OZONE CHEMISTRY AND TECHNOLOGY

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

OZONE CHEMISTRY AND TECHNOLOGY

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