Morphology and Mechanical Properties of High Density Polyethylene Exposed to Chlorinated Water

Abstract

High density polyethylene (HDPE) is often used in applications that include both structural loads and oxidative environmental conditions. In this study, the effect of an oxidative environment on HDPE morphology and mechanical performance is evaluated. Extruded thin 70 micron HDPE samples (as-extruded and heat treated) are exposed to 5ppm chlorinated water at 70 degrees Celsius for up to 1250 hours. Changes in semicrystalline polymer morphology as a function of exposure time are evaluated and compared with tensile and creep test data. Additionally, data are evaluated as a function of molecular weight, to eliminate variations in sample thickness, processing, or variation in oxidative environment conditions. IR spectrum data indicate an increase in the carbonyl functional groups with increased exposure time and decreased molecular weight, with the largest peak change in the carbonyl band being the ketone group, and a 300% increase in the carbonyl index. Molecular weight data reveal no change in molecular weight of 200 kg/mol from 0-250 hours, with over a 50% decrease in Mw after 750 hours, and a slower decrease with further exposure. Molecular weight distribution data and determination of chain scissions and crosslinking reveal preferential chain scission of high molecular weight chains as the underlying mechanism for this decrease in molecular weight. Heat treated samples exhibit more resistance to a decrease in molecular weight. The critical molecular weight is observed at ~83-86 kg/mol for the as-extruded HDPE. Crystallinity measurements using Differential Scanning Calorimetry and X-Ray Diffraction (XRD) differ by up to 10% but show a steady increase in crystallinity as exposure time increases, due to chemicrystallization. Molecular weight is determined to be the primary characteristic governing ultimate tensile strength, while percent crystallinity and molecular weight both govern the strain at break. In a preliminary study of lamellar morphology, a decrease in the interlamellar spacing, lamella thickness, and long period is observed with increased degradation. The critical interlamellar spacing from the ductile to brittle transition for as-extruded HDPE is observed to occur at ~85Å, determined using the XRD crystallinity results, and ~38Å determined using the DSC results. The data presented in this study demonstrate the correlation between morphology changes and a loss in ductility in HDPE. These data are the basis for a predictive model for lifetime of HDPE components exposed to oxidative environments.