Epoxy Resin Chemistry II - American Chemical Society

15 Water Dispersible Epoxy-g-Acrylic Copolymer for Container Coating Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 5, 2015 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch015

1

2

JAMES T. K. WOO, VINCENT TING, JAMES EVANS, CARLOS ORTIZ, GARY CARLSON, and RICHARD MARCINKO 3

Glidden Coatings & Resins, Dwight P. Joyce Research Center, Strongsville, OH 44136

The role of the interior coatings for cans in contact with food or beverage is demanding indeed. Some of the general requirements for food and beverage can coatings are listed in the following: 1) It must be non-toxic and free from flavors and odors which would adversely affect the quality of the products packed. 2) It must not be affected by the products packed and must prevent reaction between the contents and the metal of the can. 3) It must readily be applicable and rapidly be cured on various types of cans. The product should also be suitable for the type of equipment used. 4) It must have good adhesion and be flexible and tough enough to maintain continuous protective film in subsequent use. 5) It must be economical. For beer application, special care must be taken with its preservation and it is particularly important with beer cans that coverage should be complete. The critical flavor nature for beer demands the above. The resins used for the manufacture of food and beverage can lacquers may be either natural or synthetic. Resins combined with drying or semi-drying oils form a class of coatings known as oleoresinous. Other coatings for food and beverage cans are obtained using synthetic resins such as phenolics and epoxies, and acrylic, vinyl, and butadiene polymers. For a brief summary as to what type of resins are used for cans, see Table I. 1 2

3

Current address: IBM Corporation, Boulder, CO 80301 Current address: Glidden Organic Chemicals, Jacksonville, FL 32201 Current address: Tintas Yperanga, Rio de Janeiro, Brazil

0097-6156/83/0221-0283$06.00/0 © 1983 American Chemical Society

In Epoxy Resin Chemistry II; Bauer, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

284

EPOXY RESIN CHEMISTRY

TABLE I . 3 p i e c e beverage can 3 piece s a n i t a r y can

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 5, 2015 | http://pubs.acs.org Publication Date: June 8, 1983 | doi: 10.1021/bk-1983-0221.ch015

2 piece D & I can

V i n y l r e s i n , butadiene polymer. Epoxyphenolic, oleoresinous. Epoxy

A l l these are s o l v e n t based m a t e r i a l s , and normally low i n s o l i d s . With the i n t r o d u c t i o n of p o l l u t i o n l e g i s l a t i o n s , i t i s imperative that n o n - p o l l u t i n g polymers be developed f o r food and beverage a p p l i c a t i o n s . Many approaches have been described i n the patent l i t e r a t u r e . They i n c l u d e water-borne systems powder , high s o l i d s , r a d i a t i o n curable , electrocoat , e t c . The types of r e s i n described i n c l u d e a c r y l i c , p o l y e s t e r , epoxy, v i n y l and combina t i o n s of these. In t h i s chapter, we w i l l d i s c u s s some of the chemistry of a novel approach to i n t e r i o r can c o a t i n g . ^) approach i n v o l v e s g r a f t i n g of c a r b o x y l c o n t a i n i n g monomers onto high molecular weight epoxy r e s i n . Epoxy r e s i n s ( e p i bisphenol A type) are known f o r t h e i r adhesion, c o r r o s i o n r e s i s t a n c e and i n e r t n e s s p r o p e r t i e s . A d d i t i o n a l l y , they are hydrophobic. E s t e r g r a f t i n g of epoxy r e s i n s by r e a c t i o n of cgrboxylepoxide f u n c t i o n a l i t i e s has been known f o r many years The c a r b o x y l bearing molecule can be an anhydride, f a t t y a c i d or a c r y l i c copolymer c o n t a i n i n g carboxyl f u n c t i o n a l i t i e s . Upon n e u t r a l i z a t i o n w i t h base, the epoxy r e s i n i s rendered water d i s p e r s i b l e . The obvious weak l i n k i n t h i s approach i s the presence of e s t e r l i n k a g e which w i l l hydrolyze w i t h time, causing i n s t a b i l i t y . In order not to have t h i s weak l i n k , the carboxyl c o n t a i n i n g monomer was g r a f t e d onto high mol. wt. epoxy r e s i n (by carbon-carbon bond f o r m a t i o n ) . The r e s u l t a n t a c i d c o n t a i n i n g g r a f t copolymer can be dispersed i n t o water upon n e u t r a l i z a t i o n w i t h base. Due to the presence of g r a f t copolymer and absence of e s t e r l i n k a g e the emulsion formed has e x c e l l e n t s t a b i l i t y . For most polymers , i t i s o f t e n thermodynamically unfavorable f o r them to form homogeneous mixtures with each other. The key i s to produce microheterogeneous polymeric systems so that each component polymer can s t i l l r e t a i n most of i t s i n d i v i d u a l p r o p e r t i e s while c o n t r i b u t i n g i n a s y n e r g i s t i c way to provide new macroscopic p r o p e r t i e s f o r the m a t e r i a l s as a whole. Often times, polymeric emulsions are formed. A l l emulsions are thermodynamically unstable because t h e i r i n t e r f a c i a l area i s orders of magnitude greater than the i n t e r f a c i a l area of the corresponding coagulated systems. A s o - c a l l e d " s t a b l e emulsion" i s i n r e a l i t y a meta-stable system. The input of a c e r t a i n a c t i v a t i o n energy i s necessary f o r c o a g u l a t i o n to occur, and the T n



