The characteristic changes of rice straw fibers in anaerobic digestion and its effect on rice straw-reinforced composites

Rice straw (RS) was utilized to produce biogas, and its biogas residues (BR) were studied on different degradation time. When rice straw was digested, much biogas was produced with average 50% methane. In anaerobic condition, non-fiber contents were mainly digested in the initial 10 days. Meanwhile, the surface of rice straw fibers became rough and its onset decomposition temperature also rose from 170 °C to 210 °C. Cellulose and hemicellulose were digested from 10 days to 30 days, and it determined that the cellulose crystallinity declined from 44.9% to 40.1% and hydroxyl groups was also seen to decrease, which conduces to the reduction of polarity and hydroscopicity. However, lignin was hardly digested in the whole anaerobic process. Inevitably, lignin was accumulated to contribute to the thermal stability and mechanical properties of rice straw fibers. Furthermore, rice straw/low-density polyethylene (RS/LDPE) and biogas residues/low-density polyethylene (BR/LDPE) composites were prepared. Compared with RS/LDPE composites, BR/LDPE composites showed obviously better tensile and flexural properties. Consequently, biogas residues of rice straw have better chemical and physical properties than the undigested rice straw.