A thermally expandable microcapsule with core-shell structure was newly developed for foaming thermoplastic polymers. It can be utilized in the injection molding or extrusion processes of the thermoplastic polymers typically operated at temperatures above 200℃, which the conventional thermally expandable microcapsules could not be applied to. The core-shell structure consists of polymer shell and hydrocarbon in core, which is used as a physical blowing agent. A mathematical model was developed to describe the expansion behavior and it was used to find key factors for controlling expandability of the capsule and to design the new capsule. The visual observation of batch foaming experiments of an existing microcapsule and numerical simulation calculations with the developed models elucidated that the rheological properties of the shell polymer and permeability of blowing agent in the shell polymer are two of the important factors to be tuned for developing the new capsule. For the new microcapsule, both the viscosity and permeability of the polymer shell were controlled by optimizing the composition in the shell polymer. Furthermore, in the final stage of developments, we investigated rheological properties, gas permeability, expandability of the newly developed microcapsule and the surface appearance of foamed products injection-molded and extruded with the microcapsule. The new capsule could achieve more than 30％ density reduction while maintaining a smooth surface at PP foam injection molding and extrusion.
