Controlled Release Pesticides - American Chemical Society

13 Effects of W a l l Parameters on the Release of Active Ingredients from Microencapsulated Insecticides J. R. LOWELL, JR. and W. H. CULVER Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0053.ch013

Pennwalt Corp., 1630 E. Shaw Ave., Suite 179, Fresno, Calif. 93710 C. B. DeSAVIGNY Pennwalt Corp., P. O. Box 3608, Bryan, Tex. 77801

A p p l i c a t i o n of Pennwalt microencapsulation technology t o methyl parathion has produced the first commercial microencapsulated p e s t i c i d e , PENNCAP-M I n s e c t i c i d e . In the course of our encapsulation development programs a s u b s t a n t i a l body o f i n f o r m a t i o n has been c o l l e c t e d regarding the e f f e c t s of v a r i o u s w a l l parameters on r e l e a s e r a t e s o f microencapsulated formulations. The polymer system used in preparation of PENNCAP-M is a c r o s s l i n k e d nylon-type polymer produced from sebacoyl c h l o r i d e , ethylenediamine (EDA), d i e t h y l e n e t r i a m i n e (DETA), and polymethylenepoly-phenylisocyanate ( P A P I ) . The a c t i v e i n g r e d i e n t , d i a c i d c h l o r i d e and PAPI form a homogeneous mixture, which is dispersed in an aqueous medium. When brought i n t o contact w i t h an aqueous s o l u t i o n of the diamines, a shell of c r o s s - l i n k e d nylon forms around each i n d i v i d u a l d r o p l e t . The product is typically formulated as an aqueous suspension, as w i t h PENNCAP-M. While m a t e r i a l s encapsulated may i n c l u d e liquids, s o l u t i o n s , suspensions, o r solids, our efforts have been most s u c c e s s f u l w i t h the two former c a t e g o r i e s . Encapsulation in similar polymers has produced e f f e c t i v e i n s e c t i c i d e formulations of Diazinon®, parathion, v a r i o u s p y r e t h r o i d s , and malathion. In this paper some observations will be presented on the e f f e c t s of capsule w a l l t h i c k n e s s , c r o s s l i n k i n g , and a c i d c h l o r i d e s on r e l e a s e r a t e s o f active ingredients. I v y has p r e v i o u s l y reported on the r e d u c t i o n i n mammalian t o x i c i t y and the extension o f i n s e c t i c i d a l a c t i v i t y r e s u l t i n g from encapsulation of methyl parathion. As p r e s e n t l y marketed, PENNCAP-M i s a t l e a s t 6 times l e s s t o x i c o r a l l y and 12 times l e s s t o x i c dermally than e m u l s i f i a b l e concentrate formu®

®

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145 In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0053.ch013

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Figure 1.

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Effect of wall thickness on bioactivity of encapsulated parathion

In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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l a t i o n s o f methyl parathion, on an a c t i v e i n g r e d i e n t basis. I n s e c t i c i d a l a c t i v i t y i s e x t e n d e d , u p t o 2% times, depending on t h e crop and weather c o n d i t i o n s . DeSavigny and I v y have r e p o r t e d f u r t h e r on t h e Pennwalt microencapsulation process, r e v i s i n g t h e t o x i c o l o g y f i g u r e s a s p r o d u c t i o n e x p e r i e n c e was o b tained. Additional insect control reports confirmed t h e g r e a t e r r e s i d u a l a c t i v i t y o f PENNCAP-M a s c o m p a r e d w i t h EC f o r m u l a t i o n s . We h a v e a l s o r e p o r t e d o n t h e u s e o f PENNCAP-M f o r e f f e c t i v e i n s e c t c o n t r o l on tobacco p l a n t s , and on t h e e c o n o m i c s o f m i c r o e n c a p s u l a t i o n a s d e m o n s t r a t e d b y o u r e x p e r i e n c e w i t h PENNCAP-M . K o e s t l e r has proposed a mechanism f o r t h e a c t i o n o f m i c r o e n c a p s u l a t e d h e r b i c i d e s and i n s e c t i c i d e s , suggesting t h a t t h e h i g h number o f p a r t i c l e s , c l o s e p a r t i c l e s p a c i n g , and h i g h l o c a l c o n c e n t r a t i o n s o f t o x i c a n t a r e responsible f o rthe e f f e c t i v e n e s s o f t h e microencapsulated products. D u r i n g d e v e l o p m e n t o f a new m i c r o e n c a p s u l a t e d pesticides i n our l a b o r a t o r i e s , candidate preparat i o n s a r e s c r e e n e d b y means o f a b i o a s s a y technique. The b i o a s s a y c o n s i s t s o f s p r a y i n g a s u i t a b l e s u r f a c e , u s u a l l y bean f o l i a g e o r cardboard, w i t h a standard amount o f t h e i n s e c t i c i d e . The s u r f a c e i s t h e n infested a t suitable intervals with insects, generally crickets. Percent m o r t a l i t y i s determined as a function o f time a f t e r spraying. Corrected percent m o r t a l i t y f i g u r e s f o rt h e d u r a t i o n o f t h e t e s t a r e averaged, g i v i n g a " B i o a c t i v i t y " f i g u r e which a p p r o a c h e s 100 f o r a m a t e r i a l w h i c h i s i n s e c t i c i d a l l y a c t i v e and p e r s i s t s f o rt h e d u r a t i o n o f t h e t e s t . Low b i o a c t i v i t y r e s u l t s m o s t o f t e n i n d i c a t e a r a p i d r e l e a s e and lowp e r s i s t e n c e , b u t o c c a s i o n a l l y a formulation w i l l produce a lowpercent m o r t a l i t y which p e r s i s t s f o rthe duration o f the t e s t , i n d i c a t i n g very slow r e l e a s e . 2


