Solid-State Shear Pulverization of Post-Industrial Uhmwpe: Particle Morphology and Molecular Structure Modifications Toward Conventional Mechanical Recycling

Ultra-high molecular weight polyethylene (UHMWPE) is one of the most prominent high-performance thermoplastics for biomedical, leisure, and coating applications. Large-scale recycling of UHMWPE is extremely difficult due to the high melt viscosity of the material as well as its exceptional chemical resistance and impact strength. There is a need for a commercially scalable methodology that can process the waste feedstock for mechanical recycling while sustaining the outstanding physical properties of the material. Solid-state shear pulverization (SSSP) is a continuous, twin-screw extruder-based processing technique in which the low-temperature application of shear and compressive forces impart changes in structure at different length scales to overcome the challenges of difficult-to-recycle polymers. This paper investigates the use of SSSP in mechanically recycling post-industrial scrap UHMWPE (rUHMWPE) material from a local ski and snowboard manufacturer. The SSSP-processed particles are flat, micron-scale flakes with enhanced surface area, which can sinter very quickly when compression molded. The molded rUHMWPE samples in turn exhibit enhanced ductility and toughness compared to the as-received scrap material, based on the tunable mechanochemical modification of the ethylene chains. Polymer characterization is performed on both rUHMWPE samples along with a control UHMWPE in order to determine the effects the SSSP process has on the molecular nature of the molecule. The results show that post-industrial recycling of engineering and high-performance plastics can achieve a highly sustainable and value-added life cycle when executed with a proper processing technique. The rUHMWPE could be easily compression-molded in a conventional way as the molecular architecture of rUHMWPE has been modified to flow and interdiffuse effectively in its sintering mechanism. However, there were no disruptions to the robust ethylene crystal structure. These results have positive implications for sustainability, which can be extended to other types of high melt-viscosity or otherwise difficult-to-recycle engineering and high-performance thermoplastic materials. As society continues to use more advanced thermoplastic materials in real-world applications, the industry has a responsibility to find a more robust, systematic, and universal recycling methodology for these polymeric systems. While this paper focused on the recycling of neat systems, the SSSP technology can be applied to other polymer sustainability solutions. It has the potential to improve the compatibilized blending of mixed polymer streams and compounding of bio-based fillers. Different pathways to sustainability need to be curated to make a significant difference in this polymer-laden society.