12 Microencapsulated Pesticides
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HERBERT B. SCHER Formulations Section, Chemical Research Department, Stauffer Chemical Co., 1200 So. 47th St., Richmond, Calif. 94804
Microcapsules are 1-200 micron p a r t i c l e s composed o f a s o l i d or l i q u i d core surrounded by a w a l l . The wall i s g e n e r a l l y polymeric i n nature (Figure 1) and c o n s t i t u t e s 5-25 percent o f the microcapsule by weight. The wall i s o l a t e s and protects the core material i n storage but i s designed to r e l e a s e the core m a t e r i a l i n a c o n t r o l l e d fashion when the microcapsules are e x posed t o the environment. The core material can be released from the microcapsules by crushing the w a l l , breaking the wall by pressure from w i t h i n , d i s s o l v i n g the w a l l , hydrolyzing the wall o r by d i f f u s i n g through the w a l l . Controlled release of pesticides ( i n s e c t i c i d e s , herbicides, f u n g i c i d e s , fumigants, j u v e n i l e hormone mimics, i n s e c t sex a t t r a c t a n t s and animal health compounds) can be achieved by microencapsulation. P e s t i c i d e microcapsule systems can be designed t o : 1. Reduce mammalian t o x i c i t y and extend a c t i v i t y . 2. Reduce evaporative l o s s e s . 3. Reduce p h y t o t o x i c i t y . 4. Protect p e s t i c i d e s from environmental degradation. 5. Reduce l e a c h i n g . 6. Reduce p e s t i c i d e l e v e l s i n the environment. An aqueous d i s p e r s i o n o f p e s t i c i d e microcapsules i s a p a r t i c u l a r l y useful c o n t r o l l e d r e l e a s e p e s t i c i d e formulation because: 1. It i s composed o f d i s c r e t e microcapsules as opposed to aggregates. 2. I t can be d i l u t e d with water or l i q u i d f e r t i l i z e r s and sprayed using conventional equipment. Uniform f i e l d coverage o f p e s t i c i d e i s p o s s i b l e . 3. I t requires l e s s polymeric component per pound o f p e s t i c i d e than m o n o l i t h i c d e v i c e s . 4. I t i s capable o f e s t a b l i s h i n g a constant p e s t i c i d e r e l e a s e rate (See Figures 2 and 3 ) .
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In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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12.
SCHER
Microencapsulated
127
Pesticides
Gelatin
Polyethers
Gum A r a b i c
Polyesters
Starch
Polyamides
Sugar
Polyureas
Ethyl
Polybutadiene
Cellulose
Carboxymethyl
Cellulose
Polysiloxanes
Paraffin Polyvinyl
Polyisoprene
alcohol
Polyurethanes
Polyethylene
Epoxy r e s i n s
Polypropylene
inorganic
silicates
Polystyrene Polyacrylamide Encyclopedia of Polymer Science and Technology
Figure 1. Common microcapsule wall materials
In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
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128
C O N T R O L L E D R E L E A S E PESTICIDES
0 * Diffusion Coefficient Κ » Distribution Coefficient DK * Permeability AC
s
Concentration Difference Across Wall
I n i t i a l high release due to migration of p e s t i c i d e
approaches exhaustion
Time Chemical Technology Figure 2.
Pesticide release rate from microcapsule (2)
In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.
Microencapsulated
SCHER
Pesticides
129
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(SLAB GEOMETRY)
Dissolved P e s t i c i d e dM dt
dM dt
2M„
Dispersed P e s t i c i d e 1/2
M
dM dt
π l t 2
f o r 0.4