15.

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ET AL.

Water

Dispersible

Epoxy-g-Acrylic

Copolymer

285

higher t h i s a c t i v a t i o n energy, t h e higher i s t h e m e t a - s t a b i l i t y of the emulsion. The mechanism o f s t a b i l i z a t i o n o f p o l y m e r i c o i l - i n - o i l emulsions c o n s i s t s o f the f o l l o w i n g : 1) I n c o m p a t i b i l i t y o f p o l y m e r s c a u s e s t h e phenomenon o f phase s e p a r a t i o n o f p o l y m e r s i n s o l u t i o n , 2) t h i s c a u s e s t h e f o r c e w h i c h d r i v e s t h e g r a f t copolymer i n t o t h e i n t e r f a c e o f polymeric o i l - i n - o i l emulsions, and 3) t h i s c a u s e s t h e f o r m a t i o n o f c o a l e s c e n c e b a r r i e r s . W i t h o u t t h e phenomenon o f phase s e p a r a t i o n w h i c h l e a d s t o a r e p u l s i o n o f d i s s i m i l a r p o l y m e r c h a i n s , t h e r e w o u l d be no r e a s o n f o r a g r a f t c o p o l y m e r t o a c c u m u l a t e i n t h e i n t e r f a c e o f an i m m i s c i b l e polymer s o l u t i o n . In order t o s t a b i l i z e a polymeric o i l - i n - o i l e m u l s i o n , a g r a f t c o p o l y m e r must a c c u m u l a t e i n t h e i n t e r f a c e o f t h e e m u l s i o n and f o r m a p r o t e c t i v e c o a t i n g a r o u n d the e m u l s i o n d r o p l e t s ; a s o - c a l l e d c o a l e s c e n c e b a r r i e r . I n t h e c a s e o f t h e e p o x y - a c r y l i c g r a f t c o p o l y m e r , t h e epoxy i s n o t s o l u b l e i n t h e monomer m i x t u r e , even a s l o w a s a 10% solution. However, t h e epoxy r e s i n b e i n g t h e m a j o r i t y component a c t s a s t h e c o n t i n u o u s p h a s e . The p u r p o s e o f t h e g r a f t e p o x y - s t y r e n e - m e t h a c r y l i c a c i d copolymer i s t o lower t h e b a r r i e r a t t h e i n t e r f a c e so t h a t a s t a b l e o i l - i n - o i l e m u l s i o n i s f i r s t o b t a i n e d and upon n e u t r a l i z a t i o n w i t h a t e r t i a r y a m i n e , d i m e t h y l e t h a n o l amine, a s t a b l e o i l - i n - o i l e m u l s i o n i n w a t e r i s t h e n obtained. S y n t h e s i s . a n d c h a r a c t e r i z a t i o n o f t h e g r a f t copolymer has been r e p o r t e d . The Epoxy r e s i n u s e d i s a h i g h m o l e c u l a r w e i g h t (Mn=10,000) e p i c h l o r o h y d r i n - B i s p h e n o l t y p e , and t h e a c r y l i c i s a p p r o x i m a e l y a 2:1 m o l a r r a t i o o f m e t h a c r y l i c acid/styrene. The E p o x y / a c r y l i c r a t i o i s a p p r o x i m a t e l y 80/20 by w e i g h t . G r a f t i n g i s a c h i e v e d , t ^ . f r e e r a d i c a l means. In f r e e r a d i c a l g r a f t i n g , t h e r e a r e two p o s s i b l e processes taking place: (a) G r a f t i n g f r o m : t h e f r e e r a d i c a l ( o r o t h e r a c t i v e s i t e ) i s g e n e r a t e d on t h e backbone and s u b s e q u e n t l y i t i n i t i a t e s t h e p o l y m e r i z a t i o n o f monomers t o p r o d u c e branches. (b) G r a f t i n g o n t o : a g r o w i n g f r e e r a d i c a l ( o r o t h e r a c t i v e s p e c i e s ) a t t a c k s another preformed polymer p r e f e r e n t i a l l y c a r r y i n g s u i t a b l e s u b s t i t u e n t s and t h e r e b y p r o d u c e s a branch o f t h e preformed backbone. W i t h a few e x c e p t i o n s , most g r a f t c o p o l y m e r s f r o m f r e e r a d i c a l induced g r a f t i n g processes u s u a l l y produce n o t o n l y t h e d e s i r e d g r a f t c o p o l y m e r s , b u t a l s o homopolymers and o t h e r s i d e reactions. C o n s e q u e n t l y , t h e e x p l o r a t i o n and d e t a i l e d c h a r a c t e r i z a t i o n o f g r a f t s p r o d u c e d by f r e e r a d i c a l methods i s o f t e n cumbersome. C h a r a c t e r i z a t i o n o f t h e g r a f t c o p o l y m e r by s o l v e n t e x t r a c t i o n i n d i c a t e s t h a t a s u b s t a n t i a l amount o f t h e epoxy r e s i n and o f t h e a c r y l i c are free. About 4 7 % o f t h e epoxy r e s i n i s u n g r a f t e d , 6 1 %




