Energetic studies on epoxy-polyurethane interpenetrating polymer networks

A blend system prepared from epoxy resin (EP) and polyurethane (PU) was investigated in terms of glass-transition temperature (T_g), contact angle, mechanical interfacial, and mechanical properties. Deionized water and diiodomethane were chosen as the angle testing liquids. In this work, the models of Owens-Wendt and Wu, using a geometric mean, were studied to analyze the surface free energy of the EP/PU blend system. Fourier transform infrared (FTIR) spectroscopy was employed to investigate the intermolecular hydrogen bonding and functional group changes. The impact test was carried out at room and cryogenic temperatures to determine the low-temperature performance of PU. As a result, mechanical interfacial and mechanical properties give a maximum value at 40 phr of PU, and the deviation of T_g of EP/PU was the closest at 40 phr of PU. Thus it is concluded that EP and PU have the best compatibilities at this ratio. Furthermore, the specific (or polar) component of the surface free energy of the blend system was largely influenced on the addition of the PU, resulting in increasing the critical stress intensity factor (K_IC) and the impact strength for the excellent low-temperature performance. These results could be explained by means of improvement of hydrogen bonding between the hydroxyl group in EP and isocyanate group in PU.