The Effects of Hostile Environments on Coatings and Plastics

2 Degradation of Polymers in Hostile Environments HERMAN F. MARK

Downloaded by UNIV OF SYDNEY on January 22, 2016 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch002

Polytechnic Institute of New York, Brooklyn, NY 11201

The influences o f "hostile environment" on polymers heat, irradiation, mechanical and e l e c t r i c a l abuse, the action of aggressive chemicals - may be generally c l a s s i f i e d in:, a) changes o f the chemical character and microstructure of the individual macromolecules, and b) changes in the supermolecular structure and texture of the system. a) These changes include: decrease o f the molecular weight by chain scission which causes a r e duction of tensile strength, modulus and toughness, introduction of reactive (polar) groups which cause changes in compatibility, electrical and optical behavior, introduction o f light-absorbing groups which cause discoloration, and internal cyclization of the chains which may r e sult in hardening and decrease in toughness. b) These changes include: crosslinking between hitherto independent macromolecules; a moderate extent may have favorable consequences but too much o f it causes reduced impact strength and brittleness, additional crystallization; here again a certain degree of this change may be advantageous whereas too much may cause opacity, haze and crack formation, and release of gaseous fragments causing bubble formation and reducing strength. A few examples for p a r t i c u l a r l y drastic influences of hostile environment on polymeric systems will be discussed. I t i s a p l e a s u r e a n d a n h o n o r f o r me t o b e c o n t r i b u t i n g t o t h e p r o c e e d i n g o f a s y m p o s i u m h e l d i n h o n o r o f my g o o d f r i e n d ,

0097-6156/ 83/0229-0011 $06.00/0 ® 1983 American Chemical Society

In The Effects of Hostile Environments on Coatings and Plastics; Garner, David P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

12

EFFECTS OF HOSTILE ENVIRONMENTS

Ray S e y m o u r . We f i r s t met a t G i b s o n I s l a n d some f o r t y y e a r s ago at a Gordon Research Conference. S i n c e t h e n , Ray h a s had a n i l l u s t r i o u s a n d e v e n t f u l c a r e e r ; h e has b e e n a n u n d e r s t a n d i n g e x p e r i m e n t a l c h e m i s t , a v e r y s u c c e s s f u l m a n a g e r a n d a d v i s o r t o chemi s t s , a n d now a n e x c e l l e n t e d u c a t o r o f c h e m i s t s . I t h i n k t h a t a n y o n e who has f o l l o w e d h i s c a r e e r i s a w a r e o f h i s n u m e r o u s c o n t r i b u t i o n s t o c h e m i s t r y a n d we a l l h o p e t h a t i n h i s c u r r e n t p o s i t i o n a t the U n i v e r s i t y o f Southern M i s s i s s i p p i he w i l l c o n t i n u e t o p r o m o t e o u r s c i e n c e a s w e l l a s make t h e w o r l d a b e t t e r p l a c e t o l i v e . Ray i s a t r u e g e n t l e m a n a s w e l l a s a s h i n i n g e x a m p l e f o r a fine scientist. Downloaded by UNIV OF SYDNEY on January 22, 2016 | http://pubs.acs.org Publication Date: September 29, 1983 | doi: 10.1021/bk-1983-0229.ch002

