Tensile Stress Relaxation of Thermosetting Polyurethane Solid and Foam

Stress relaxation behavior of thermosetting polyurethane (PU) solid and foam were investigated in tensile mode using a dynamic mechanical analyzer (DMA). PU solid samples were manufactured in a closed mold under compression to avoid any foam formation, whilst the foam samples were manufactured inside a woven using a silicone mold. Effects of cure and the post-cure temperature were also investigated on the stress relaxation behavior. Samples in the form of rectangular cross-section were subjected to a predetermined amount 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 dominant with increasing test temperature. The experimental data was precisely modeled using a generalized Maxwell’s model and the characteristic relaxation time was identified with the corresponding relaxation process. Although the stress relaxation behavior of PU solid and PU foams were found similar at room temperature, the relaxation behavior of the foam was found to be influenced by the cell morphology at higher temperature.