Thermal insulation properties of a rigid polyurethane foam synthesized via emulsion-template

Abstract

One approach to reducing the thermal conductivity of polymer foams, commonly employed as insulation materials, involves decreasing the pore size. By reducing the pore size to a few microns or less, the Knudsen effect can occur, leading to a decrease in the thermal conductivity of the gas within the pores. Consequently, there has been significant research on reducing pore size to this scale. However, the majority of studies have focused on thermoplastic polymer foams, leaving cross-linked thermoset foams relatively understudied. Rigid polyurethane foam, a typical thermoset polymer foam, generally exhibits pore sizes primarily above 100 µm. Unlike thermoplastic foams, rigid polyurethane foams are produced by mixing prepolymers, leading to the formation of numerous bubbles inside. This characteristic makes it challenging to produce foams with pore sizes smaller than the initial bubbles due to non-classical nucleation via these bubbles. In this study, a novel emulsion-template process was employed to address these limitations. This process involved dispersing oil droplets of several microns in the prepolymer to form an emulsion, initiating a urethane reaction, and subsequently removing the dispersed phase. As a result, rigid polyurethane foams with pores of several microns were successfully produced. Furthermore, it was confirmed that these micropores positively impacted the foam’s thermal insulation performance.

Graphical abstract

Rigid polyurethane foam with bimodal pore size distribution can be made via emulsion template. Gas filling small pores (~ 1 μm) has a lower thermal conductivity than gas filling large pores (> 100 μm), even if the gas composition is the same.