Coating Performance of Aqueous Composite Latices with N-Isopropylacrylamide Shell and Thermosensitive Permeation Properties of Their Microcapsule Membranes.

Abstract

An aqueous composite latex which suppressed particle adhesion by electrostatic charge and agglomeration in coating and had self-film-formability in dissolution was developed as a coating meterial for subsieve-sized particles using the Wurster process. This composite latex was composed of a poly(ethyl acrylate (EA)/methyl methacrylate(MMA)/2-hydroxyethyl methacrylate (HEMA)) core and a thermosensitive poly(N-isopropylacrylamide (NIPAAm)) shell. The polymer yield in coating of 53-63μm lactose with homogeneou latices without poly(NIPAAm) shell exhibited a remarkable particle size dependency because of particle adhexion arising form the electrostatic charge. On the contrary, composite latices reduced the production of poorly particles and the particle size dependency of polymer yield, when the coating operation was done at temperatures where poly(NIPAAm) shells were swellable, i.e., below the lower critical solution temperature (LCST; 32°C) of poly(NIPAAm). Agglomeration tendency strongly depended on the hardness of the poly(EA/MMA/HEMA) cores. Microcapsules coated with composite latices exhibited a temperature-sensitive release of water-soluble lactose, while with homogeneous latices the release was simply enhanced with rise in temperature. When the microcapsules coated with composite latices were exposed to a critical temperature around LCST during the dissolution process, the lactose release from them was most suppressed, suggesting that shrinkage of the dehydrated poly(NIPAAm) shells in the coat of the microcapsules at the critical temperature would contribute to formation of a compact film which could strongly suppress water- and lactose-permeation.