Multiple-layer coating of solids with impermeable film dispersing permeable particles for controlled release

For precise control of the release rate for fertilizers, pharmaceuticals, etc., the release characteristics were investigated by preparing active core particles coated with multiple layers of insoluble but permeable particles dispersed inside impermeable wax. The release profiles were described by a mathematical model which was successfully verified with experimental data. The two release stages were taken into account; the initial stage corresponds to a constant concentration of the inner solution during the existence of the solid core, and the final stage to a decreasing inner concentration with the elapse of time without the core. As a result, the low release rate was attained by involving the layer of a low volume fraction of permeable particles most closely to the core or by a thicker layer. Also, the effect of the layer thickness was verified to be lower than that of the volume fraction. Thus, the present results confirmed the potentiality of the multiple layer coating to achieve the desired release rate characteristics by choosing the proper combination of the volume fraction of permeable particles, the layer thickness and the position of the layer.