Cross-Linking of Ethylene—Propylene Copolymer Rubber with

Chapter 31

Cross-Linking of Ethylene-Propylene Copolymer Rubber with Dicumyl Peroxide-Maleic Anhydride Norman G. Gaylord, Mahendra Mehta, and Rajendra Mehta Downloaded by PURDUE UNIV on August 8, 2016 | http://pubs.acs.org Publication Date: December 22, 1988 | doi: 10.1021/bk-1988-0364.ch031

Gaylord Research Institute, Inc., New Providence, ΝJ 07974 The reaction of EPR with dicumyl peroxide (DCP) at 180°C yielded a fraction insoluble in cyclohexane at 22°C. The presence of maleic anhydride (ΜΑΗ) in the EPR-DCP reac­ tion mixture increased the amount of cyclohexane-insol­ uble gel. However, the gel concentration decreased as the DCP concentration increased. The ΜΑΗ content of the soluble polymer increased when either the ΜΑΗ or the DCP concentration increased. The molecular weight of the soluble polymer increased with increasing ΜΑΗ concentra­ tion and decreased with increasing DCP concentration in the reaction mixture. The products from the EPR-DCP and EPR-MAH-DCP reactions were soluble in refluxing xylene and were fractionated by precipitation with acetone. The presence of stearamide in the EPR-MAH-DCP reaction in­ creased the amount and the molecular weight of the cyclo­ -hexane-soluble polymer. The c r o s s l i n k i n g o f e t h y l e n e - p r o p y l e n e c o p o l y m e r r u b b e r (EPR) i n t h e p r e s e n c e o f o r g a n i c p e r o x i d e s has been i n v e s t i g a t e d by N a t t a and/or h i s coworkers (1-3) and o t h e r s ( 4 , 5 ) . C o - a g e n t s s u c h a s s u l f u r (3,4) and u n s a t u r a t e d monomers ( 6 j , i n c l u d i n g m a l e i c a n h y d r i d e (ΜΑΗ)(3,7) have been u t i l i z e d i n an e f f o r t t o i n c r e a s e t h e c r o s s l i n k i n g e f f i ­ c i e n c y i n t h e E P R - p e r o x i d e system. As p a r t o f o u r c o n t i n u i n g s t u d y o f t h e p e r o x i d e - c a t a l y z e d r e a c ­ t i o n s o f ΜΑΗ w i t h s a t u r a t e d p o l y o l e f i n s , t h e p r e s e n t i n v e s t i g a t i o n was u n d e r t a k e n t o d e t e r m i n e t h e e x t e n t o f c r o s s l i n k i n g and/or d e g r a d ­ a t i o n which accompanies t h e EPR-MAH r e a c t i o n . The g e l c o n t e n t , p r e ­ sumably i n d i c a t i v e o f c r o s s l i n k i n g , was d e t e r m i n e d by e x t r a c t i o n w i t h c y c l o h e x a n e a t room temperature (22°C) f o r 60 h r . EXPERIMENTAL EPR REACTIONS. V i s t a l o n 404 EPR (Exxon C h e m i c a l Co.)(40/60 wt r a t i o E/P, / M 37) was f l u x e d f o r 2 min i n t h e m i x i n g chamber o f a B r a bender P l a s t i c o r d e r a t 60 rpm and 180°C. A m i x t u r e o f DCP, ΜΑΗ and s t e a r a m i d e ( S A ) , when p r e s e n t , was added i n f o u r p o r t i o n s a t 2 min i n t e r v a l s t o t h e 40g f l u x i n g EPR. A f t e r t h e l a s t a d d i t i o n , m i x i n g M

w

n

0097-6156/88/0364-0438$06.00/0 © 1988 American Chemical Society

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

31.

GAYLORDETAL.

Ethylene-Propylene

Copolymer Rubber

439

was c o n t i n u e d f o r 2 min ( t o t a l 10 min) and then t h e r e a c t i o n m i x t u r e was q u i c k l y removed from the chamber.