286

o f t h e a c r y l i c monomer p o l y m e r i z e s t o f o r m f r e e , u n g r a f t e d a c r y l i c c o p o l y m e r s . The f r e e epoxy r e s i n i s o l a t e d i s o f l o w e r m o l e c u l a r w e i g h t t h a n t h a t o f t h e s t a r t i n g e p o x y r e s i n and the f r e e a c r y l i c copolymer i s h i g h e r m o l e c u l a r weight than t h a t of t h e a c r y l i c c o p o l y m e r made u n d e r t h e same c o n d i t i o n s i n t h e a b s e n c e o f epoxy r e s i n . We were a l s o a b l e t o d e d u c e t h e following: ( I ) The amount o f g r a f t i n g between 3% and 15% f r e e r a d i c a l i n i t i a t o r b a s e d on monomer i s r e l a t e d t o t h e amount o f s u c h i n i t i a t o r u s e d . From f r a c t i o n a l p r e c i p i t a t i o n d a t a o f g r a f t c o p o l y m e r s p r e p a r e d w i t h i n c r e a s i n g amounts o f f r e e r a d i c a l i n i t i a t o r , t h e r e i s an i n c r e a s i n g l e v e l o f an a c i d c o n t a i n i n g epoxy f o r m e d . In f r a c t i o n a l p r e c i p i t a t i o n , the epoxy-ga c r y l i c c o p o l y m e r m i x t u r e was d i s s o l v e d i n N - m e t h y l - p y r r o l i done (NMP), a s o l v e n t t h a t w i l l d i s s o l v e a l l t h r e e c o m p o n e n t s , f r e e epoxy r e s i n , f r e e a c r y l i c c o p o l y m e r and g r a f t c o p o l y m e r . Upon a d d i n g t o l u e n e (a n o n s o l v e n t f o r t h e a c r y l i c c o p o l y m e r ) , t h e a c r y l i c p o r t i o n t e n d s t o s t a y i n t h e NMP r i c h l a y e r and t h e f r e e epoxy w i l l m i g r a t e t o t h e t o p t o l u e n e r i c h l a y e r , w h i c h s h o u l d have an a b s e n c e o f a c i d f u n c t i o n a l i t i e s . However, epoxy r e s i n t h a t has few c a r b o x y l f u n c t i o n a l i t i e s w i l l behave l i k e f r e e epoxy r e s i n and m i g r a t e t o t h e t o l u e n e rich layer. W i t h t h i s t e c h n i q u e , we were a b l e t o d e t e r m i n e t h a t s a m p l e s made w i t h h i g h l e v e l o f f r e e r a d i c a l i n i t i a t o r have h i g h e r amounts o f c a r b o x y l c o n t a i n i n g epoxy r e s i n s . T h i s i n c r e a s e d a c r y l i c s o l u b i l i t y or r e l a t i v e p e r c e n t g r a f t i n g can be shown t o be a s t r a i g h t l i n e f u n c t i o n s w i t h percent free r a d i c a l i n i t i a t o r . See F i g u r e I .