Discussion I have been asked t o g i v e an o v e r v i e w o f the e f f e c t s o f host i l e environment on p l a s t i c s . Emphasizing not the degradation o r d e s t r u c t i o n o f t h e b u l k p o l y m e r i c m a t e r i a l and i t s p r o p e r t i e s , but r a t h e r what i s happening t o i n d i v i d u a l m a c r o m o l e c u l e s i n h o s t i l e e n v i r o n m e n t s , a n d a l s o t o l a r g e r a s s e m b l i e s o f t h e m . The h o s t i l e environments t o be d i s c u s s e d are heat, r a d i a t i o n , mechanical s t r e s s e s and t h e p r e s e n c e o f o x y g e n i n c o m b i n a t i o n w i t h h e a t and radiation. Heat D e g r a d a t i o n . Heat i s m o l e c u l a r motion and f o r s o l i d b o d i e s may b e r e p r e s e n t e d b y w a v e s w h i c h t r a v e r s e t h e s y s t e m i n d i r e c t i o n s . Waves c a n b e v e r y c o m p l e x a n d t h e r e c a n b e i n t e r f e r e n c e p h e n o m e n a a n d f l u c t u a t i o n s a s s o c i a t e d w i t h t h e m . A s i m p l e example i s the white caps on a l a k e . White caps are the c o n s t r u c t i v e i n t e r f e r e n c e o f water waves. Each i n d i v i d u a l wave would n o t have the e n e r g y t o form a w h i t e c a p , but a c o m b i n a t i o n o f waves does y i e l d them. Thus, i t i s the c o n c e n t r a t i o n o f energy a t a g i v e n p o i n t t h a t i s i m p o r t a n t . A s i t i s i n water, s o can i t be i n a macromolecule. I t i s n o t t h e m o t i o n due t o t h e a v e r a g e t e m p e r a t u r e o f the p o l y m e r i c m a t e r i a l , but the f l u c t u a t i o n s t h a t are c r i t i c a l . I f t h e f l u c t u a t i o n s a r e f r e q u e n t and t h e r e i s a c o n c e n t r a t i o n o f energy through c o n s t r u c t i v e i n t e r f e r e n c e , then there i s a c o n c e n t r a t i o n of energy at a point along the chain that i s higher than the s t r e n g t h o f the c h a i n can s u p p o r t . In a l o n g f l e x i b l e c h a i n m o l e c u l e t h e r e a r e v a r i o u s s e g m e n t a l motions. E v e n t u a l l y a t one p o i n t t h e r e w i l l be s u c h a c o n c e n t r a t i o n o f energy that the chain breaks. Inthe simplest case, that o f p o l y e t h y l e n e ( P E ) w i t h o n l y C-C a n d C-H b o n d s , t h e d i s a s s o c i a t i o n e n e r g y f o r t h e s e bonds i s a b o u t 80 and 9 0 kcal/mol, resp e c t i v e l y . S o when t h e e n e r g y e x c e e d s t h e a m o u n t , t h e c h a i n b r e a k s , t h e two s i g m a b o n d i n g e l e c t r o n s a r e s e p a r a t e d a n d two l o n e e l e c t r o n s ( f r e e r a d i c a l s ) are formed. A v e r y p r o b a b l e r e a c t i o n would be f o r the r a d i c a l s t o recomb i n e . They are a l r e a d y i n a bonding s i t u a t i o n . Since t h a t i s occ u r r i n g i n s o l i d system (or c o n c e n t r a t e d s o l u t i o n ) , there i s a p o s s i b i l i t y o f a cage e f f e c t . That i s , a f t e r s e p a r a t i o n , the f r e e r a d i c a l s a r e h e l d i n a r e l a t i v e l y c o n f i n e d a r e a and a s a r e s u l t



2.

