The injection molding of large industrial ultrahigh molecular weight polyethylene products: A microbeam WAXD/SAXS investigation of the impaction of bimodal polyethylene on the structure and mechanical properties

Abstract

The injection molding of ultra-high molecular weight polyethylene (UHMWPE) large industrial products has always been a significant challenge. This study demonstrated that the addition of bimodal polyethylene (BPE) can reduce the melt viscosity of UHMWPE, enabling the injection molding of UHMWPE products with high-performance, large size, and complex shapes. Detailed analyses including microbeam wide-angle X-ray diffraction (WAXD)/small-angle X-ray scattering (SAXS), differential scanning calorimeter (DSC), and scanning electron microscope (SEM) were conducted to investigate the crystal structural differences across various regions of UHMWPE products with different BPE contents. This study revealed that those products with low BPE content exhibited larger lamellar crystal sizes and higher tensile strength, albeit with poor microstructure uniformity, resulting in low elongation at break. Conversely, products with high BPE content had more uniform microstructures but low degrees of crystal perfection, significantly improving elongation at break. Impact performance tests revealed that adding BPE did not damage the mechanical properties of UHMWPE, showing an ultra-high impact strength of 500 kJ/m². Furthermore, this study identified the significant effect of the complex flow field and molecular weight fraction of BPE on the crystal structure of the products. At low BPE concentrations, the low molecular weight portion of BPE predominated in the plasticizing effect during injection mold. Under weak flow conditions, this increased molecular chain relaxation in UHMWPE, making it difficult to maintain the oriented structure. Conversely, strong flow conditions promoted the formation of the oriented structure. However, at high BPE concentrations, the high molecular weight portion of BPE predominantly hindered the movement of UHMWPE, resulting in chain entanglement that does not improve significantly. In this context, the differences in the injection-molded samples under weak and strong flow conditions are minimal.