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CYCLOHEXANE EXTRACTION. A 5-6g p o r t i o n o f t h e p r o d u c t was c u t i n t o s m a l l p i e c e s and s t i r r e d i n 250 ml c y c l o h e x a n e a t room temperature f o r 60 h r . The i n s o l u b l e f r a c t i o n was s e p a r a t e d by f i l t e r i n g the s o l u t i o n t h r o u g h c h e e s e c l o t h . The c y c l o h e x a n e - s o l u b l e f r a c t i o n was r e c o v e r e d by d i s t i l l i n g t h e s o l v e n t i n vacuo and the polymer was d r i e d i n vacuo a t 40 C f o r 24 h r . XYLENE FRACTIONATION. A 5-6g p o r t i o n o f the p r o d u c t was c u t i n t o s m a l l p i e c e s and h e a t e d i n 200 ml r e f l u x i n g x y l e n e f o r 5 h r . The h o t s u s p e n s i o n was f i l t e r e d t h r o u g h c h e e s e c l o t h i n t o 800 ml a c e t o n e . The polymer a d h e r i n g t o the s i d e s o f t h e f l a s k and t h e s m a l l amount f i l ­ t e r e d from t h e s u s p e n s i o n were r e f l u x e d w i t h an a d d i t i o n a l 150 ml x y l e n e u n t i l complete s o l u t i o n was n o t e d and then p r e c i p i t a t e d i n acetone. The p r e c i p i t a t e d polymers were combined as F r a c t i o n I . The combined f i l t r a t e s were c o n c e n t r a t e d on a r o t a v a p o r a t o r and t h e r e s i ­ d u a l polymer, a s w e l l as t h e polymer r e c o v e r e d by p r e c i p i t a t i o n , were d r i e d i n vacuo a t 140 C f o r 4 h r . The polymer r e c o v e r e d from the f i l t r a t e s was i d e n t i f i e d as F r a c t i o n I I . CHARACTERIZATION. The i n t r i n s i c v i s c o s i t y o f t h e s o l u b l e f r a c t i o n s was d e t e r m i n e d i n t o l u e n e a t 30 C. The ΜΑΗ c o n t e n t o f t h e s o l u b l e f r a c t i o n s was d e t e r m i n e d by h e a t i n g a 0.5-1.0g p o r t i o n i n r e f l u x i n g w a t e r - s a t u r a t e d x y l e n e f o r 1 h r and t i t r a t i n g t h e h o t s o l u t i o n w i t h 0.05N e t h a n o l i c Κ 0 Η u s i n g 1% thymol b l u e i n DMF a s i n d i c a t o r . RESULTS The [y] o f EPR d e c r e a s e d when mixed f o r 10 min i n a Brabender P l a s t i c o r d e r a t 180 C. The complete s o l u b i l i t y o f t h e EPR i n c y c l o h e x a n e a t 22 C was unchanged. The p r e s e n c e o f 0.25-0.5 wt-% DCP a t 180°C r e s u l t e d i n t h e f o r ­ m a t i o n o f about 20% o f a c y c l o h e x a n e - i n s o l u b l e f r a c t i o n . The p r e ­ sence o f 5 wt-% ΜΑΗ (based on EPR) i n c r e a s e d t h e amount o f c y c l o h e x ­ a n e — i n s o l u b l e g e l , whose c o n c e n t r a t i o n d e c r e a s e d from 65% t o 27% a s the DCP c o n t e n t i n c r e a s e d from 0.25 t o 1.0 wt-% (based on EPR), r e s ­ pectively. The c y c l o h e x a n e - s o l u b l e polymer c o n t a i n e d about 1 wt-% ΜΑΗ which a p p a r e n t l y i n c r e a s e d w h i l e t h e [rç] d e c r e a s e d as t h e DCP content i n c r e a s e d (Table I ) . When t h e DCP c o n c e n t r a t i o n was k e p t c o n s t a n t a t 1 wt-% w h i l e t h e ΜΑΗ c o n c e n t r a t i o n i n the c h a r g e i n c r e a s e d from 5 t o 20 wt-%, t h e amount o f c y c l o h e x a n e - i n s o l u b l e polymer d e c r e a s e d from 29 t o 23%. The o f t h e c y c l o h e x a n e - s o l u b l e polymer i n c r e a s e d and i t s ΜΑΗ c o n t e n t d e c r e a s e d , a s the ΜΑΗ c o n c e n t r a t i o n i n t h e c h a r g e i n c r e a s e d ( T a b l e I I ) . When s t e a r a m i d e (SA) was p r e s e n t i n t h e DCP-MAH m i x t u r e added t o EPR a t 180 C, t h e amount o f c y c l o h e x a n e - i n s o l u b l e EPR-g-MAH d e c r e a s e d , a n a l o g o u s t o t h e e f f e c t o f SA i n r e d u c i n g c r o s s l i n k i n g i n t h e PE-MAHp e r o x i d e r e a c t i o n ( 8 , 9 ) . The [η] o f t h e c y c l o h e x a n e - s o l u b l e EPR-g-MAH i n c r e a s e d when SA was p r e s e n t i n t h e r e a c t i o n m i x t u r e , a n a l o g o u s t o the e f f e c t o f SA i n r e d u c i n g d e g r a d a t i o n i n t h e PP-MAH-peroxide r e ­ a c t i o n (Table I I I ) .

