Green polypropylene/waste paper composites with superior modulus and crystallization behavior: Optimizing specific energy in solid-state shear pulverization for filler size reduction and dispersion

Solid-state shear pulverization (SSSP) is a continuous process that overcomes challenges in producing well-dispersed polymer composites that cannot be made by twin-screw melt extrusion. We use SSSP to produce 85/15 wt% polypropylene/waste paper biocomposites with polypropylene pellets and 2-cm-square waste paper pieces as starting material. Single-pass SSSP achieves effective filler size reduction and dispersion within the polypropylene matrix. We determine how waste paper size reduction and composite properties are functions of specific energy input and tune specific energy input by SSSP screw design and throughput. Composites made at moderate to high specific energy input (14-35 kJ/g) have 25 to nearly 50% of filler particles at sub-micron size; relative to neat polypropylene, composites exhibit a 70% increase in Young's modulus, retention of neat polypropylene yield strength, and a ∼50% reduction in crystallization half-time. Estimates indicate that the cost of such biocomposite materials made by SSSP is less than that of virgin polypropylene.