Microcapsules containing insecticides

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

Capsule diameter (pm) Figure 5.5 Size distributions of microcapsules containing insecticide [28]. 

The delivery vehicle of fenvalerate-containing insecticides may account for wide variations in toxic action. For example, fenvalerate microcapsules used to control caterpillar pests (Plutella xylostella, Spodoptera liturd) were most effective with tMn-walled capsules and small particles however, significant protection of nontarget organisms, such as fish, occurred with thicker-walled capsules and larger particles. The popularity of commercial synthetic pyrethroids and their widespread replacement of older, more-toxic compounds in various settings... 

Ohtsubo et al. have described a method to identify those parameters which determine the physical strength of PU microcapsules containing liquid insecticide (viz., fenitrothion) [45]. These microcapsules have been found to be effective in con-troUing household pests such as cockroaches. As expected, the strength of the microcapsule depends mainly on the microcapsule mass mean diameter (D), the thickness of the membrane wall (T), and particle size distribution expressed as polydispersity (e.g., D /DJ. The coefficient of variation of the particle size distribution (CV) is first determined using any suitable instrument (particle size analyzer) and method. Wall thickness T is calculated from the following equation ... 

Aminoplast (MF) has been used to prepare microcapsules of hquid pesticides, namely methyl parathion [71] and metachlor [72]. Other selected examples of microcapsules containing agrochemicals such as pesticides, herbicides, fertihzers and insecticides have been reported and are summarized in Table 5.10. 

The widely used organophosphate Insecticide methyl parathlon was the first material to be formulated as a microencapsulated pesticide. This formulation, sold under the tradename PENNCAP-M Insecticide (a registered trademark of Pennwalt Corporation), consists of nylon-type microcapsules which contain the active Ingredient. The capsules are suspended In water and typically have an average particle size of approximately 25 microns (fifty percent by weight of the capsules have a particle size of 25 microns or more). Upon application by conventional spray equipment the water evaporates, and the active Ingredient Is slowly released over an extended period of time. 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Another innovation in in-situ microencapsulation is aminoplast shellwalls containing base-cleavable ester moiety [15]. Polyols reacted with diacids that contain thiol or hydroxy functionality produce crosslinking groups. These crosslinking groups along with urea-formaldehyde prepolymer are dissolved into the pesticide and the in-situ microencapsulation process is completed. The resultant microcapsules may contain an insecticide that is safer to handle under acidic conditions yet will rapidly release the insecticide in the alkaline gut of an insect. Alternately, base could be added to the spray tank to rapidly release capsule contents prior to application. 

Microencapsulation. Microencapsulation (qv) has now been commercially practiced for more than 30 years, following the first application of the technology to carbonless copying paper. Pesticide formulations based on microcapsules appeared in 1974 with the product Penncap-M containing the insecticide methyl parathion (4). Since then many microcapsule suspension formulations have been introduced and form the major group of CRF. 

Example 19.1-2 Bollworm pheromones You have been using 1.0-mm-diameter microcapsules filled with a saturated solution of the pheromone gossyplure, for the pink bollworm. When the capsules contain excess solid pheromone, they attract bollworm moths into insecticide-loaded traps for 27 days. 

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