Microencapsulation by both simple and complex coacervation is known. U.S. Pat. No. 2,800,457 discloses the basic process of complex coacervation including the steps of (1) emulsifying an oily core ingredient in an aqueous solution of a first colloid, (2) mixing the emulsion with an aqueous solution of a second colloid having opposite electrical charge to that of the first colloid, (3) causing coacervation of the colloids around the oil droplets by dilution with water or by pH adjustment, (4) cooling the mixture to about 0.degree. C. to 10.degree. C. to cause gelation of the coacervate, and (5) hardening the gelled coacervate by pH or temperature adjustment or by treatment with aldehydes. U.S. Pat. No. 3,341,466 discloses a complex coacervation process enabling production of large microcapsules with thick walls. U.S. Pat. No. 3,549,555 discloses a simple coacervation process for encapsulation of oil-in-water emulsions. U.S. Pat. No. 3,956,172 discloses a complex coacervation process wherein the microcapsules are hardened rapidly, without yellowing, and without an increase in the viscosity of the microencapsulation mixture. British Patent No. 929,406 discloses a method of increasing the hardness and decreasing the premeability of microcapsules by treating them with trivalent iron salts and gallic, digallic or tannic acids, using polyoxyethylene sorbitan monostearate in concentrations well below 1% as a surfactant to oppose the tendency of the microcapsules to aggregate.
However, owing to phase inversion, the requirement for dilution and for proper pH, all of the foregoing processes suffer from the drawbacks of requiring constant attention and adjustment of stirring conditions, solution viscosity, pH, and temperature, often producing microcapsules with a significant degree of agglomeration. Furthermore, such prior art processes are particularly unsatisfactory for the production of microcapsules of certain core ingredients that are appreciably soluble in the capsule wall material such as the mosquito repellent, N,N-diethyl-m-toluamide (deet). Typically, microcapsules of such core ingredients are weak, and are composed of less than 50% by dry weight of the core ingredient. In addition, such microencapsulation processes have a poor yield of core ingredient, with typically only about 10% to 40% of the core ingredient being encapsulated.
What is needed, therefore, is a simple highyield microencapsulation process capable of producing stable, relatively high-core-content microcapsules rapidly and without agglomeration, especially with respect to core ingredients that are appreciably soluble in the microcapsule wall material.