Preparation and Characterization of Nanocrystalline Cellulose by Acid Hydrolysis of Cotton Linter

The purpose of this study was to using acid hydrolysis of cotton linter to generate nanocrystalline cellulose (NCC). Based on a 24 factorial design, the effects of sulfuric acid concentration, temperature, hydrolysis time and solid-to-liquid ratio on NCC yield were examined. The NCC specimens obtained from different sulfuric acid concentrations were subjected to a battery of analyses, including dynamic light scattering (DLS), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), small-angle X-ray diffraction (SAXRD), 13C nuclear magnetic resonance (13CNMR), and thermo gravimetric analysis (TGA) to probe the particle size distribution, morphology, functional group shifts, crystalline structure, crystallinity, position of carbon and thermal degradation properties of the ensuing NCC. The results indicated that sulfuric acid concentration and solids to liquor ratio at the higher levels, and temperature and reaction time at the lower levels were significantly conducive to NCC yield increases. The main effects in diminishing order were acid concentration, temperature, hydrolysis time, and solids to liquor ratio. Results of DLS and TEM observation suggested that the NCC had a size distribution center around 20~200 nm, with the length to width ratios ranging from 1: 1 to 1: 30. The FTIR analysis indicated that absorption peaks at 1150~1260, and 1300~142o cm-1 were derived from sulfonic ester bonds on the cellulosic chains. The SAXDR of the NCC samples confirmed that a portion of the crystalline cellulose changed from cellulose I to cellulose II. Along with increased sulfuric acid concentration, the crystallinity of NCC decreased, although the 85.5% NCC crystallinity was still higher than the 81.9% crystallinity of the original cotton linter. Solid state 13CNMR spectra indicated that the C6 atoms along the cellulosic chain were shifted from 65.28 ppm to a higher magnetic domain (to a lower ppm), again confirming the change in NCC crystalline morphology. The TGA indicated that the lower sulfuric acid NCC specimen began the step 1 weight loss at ca. 149℃, whereas its starting temperature of step 2 weight loss was generally higher than the mid- and high-acid NCC, being 337 and 205℃, respectively. The high-acid NCC only showed marked weight loss at 243℃. The study found that a sulfuric acid concentration of 60%, solids to liquor ratio of 1: 20, hydrolysis temperature of 45℃ and hydrolysis time of 5 min produced the best yield of 54.4%.