Environmental stress cracking of high density polyethylene packages

High-density polyethylene (HDPE) is a polymer widely used for packaging applications, such as bottles for household detergents. During the service life of a bottle, the material can be exposed to several substances that can affect it both chemically and physically. One of such (physical) phenomena is called Environmental Stress Cracking (ESC) which may occur in polymeric materials by the combined action of stress and the presence of certain substances. It is related to local plasticization, craze/crack initiation and propagation and it can lead to the premature failure of the component. In this work the behaviour of two blow-moulding HDPE grades, employed for the manufacturing of bleach bottles, was investigated. During the product distribution, bottles are stored in pallets which may be stacked. The containers located at the bottom are subjected to a high mechanical stress due to the weight of the surmounting bottles; the presence of bleach can cause the bottle to fail at lower stress values with respect to the material strength measured in air. The composition of commercial bleach (an alkaline aqueous solution containing about 3 % sodium hypochlorite, up to 1.2 % surfactants and up to 0.1 % perfume) is such that chemical interaction with HDPE (e.g. oxidation) could occur alongside ESC [1]. In order to investigate the effect of these (physical and/or chemical) interactions on the mechanical properties of the polymer, several solutions containing combinations of the various bleach components were considered as potentially active environments. Preliminarily, interaction tests were performed and the change in specimen mass, tensile properties and scratch hardness were evaluated at different soaking times (from 1 week to 6 months). The fracture behaviour of the two materials was studied using a fracture mechanics based approach, as proposed in [2]. Test on notched samples were conducted with three and four point bending and double cantilever beam configurations, using constant load and constant speed loading histories. First, testing in air was performed to identify effects of temperature, sample size and processing conditions on the apparent fracture toughness KIC. Then, ESC resistance of the two material was investigated performing fracture tests in different active environments (some of the solutions used for the interaction tests); temperature was used as an accelerating factor to decrease testing times. The combined effects of bleach components on the toughness during the crack initiation and propagation phases were studied and relevant critical interaction times were determined.