High-density polyethylene (HDPE) can be blow-molded and used for the production of bottles for aggressive products. These products can interact both chemically and physically with the polymer constituting these containers, leading to a decrease in the performance of the material and undermining the structural integrity of the component.\ud A fracture mechanics approach was adopted to evaluate the Environmental Stress Cracking Resistance (ESCR) of two HDPE commercial grades used for bleach containers; two different solutions, with and without sodium hypochlorite (the main ingredient of commercial bleach solutions), were considered as aggressive environments. Size effects were studied using different test configurations and loading histories in air. The correlation between the stress intensity factor and the initiation time was found. A clear effect of the aggressive solutions on the fracture resistance of the two HDPEs was observed, irrespective of the presence of sodium hypochlorite; the effect therefore has to be ascribed to other bleach components

In this work the environmental stress cracking resistance of two high-density polyethylene grades, employed for the manufacturing of bleach bottles, was investigated by using a fracture mechanics approach. Two aqueous solutions similar to common bleach products (with and without sodium hypochlorite) were considered as the active environment, whose effect on both the initiation and propagation phases of fracture was evaluated for the two materials. Tests at different temperatures were performed and a time-temperature superposition scheme was applied both in air and in the active environment. Temperature was shown to influence fracture behaviour only through the material inherent viscoelasticity.

According to Linear Elastic Fracture Mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson's ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the Slow Crack Growth and the Environmental Stress Cracking resistance of two highdensity polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behaviour of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behaviour exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

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