Figure

1. S o l u b i l i t y data of g r a f t copolymers p r e p a r e d w i t h d i f f e r e n t l e v e l s of f r e e r a d i c a l i n i t i a t o r .


15.

woo

ET AL.

Water

Dispersible

Copolymer

287

The minimum amount o f f r e e r a d i c a l i n i t i a t o r u s e d h a s t o be a p p r o x i m a t e l y 3% b a s e d on monomer w e i g h t . H i g h e r l e v e l s o f f r e e r a d i c a l i n i t i a t o r seems t o g i v e h i g h e r l e v e l o f g r a f t i n g . O r d i n a r i l y , h i g h l e v e l s o f i n i t i a t o r e i t h e r cause homopolymeri z a t i o n o f monomer (°Ç) recombination termination processes . During r a d i a t i o n g r a f t i n g , a s i m i l a r phenomenon i s o b s e r v e d , t h a t i s t h e p e r c e n t g r a f t i n g i n c r e a s e s w i t h dosage up t o a c e r t a i n p o i n t t h e n l e v e l s o f f . A l l o f o u r d a t a seems t o v e r i f y t h e f r a c t i o n a t i o n r e s u l t s , t h a t i s the h i g h e r l e v e l o f f r e e r a d i c a l i n i t i a t o r produces h i g h e r l e v e l of g r a f t i n g . When t h e g l a s s t r a n s i t i o n t e m p e r a t u r e o f t h e g r a f t c o p o l y m e r s p r e p a r e d w i t h d i f f e r e n t amounts o f f r e e r a d i c a l i n i t i a t o r were c o m p a r e d , t h e d a t a showed a d i s t i n c t n a r r o w i n g t r e n d i n t h e g l a s s t r a n s i t i o n t e m p e r a t u r e r a n g e . T h i s seems t o i n d i c a t e t h a t w i t h h i g h e r amounts o f f r e e r a d i c a l i n i t i a t o r , t h e g r a f t c o p o l y m e r i s becoming more u n i f o r m o r i t may i n d i c a t e t h a t t h e r e may be a h i g h e r l e v e l o f g r a f t i n g p r e s e n t . See F i g u r e I I . i s


Epoxy-g-Acrylic

w

a

s

t

e

d

b

v

P a r t i c l e S i z e Data The p a r t i c l e s i z e o f t h e aqueous d i s p e r s i o n made w i t h i n c r e a s i n g l e v e l s o f f r e e r a d i c a l i n i t i a t o r show a s i g n i f i c a n t d e c r e a s e . The p a r t i c l e s i z e d a t a i s summarized i n T a b l e I I .

40 u ο

30

10

1

2

4

6

8

% free radical F i g u r e 2.

Tg range v s f r e e

radical

10

12

14

initiator initiator

concentration.



288 TABLE I I .


P a r t i c l e s i z e D i s t r i b u t i o n o f D i s p e r s i o n from Grafted Copolymers Made With D i f f e r e n t L e v e l s o f Free R a d i c a l I n i t i a t o r Concentration

