Aminoplast microcapsules

Acrylate capsules produced by radical-induced polymerization. Gelatin capsules manufactured by coacervation. Polyurethane capsules produced by polyaddition of amines with diisocyanates. Aminoplast microcapsules made by polycondensation. 

FIGURE 60.1 Aminoplastic microcapsules, definition of capsule size. 

When mortality assessment experiments were carried out with Helicoverpa zea (a foliar-feeding lepidopteran with an alkaline gut), the LCjq values at 2 DAT for Lorsban 4E, standard aminoplast microcapsules and microcapsules with >90% disulfide Hnkages were 14.5, 96.4 and 14.7 ppm, respectively. The results indicated that microcapsules made only of aminoplast resin did not have disulfide linkages and thus did not break down in the gut of the insect, as indicated by the higher LC5Q value. However, those microcapsules with disulfide linkages ruptured in the insect gut and provided comparable control to that with Lorsban 4E. 

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. 

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. 

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