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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CHEMICAL REACTIONS ON POLYMERS

Table I .

E f f e c t o f DCP a t C o n s t a n t ΜΑΗ C o n c e n t r a t i o n a t 180°C C yc1ohexane E x t r a c t i o n

ΜΑΗ wt-% on EP

DCP wt-% on EP on ΜΑΗ

0 0 0

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a

c

d

0 0.25 0.5

0 0 0

Soluble

b

Insoluble

c

[yi] %

dl/g

100 77 80

1.20 1.42 1.23

a

ΜΑΗ wt-%

%

0 0 0

0 20 16

5 0.25 5 33 1.10 0.9 5 0.5 10 56 0.56 1.0 5 1^0 20 73 0.89 1^3 E x t r a c t i o n w i t h c y c l o h e x a n e a t 2 2 ^ f o r 60 h r R e c o v e r e d by s o l v e n t removal from f i l t r a t e T o l u e n e , 30°C U n t r e a t e d EPR fyj] 1.42 d l / g Table I I .

E f f e c t o f ΜΑΗ a t C o n s t a n t DCP C o n c e n t r a t i o n a t 180°C Cyclohexane

ΜΑΗ wt-% on EP

DCP wt-% on EP on ΜΑΗ

5 10 20 a,b,c

1.0 20 1.0 10 1.0 5 Footnotes t o Table I

T a b l e :I I I .

65 43 27

E f f e c t of

Soluble

Extraction

Insoluble

b

%

dl/g

ΜΑΗ wt-%

70 72 75

1.31 1.34 1.64

0.7 0.5 0.3

[71]°

Stearamide

3

% 29 26 23

i n EPR-MAH-DCΡ R e a c t i o n a t 180°C Cyclohexane E x t r a c t ! o n

ΜΑΗ wt-% on EP 5 5

DCP wt-% on EP on ΜΑΗ 0.25 0.25

5 5

5 0.5 10 5 0.5 10 a,£>,c F o o t n o t e s t o T a b l e I d

d

SA mole-% on ΜΑΗ

Soluble

a

Insoluble

b

%

dl/g

ΜΑΗ wt-%

0 20

33 43

1.10 1.18

0.9 0.5

65 55

0 20

56 68

0.56 1.02

1.0 1.6

43 29

%

SA-MAH-DCP m i x t u r e added i n 4 e q u a l p o r t i o n s t o m o l t e n EP

The polymer formed i n t h e r e a c t i o n o f EPR w i t h 0.5 wt-% DCP a t 180 C, i n t h e p r e s e n c e o r absence o f 5 wt-% Μ Α Η , was c o m p l e t e l y s o l ­ uble i n r e f l u x i n g xylene, although i t contained a f r a c t i o n i n s o l u b l e i n c y c l o h e x a n e a t 22 C. The EPR and EPR-g-MAH were f r a c t i o n a t e d by a d d i t i o n o f the xylene s o l u t i o n t o acetone. Both t h e a c e t o n e - p r e c i p i t a t e d polymer ( I ) and t h e polymer r e c o v ­ e r e d by removal o f t h e s o l v e n t from t h e a c e t o n e - x y l e n e f i l t r a t e ( I I ) had a h i g h e r [Ή] when ΜΑΗ was p r e s e n t i n t h e c h a r g e , a l t h o u g h t h e amount o f a c e t o n e - p r e c i p i t a t e d polymer ( I ) was d e c r e a s e d when ΜΑΗ had

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

31.

GAYLORDETAL.

Ethylene-Propylene

Copolymer Rubber

441

been p r e s e n t . The p r e s e n c e o f s t e a r a m i d e i n t h e EPR-MAH-DCP r e a c t i o n m i x t u r e i n c r e a s e d t h e amount and [yi] o f t h e acetone p r e c i p i t a t e d polymer ( T a b l e I V ) . T a b l e IV.

