Characterizing degradation of Bimodal Pipe Grade HDPE: The Role of Short Chain Branching

Bimodal HDPE with a wider molecular weight range and comonomer (α-olefin) inclusion, which forms short-chain branching, has been the focus of recent research to provide long-term durability and improved resistance to cracking. This study aims to identify changes in pipe-grade bimodal HDPE structure when exposed to an oxidative environment and investigate the role of short-chain branching on mechanical properties. Bimodal pipe-grade polyethylene samples were exposed to chlorinated water for up to 3000 hours at 65 C, with 5 ppm Cl and an oxidative reduction potential of 825 mV. The samples were characterized using a differential scanning calorimeter (DSC), attenuated total reflection spectrometer with IR detection and gel permeation chromatography with IR detector (GPC-IR). Additional tests were performed on unexposed, and samples exposed for 2500 hours: temperature rising elution chromatography (TREF) to measure the short chain branching distribution (SCBD) and cross fractionation chromatography to relate the MWD with the SCBD profile. GPC-IR of undegraded and degraded samples showed a bimodal and unimodal molecular weight distribution, respectively. The GPC-IR data shows a noticeable shift in the branching distribution towards lower molar masses. However, the average SCB values and distributions remain similar, with no significant changes in branching. The degraded sample has a higher average methyl group frequency (7.8 vs. 5.3 CH3/1000TC) due to the lower molar mass and reduced average chain length. TREF of an undegraded sample had a unimodal distribution compared to a bimodal distribution of the degraded (2500 hour) sample. The analysis of the TREF data indicates that while the MWD for the degraded sample is homogeneous in molar mass, the sample is heterogeneous with respect to chemical composition (i.e., a bimodal CCD). This result can be interpreted as a creation of an interphase with crystallizable fractions known to be effective tie chains and increase chain entanglements, thereby increasing the time for HDPE to transition from ductile to brittle phase. The DSC result indicates that the sample exposed for 2300 hrs had a mass crystallinity value of 75% compared to undegraded sample mass crystallinity of 54%. This result further supports the TREF results which indicate the formation of new crystalline regions.