The detergent stress-cracking of polyethylene

Abstract

Many plastic materials fail under plain strain conditions by a process of micro-voiding in sheet-like structures known as crazes. These crazes are generally weak though their strength and stability may be improved in materials where orientation (strain) hardening occurs. Previously we have used finite element analysis, with a model consisting of an assembly of cylindrical holes, to study this type of plastic failure. With simple plasticity such models fail soon after yield, though they also would be stabilized by strain hardening. In this study we have introduced a surface-tension term which increases the resistance to plastic deformation. The effect is greatest when the holes are small and the yield strength of the material low. It is shown that when values of these two parameters representative of polyethylene at 50 °C are used surface tension should have a marked stabilizing effect for holes of < 25 nm radius. Even above this size range the failure stress can be affected at the 5-20 % level. An electron micrographic study was then carried out on polyethylenes cracked in the presence of an aqueous detergent (detergent stress cracking). It is shown that numerous small cavities and fibrils are formed on a scale small enough for the effect of surface tension to be significant. The rôle of the detergent in reducing surface tension and so promoting the fracture of polyethylene may therefore be understood. The marked influence of molecular mass on environmental stress cracking behaviour may be correlated with the greater development of orientation hardening at high strains with high molecular mass polymers. This increases the amount of plastic work preceding fracture and tends to stabilize a craze-like system.