Strategies for enhancing the processability of UHMWPE

Abstract

Four strategies to enhance the processability of UHMWPE are explored using heterogenised metallocene catalysts to produce polymers with decreased melt viscosity and excellent mechanical performance. Ultra-high molecular weight polyethylene (UHMWPE, M w > 10 6 g mol −1 ) has been prepared using slurry-phase titanium permethylindenyl-phenoxy (PHENI*) catalysts. Four strategies have been investigated for improving the melt processability of UHMWPE, which is the chief limiting factor to the applications of this high-performance polymer. 1) Active site engineering was used to explore the entanglement density in the resulting polymer, with substantially disentangled PE identified through thermal and rheological characterisation. 2) Hydrogen and ZnEt 2 were employed as chain transfer agents to modulate the polyethylene molecular weight and distribution (MWD). A sequential reactivity protocol using ZnEt 2 was able to produce bimodal UHMWPE with improved processability. 3) MWD tuning was further investigated using multisite catalysts, with the reaction conditions and Ti : Zr ratio able to control MWD to essentially arbitrary shapes. The inclusion of low molecular weight fractions into UHMWPE improves the processability without compromising mechanical characteristics. 4) Polymer-reinforced composite blends of UHMWPE with either HDPE or LDPE as a highly processable matrix were extruded and explored, with polymer miscibility and mechanical properties studied in detail.