MARK

Degradation of Polymers

13

recombination i s l i k e l y . This i s p a r t i c u l a r l y true i f the chains a r e r i g i d and t y p i c a l l y r i g i d c h a i n s c o n t a i n i n g c y c l i c a r o m a t i c u n i t s i n the c h a i n have h i g h heat s t a b i l i t y . Another example o f chains being held i n very r i g i d p o s i t i o n i s i n diamond. A carbonc a r b o n b o n d i n d i a m o n d i s no s t r o n g e r t h a n one i n PE o r a n y o t h e r o r g a n i c compound. But i n a diamond w i t h i t s t h r e e d i m e n s i o n a l s t r u c t u r e , when a b o n d b r e a k s t h e r e i s l i t t l e t h a t t h e r a d i c a l s c a n do b u t r e c o m b i n e . S o , e v e n a v e r y w e a k , s e n s i t i v e b o n d may be p r o t e c t e d t o some d e g r e e by a c a g e e f f e c t i f i t i s i n a s u f f i c i e n t l y r i g i d environment. On t h e o t h e r h a n d , i f t h e c h a i n s a r e f l e x i b l e l i k e PE a n d t h e temperature i s high with the segments moving around q u i t e r a p i d l y , t h e n t h e two e l e c t r o n s on t h e c h a i n e n d may become s o s e p a r a t e d t h a t i t i s u n l i k e l y t h a t they would recombine. Two f r e e r a d i c a l s a r e t h e n d o r m a n t . A r a d i c a l may p i c k up a h y d r o g e n f r o m an a d j a c e n t c h a i n and t h e r a d i c a l i s t r a n s f e r r e d . However, even though the c h a i n s a r e b r o k e n , the amount o f m a t e r i a l p r e s e n t remains the s a m e . T h a t i s , t h e r e i s a r e d u c t i o n o f m o l e c u l a r w e i g h t (MW) b u t n o t o f w e i g h t . The n u m b e r o f c h a i n s go up a n d t h e MW o f t h e c h a i n s goes down. T h e r e i s a r e d u c t i o n i n t h e p r o p e r t i e s o f t h e m a t e r i a l , b u t a g a i n t h e t o t a l w e i g h t o f t h e s a m p l e d o e s n o t go down. What h a p p e n s t o t h e f r e e r a d i c a l s ? I f t h e y a r e on a f l e x i b l e m o l e c u l e l i k e PE w h i c h i s m o v i n g a r o u n d q u i t e r a p i d l y , t h e n t h e r e i s a h i g h p r o b a b i l i t y t h a t i t may p i c k o f f a h y d r o g e n o f i t s own c h a i n f i v e t o s e v e n c a r b o n a t o m s b a c k a l o n g t h e c h a i n . So t h e n t h e r e i s a t r a n s f e r o f t h e r a d i c a l t o a p o s i t i o n away f r o m t h e c h a i n e n d a n d a t e r m i n a t i o n o f a c t i v i t y a t a c h a i n e n d . The f r e e r a d i c a l may t h e n p i c k up a monomer a n d t h e n c o n t i n u e g r o w i n g f r o m t h e new r e a c t i v e p o s i t i o n . The n e t e f f e c t i s t h a t a s h o r t b r a n c h has b e e n f o r m e d . In p r i n c i p a l , t h i s i s s i m i l a r t o t h e w e l l known s h o r t c h a i n b r a n c h i n g o f PE when p r o d u c e d by t h e h i g h t e m p e r a t u r e h i g h p r e s s u r e m e t h o d . In t h i s c a s e , many s h o r t b r a n c h e s a r e f o r m e d . T h e r e i s a l s o a p r o b a b i l i t y t h a t t h e f r e e r a d i c a l may a t t a c k an a t o m f a r t h e r r e m o v e d a l o n g t h e same c h a i n a n d t h e n c y c l i z a t i o n may o c c u r . S o , i n t h e s i m p l e s t c a s e we may e x p e c t d e g r a d a t i o n , MW l o s s , b r a n c h i n g and c y c l i z a t i o n . Now t h i n g s g e t m o r e c o m p l i c a t e d a s s o o n a s t h e r e a r e s u b s t i t u e n t s on t h e c h a i n . A g a i n , a b o n d may be b r o k e n t h i s t i m e b e t w e e n a CH2 u n i t a n d a CHR u n i t w h e r e R i s t h e s u b s t i t u e n t . Two d i f f e r ent types o f r a d i c a l s are then formed. The CH2 r a d i c a l e n d g r o u p i s a v e r y r e a c t i v e r a d i c a l a n d s o o n w i l l p i c k up a h y d r o g e n . The CHR r a d i c a l i s m o r e s t a b l e a n d may e v e n be v e r y s t a b l e d e p e n d i n g on t h e g r o u p R. I f i t i s r e l a t i v e l y s t a b l e , t h e n i t may d i s a s s o c i a t e i n t o a monomer a n d i n i t i a t e a s t e p w i s e c h a i n d e p o l y m e r i z a t i o n . There are s e v e r a l cases where t h i s i s v e r y pronounced, part i c u l a r l y i f R i s v e r y s t a b i l i z i n g as i n t h e c a s e o f a l p h a - m e t h y l s t y r e n e . The d e g r a d a t i o n i n t h i s c a s e i s o f a d i f f e r e n t n a t u r e . The monomer i s e l i m i n a t e d a n d t h e r e f o r e t h e w e i g h t o f t h e s a m p l e g o e s down w h i l e t h e MW o f t h e i n d i v i d u a l m o l e c u l e r e m a i n s a p p r o x i -