F r a c t i o n a t i o n o f EPR-MAH-DCP P r o d u c t i n X y l e n e Xylene

ΜΑΗ wt-% on EP

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0 0 5 5

DCP SA CH wt-% mole-% I n s o l on EP on ΜΑΗ on ΜΑΗ %

0.0 0.5 0.5 0.5

0 0 10 10

0 0 0 20

0 16 43 29

Fractionation e

b

Fraction I * ΜΑΗ [>|] % d l / g wt-% r

82 85 48 68

2.44 1.17 1.38 1.71

0 0 1.9 1.2

a

0

e

Fraction I I [^] ΜΑΗ % d l / g wt-% £

15 15 46 28

0.60 0.37 1.23 0.94

0 0 2.9 2.7

a

EPR r e a c t i o n s c o n d u c t e d i n P l a s t i c o r d e r a t 180°C k E x t r a c t i o n w i t h c y c l o h e x a n e a t 22°C f o r 60 h r E x t r a c t i o n with r e f l u x i n g xylene f o r 5 h r F r a c t i o n I r e c o v e r e d by p r e c i p i t a t i n g x y l e n e s o l u t i o n i n acetone F r a c t i o n I I r e c o v e r e d by removal o f s o l v e n t from f i l t r a t e T o l u e n e , 30°C

c

d

e

f

DISCUSSION The complete s o l u b i l i t y i n r e f l u x i n g x y l e n e o f t h e EPR and EPR-g-MAH produced by r e a c t i o n w i t h DCP a t 180°C s u g g e s t s t h a t t r e a t m e n t w i t h c y c l o h e x a n e a t 22°C does n o t s e p a r a t e t h e c r o s s l i n k e d polymer from the u n c r o s s l i n k e d polymer b u t r a t h e r f r a c t i o n a t e s t h e u n c r o s s l i n k e d polymer based on m o l e c u l a r w e i g h t . A l t e r n a t i v e l y , the l i g h t l y c r o s s l i n k e d polymer i s s w o l l e n and t h e c r o s s l i n k s a r e broken i n r e ­ f l u x i n g xylene. The c r o s s l i n k i n g o f EPR i n t h e p r e s e n c e o f r a d i c a l s from p e r o x i d e d e c o m p o s i t i o n i s a t t r i b u t a b l e t o a t t a c k on t h e s e c o n d a r y CH2 m o i e t i e s and t h e g e n e r a t i o n o f polymer r a d i c a l s which c o u p l e e i t h e r w i t h s i m i ­ l a r s e c o n d a r y polymer r a d i c a l s o r polymer r a d i c a l s g e n e r a t e d by a t t a c k on t h e t e r t i a r y CH m o i e t i e s on t h e p r o p y l e n e u n i t s i n t h e chain. The g e n e r a t i o n o f t e r t i a r y r a d i c a l s by t h e l a t t e r r o u t e i s t h e predominant r e a c t i o n . The t e r t i a r y r a d i c a l s p r e f e r e n t i a l l y undergo d i s p r o p o r t i o n a t i o n , r e s u l t i n g i n degradation r a t h e r than c r o s s l i n k i n g . I n c r e a s i n g t h e DCP c o n c e n t r a t i o n r e s u l t s i n an i n c r e a s e i n t h e amount o f s o l u b l e polymer a n d a d e c r e a s e i n t h e m o l e c u l a r weight o f t h e polymer due t o i n c r e a s e d t e r t i a r y CH a t t a c k , f o l l o w e d by d i s p r o p o r ­ tionation. The r a p i d d e c o m p o s i t i o n o f a p e r o x i d e i n t h e p r e s e n c e o f ΜΑΗ r e s u l t s i n t h e e x c i t a t i o n and h o m o p o l y m e r i z a t i o n o f ΜΑΗ. When t h e l a t t e r i s c o n d u c t e d i n t h e p r e s e n c e o f p o l y p r o p y l e n e , the l a t t e r undergoes d e g r a d a t i o n (10) w h i l e p o l y e t h y l e n e i s c r o s s l i n k e d under the same c o n d i t i o n s ( 1 1 ) . T h i s has been a t t r i b u t e d t o t h e p r e s e n c e o f e x c i t e d ΜΑΗ which i n c r e a s e s the r a d i c a l g e n e r a t i o n on t h e polymer beyond t h a t due t o t h e r a d i c a l s from t h e p e r o x i d e . A t a f i x e d ΜΑΗ c o n c e n t r a t i o n , i n c r e a s i n g t h e p e r o x i d e c o n t e n t r e s u l t s i n a h i g h e r c o n c e n t r a t i o n o f e x c i t e d ΜΑΗ and a g r e a t e r gen­ e r a t i o n o f polymer r a d i c a l s . The l a t t e r undergo d i s p r o p o r t i o n a t i o n and t h e amount o f s o l u b l e polymer i n c r e a s e s w h i l e i t s m o l e c u l a r weight decreases. Notwithstanding, a t a given peroxide concentration,

