Structural Approaches of Polyethylene Environmental Stress-Crack Resistance

J Cazenave,R Seguela,B Sixou

doi:10.2516/ogst:2006011

Abstract

Structural Approaches of Polyethylene Environmental Stress-Crack Resistance This work was aimed at understanding and predicting of the long-term mechanical behavior of polyethylene in relation to molecular architecture. An insight into the semi-crystalline microstructure and the chain topology consisting of intercrystalline tie chains and loose folding loops was the main objective of the study. A two-stage crystallization segregation was used for separating the chain species which can easily crystallize by regular folding from those which cannot. The natural draw ratio that develops beyond the yield point proved to be a highly sensitive parameter for probing the chain topology via the crystalline lamella fragmentation and chain unfolding under tensile drawing. Small-angle X-ray scattering provided semi-quantitative evaluation of the stacking disorder of the lamellar microstructure. The data from the three approaches provided correlations with chain architecture in very good agreement with the stress crack resistance dependency. These correlations were discussed in terms of crystallization kinetics and molecular topology. A practical issue of the work is to provide relevant tools for predicting stress cracking resistance.

Highlights

During the last two decades, polyethylene-based materials (PE) have been increasingly used in application domains demanding long life-time in service, such as pipes for gas distribution or fuel tanks for land vehicles [1]
The materials are labeled according to decreasing density which results from co-unit concentration increase and/or molar weight distribution (MWD) broadening
ZN polymers have a bimodal MWD which results from to the tandem-reactor synthesis. This polymer series display a preferred incorporation of the co-units in the long chains which is another factor of improvement of inter-crystalline tie molecules. All these molecular peculiarities have conspicuous incidence on the stress crack resistance via the chain topology that will be evaluated through the study of the crystallization kinetics, the natural draw ratio, and the stacking disorder of the lamellar structure

Summary

Introduction

During the last two decades, polyethylene-based materials (PE) have been increasingly used in application domains demanding long life-time in service, such as pipes for gas distribution or fuel tanks for land vehicles [1]. Slow crack growth is the major failure mechanism of PE under stress below the yield point This phenomenon determines the life-time in service. Localized plastic yielding occurs about stress-nucleated cavities in the inter-crystallite amorphous layers [2, 3] This is accompanied with fragmentation and partial chain unfolding of the crystalline lamellae, a process which obeys the fibrillar transition model described by Peterlin [4, 5]. The resistance to stress cracking is generally ascribed to the tensile resistance of the microfibrils It is mainly governed by the concentration of the load-bearing inter-crystalline tie molecules linking the crystal blocks in the microfibrils [8, 9] that slow down the crack opening

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