% Free Radical Initiator Concentration 1) 1 2) 2 3) 3 4) 5 5) 15

Surface Diameter

Volume Diameter

Specific Surface

Wt. Ave. Diameter

2.5675 1.6910 0.8716 0.5529 0.3108

2.7826 1.8393 0.9537 0.5761 0.3158

18000 27030 51530 94060 180400

2.1953 2.6304 2.8190 1.6759 1.1457

The smaller p a r t i c l e s i z e d i s p e r s i o n made with higher l e v e l of f r e e r a d i c a l i n i t i a t o r concentration suggests that higher l e v e l s o f g r a f t copolymer are formed with higher l e v e l s o f f r e e radical initiator. T h i s suggests that the g r a f t copolymer behaves as a s u r f a c t a n t . ( I I ) Higher molecular weight epoxy r e s i n tends to be p r e f e r e n t i a l l y g r a f t e d . This i s i n c o n t r a s t with g r a f t i n g on polystyrene where the r e l a t i v e r e a c t i v i t y o f polystyrene was found to increase with decreasing molecular weight. This suggests that the r e a c t i v i t y o f polystyrene depends mainly on the nature of i t s c o i l e d conformation i n solution. In the epoxy r e s i n case, the molecular weight o f the epoxy r e s i n i s low compared to that o f polystyrene. The higher molecular weight epoxy r e s i n tends to have greater r e a c t i v i t y towards g r a f t i n g . One o f the reasons could be that there are more g r a f t i n g s i t e s a v a i l a b l e i n a higher molecular weight epoxy r e s i n . This can be demonstrated as f o l l o w s : In a high molecular weight epoxy r e s i n (Mn approx. 10,000) there are roughly 34 repeating u n i t s of OH -0-CH -CH-CH -0 i n the backbone. There a r e , t h e r e f o r e , a l s o 34 χ 5 = 170 hydrogens that can be abstracted by f r e e r a d i c a l to form a r a d i c a l s i t e where i n i t i a t i o n of monomers can occur. 2

2


15.

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ET AL.

Water

Dispersible

Epoxy-g-Acrylic

OH


289

OH

-0-CH -CH-CH -0 2

Copolymer

» -0-CH -C-CH -0- + RH

2

2

2

OH VM I -O-CH2-C-CH2-OI M M "MX

Now i f a l o w m o l e c u l a r w e i g h t epoxy r e s i n i s u s e d , e.g. Mn a p p r o x . 1000, (Epon 1001 o r DER 661) t h e r e a r e a b o u t 2 OH

I -0-CH -CH -CH - 0 - i n t h e epoxy b a c k b o n e , and o n l y 10 a b s t r a c t a b l e hydrogens. ( I l l ) Epoxy f u n c t i o n a l i t i e s a r e n o t e s s e n t i a l f o r g r a f t i n g . F o r a g r a f t c o p o l y m e r made by t h e r e a c t i o n o f epoxy w i t h c a r b o x y l f u n c t i o n a l i t i e s , t h e p r e s e n c e o f epoxy functionalities i s essential. S i n c e t h i s approach r e q u i r e s c a r b o n - c a r b o n bond f o r m a t i o n , e s t e r l i n k a g e s a r e n o t necessary. T h e r e f o r e epoxy f u n c t i o n a l i t i e s c a n be capped w i t h phenol or benzoic a c i d , e t c . , o r bisphenol-A t e r m i n a t e d epoxy r e s i n c a n be p r e p a r e d f o l l o w e d by g r a f t i n g w i t h a c i d c o n t a i n i n g monomers, and r e s u l t a n t e p o x y - g - a c r y l i c c o p o l y m e r c a n be p r e p a r e d . 2

MECHANISM OF GRAFTING From c l a s s i c a l p o l y m e r i z a t i o n scheme, t h e r e i s : a) I n i t i a t i o n b) P r o p a g a t i o n c) Chain T r a n s f e r d) T e r m i n a t i o n I n an i d e a l i z e d c a s e , where t r a n s f e r mechanism o n l y o c c u r s t o a " F o r e i g n P o l y m e r " , i . e . t r a n s f e r t o t h e monomer i s n e g l i g i b l e , an e q u a t i o n c a n be d e r i v e d t o show t h a t t h e r a t e o f grafting i s : Vr = k

r

[ P r . ] [R]

[M]