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The t h i c k n e s s o f t h e c a p s u l e w a l l c a n b e estimated by simple geometry c a l c u l a t i o n s t o be d i r e c t l y p r o p o r t i o n a l t o t h e percentage o f polymerforming i n g r e d i e n t s i n t h e formulation. F o r any f o r m u l a t i o n , then, t h e "percent w a l l " c a nbe r e p r e sented as a r e l a t i v e w a l l thickness. I n F i g u r e 1, the b i o a c t i v i t y o f parathion encapsulated i n three d i f f e r e n t w a l l systems i s d e p i c t e d as a f u n c t i o n o f wall thickness. I ti s r e a d i l y apparent t h a t i n each case t h e b i o a c t i v i t y dropped as t h e w a l l t h i c k n e s s increased. I n t h e c a s e o f W a l l A, i n p a r t i c u l a r , t h e d a t a showed t h a t t h e p e r c e n t m o r t a l i t y was i n c r e a s i n g with time f o ra l l three w a l l thicknesses, i n d i c a t i n g American Chemical Society Library

1155 18th St.,

N.W.

In Controlled Release Pesticides; Scher, H.; Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

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t h a t t h e l o w b i o a c t i v i t y f o r t h i s w a l l was due t o very slow release. I n F i g u r e 2, i t c a n b e s e e n t h a t f o r D i a z i n o n encapsulated i n Wall D b i o a c t i v i t y on f o l i a g e increased with increasing w a l l thickness u n t i l a p l a t e a u was r e a c h e d . When t e s t e d i n s o i l , h o w e v e r , t h e same f o r m u l a t i o n s h o w e d i n c r e a s e s i n b i o a c t i v i t y w i t h i n c r e a s i n g w a l l t h i c k n e s s u n t i l a p e a k was reached, a f t e r which the b i o a c t i v i t y decreased. F o l i a r b i o a c t i v i t y with malathion encapsulated in Wall D increased with increasing wall thickness. In F i g u r e 3, r e s u l t s f o r e n c a p s u l a t e d m e t h y l p a r a t h i o n s h o w e d a n i n s e n s i t i v i t y t o w a l l t h i c k n e s s when e n c a p s u l a t e d i n W a l l D. Another major f a c t o r i n f l u e n c i n g the r e l e a s e of a c t i v e i n g r e d i e n t from capsules i s crosslinking. I n F i g u r e 4, t h e d a t a o b t a i n e d s h o w e d a p e a k i n b i o a c t i v i t y a t 2 5 % c r o s s l i n k i n g f o r W a l l E, u s i n g b o 1 1 w o r m s a s t h e t e s t i n s e c t . The increased b i o a c t i v i t y i n t h i s c r o s s l i n k i n g s t u d y was associated w i t h lower release r a t e s . DeGennaro e t a l . h a v e shown t h a t t h e r e l e a s e o f sodium p e n t o b a r b i t a l i n c r e a s e d w i t h i n c r e a s i n g c r o s s l i n k i n g b y i n c r e a s i n g p r o p o r t i o n s o f DETA. The i n c r e a s e was a t t r i b u t e d t o i n c r e a s i n g p o r o s i t y a s t h e p r e p a r a t i o n became more h i g h l y c r o s s l i n k e d . It s h o u l d be n o t e d , h o w e v e r , t h a t t h e i r s t u d i e s u t i l i z e d an i o n i c a c t i v e i n g r e d i e n t , w h i l e o u r preparations have used r e l a t i v e l y n o n p o l a r o r g a n i c m a t e r i a l s . C r o s s l i n k i n g i n t h e i r s y s t e m was a c h i e v e d w i t h a p o l y f u n c t i o n a l amine, w h i l e i n ours, a p o l y f u n c t i o n a l isocyanate i s the p r i n c i p a l source of c r o s s l i n k i n g . E f f e c t s of the i d e n t i t y of the a c i d c h l o r i d e have not been e x t e n s i v e l y i n v e s t i g a t e d i n our l a b o r a tories. As shown i n F i g u r e 5, t h e b i o a c t i v i t y v a r i e d w i t h the chain length of the d i c a r b o x y l i c a c i d . In W a l l F, a s s h o w n , t h e b i o a c t i v i t y was l o w e s t w i t h a z e l a o y l c h l o r i d e , w h i l e a d i p o y l and s e b a c o y l c h l o r i d e b o t h gave s i g n i f i c a n t l y g r e a t e r a c t i v i t y i n t h i s w a l l formulation. In c o n c l u s i o n , the data presented here have shown t h a t b i o a c t i v i t y v a r i e s i n a n i r r e g u l a r m a n n e r w i t h i n c r e a s i n g w a l l t h i c k n e s s and i n c r e a s i n g c r o s s linking. The a c i d c h l o r i d e c h o s e n a l s o a f f e c t e d a c t i v i t y , b u t t e s t i n g has been i n s u f f i c i e n t t o draw any c o n c l u s i o n s . I t m u s t t h u s be c o n c l u d e d , t h e n , t h a t p r o d u c i n g an optimum e n c a p s u l a t e d p e s t i c i d e f o r m u l a t i o n by t h e P e n n w a l t p r o c e s s s t i l l r e q u i r e s an e m p i r i c a l approach. F o r each p r o d u c t w h i c h has reached the f i e l d development stage, a p a i n s t a k i n g l a b o r a t o r y i n v e s t i g a t i o n of each of these v a r i a b l e s


