Extraction of Rice Straw Cellulose Nanofibers by Optimizing a Chemo-Mechanical Method, Development of Thin Films, and Characterization

Rice straw is identified as one of the most abundant and sustainable natural sources to extract cellulose since the annual rice straw yields 3.4 million metric tons on average in Sri Lanka. Rice straw raw material obtained from the native Sri Lankan rice cultivar BW 372 was used as the primary cellulosic matter in this study. A combination of chemical and mechanical protocols, including the main steps of accelerated caustic immersion, acid hydrolysis, alkaline treatment, bleaching, and ultrasonication was implemented to extract nanocellulose from rice straw. The removal of hemicellulose, pectin, lignin, and extractive materials is demonstrated through thermogravimetric analysis (TGA), X-ray diffraction (XRD), and color profile analysis at each processing step. The TGA curves attributed to thermal properties indicated a significant enhancement and around an 11% increase in degradation temperature in chemically treated rice straw samples compared to raw materials. The crystallinity index of the purified samples was increased by 27% due to the bleaching process in contrast to the raw material. Ultra-sonic homogenization was optimized to prepare cellulose nanofiber dispersion in an aqueous medium. The most stable pH value of the dispersion (0.5 v/v%) was recorded as pH-7 with -27 mV Zeta-potential. Cellulose nanofiber films developed using a casting method in varying concentrations were characterized based on thickness indicators, color parameters, scanning electron microscopic images, XRD, and TGA. The average thickness of the films was recorded in (8–20) μm range. The grams per square meter (GSM) values of films were significantly (p < 0.05) different depending on varying concentrations. Thin films prepared using higher concentrations of the dispersion exhibited a significant (p < 0.05) increase in total color difference (∆E) and a significant (p < 0.05) decrease in the whiteness index. These nanofiber films present an excellent edible and biodegradable alternative to polythene and nonbiodegradable plastics in the packaging industry.