14

EFFECTS OF HOSTILE ENVIRONMENTS

m a t e l y t h e same. T a k i n g two m e a s u r e m e n t s , n a m e l y t h e w e i g h t o f t h e m a t e r i a l a n d t h e MW o f t h e c h a i n s , a l l o w s u s t o d i s t i n g u i s h b e t w e e n t h e two d e g r a d a t i o n m e c h a n i s m s o f c h a i n s c i s s i o n a n d d e p o l y m e r i z a t i o n i n t e r m s o f r e l a t i v e f r e q u e n c y o f t h e two m e c h a nisms. F o r e x a m p l e , b a s e d o n t h i s a p p r o a c h , we know t h a t p o l y ( m e t h y l m e t h a c r y l a t e ) (PMMA) c a n b e d e g r a d e d a t h i g h t e m p e r a t u r e s t o g i v e a g o o d y i e l d o f MMA m o n o m e r . A t h i r d p o s s i b i l i t y i s that the s u b s t i t u e n t r e a c t s with a n e i g h b o r i n g h y d r o g e n a n d RH i s r e l e a s e d . W h e n e v e r t h e b o n d s t r e n g t h s o f RH a r e l a r g e r t h a n t h e b o n d s t r e n g t h s o f t h e c h a i n and t h e t e m p e r a t u r e i s h i g h enough, t h i s w i l l be a n a d d i t i o n a l means o f d e c o m p o s i t i o n . A l l three types of decomposition discussed t h u s f a r a p p l y t o a s i n g l e c h a i n , and under g i v e n c o n d i t i o n s , may o c c u r s i m u l t a n e o u s l y . I f we c o n s i d e r t h a t t h e c h a i n i s one o f m a n y , t h e n when a f r e e r a d i c a l i s formed because o f c h a i n s c i s s i o n somewhere i n the system a hydrogen i s a b s t r a c t e d w i t h the r e s u l t t h a t the r a d i c a l may b e t r a n s f e r r e d t o a n o t h e r c h a i n . I f a c h a i n has m o r e t h a n one f r e e r a d i c a l a n d i f t h e c h a i n s a r e i n r a p i d m o t i o n , t h e n t h e r a d i c a l s may be s h i f t e d a l o n g t h e c h a i n and can p o s s i b l y w i n d up n e x t t o one a n o t h e r . The r a d i c a l s may t h e n c o m b i n e t o f o r m a double bond. The m a i n f e a t u r e s o f d e g r a d a t i o n f o r i n d i v i d u a l c h a i n s due to high temperature then are: chain s c i s s i o n , d e p o l y m e r i z a t i o n , b r a n c h i n g , s e l f b r a n c h i n g , s e l f c y c l i z a t i o n , removal o f a s u b s t i t u e n t , and f o r m a t i o n o f d o u b l e b o n d s . R a d i a t i o n D e g r a d a t i o n . What i f i r r a d i a t i o n o f t h e m o l e c u l e o c c u r s , and what i s t h e d i f f e r e n c e between h e a t i n g a m a t e r i a l and i r r a d i a t i n g i t ? Heat e s s e n t i a l l y works o n the v i b r a t i o n s o f atoms; i t i s uniform throughout the system. R a d i a t i o n works on the e l e c t r o n s a n d o n l y a few s i t e s w i l l a b s o r b e n e r g y . The e n e r g y i n the form o f photons does not go d i r e c t l y i n t o the v i b r a t i o n a l modes o f t h e m o l e c u l e s , b u t i n s t e a d i n t o t h e e x c i t a t i o n o f c e r t a i n o f i t s e l e c t r o n s . A l l o t h e r s i t e s r e m a i n u n e f f e c t e d . From t h e t h e o r y o f e l e c t r o n i c e x c i t a t i o n we know t h a t i f s o m e w h e r e i n a molecule an e l e c t r o n i s put i n t o an e x c i t e d s t a t e , there are three w a y s t o g e t r i d o f t h e e x c e s s e n e r g y . One i s f l u o r e s c e n c e , one i s p h o s p h o r e s c e n c e , and t h e l a s t and most i m p o r t a n t f o r t h i s d i s c u s s i o n i s t h e c a s c a d i n g down o f t h e e l e c t r o n i c e n e r g y i n t o v i b r a t i o n a l and r o t a t i o n a l e n e r g y ; e n e r g y e m b o d i e s i n t h e s e modes o f atoms can cause the b r e a k i n g o f a bond. R a d i a t i o n covers a wide range o f f r e q u e n c i e s from r a d i o t o gamma r a y s . The f r e q u e n c i e s o f r a d i a t i o n t h a t a r e m o s t h a r m f u l to polymeric systems are those from the blue p a r t o f the v i s i b l e s p e c t r u m a n d t h e n e a r u l t r a v i o l e t . The l o n g e r w a v e l e n g t h s a r e n o t e n e r g e t i c enough t o harm m o l e c u l e s and most o f t h e o t h e r p o t e n t i a l l y harmful high frequency rays are screened out by our atmosphere. H o w e v e r , a t 300 nm a n d a b o v e i n t o much r a d i a t i o n p e n e t r a t e s t h e a t m o s p h e r e and can a c t o n t h e m a t e r i a l . Here groups such a s


2.

