A comparative study of multimodal vs. bimodal polyethylene pipe resins for PE-100 applications

In this paper several high density polyethylene pipe resins are compared to polyethylene resins made in the Phillips Loop‐Slurry Process (single‐reactor), using a catalyst of chromium on modified aluminophosphate (Cr/AlPO4). A brief description of the Cr/AlPO4 system is presented along with polymer chain architecture (i.e., topology) and other structural aspects of polymers made from these catalysts. The physical properties of these resins and their fabricated pipe processing/performance properties were compared to both PE80 and conventional bimodal type PE‐100 resins. Results from this study show that high‐density polyethylene resins made from Cr/AlPO4 catalysts possess primary structural attributes that are truly unique for chromium‐catalyzed resins. These multimodal resins have very high molecular weights (MW; Mw > 400 kg/mol) and exceptionally broad MW distributions (MWD; Mw/Mn > 50). Topologically, these resins were found to have uniform short chain branching distributions across the MWD; as in polymers made from single site metallocene catalysts) and significantly reduced levels (10 times less) of long chain branching when compared to resins made using chromium catalysts on conventional supports. Furthermore, results from rheological, fabricated pipe processing, and performance studies suggest that these resins should be especially well suited for high performance pipe applications. Pipe samples made from these resins were found to meet PE‐100 requirements including low temperature impact properties. POLYM. ENG. SCI., 45:1203–1213, 2005. © 2005 Society of Plastics Engineers