Cold-Drawn Material As Model Material for the Environmental Stress Cracking (ESC) Phenomenon in Polyethylene. A Raman Spectroscopy Study of Molecular Stress Induced by Macroscopic Strain in Drawn Polyethylenes and Their Relation to Environmental Stress Cracking

Abstract

Raman spectroscopy was used to study strain-induced molecular stress in cold-drawn polyethylenes, which were being used as a model system for fibrils present in the crazes formed during environmental stress crack resistance (ESCR) tests. The molecular stress was measured at 240 K in order to minimize relaxation phenomena. Molecular stress was related to macroscopic strain and, by correcting for differences in E-moduli, to true stress. In this paper, the measured molecular stress is related to ESCR values and sample characteristics. It was observed that good ESCR materials showed a lower molecular stress than worse ESCR materials at the same macroscopic strain level. It was also observed that the molecular weight has a major effect on the observed molecular stress per macroscopic strain (molecular stress per macroscopic strain decreases with increasing M w), whereas the effect of chain branching is smaller (molecular stress per macroscopic strain decreases with chain branching).

The structural evolution of high-density polyethylene during crazing in liquid medium

“…In this situation, physically active media, such as hydrocarbons, oils, alcohols, and surfactants, which do not modify the chemical structure of the polymer, can affect the mechanical properties of polyethylene and decrease its endurance. Since the breakage of a polymer being deformed in a liquid medium is preceded by nucleation of crazes and cracks, the functional properties of polymers are estimated by monitoring their appearance upon the complex action of an ambient liquid and stress [11][12][13][14][15][16]. In the literature, this phenomenon is referred to as the environmental stress cracking/crazing (ESC), and the main efforts of researchers have been focused on the suppression or deceleration of the craze/ crack development.…”

Polyethylene blends: A correlation study between morphology and environmental resistance

“…Environmental stress cracking (ESC) is the result of a slow process induced by the continuous action of a small load around ambient temperature in an aggressive environment. The visible effect is the appearance of macroscopic cracks in the material; often microscopic fibrils are found around the fracture surface and crack tip [2,3]. This process is considered to be responsible for 20e30% of the failures in engineering plastics [4e6], mainly for amorphous polymers in their glassy state.…”

Structural Approaches of Polyethylene Environmental Stress-Crack Resistance

“…The creep behavior of such drawn samples revealed very good correlations with life-time data, at both macroscopic [10,11] and microscopic scales [12]. Although elegant, these methods proved to be not sensitive enough to discriminate materials of very long lifetime [10][11][12].…”

“…This paper addresses the question of different methods for determining PE ability to develop tie molecules, with the goal of predicting the environmental stress crack resistance, one of the most widely used method for long-term mechanical property evaluation [10,12]. Three approaches have been put to the test in that aim.…”