Nanocrystalline Cellulose Prepared from Softwood Kraft Pulp via Ultrasonic-assisted Acid Hydrolysis

Nanocrystalline cellulose (NCC) with small particle size and high crystallinity was prepared via the combined method of ultrasonication and acid hydrolysis from bleached softwood kraft pulp (BSKP). Scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy were used for determination of morphology, crystal structure, and surface chemical groups. Thermal behavior was analyzed by thermogravimetric analysis. The analyses revealed that rod-shaped NCC particles with diameter of 10 to 20 nm can be obtained. Ultrasonication can induce cellulose folding, surface erosion, and external fibrillation of BSKP, together with the shorter average length of NCC (96 nm) than that prepared without ultrasonication (150 nm). Due to the smaller size and larger number of free ends of chains, the thermal stability of NCC was lower than BSKP. The degradation of BSKP exhibited one significant pyrolysis stage within the range of 300 to 420 °C. In contrast, UH-NCC exhibited three pyrolysis stages within the range of 210 to 450 °C. NCC prepared with ultrasonication decomposed at lower temperature and over a wider temperature range, together with higher char yield of 43% (compared with 27% for that without ultrasonication). The obtained NCC had similar surface chemical structures but higher crystallinity (82%) compared with that of the starting BSKP (74%).