Thermosetting polyurethane foams by physical blowing agents: Chasing the synthesis reaction with the pressure

[Display omitted] •A two-stage process was designed to gas-foaming thermosetting polyurethane.•In the first stage, a fast pressure quench to a non-zero pressure is utilized to induce bubble nucleation.•In the second stage, pressure was slowly decreased to ambient pressure to induce bubble growth.•This second stage is designed to chase the synthesis kinetics, not to induce excessive stress on the expanding matter.•Rigid microcellular PU (20 μm and 150 kg/m3) could be successfully produced. We developed a novel method to produce microcellular thermosetting polyurethane foaming by the gas-foaming technique using high-pressure physical blowing agents. In particular, to tackle the inherent difficulties of imposing a rapid pressure quench O(10−2 s) to a material whose synthesis timing is much larger O(102 s), we utilized a two-stage foaming. In the first stage, a rapid pressure quench O(10−2 s) from the saturation pressure to an intermediate pressure, was imposed to nucleate a large amount of bubbles; in the second stage, the growth of the nucleated bubble is controlled by slowly O(102 s) decreasing the pressure to ambient pressure. In this way, by separating the nucleation from the growth stage and by chasing the synthesis reaction with the pressure to avoid excessive stress to the curing polymer, we achieved fine-celled (size diameter of 20 μm), medium-to-low density (150 kg/m3) thermosetting polyurethane foams.