Anisotropic thermal expansion of bio-based rigid low-density closed-cell polyurethane foams

Rigid low-density polyurethane (PU) foams containing bio-based components are being increasingly considered as a sustainable substitute for petrochemical resources-based foams for thermal insulation in ambient conditions. However, for applications involving high temperature gradients, the thermal expansivity of foams also has to be characterised. With this aim, rigid low-density polyurethane foams have been manufactured using polyols derived from renewable resources (tall oil fatty acids) and their thermal expansion in the temperature range from 20 to −150 °C was studied. The mean coefficients of linear thermal expansion (CTE) in this range and the differential CTEs at 20 and -150 °C were determined. Increasing the cross-link density of the PU polymer matrix resulted in reduction of the CTEs of both monolithic polymer and closed-cell PU foams. Changing the molecular weight per branching unit of PU from 442 to 318 g/mol caused reduction of the differential CTE of the monolithic PU by about 30% at 20 °C and by 11% at −150 °C. The CTEs of anisotropic foams were expressed in terms of CTE of the polymer and mechanical characteristics of the foams. The predicted CTEs were found to be in good agreement with experimental ones within the temperature range considered.