Preparation of cellulose nanofibrils from bamboo pulp by cellulase pretreatment combined with ultrasound method

Abstract:Cellulase enzymatic hydrolysis uses cellulase to selectively enzymatic hydrolyze the amorphous and imperfect crystalline region of cellulose, and finally obtains cellulose nano?brils (CNF). During the process of cellulase pretreatment, the fiber surface is corroded, and the fiber is cut and shortened, which leads to the degradation of the cellulose macromolecule into the segments of low degree of polymerization, which is further degraded into CNF under the action of mechanical force. In this study, CNF were achieved from bleached sulphate bamboo pulp by cellulase pretreatment combined with ultrasonic treatment, which is a green and efficient method. The effects of enzyme dosage, enzymolysis temperature, enzymolysis time and ultrasonic time on the yield of CNF were investigated to obtain the optimum process condition of CNF. The morphology, spectra properties, rheology properties, crystal properties and thermal stable properties of CNF were characterized by using transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), rotary rheometer, X-ray diffractometer (XRD), ultraviolet-visible spectrophotometer and thermogravimetric analyzer (TGA). Under the optimal conditions, i.e. cellulase dosage of 8% (the mass ratio of cellulase to raw material), enzymolysis temperature of 50 ℃, enzymolysis time of 10 h, and ultrasonic time treatment of 6 h, the yield of CNF could reach 62.6％, and the light transmittance could reach the lowest value, which is due to the best dispersion, the maximum crystallinity, and the most regular arrangement intensity of CNF. The prepared CNF have a diameter of 2-24 nm, a length of 50-450 nm, the crystallinity of 73.05% and have good dispersibility in water. The rheological behavior shows that the CNF colloid is a kind of shear thinning pseudo plastic fluid. With the increase of temperature, the viscosity of CNF colloid gradually decreases and finally becomes gentle, indicating that the CNF colloid has good stability. The XRD suggests that the prepared nanocellulose still belong to cellulose of Type I. The TGA analysis shows that both the initial degradation temperature and the maximum weight loss rate temperature of CNF are lower than that of the bamboo pulp fiber, but the residual mass of the CNF is larger than that of the bamboo pulp fiber at the end of the thermal decomposition and shows different thermal stability in different temperature ranges. The residual mass fraction is still up to 15.3% even at temperature of 700 ℃, which indicates that under cellulase pretreatment combined with ultrasonic cavitation, the preparation conditions of CNF are mild and the damage to the crystalline area is low. The crystallinity of the sample after ultrasonic treatment for 4 h increased from 63.7% to 69.21% compared with the raw materials of fiber. After the ultrasonic time was increased to 6 h, the crystallinity reached 73.05%. The CNF prepared in this paper is suitable for assembling functional stable cellulose-based nanomaterials and have potential application value in the fields of food and medical packaging materials, and might have a promising application prospect in the field of heat resistant biomass-based composites attributing to its good thermal stability.