Much effort goes into developing durable polyethylene resins for use in long term service applications such as geomembrane and pipe products. The major failure mechanism in these fabricated articles is slow crack growth (SCG). Typically an induced stress, say for example an impinged rock against a buried pipe, causes a localized, irreversible stretching of molecular structure (plastic yielding) about the stress nucleated cavities in the amorphous segments of the fabricated article. This visco-elastic creep is followed by crack initiation caused by molecular disentanglement. Lastly the crack propagates through the pipe in a brittle manner but may transition to a ‘ductile’ mode as crack grows. Many resin and pipe tests have been developed to help better understand and predict the SCG resistance of resin and pipe products. Due to its ease, and cited correlations to both large scale pipe and small scale resin tests, tensile properties measured on resins drawn past their yield points have gain acceptance as methods to assess the SCG resistance of polyethylene resins. These methods focus on values for the natural draw ratio (NDR ) measured at room temperature and the strain hardening modulus (SHM) measured at 80oC (ISO 18488:2015E). In this paper we examined the structural effects of molecular weight and topology (short and long chain branching) on both these post yield tensile values. Using the Primary Structure Parameter (PSP2) we found that linear correlations can be constructed between PSP2 values and both NDR and SHM values for a variety of resin architectures. However, our data also shows that such plot between PSP2 and NDR values could not distinguish between branch types whereas similar plots using the SHM could. These results will be presented and discussed.

Assessing the molecular weight and topology effects on post-yield tensile tests for slow crack-growth resistance in polyethylene resins

251st National Meeting (2016)

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