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

•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.