MARK

Degradation of Polymers

15


C=0, C=C, a n d N=C b e g i n t o a b s o r b a n d e l e c t r o n s a r e l i f t e d i n t o h i g h e r e n e r g y l e v e l s . When t h e s e e l e c t r o n s d e c a y t o l o w e r e n e r g y s t a t e s , e n e r g y i s r e l e a s e d i n t h e f o r m o f v i b r a t i o n a l modes w h i c h may c a u s e a b o n d t o b r e a k a n d c r e a t e f r e e r a d i c a l s . A f t e r t h e r a d i c a l s a r e formed t h e y w i l l a g a i n r e a c t a s has been p r e v i o u s l y d e s c r i b e d . The o n e a d d i t i o n a l d a n g e r i s t h a t t h e e x c i t e d s t a t e may b e l o n g l i v e d , w h i c h c a u s e s a d e g r a d a t i o n a f t e r a l o n g p e r i o d of time. Mechanical Degradation. A t h i r d danger i s mechanical d e g r a d a t i o n . Any k i n d o f m e c h a n i c a l a c t i o n w h i c h c h a n g e s s u b s t a n t i a l l y t h e s h a p e o r s i z e o f a p o l y m e r i c s a m p l e has t h e p o t e n t i a l t o b r e a k s i n g l e b o n d s . The q u e s t i o n i s how f a s t d o t h e y f o r m a n d how f a s t d o t h e y d i s a p p e a r by p i c k i n g up hydrogen o r b y c y c l i z a t i o n . A n y t h i n g t h a t c u t s , g r i n d s , o r s h e a r s has t h e c a p a b i l i t y o f g e n e r a t i n g f r e e radicals. The m a s t i c a t i o n o f r u b b e r i s a g o o d e x a m p l e . The p u r p o s e o f m a s t i c a t i o n i s c h a i n s c i s s i o n a n d t h e r e d u c t i o n o f t h e a v e r a g e MW of the rubber. A n o t h e r example i n a d i f f e r e n t s y s t e m would be t h e f o l l o w i n g : S u p p o s e we h a v e a s e m i c r y s t a l 1 i n e m a t e r i a l a n d t h e r e a r e c h a i n s which connect two c r y s t a l l i n e domains o f t h a t m a t e r i a l w i t h each o t h e r . T h e r e i s no r e a s o n why t h e l e n g t h s o f t h o s e c h a i n s w h i c h a r e i n t h e amorphous a r e a between t h e c r y s t a l l i n e s i t e s h o u l d be t h e same. T h e r e w i l l a l w a y s be a d i s t r i b u t i o n o f c h a i n l e n g t h s . Then i f one keeps one c r y s t a l c o n s t a n t and moves t h e o t h e r as soon as t h e s t r e s s i s g r e a t e r t h a n t h e bond s t r e n g t h o f t h e s h o r t e s t c h a i n , a r u p t u r e o f t h i s c h a i n w i l l o c c u r . A s t h e d i s t a n c e between t h e c r y s t a l s i n c r e a s e s , more and more c h a i n s w i l l b r e a k . E v e n t u a l l y , a c r a c k w i l l f o r m w h i c h may t h e n p r o p a g a t e b y a d d i tional processes. The R u l e o f O x y g e n i n P o l y m e r D e g r a d a t i o n . A l a s t p o i n t t o c o n s i d e r i s t h a t i n most c a s e s heat o r r a d i a t i o n does not a c t a l o n e , but a c t s i n c o n j u n c t i o n w i t h o x y g e n . The a d d i t i o n a l p r o b l e m i s t h a t oxygen i s a v e r y r e a c t i v e m o l e c u l e and i f a f r e e r a d i c a l forms i n i t s presence then i t can combine i m m e d i a t e l y w i t h i t t o f o r m a d i f f e r e n t r a d i c a l . T h i s r a d i c a l may t h e n a b s t r a c t a h y d r o gen a n d f o r m a h y d r o p e r o x i d e . The h y d r o p e r o x i d e c a n t h e n d e c o m pose i n t o two r a d i c a l s and s o from t h e i n i t i a l two r a d i c a l s a t o t a l o f s i x p o s s i b l e r a d i c a l s may f o r m . T h i s e x p l a i n s t h e d a n g e r in c h a i n s c i s s i o n i n the presence o f oxygen l e a d i n g t o a c h a i n reaction. Summary So, i n e s s e n c e , a l l f o u r t y p e s o f d e g r a d a t i o n p r o c e e d through f r e e r a d i c a l r e a c t i o n s . Depending on the c o n d i t i o n s , they c a n b e r e l a t i v e l y h a r m l e s s when a c a g e e f f e c t i s p o s s i b l e o r t h e r e can be a whole sequence o f s e c o n d a r y and t e r t i a r y r e a c t i o n s which lead t o polymer d e g r a d a t i o n . RECEIVED April 11, 1983


The Effects of Hostile Environments on Coatings and Plastics

Recommend Documents