290


Where [ P r . ] i s t h e c o n c e n t r a t i o n o f g r o w i n g p o l y m e r c h a i n s formed f r o m t h e i n i t i a t o r . [ R ] i s t h e t r a n s f e r p o l y m e r added t o t h e s y s t e m [M] i s t h e c o n c e n t r a t i o n o f monomer. T h e r e f o r e , t h e h i g h e s t y i e l d s o f g r a f t e d copolymer should be o b t a i n e d u n d e r t h e f o l l o w i n g c o n d i t i o n s : 1) i n c r e a s i n g c o n c e n t r a t i o n o f t r a n s f e r polymer t o a l i m i t i n g value. 2) h i g h r a t e s o f i n i t i a t i o n o b t a i n e d by a) increasing i n i t i a t o r concentration b) i n c r e a s i n g the p o l y m e r i z a t i o n temperature In p r a c t i c e i t i s n o t u s u a l l y p o s s i b l e t o t a k e advantage o f t h e s e c o n d i t i o n s s i n c e t h e m a j o r i t y o f polymers have l i m i t e d s o l u b i l i t y i n " f o r e i g n " monomer s o l u t i o n s , p a r t i c u l a r l y a t e l e v a t e d t e m p e r a t u r e s where t h e h i g h i n i t i a t i o n r a t e s l e a d t o g e l a t i o n and p h a s e s e p a r a t i o n . However, s u b s t i t u t i o n o f a r e s i n o f l o w m o l e c u l a r weight f o r t h e c o n v e n t i o n a l backbone polymer o f h i g h m o l e c u l a r w e i g h t e n a b l e s t h e s e d i f f i c u l t i e s t o be overcome to a l a r g e e x t e n t . " R e s i n s " o f l o w m o l e c u l a r w e i g h t s u c h as c o n v e n t i o n a l epoxy r e s i n s , p o l y e t h y l e n e g l y c o l s , p o l y ( m e t h o x y a c e t a l s ) , e t c . a r e r e a d i l y s o l u b l e i n s u c h monomers a s m e t h y l methacrylate, styrene, v i n y l acetate, etc., to give high c o n c e n t r a t i o n s o f " g r a f t i n g polymer". Even a t c o m p a r a t i v e l y h i g h r a t e s o f i n i t i a t i o n t h e s e s y s t e m s i n many c a s e s r e m a i n c o m p l e t e l y c o m p a t i b l e and p o l y m e r i z a t i o n c a n be t a k e n t o c o m p l e t e c o n v e r s i o n w i t h o u t phase s e p a r a t i o n on a m a c r o s c a l e . (18-21) T h e r e a r e numerous e x a m p l e s i n t h e l i t e r a t u r e " where f r e e r a d i c a l s a r e g e n e r a t e d on t h e p o l y m e r b a c k b o n e f o l l o w e d b y g r a f t i n g o f monomer o n t o t h e p o l y m e r , i . e . i n i t i a t i o n of monomer occurring after chain transfer reaction. F o r example, in g r a f t i n g monomer o n t o p o l y e s t e r f i b e r , a c t i v e c e n t e r s seem t o be c r e a t e d by d i r e c t hydrogen a b s t r a c t i o n from t h e p o l y e s t e r m o l e c u l e s by t h e p r i m a r y f r e e r a d i c a l s p e c i e s b e n z o y l o x y r a d i c a l o r by t h e s e c o n d a r y f r e e r a d i c a l s p e c i e s p h e n y l r a d i c a l . Benzol p e r o x i d e (BPO) was u s e d a s i n i t i a t o r . T h e r e was no m e n t i o n o f b e n z o i c a c i d o r benzene f o r m a t i o n . The o t h e r mechanism m e n t i o n e d was by o x i d i z i n g t h e p o l y e s t e r t o h y d r o p e r o x i d e a t s e v e r a l p o i n t s a l o n g t h e c h a i n i n a random manner. The h y d r o p e r o x i d e decomposes i n t o t h e a c t i v e form a t h i g h temperature t o produce u l t i m a t e l y m a c r o r a d i c a l s one o f w h i c h may be r e p r e s e n t e d a s q

0

0

These r a d i c a l s i t e s p e r m i t attachment w h i c h may grow i n t o s h o r t c h a i n s . T h i s i • ' g r a f t i n g f r o m " ( ) p r o c e s s where t h e f r e e a c t i v e s i t e ) i s g e n e r a t e d on t h e backbone A Z

o f monomer m o l e c u l e s s an example o f r a d i c a l (or other and s u b s e q u e n t l y i t



15.

woo

ET AL.

