Effect of the calcination temperature on the mechanical properties of polypropylene composites prepared with rice husk ash filler

Rice husks can be calcined to produce rice husk ash (RHA), which serves as a bio-based filler for plastics. In this study, we investigated the use of RHA as a hybrid silica–carbon and porous filler to improve the mechanical properties of polypropylene (PP). RHA was produced by calcining rice husks at temperatures ranging from 100 °C to 800 °C, and the resulting ash was mixed with PP to create composite materials. Elemental analysis revealed that the Si/C ratio of RHA increased with higher calcination temperatures. High silica content of RHA calcined at 800 °C (RHA800) lowered the hydrophobicity, causing the aggregate formation in the composite with PP. Measurements of nitrogen adsorption–desorption isotherms showed that RHA prepared at 400 °C (RHA400) had the highest specific surface area, but tensile testing of the PP/RHA composites found that those containing RHA calcined at 300 °C (PP/RHA300) exhibited the best mechanical performance, likely because of the high hydrophobicity of RHA300, resulting in superior elongation of the PP/RHA300 composite. Flash differential scanning calorimetry showed that the crystallization rate of PP/RHA was slower than that of PP/CB (carbon black). In comparison with PP/CB and PP/talc, wide-angle X-ray diffraction (WAXD) measurements confirmed that PP/RHA promoted the formation of the β-crystalline phase. Pyrolysis GC-MS analysis of RHA300 suggested that benzoic acid derivatives present in the ash contribute to the promotion of β-crystal formation. Our results indicate that the elongation of PP/RHA300 composites is influenced by both the porous structure of RHA and its role as a β-crystal nucleating agent. In this study, the RHA prepared using a unique ashing process involving washing displayed high compatibility with PP and acts as a biomass filler with a β-crystal nucleating effect. RHA is expected to be useful as a compatibilizer for controlling crystallization rates of polymers, which is an important challenge in plastic recycling.