Preparation And Characterization Of Nanocellulose And Self-assembly Nanocellulose Films Based On Formic Acid Hydrolysis

In recent years, along with the increasing concerns derived from fossil resource dependence, lignocellulosic biomass has been regarded as the alternative of fossil resources because of its abundance and renewability. Recently, there has been a growing interest in biorefinery of lignocelluloses, in light of their advantages in efficient fractionation of biomass into cellulose, hemicelluloses, and lignin, which could be converted to fuels,power,heat,functional materials and value-added chemicals,etc. As a kind of intermediate product during biorefinery process, nanocelluloses are receiving great attention due to their excellent mechanical and chemical properties, such as high tensile strength and elastic modulus, high specific surface area, low density, low thermal expansion combined with biodegradability and renewability. Because of their special intrinsic nanostructure and distinctive properties, nanocelluloses have broad application prospects such as reinforcing fillers, electroconductive materials, packaging materials, and biomedical materials. Many methods have been developed for production of nanocellulose, such as strong acid hydrolysis, enzymatic hydrolysis, TEMPO-mediated oxidation, mechanical shearing, etc.However, among these traditional methods, some issues have to be addressed, such as relatively severe equipment corrosion, pollution of environment, expensive and unrecyclable chemicals used, and high energy consumption. In this work, we developed a sustainable method for the preparation of functional cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF) using formic acid (FA) hydrolysis plus high-pressure homogenization. The cationic modification of CNC and the self-assembly film of CNF in water and organic solvents were also investigated.In the first part, CNC was produced from bleached eucalyptus kraft pulp with a high yield over 75% via FeCl3-catalyzed FA hydrolysis process. It was found that the particle size of resultant CNC products (F-CNC) decreased with the increase of FeCl3 dosage in FA hydrolysis, and a maximum crystallinity index (CrI) of about 75% could be achieved when the dose of FeCl3 was 0.015 M (i.e. about 7% based on the weight of starting material).Thermogravimetric analyses revealed that F-CNC exhibited a much higher thermal stability(the decomposition temperature was over 310 0C) than S-CNC prepared by typical sulfuric acid hydrolysis. In the FeCl3-catalyzed FA hydrolysis process, FA could be easily recovered and reused, and FeCl3 could be transferred to Fe(OH)3 as a high value-added product. Thus, the FeCl3-catalyzed FA hydrolysis process could be sustainable and economically feasible. In addition, F-CNC could be well dispersed in DMSO and its dispersibility in water could be improved by a cationic surface modification.In the second part, we described a sustainable route, which combined relatively mild FA hydrolysis pretreatment of bleached softwood pulp with high-pressure homogenization,to prepare cellulose nanofibrils (F-CNF). Results showed that the prepared F-CNF had high thermal stability (e.g. the onset decomposition temperature was over 290 ?). Also, it was found that the prepared F-CNF were more homogeneous and the length of the F-CNF became shorter with the increase of hydrolysis time in FA hydrolysis, and a maximum CrI of about 52.9% could be achieved when the hydrolysis time was 6 h. Compared to H-CNF(prepared by HC1 pretreatment), F-CNF showed better dispersibility in DMSO, DMF and DMAC respectively because of the introduction of ester groups on the surface of F-CNF.This high compatibility leads to that the F-CNF may have great potential in reinforcing polymeric matrix materials. In addition, FA could be easily recovered, and the recovery rate was higher than 90%. Thus, FA hydrolysis pretreatment plus high-pressure homogenization could be a sustainable and economically feasible process for the production of CNF.In the third part, the self-assembly films of F-CNF in water and organic solvent (e.g.DMAC) were prepared by solvent casting method, and resultant films in water and DMAC were called W-CNF flim and D-CNF film respectively. Then, the two types of film were characterizated and compared. It was found that the D-CNF film showed excellent mechanical strength with tensile strength of 227.97 MPa and Young's modulus of 10.80 GPa, which was much higher than that of W-CNF flim with tensile strength of 34.96 MPa and Young's modulus of 1.92 GPa. Due to hydrophobicity and compact structure, D-CNF film showed excellent water resistance and very low oxygen and water vapor permeability.This self-assembly film could be used in the fields of electron device, high-end packaging,etc.