Relationship between morphological changes and mechanical properties in HDPE films exposed to a chlorinated environment

The effect of an oxidative environment on HDPE morphology and mechanical performance were studied. Extruded HDPE film samples of thickness (~70 μm) (as-extruded and heat treated) were exposed to 5 ppm chlorinated water at 70 °C for up to 1250 h. As extruded samples had an initial crystallinity of 75% and heat treated samples an initial crystallinity of 83%. The initial molecular weight for all samples was approximately 200 kg-mol⁻¹. Changes in morphology and mechanical properties as a function of exposure time were evaluated. The most significant changes occur after 500 h exposure. Fourier-Transform Infrared Spectroscopy (FTIR) spectrum data indicate an increase in the carbonyl functional groups with increased exposure time. As the exposure time increased, the molecular weight (M_w) of the samples decreased, with M_w of the as extruded samples being consistently less than that of the heat treated samples. Crystallinity increased nearly linearly with exposure time. XRD data reveal that the interlamellar spacing decreases significantly between 500 and 750 h exposure, from 102 to 85 Å. Mechanical test data show a similar trend with tensile strength and strain at break. After exposure time of 500 h the samples become increasingly brittle. The combined morphology and mechanical data demonstrate, independent of exposure time, that there is a critical combination of molecular weight (<85 kg-mol⁻¹) and interlamellar spacing (<85 Å) corresponding to the transition from ductile to brittle behavior.