Water

Dispersible

Epoxy-g-Acrylic

Copolymer

291

i n i t i a t e s t h e p o l y m e r i z a t i o n o f monomers t o p r o d u c e b r a n c h e s . The o t h e r g r a f t i n g p r o c e s s i s t h e " g r a f t i n g o n t o " where a g r o w i n g f r e e r a d i c a l (or other a c t i v e s p e c i e s ) a t t a c k s another preformed p o l y m e r p r e f e r e n t i a l l y c a r r y i n g s u i t a b l e s u b s t i t u e n t s and t h e r e b y p r o d u c e a b r a n c h on t h e p r e f o r m e d b a c k b o n e . G r a f t e p o x y - a c r y l i c copolymer prepared w i t h a f r e e r a d i c a l i n i t i a t o r i s an example o f t h e " g r a f t i n g f r o m " p r o c e s s . I n t h e c a s e where b e n z o y l p e r o x i d e was u s e d as t h e f r e e r a d i c a l i n i t i a t o r , i t i s d e t e r m i n e d t h a t a b o u t 77% o f t h e f r e e r a d i c a l i n i t i a t o r i n s t e a d o f c a u s i n g i n i t i a t i o n o f monomers, c h a i n t r a n s f e r s w i t h t h e epoxy r e s i n b a c k b o n e , f o l l o w e d by t h e " g r a f t i n g f r o m " ofpmonomers o n t o t h e epoxy r e s i n . Benzoyl p e r o x i d e i s known t o decompose m o s t l y (90%) to the benzoyloxy r a d i c a l and t h e (10%) phenyl r a d i c a l . Mechanisms o f g r a f t i n g can be d e m o n s t r a t e d i n t h e f o l l o w i n g two schemes. (Schemes A and B ) . T h e r e i s p r o b a b l y a v e r y s m a l l amount o f Scheme Β p r e s e n t as t h e a l i p h a t i c p r o t o n s o f t h e epoxy r e s i n a r e much more a c t i v a t e d t o w a r d s f r e e r a d i c a l a b s t r a c t i o n t h a n t h e h y d r o g e n o f t h e 2butoxy-ethanol-1. The 23% o f t h e b e n z o y l p e r o x i d e t h a t d i d n o t c h a i n t r a n s f e r w o u l d i n i t i a t e p o l y m e r i z a t i o n o f monomers t o f o r m u n g r a f t e d styrene-methacrylic a c i d copolymer. T h i s c o u l d be p a r t o f t h e reason t h a t the ungrafted or f r e e s t y r e n e - m e t h a c r y l i c a c i d copolymer i s o f h i g h e r m o l e c u l a r weight than t h a t o f the s t y r e n e - m e t h a c r y l i c a c i d c o p o l y m e r made u n d e r t h e same c o n d i t i o n s i n t h e a b s e n c e o f t h e epoxy r e s i n . T h i s l o w e r amount o f f r e e r a d i c a l i n i t i a t o r would r e s u l t i n a h i g h e r m o l e c u l a r weight copolymer. The o t h e r r e a s o n i s p o s s i b l y due t o v i s c o s i t y e f f e c t , s u c h as Trommsdorf e f f e c t where h i g h e r m o l e c u l a r p o l y m e r i s o b t a i n e d due t o l e s s e r c h a n c e o f t e r m i n a t i o n i n a more v i s c o u s medium ( i . e . i n t h e p r e s e n c e o f h i g h m o l e c u l a r w e i g h t epoxy resin). From t h e d a t a g e n e r a t e d so f a r , an a t t e m p t was made t o determine the e p o x y - a c r y l i c g r a f t copolymer composition. Seventy-seven p e r c e n t o f t h e b e n z o y l p e r o x i d e formed b e n z o i c a c i d by h y d r o g e n a b s t r a c t i o n . T h i s was done by u s i n g s t y r e n e as t h e o n l y monomer and t h e r e s u l t a n t b e n z o i c a c i d t i t r a t e d i s due t o a b s t r a c t i o n o f h y d r o g e n by b e n z o y l o x y r a d i c a l . 11.6

χ 0.77

= 9 m mole o f BPO

i s involved in grafting

S i n c e a b o u t h a l f o f t h e epoxy r e s i n and a b o u t t w o - t h i r d s o f t h e a c r y l i c a r e f r e e , t h e g r a f t e d c o m p o s i t i o n i s now l i s t e d i n Table ( I I I ) .



Ο

• r

OH 2

2

MMMMM

1

OH

Monomer

M

The l a t t e r i s probably more l i k e l y due to proximity and concentration of the a l i p h a t i c hydrogens of the epoxy backbone.

Solvent

C-OH+

OH I 0-CH-CH-CH -0

-O^^^^O^H CH-CH .0-^gK

Epoxy Resin Chemistry II - American Chemical Society

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