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Abstract
AbstractIn the present study, the influence of electron‐beam irradiation on plasticity‐controlled and crack‐growth‐controlled failure in high‐density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross‐linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross‐linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission.
Highlights
The gel fraction, obtained from Size exclusion chromatography (SEC) recovery, is the lowest for PE-8.4-51-230, presumably due to the presence of short chain branching (SCB), while PE-4.4-197-1790 showed the highest gel fraction, which could be attributed to a larger fraction of long chains, as well as a lower crystallinity
An increase in both yield stress and Young's modulus is observed as a function of gel fraction, which could be attributed to an increase in the cross-link density of the amorphous phase, since the crystalline phase remains unaltered
Small differences are observed in the crack growth kinetics as a function of irradiation dose, which is quite remarkable, since the molecular weight distribution (MWD) changes drastically with irradiation dose
Summary
High-density polyethylene (HDPE) is a subclass of polyethylene that has seen increasing use in pressure pipe applications since its conception in the 1950's.1–3 Since a service life-time of more than 50 years is expected for HDPE used in these applications, it's important to determine which molecular and morphological parameters govern the long-term performance.[1,4,5,6] As discussed extensively in literature, long-term failure under service conditions is predominantly caused by two failure processes: plasticity-controlled failure and crack-growth-controlled failure.[1,5,7,8,9,10] In previous research, it was shown that crystallinity was one of the main parameters governing the. The influence of MWD and SCB content on the long-term failure properties will subsequently be compared as a function of dose Both plasticity-controlled failure and crack-growth-controlled failure will be investigated in this publication, since it has been shown that both failure processes can govern long-term failure.[9,31] size exclusion chromatography (SEC) will be used to analyze the changes in MWD of the sol fraction (as far as they can be observed) as function of irradiation dose. These results will subsequently aid in clarifying the changes in the structure of the amorphous phase that cause the difference in long-term failure behavior. This would imply that SCB have a stronger inhibiting effect on cross-linking at (relatively) low doses
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