The effects of pre-immersion in hostile environments on the ESC behaviour of urethane-acrylic polymers

Abstract

Cross-linked urethane acrylics are being developed for a range of applications, in particular, for automotive components. In such situations, they are likely to encounter hostile liquid environments and so environmental stress cracking (ESC) has become an important issue. In a study of the ESC behaviour of such materials, it has been found that diffusion effects are important, and has led to a series of experiments involving immersion of the samples in the environments prior to loading. The ESC behaviour of two urethane-acrylic materials of different cross-link densities were studied. The effects of prior immersion in a range of environments, including petrol, ethanol, acetic acid and ethylene glycol were determined. This was carried out with a combination of three-point bend testing to determine the critical strain needed for craze formation and with constant strain rate tensile testing. Fracture surfaces were examined using scanning electron microscopy and accurate weight change measurements were made to assess relative diffusion rates. It was found that pre-immersion caused a reduction in the measured strain to crazing with three-point bend tests, due to relaxation of the surface stresses by swelling and plasticisation and to blunting of surface flaws. The effects on tensile tests were more complex, with crack blunting, surface swelling and global softening due to plasticisation all playing an important part in addition to the conventional ESC effect.