Morphological and Mechanical Performance of Unstressed and Stressed Pipe-Grade High-Density Polyethylene Exposed to Chlorinated Environment

Abstract

High-density polyethylene (HDPE) is used in various industries, including pipes for carrying potable water, which creates an oxidative environment. This work investigates oxidative degradation of bimodal pipe-grade HDPE with short chain branches placed on the higher molecular weight chains. These short chain branches are intended to enhance pipe resistance to environmental stress corrosion cracking and are hypothesized to prevent oxidized polymer chains from migrating from the amorphous region to the crystalline region. This study evaluates the effect of stress on the morphological and mechanical performance of pipe-grade HDPE exposed to an oxidative environment. Extruded film HDPE samples are exposed to a 5ppm chlorinated water bath at 60℃ for up to 1500 hours. An apparatus is designed to keep the samples submerged in the bath while applying a constant load. During exposure, samples are subjected to a continuous stress (creep loading) of 0, 3, 5 or 6 MPa. Mechanical performance of the samples is evaluated by creep behavior during exposure (percent elongation vs. exposure time) and tensile tests after exposure (strain at break, strength, and modulus of toughness, which is the material’s capacity to absorb energy in plastic deformation). Morphology is assessed by Fourier-Transform infrared spectroscopy (carbonyl index), differential scanning calorimetry (crystallinity), and strain hardening modulus (entanglement molecular weight). The data is analyzed by grouping the samples into two sets based on thickness: “thin” (49µm to 63µm) and “thick” (79µm to 109µm). To account for variation in thickness among samples, a normalized exposure time (a function of Tref/T^2), referenced to a 90µm thick sample, is calculated. Key findings of this study are as follows: All samples, regardless of the application of stress, exhibit surface degradation. The carbonyl index (CI) increases up to 189% after 1500 hours of exposure. For thick samples, crystallinity remains constant, ranging from 62% to 64%, regardless of the application of stress. Only thick samples exposed for 1500 hours with an applied stress of 5MPa show evidence of degradation through the thickness. Entanglement molecular weight and mechanical performance are “bulk” (through-the-thickness) properties. For these thick samples with an applied stress of 5MPa, after 1500 hours of exposure, entanglement molecular weight increases by 58%, ultimate tensile strength decreases by 48%, and percent elongation at break decreases by 50%. This pilot study demonstrates the feasibility of constant loading during exposure and the importance of sample thickness. The thin sample set may have exhibited signs of degradation in some regions, but due to the variation in thickness across the film, the data are inconsistent. Data from the thick samples show greater consistency; however, the exposure time is insufficient to produce a brittle sample in the absence of applied stress. Future studies should include longer exposure times and additional molecular weight measurements, including molecular weight distribution and polydispersity index using gel permeation chromatography; due to the limited number of testing stations, a single stress level of 5MPa should be applied.