Tensile Stress Relaxation Behavior of Thermosetting Polyurethane Solid and Foams: Experiment and Model Prediction

Stress relaxation behavior of thermosetting polyurethane (PU) solid and foams were investigated in tensile mode using a dynamic mechanical analyzer (DMA). PU solid samples were manufactured in a closed mold to avoid any foam formation, whilst PU foam samples were manufactured inside a woven using a silicone mold. Samples with rectangular cross-section were subjected to a predetermined amount of tensile strain and the tensile force was recorded as a function of time. Relaxation modulus was determined for different temperatures up to near the glass transition temperature. It was found that the viscous part becomes more dominant with increasing test temperature. Although the stress relaxation behavior of PU solid and foam were found similar at lower temperature, the relaxation behavior of the foam was influenced by the cellular structure especially at higher temperature due to the combination of gas expansion and cell wall softening. Different stress relaxation models such as Maxwell model, Burgers model, Generalized Maxwell (GM) model, and Stretched exponential model were employed to predict the relaxation behavior of PU solid and foams. It was found that the GM model (with three or more elements) and the Stretched exponential model were in good agreement with the experimental data in predicting the stress relaxation behavior of both solid and foams. The predicted relaxation time and equilibrium modulus were found to decrease with increase in temperature.