Aging properties of a vegetable-based polyurethane foam under high relative humidity and different temperatures

The objective of this study is to assess, characterize, and forecast the aging effects on the mechanical properties of a vegetable-based polyurethane foam (PUF) derived from castor oil under elevated relative humidity conditions and at two distinct load orientations (aligned with the expansion direction and perpendicular to it). Ten specimens were subjected to temperatures of 60, 75, and 90°C, along with a relative humidity of 90%, for a duration of up to 60 days. Compression tests, following ASTM D1621-16 standards and supplemented by 3D digital image correlation, revealed nearly isotropic behavior with marginally lower compression strength in the expansion direction. On the other hand, the strength of the foams at 90°C was higher in that direction. This led to a smaller activation energy (Ea) in this property, which ended up showing a shorter lifespan prediction considering the strength retention in this direction. The stiffness values were proportionally more similar in both directions than the strength ones and this was observed in the Ea for those curves. In contrast to several other foams and polyurethanes, the one examined in this study exhibited consistently higher Ea values for its mechanical properties, particularly in the transverse direction to expansion, surpassing 94 kJ/mol. This indicates a robust diffusion and chemical stability, suggesting its potential for prolonged utilization compared to conventional oil-based foams. A prediction was made that this foam would lose 50% of its stiffness and strength in around 9.8 and 9.5 years at 20°C, respectively, considering the directions more sensitive to the aging process. This research offers valuable insights for predicting lifespan based on operating temperatures and property retention needs, contributing to the advancement of sustainable materials in structural applications.