f

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M - Wall D -Foliage ID - Wall D -Foliage

D - Wall D - S o i l

l

ι 1

ι 2

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ι 4

Relative Wall Thickness

Figure 2. Effect of wall thickness on bioactivity of encapsulated Diazinon (D) and mahthion (M)

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Figure 3.

Effect of wall thickness on bioactivity of encapsulated methyl parathion



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Figure 5. Effect of acid chloride chain length on bioactivity of encapsulated parathion


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has

been necessary. One a s p e c t o f t h e e f f e c t o f c r o s s l i n k i n g i s worth noting here. With regard to the suggestion of DeGennaro e t a l . t h a t t h e e f f e c t o f i n c r e a s i n g c r o s s l i n k i n g i n t h e i r capsule formulations increased the r e l e a s e r a t e b y i n c r e a s i n g p o r o s i t y , we generally have found the o p p o s i t e e f f e c t w i t h the r e l a t i v e l y n o n p o l a r p e s t i c i d e s w h i c h we h a v e e n c a p s u l a t e d , when c r o s s l i n k i n g was e f f e c t e d b y a p o l y f u n c t i o n a l isocyanate. I t may b e t h a t t h e e f f e c t s o b s e r v e d w i t h i n c r e a s i n g amounts o f p o l y f u n c t i o n a l isocyanate r e l a t e t o changes i n the s o l v e n t p r o p e r t i e s of the capsule w a l l polymer, a l t e r i n g the s o l u b i l i t y of the d i f f u s i n g s u b s t a n c e i n t h e w a l l , and t h u s c h a n g i n g i t s d i f f u s i o n c h a r a c t e r i s t i c s . U n l e s s a f f e c t e d by s w e l l i n g or h y d r a t i o n e f f e c t s , the e f f e c t s of changes i n m i c r o p o r o s i t y s h o u l d be s i m i l a r i n d i r e c t i o n f o r i o n i c or nonionic materials. I f the e f f e c t of the polymer composition i s to a l t e r the s o l u b i l i t y of the encapsulated m a t e r i a l i n the polymer, then opposite e f f e c t s f o r i o n i c and n o n p o l a r s p e c i e s , as o b s e r v e d h e r e , m i g h t be e x p e c t e d . We o f f e r t h i s a s a w o r k i n g hypothesis deserving of further study. PAPI i s a trademark o f Upjohn Diazinon i s a trademark of Ciba-Geigy

Literature Cited 1. Ε. E. Ivy, J . Econ. Entomol., 65(2), 473 (1972). 2. C. B. DeSavigny and Ε. E. Ivy, in J . E. Vandagaer, E d i t o r , "Microencapsulation: Processes and A p p l i c a t i o n s , " Plenum P u b l i s h i n g Corp. 3. J . R. L o w e l l , J r . and J . L. Frizzell, Reprints of Papers from the Symposium "Chemical Require ments of the Tobacco Industry," 169th Meeting of the American Chemical Society, P h i l a d e l p h i a , PA, April, 1975. 4. J . R. L o w e l l , J r . and J . J . Murnighan, Reprints of Papers from the Symposium "Economics and Market Opportunities f o r C o n t r o l l e d Release Products," 172nd Meeting of the American Chemical S o c i e t y , San F r a n c i s c o , CA, August, 1976. 5. R. C. K o e s t l e r , i n N. F. Cardarelli, E d i t o r , "Proceedings 1976 C o n t r o l l e d Release P e s t i c i d e Symposium," Akron, OH, September, 1976. 6. M. DeGennaro, et al., i n D. R. Paul and F. W. H a r r i s , Ed., " C o n t r o l l e d Release Polymeric Formulations," ACS Symposium S e r i e s , American Chemical Society, Washington, D.C., 1976, pp. 195-207.


Controlled Release Pesticides - American Chemical Society

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