Effect of calcium stearate and a fluoroelastomer on the flowability of UHMWPE/HDPE composites and the underlying mechanism

The melt viscosity of ultra-high molecular weight polyethylene (UHMWPE) is extremely high, rendering its processing exteremely difficult; thus, flow modification is required to improve its flow properties. In this study, calcium stearate (CaSt) and a fluoroelastomer (FP) were used as flow modifiers to improve the flow properties of UHMWPE. The experimental results revealed that both lubricants improved the flowability of UHMWPE: CaSt improved the melt flow index of UHMWPE, while the FP had no significant effect on the melt flow index but significantly reduced the torque. The underlying mechanisms for the above effects were investigated in terms of the structure–property relationships. Rheological and dynamic mechanical analyses revealed that CaSt acted as an internal lubricant, reducing the degree of entanglement among HMWPE molecular chains. Torque rheological measurements revealed that the FP primarily acted as an external lubricant, precipitating to form an external lubricant layer during extrusion, which reduced the torque and die pressure.