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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CHEMICAL REACTIONS ON POLYMERS

the p r e s e n c e o f ΜΑΗ i n c r e a s e s t h e amount o f c y c l o h e x a n e - i n s o l u b l e polymer a p p r o x i m a t e l y t h r e e f o l d . A t a f i x e d DCP c o n c e n t r a t i o n , i n ­ c r e a s i n g t h e ΜΑΗ c o n t e n t has l i t t l e e f f e c t on t h e c o n c e n t r a t i o n o f e x c i t e d ΜΑΗ and t h e r e f o r e a n e g l i g i b l e e f f e c t on t h e amount o f s o l u b l e polymer. F u r t h e r , t h e ΜΑΗ c o n t e n t o f the polymer d e c r e a s e s w i t h an i n c r e a s e i n t h e amount o f ΜΑΗ i n t h e c h a r g e due t o t h e q u e n c h i n g o f e x c i t e d ΜΑΗ by e x c e s s ground s t a t e Μ Α Η . A l t h o u g h i t has been s u g g e s t e d t h a t c r o s s l i n k i n g i n t h e p r e s e n c e o f ΜΑΗ i n v o l v e s c o u p l i n g o f appended ΜΑΗ r a d i c a l s w i t h o t h e r appended ΜΑΗ r a d i c a l s o r w i t h polymer r a d i c a l s (7), the former i s i m p r o b a b l e due t o t h e tendency f o r d i s p r o p o r t i o n a t i o n r a t h e r t h a n c o u p l i n g between r a d i c a l s d e r i v e d from s t r o n g e l e c t r o n a c c e p t o r monomers s u c h as Μ Α Η . The i n c r e a s e d c r o s s l i n k i n g may be a t t r i b u t e d t o t h e p r e f e r e n t i a l g e n e r a t i o n o f s e c o n d a r y polymer r a d i c a l s due t o t h e p r e s e n c e o f e x c i t e d ΜΑΗ and t h e r e f o r e an i n c r e a s e i n t h e amount o f c o u p l i n g o f s e c o n d a r y polymer r a d i c a l s r a t h e r t h a n o f pendant ΜΑΗ r a d i c a l s . Stearamide i s one o f many e l e c t r o n donors which donate an e l e c ­ t r o n t o t h e c a t i o n i c m o i e t y i n e x c i t e d Μ Α Η o r i n p r o p a g a t i n g -ΜΑΗ c h a i n s . T h i s r e s u l t s i n t h e i n h i b i t i o n o f the h o m o p o l y m e r i z a t i o n o f ΜΑΗ and d e c r e a s e s t h e c r o s s l i n k i n g o f p o l y e t h y l e n e and the d e g r a d a ­ t i o n o f p o l y p r o p y l e n e which accompany t h e p e r o x i d e - c a t a l y z e d r e a c t i o n o f ΜΑΗ w i t h t h e s e p o l y o l e f i n s ( 8 , 9 ) . The p r e s e n c e o f s t e a r a m i d e i n t h e EPR-MAH-DCP r e a c t i o n i n c r e a s e s the y i e l d o f s o l u b l e polymer, i . e . d e c r e a s e s t h e c r o s s l i n k i n g , which polymer has a h i g h e r m o l e c u l a r w e i g h t , i . e . d e c r e a s e s t h e d e g r a d a t i o n , t h a n n o t e d i n t h e absence o f s t e a r a m i d e .

Literature Cited 1. Crespi, G.; Bruzzone, M. Chim. e Ind. (Milan) 1959, 41, 741. 2. Natta, G.; Crespi, G.; Bruzzone, M. Kautschuk u. Gummi 1961, 14 (3), WT54. 3. Natta, G.; Crespi, G.; Valvassori, A.; Sartori, G. Rubber Chem. Technol. 1963, 36, 1583. 4. Loan, L. D. J. Polym. Sci. 1964, A2, 3053. 5. Harpell, G. A.; Walrod, D. H. Rubber Chem. Technol. 1973, 46, 1007. 6. Lenas, L. P. Ind. Eng. Chem., Prod. Res. Dev. 1963, 2, 202. 7. Natta, G.; Crespi, G.; Borsini, G. (to Montecatini Soc. Gen.) U.S. Patent 3 236 917, 1966. 8. Gaylord, N. G. (to Gaylord Research Institute, Inc.) U.S. Patent 4 506 056, 1985. 9. Gaylord, N. G.; Mehta, R. Polymer Preprints 1985, 26 (1), 61. 10. Gaylord, N. G.; Mishra, M. K. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 23. 11. Gaylord, N. G.; Mehta, M. J. Polym. Sci., Polym. Lett. Ed. 1982, 20, 481. RECEIVED October

13, 1987

Benham and Kinstle; Chemical Reactions on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.