Rice straw fiber-reinforced high-density polyethylene composite: Effect of fiber type and loading

Abstract

Composite panels using virgin and recycled high-density polyethylene (VHDPE and RHDPE) and five types of natural fibers including four rice straw components (i.e., rice husk, rice straw leaf, rice straw stem, and whole rice straw) and wood fiber as control were made by melt compounding and compression molding. Fiber characteristics and the influences of fiber type and loading rate on HDPE crystallization behavior and composite mechanical properties were investigated. Fiber length and aspect ratio distributions for all fibers followed a lognormal distribution after milling with two parameters defining the curve location (i.e., mean fiber length/aspect ratio) and shape (i.e., mean fiber length/aspect ratio distribution). For both VHDPE and RHDPE, rice straw fiber systems had comparable mechanical properties with those of wood composites. Increase in fiber loading led to increased moduli and decreased tensile and impact strength. Composite panels with rice husk had the smallest storage moduli, but their impact strength was comparable or better than that of other straw fibers. Very little difference in mechanical properties existed among leaf, stem, and whole straw fibers. The particular recycled HDPE resin and its composites had significantly better moduli and strength properties compared to the virgin HDPE systems due to additives used during initial processing. X-ray diffraction experiments showed that introducing fiber to HDPE matrix did not change characteristic peak position, but the fiber increased crystalline thickness of HDPE system. Differential scanning calorimetry experiments showed that VHDPE had significantly larger peak heat flow during cooling run than the RHDPE, indicating higher crystallization rates for VHDPE. The use of fiber in both resin systems led to the reduced peak heat flow rate. The study showed that rice straw fibers can work well with both VHDPE and RHDPE as reinforcing filler. Future work will deal with effect of coupling treatments of the straw fibers in single phase or commingled plastics composite systems.