Application of short-term methods to estimate the environmental stress cracking resistance of recycled HDPE
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Application of short‑term methods to estimate the environmental stress cracking resistance of recycled HDPE Anna Gobetti1 · Giorgio Ramorino1 Received: 27 May 2020 / Accepted: 21 October 2020 / Published online: 31 October 2020 © The Author(s) 2020
Abstract Environmental stress cracking is a serious problem for polyethylene because it can cause failure without any visible warning due to the slow crack growth accelerated by aggressive agents. Tie molecules and entanglements are the main macromolecular characteristic increasing environmental stress cracking resistance, thus in this work mechanical and thermal properties governed by those macromolecular characteristics are determined by performing simple tests executable in the industrial laboratories for quality control on recycled high-density polyethylene. The mutual relation between the determined properties confirms their dependence on the investigated macromolecular characteristics and allows to predict in a comparative way the expected environmental stress cracking. The mechanical properties related to the environmental stress cracking resistance are the strain hardening modulus and the natural draw ratio. The strain hardening modulus is an intrinsic property that measure the disentanglement capability of the inter-lamellar links and the natural draw ratio is a highly sensitive parameter to the macromolecular network strength via the intercrystalline tie molecules. Since the measurement of these properties according to the standard ISO 18,488 requires a temperature chamber not often available in the industrial laboratories, the tensile test was performed also at room temperature and displacement rate 0.5 mm/min; a proportionality between the data obtained at different test condition emerged. The thermal property related to the environmental stress cracking resistance is the stepwise isothermal segregation ratio that state the chain fraction that generates a high rate of tie molecules responsible of environmental stress cracking resistance. Keywords Environmental stress cracking · Recycled high density polyethylene · Tie molecules · Strain hardening modulus · Natural draw ratio · Stepwise isothermal segregation ratio
Introduction The present research comes from a real business need of a company that produces recycled plastic compounds. It focuses on a specific class of recycled high-density polyethylene (R-HDPE) that is designed to be processed by extrusion, for example in producing pipes, covers for electric cables, etc. Since for some of these applications the expected service life can exceed the 50 years, maintaining product integrity is of fundamental importance. This could
* Anna Gobetti [email protected] Giorgio Ramorino [email protected] 1
Department of Mechanical and Industrial Engineering, University of Brescia, Via Branze, 38, 25123 Brescia, Italy
be translated in terms of material properties in environmental stress cracking resistance (ESCR). The purpose of this work is to verify the mutual correlation between properties indirectly
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