Microcellular Thermosetting Polyurethane Foams

Thermosetting polyurethane foams are nowadays produced with typical bubble size, d > 150 μm, with plenty of room for improvement towards the cellular structure refinement, to gain, among others, in the thermal insulation performances. We herein report a first example of a microcellular thermosetting polyurethane foam, i. e. with bubble size below 5 μm, produced via the gas foaming technology. In particular, high-pressure CO2, N2 and their mixtures were utilized as blowing agents: solubilized separately into the polymer precursors, they were brought into a supersaturated state by a pressure reduction to induce the bubble nucleation and growth. To achieve microcellular foams, we made use of a novel two-stage pressure reduction program, concurrent to the polymer curing. The first stage is a pressure quench O (10-2 s) from the saturation pressure to an intermediate pressure to induce the nucleation of a large amount of dense bubbles. The second stage is a slow O (102 s) further pressure decrease to ambient pressure, allowing for a slow bubble growth, designed to reach ambient pressure exactly when the curing reached completion.