Dispersion and Property Enhancements in Polyolefin/Soy Flour Biocomposites Prepared via Melt Extrusion Followed by Solid-State Shear Pulverization

Biocomposites of low-density polyethylene (LDPE) and polypropylene (PP) with 5-40 wt% soy flour (SF) are produced by two-step single-screw extrusion (SSE) followed by solid-state shear pulverization (SSSP). The SSE-SSSP approach overcomes limitations with melt mixing, e.g poor SF dispersion and degradation, and limitations with single-step SSSP. Microscopy shows that SF is well dispersed in SSE-SSSP composites but agglomerated and degraded in melt-mixed composites. The SSE-SSSP composites exhibit major improvements in Young's modulus relative to neat polymer, including 74 and 43% increases in 80/20 wt% LDPE/SF and 95/5 wt% PP/SF composites, respectively. Relative to neat polymer, SSE-SSSP composites exhibit the largest improvements in Young's modulus and best tensile strengths reported for polyolefin/SF composites. Crystallization and viscosity are only slightly affected by SF in the composites. At 20% and higher mass loss, char can result in greater thermo-oxidative stability of 80/20 wt% polyolefin/SF composites relative to neat polymer. A novel processing technique combining single-screw extrusion (SSE) and solid-state shear pulverization (SSSP) is employed for producing polyolefin/soy flour biocomposites with synergistic property enhancements. In comparison, SSE and melt-mixed composites show severe particle agglomeration resulting in poor material properties. The two-step processing is able to overcome processing challenges associated with SSSP alone.