Influence of morpho-structural parameters on the environmental stress cracking of polyethylene

The environmental stress cracking resistance (ESCR) of four polyethylenes in active medium (10% Tergitol solution) was investigated. The four materials were chosen to explore a broad range of performance and applications; they are listed according to increasing expected ESCR: - an injection moulding, low-molecular weight (MW) HDPE homopolymer - two rotomoulding LLDPE copolymers with a different comonomer (butene or hexene) - a blown film extrusion high MW HDPE copolymer (hexene) The fracture resistance of the hexene LLDPE copolymer was expected to be slightly higher than that of the butene one, because of the longer alkyl group. The two materials were chosen to challenge the ability of the different testing methods to discriminate between similar levels of ESCR. Several analytical techniques were employed to obtain relevant morpho-structural parameters: density, degree of crystallinity, MW distribution, short chain branch content and average lamellar thickness. ESCR was evaluated by employing three well-known but widely different approaches: - the Bell Telephone test (ASTMD1693), performed on notched specimens immersed in the active environment at 50°C - strain hardening modulus (SHM), obtained from tensile tests performed in air at 80°C - fracture mechanics (FM) tests on three- and four-point bending notched specimens, performed at varying temperatures both in air and in the active environment; two different loading histories were considered (creep and constant displacement rate) MW distribution seems to have a greater effect on ESCR behaviour with respect to other morpho-structural parameters. There was instead a strong consistency of the different ESCR testing methods, so that SHM could replace ASTMD1693 as an industrial test aimed at ranking ESCR of polyethylenes. FM, while being more complex to carry out, provides a wealth of additional information which could be used to actually predict the lifetime of products working in an ESC environment.