Hot-Compressed Water Treatment and Subsequent Binderless Hot Pressing for High-Strength Plate Preparation from Rice Husk

Treatment of herbaceous biomass in hot-compressed water (HCW) depolymerizes and extracts substantial portions of hemicellulose and lignin together with silica, which are all useful chemical feedstocks. The present study proposes processing of rice husk (RH) into high-strength material after pretreatment in HCW. A type of RH was treated in HCW at 140-200 °C in a tubular percolator, dried, pulverized, and then molded into rectangular or circular plates by hot pressing without a binder. The plates of HCW-treated RH had greater tensile and flexural strengths than those of the original RH, while having smaller water absorptivity and swellability. The plates of RH pretreated at 160 °C had the best properties such as tensile strength of 26 MPa (2.8 times that of the nontreated RH), flexural strength of 21 MPa (2.6 times), fracture energy of 1453 mJ (7.8 times), water uptake of 40 wt % (0.74 times), and swelling ratio in water of 1.37 (0.92 times). These properties were attributed mainly to greatly improved pulverizability, resulting occurrence of fibrous reinforcement material, and optimized fractions of cellulose and silica (as reinforcement materials) and those of hemicellulose and lignin (as the matrix and binder). This paper also reports and discusses combined effects of the conditions of HCW treatment, pulverization, and hot-press molding on the strength of plates.