Open-ended pores of a porous granule are filled with a liquid material which is at least partially immiscible with water, and sealed with a porous polyurea membrane which limits the release rate of the material into the surrounding medium. The membrane is formed by (a) applying to the bare granule an organic solution comprising the liquid material and an organic polyisocyanate, followed by (b) applying to the granule an aqueous solution comprising water and a catalytic amount of a catalyst selected from the group consisting of a basic organic tertiary amine and an alkyl tin carboxylate.
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