Green composites of polypropylene and eggshell: Effective biofiller size reduction and dispersion by single-step processing with solid-state shear pulverization

Eggshell (ES), a waste byproduct from food processing and hatcheries, contains ~95% calcium carbonate (CC), making it a potentially attractive, less expensive substitute for commercial CC. Past work used complex grinding-sieving and/or chemical modification steps to aid in dispersing ES in polymers such as polypropylene (PP). Both steps add to the cost and reduce the green aspect of the composite. Here, green composite materials of PP with 5-40. wt% unmodified ES shards of several centimeters in size are directly processed using continuous, single-step solid-state shear pulverization (SSSP). Electron microscopy and particle size analysis show very good dispersion with some ES particles near the nanoscale in the composite. Well-dispersed ES particles dramatically increase PP crystallization rates with a 5-7% increase in PP crystallinity. The very good dispersion leads to a major increase in Young's modulus (87% increase relative to neat PP for 40. wt% ES) and a modest increase in hardness; composites exhibit reductions in yield strength, elongation at break, and impact properties. Mechanical and crystallization properties are equal to or better than the best literature data for PP/ES composites without chemical modification made by multi-step approaches involving melt processing. In addition, the composites exhibit high thermal degradation temperatures compared to neat PP, indicating the potential for ES to improve processing stability. Composites with 20-40. wt% ES exhibit solid-like rheological response with no crossover of shear storage and loss moduli. Nevertheless, PP/ES composites retain viscosities close to that of neat PP at shear rates experienced in melt processing. Overall, property enhancements resulting from superior dispersion of ES in PP achieved by SSSP reveal ES to be a promising green filler for thermoplastics.