4 Conclusions
Residues from plants are interesting alternatives as cellulose sources for several applications. In this work a chlorine-free procedure for the isolation of cellulose from rice husk was shown to be very efficient. The overall process does not produce any toxic effluents. On the basis of the whole cellulose content expected for rice husk, this method resulted in a yield around 74%. TGA analysis performed under nitrogen showed high residual mass for RH at 700 °C. This can be partially attributed to the high silica content of the material. In our study, the ash content of RH at 1000 °C was determined to be 16 wt%. FTIR, TGA and MDSC analyses agreed well with respect to the elimination of hemicellulose and lignin from rice husk after the purification procedure used to isolate cellulose. WAXD experiments indicated that the crystallinity of RH cellulose (67%) was lower than that of MCC (79%). Lower crystallinity has been pointed out as a factor, among others, that can lower the thermal degradation temperature. The decomposition temperature of RH cellulose was found to be lower than commercial microcrystalline cellulose. Besides water elimination, the MDSC analyses showed one main endothermic event for cellulose samples (RH cellulose and MCC), which was related to the melting of cellulose crystals. The TGA and MDSC results agree well with respect to the thermal stability of rice husk cellulose and helped to improve the knowledge on the complex behavior of cellulose degradation.
Cellulose whiskers were successfully obtained by sulfuric acid hydrolysis of the rice husk cellulose. According to TEM and AFM images, it was possible to isolate needle-like structures of cellulose whiskers, with sizes varying from 6 to 14 nm in width and 100–400 nm in length. The average values of length and thickness of these whiskers give an aspect ratio around 18. Such a value of aspect ratio is adequate for application of RH whiskers as nanofillers in polymer matrices. In this way the use of rice husk as a novel material source allows to obtain new particles with nanometric dimensions widening the supply of nanostructured materials usable for polymer nanocomposites.