i n t o t h e p o l y m e r b e c o m e s t h e r a t e - c o n t r o l l i n g process. T h i s c h e m i s o r p t i o n t h e o r y w a s v e r i f i e d f u r t h e r b y c a l c u l a t i n g t h e a c t i v a t i o n e n e r g y of t h e process, w h i c h w a s 9.2 k c a l . (Figure 2). Ozonization of Polystyrenes. T h e a c t i o n of ozone o n t h e p o l y m e r s A.

(CH—CH ) 2

n

D . (CD—CD )„ Φ 2

B . (CD—CH )„ Φ

E.

(CH—CHD)„ Φ

C.

F.

(CD—CDH) Φ

2

(CH—CD ) 2

n

n


171


BEACH ELL A N D N E M P H O S - O X I DATIVE DEGRADATION OF POLYMERS

I/TX

Figure

2.

ΙΟ"*

3

Activation energy of polyethylene

ozonization

of

was s t u d i e d i n t h e t e m p e r a t u r e r a n g e f r o m 65° t o 154° C . T h e changes i n t h e s p e c t r a of o z o n i z e d p o l y s t y r e n e s were i d e n t i c a l f o r a l l p o l y m e r s s t u d i e d , d e u t e r a t e d a n d u n d e u t e r a t e d . T h i s seems t o i n d i c a t e t h a t t h e r e a c t i o n p r o c e e d s t h r o u g h t h e same mechanism i n all compounds. I t w a s seen f r o m t h e i n f r a r e d s p e c t r a t h a t p o l y s t y r e n e is n o t as s u s c e p t i b l e t o o z o n i z a t i o n as some o t h e r p o l y m e r s — i . e . , p o l y e t h y l e n e is o z o n i z e d r e a d i l y e v e n a t r o o m t e m p e r a t u r e . T h e f o l l o w i n g c o n c l u s i o n s c a n b e d r a w n f r o m a s t u d y of t h e t e m p e r a t u r e r a n g e f r o m 6 5 ° t o 155° C . T h e first c h a n g e i n t h e s p e c t r a of p o l y s t y r e n e s d u r i n g o z o n i z a t i o n i s t h e a p p e a r a n c e of a w e a k b a n d a t a p p r o x i m a t e l y 5.7 microns. I t s m a i n characteristic is t h a t i t appears i m m e d i a t e l y a t the start of t h e r e a c t i o n , i t s r a t e of i n c r e a s e c o n t i n u o u s l y d e c r e a s i n g a n d q u i c k l y d i m i n i s h i n g t o a p o i n t w h e r e n o f u r t h e r increase o f a b s o r p t i o n i s o b s e r v e d a t 5.7 m i c r o n s . A t t e m p e r a t u r e s b e l o w a p p r o x i m a t e l y 140° C . ( t h e m e l t i n g p o i n t of p o l y s t y r e n e s ) t h e i n t e n s i t y o f t h i s b a n d r e m a i n s c o n s t a n t , regardless of t e m p e r a t u r e , o v e r l o n g p e r i o d s of o z o n i z a t i o n . A t t e m p e r a t u r e s a b o v e 140° C . ( a p p r o x i m a t e l y ) , i t a g a i n r e m a i n s c o n s t a n t b u t f o r s h o r t p e r i o d s , d e p e n d i n g o n t h e r e a c t i v i t y of t h e p o l y m e r — i . e . , long period for ( C D — C H ) while ( C H — C D H ) is v e r y reactive, a n d after this 2

0 0 s t e a d y - s t a t e p e r i o d a r a p i d o x i d a t i o n t a k e s p l a c e as e v i d e n c e d b y t h e r a p i d i n c r e a s e of the 5.9-micron c a r b o n y l b a n d . These observations indicate t h a t t h e 5.7-micron b a n d is d u e m a i n l y t o a n i n t e r m e d i a t e f o r m i n g o n t h e s u r f a c e l a y e r s b e t w e e n t h e p o l y m e r s u b s t r a t e a n d t h e ozone. I t s s t r u c t u r e p r o b a b l y i n v o l v e s a c o m p l e x of p e r o x i d i c o r ozonide structure. T h i s intermediate is stable t o f u r t h e r oxidation o r other reactions at l o w temperatures, while a t higher temperatures i t decomposes r a t h e r r e a d i l y u n d e r t h e i n f l u e n c e of o x y g e n , ozone, a n d h i g h t e m p e r a t u r e t o y i e l d c a r b o n y l c o m p o u n d s a n d other secondary products. T h e r e s u l t s of t h e s t u d y o n t h e rates of o z o n i z a t i o n of d e u t e r a t e d p o l y s t y r e n e s , as m e a s u r e d b y t h e r a t e of increase of t h e 5 . 7 - m i c r o n a b s o r p t i o n b a n d i n t h e i n f r a r e d s p e c t r a , a r e o u t l i n e d i n F i g u r e s 3 t o 7.


A D V A N C E S IN

172

η

o. or ο / ω

Oxidative Degradation of Polymers in Presence of Ozone

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