6 Matrix Factors Affecting the Controlled Release of Pesticides from Elastomers N. F. CARDARELLI and S. V. KANAKKANATT
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Environmental Management Laboratory, University of Akron, Akron, Ohio 44325
I t i s w e l l known t h a t v a r i o u s o r g a n i c m a t e r i a l s u t i l i z e d as pest c o n t r o l agents a r es o l u b l ei n elastomers. I f t h es o l u t i o n l i m i t i s n o t exceeded, s o l u t e molecules a r euniformly dispersed throughout the m a t r i x i naccordance w i t h t h e p r i n c i p l e s governing s o l u t i o n e q u i l i b r i u m . During environmental exposure agent molecules adsorbed on t h eelastomer s u r f a c e o r absorbed j u s t below t h a t s u r f a c e w i l l pass v i a d i s s o l u t i o n processes into t h esurrounding a i r , s o i l , o r water. This action e f f e c t i v e l y creates a condition o f l o c a l i z e d d i s e q u i l i b r i u m and s o l u t e molecules, d r i v e n by s o l u t i o n p r e s s u r e , m i g r a t e t o w a r d s t h e d e p l e t i n g surface. Thus a c o n t i n u o u s l o s s p r o c e s s i s e s t a b lished. The mechanism h a s been t r e a t e d a t l e n g t h by v a r i o u s authors^-"^. B i o l o g i c a l l y e f f i c a c i o u s longevi t y o f s u c h s y s t e m s h a s b e e n known t o e x c e e d n i n e years.2 Present commercial a p p l i c a t i o n s a r ei n t h e areas o f a n t i f o u l i n g coatings and long l a s t i n g m o l l u s c i c i d e s . ^""7 In g e n e r a l , work w i t h c o n t r o l l e d r e l e a s e m a t e r i a l s has been keyed t o p r a c t i c a l a p p l i c a t i o n s a n d b a s e d upon an e m p i r i c a l a p p r o a c h . The d i f f u s i o n - d i s s o l u t i o n mechanism h a s been e l u c i d a t e d b u t v e r y l i t t l e p a s t e f f o r t has been devoted t o t h e development o f a fundam e n t a l u n d e r s t a n d i n g r e g a r d i n g t h e i n f l u e n c e o f compounding v a r i a b l e s andm a t r i x s e l e c t i o n on t h e loss mechanism. Such knowledge would be o f c o n s i d e r a b l e value t o the design o f controlled release pesticide formulations and theo p t i m i z a t i o n o f favorable o r d e s i r a b l e p r o p e r t i e s . The work r e p o r t e d h e r e i n i s a step i nthat d i r e c t i o n .
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6.
CARDARELLI AND
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General
KANAKKANATT
Matrix
Factors
61
Considerations
P r o p e r compounding o f t h e c o n t r o l l e d release formulation i s e s s e n t i a l t o develop various desirable properties - long h a l f - l i f e , a p p r o p r i a t e agent loss r a t e , i n t e g r i t y o f t h e p r o c e s s e d geometry and t h e like. E x p e r i e n c e h a s shown t h a t t h e n a t u r e o f t h e m a t r i x , degree o f c r o s s l i n k i n g and t h e use o f a d d i t i v e s such as c u r a t i v e s , a c c e l e r a t o r s , reinforcing agents, a n t i o x i d a n t s , e t c . g r e a t l y i n f l u e n c e agent s o l u b i l i t y and l o s s r a t e . The c u r e s y s t e m u s e d a f f e c t s both t h e r a t e o f c r o s s l i n k i n g and t h e u l t i m a t e crosslink density. Carbon b l a c k and p o s s i b l y o t h e r r e i n f o r c i n g additives tend t o decrease the surface loss rate o f the p e s t i c i d e by i n c r e a s i n g t h e d i f f u s i o n p a t h l e n g t h . I t may a p p e a r t h a t t h e p a r t i c l e s i z e o f c a r b o n b l a c k u t i l i z e d i s a s c r i t i c a l a s t h e amount. But fine b l a c k s do n o t r e t a r d o r p r e v e n t d i f f u s i o n t o any g r e a t e r degree than coarse carbon b l a c k s w i t h i n t h e l i m i t s studied. In general, increasing the overall m o l e c u l a r weight and/or d e c r e a s i n g t h e m o l e c u l a r w e i g h t between c r o s s l i n k s w i l l lower t h e d i f f u s i o n c o e f f i c i e n t thus slowing loss r a t e . The s e g m e n t a l m o t i o n o f a p o l y m e r c h a i n i n c r e a s e s as t h e t e m p e r a t u r e r i s e s o r when t h e a m b i e n t t e m p e r a t u r e i s v e r y f a r removed from 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 s (Tg) a n d t h e t e s t t e m p e r a t u r e w o u l d b e much a b o v e i t . V a r i a t i o n i n t h e e n v i r o n m e n t a l t e m p e r a t u r e s (T) a l t e r s t h e m a g n i t u d e o f t h e d i f f e r e n c e , Τ - Tg. Two d i s t i n c t v o l u m e f r a c t i o n s m u s t b e c o n s i d e r e d i n a thorough study. One c o n s i s t s o f t h e v o l u m e f r a c t i o n o f t h e rubber i n t h e s w o l l e n s t a t e , a r i s i n g from pesticide incorporation. T h i s volume f r a c t i o n reflects the o r i g i n a l concentration of the pesticide. The o t h e r f r a c t i o n c o n s i s t s o f t h a t v o l u m e o f r u b b e r containing the carbon black a d d i t i v e . The v a r i a t i o n o f t h e a b o v e p a r a m e t e r s i s n o t entirely linear. However, f o r s i m p l i c i t y and t h e narrow l i m i t s o f v a r i a t i o n e n v i s i o n e d , the assumption o f l i n e a r i t y appears t o be j u s t i f i a b l e . Serious departures from l i n e a r i t y a r e covered i n t h i s t r e a t ment by a p p r o x i m a t e c o r r e c t i o n factors. P r o p e r t i e s such as c r y s t a l l i n i t y , diffusant geometry, m o l e c u l a r weight o f t h e s t a r t i n g elastomer, and i n t e r a c t i o n s b e t w e e n a d d i t i v e s a r e n o t c o n s i d e r e d at t h i s time. Thus i t i s n e c e s s a r y t o h a v e a r e f e r ence f o r m u l a t i o n . B a s e d upon d i f f u s i o n d a t a o b t a i n e d w i t h t h e r e f e r e n c e m a t e r i a l , new f o r m u l a t i o n s c a n b e 8
In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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CONTROLLED RELEASE PESTICIDES
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Theoretical
Treatment
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