Processing of microcellular foam was studied for tough and lightweight polyurethane. In order to increase the nucleation rate in a gas supersaturated resin, ultrasonic excitation was applied to the mixture of polyol(polyether-based polyol) and isocyanate(diphenyl methane diisocyanate). Microcellular structure was produced by two sequential processes, i.e., supersaturation of the polyol resin with nitrogen gas at elevated pressure and ultrasonic bubble nucleation right after the impingement mixing of two components of the polyurethane system. Theoretical analyses based on the nucleation theories were employed to predict the rate of nucleation in the gas supersaturated polyurethane. The rate of nucleation in the resin before curing was predicted by the classical nucleation theory and the cluster theory. Ultrasonic excitation was experimentally applied to increase the rate of nucleation in the resin which had been saturated by nitrogen at low saturation pressure below 2.0 MPa. Visualization of impingement mixing in the mixing chamber demonstrated the effect of Reynolds number on the mixture.
Assuming diffusion controlled bubble growth, theoretical prediction was carried out numerically to understand the bubble growth mechanism in the mold during mold filling. Final bubble sizes were calculated by considering the gelation time and the diffusion boundary. Variation in the viscosity of polyurethane during polymerization reaction was predicted by considering reaction kinetics and the gelation time was determined. The diffusion boundary was predicted by the number of nucleated bubbles which were determined theoretically and experimentally.
