Screening of Alternative Solvents for Swelling and Continuous Microfibrillation of Cellulose Fibers

Abstract

Microfibrillated cellulose forms a tiny and dense fiber network with beneficial properties such as low density, low oxygen permeability, and excellent mechanical strength and stiffness of the elementary fibers. Common production processes utilize batch processing such as grinding, homogenization, or microfluidization in combination with chemical or enzymatic fiber pretreatment. However, the disintegration of the hierarchical cellulose microstructure requires enormously high energy input and is limited to very low solid fiber content, which renders the material economically less interesting. The need for alternative and continuous processing routes is currently high. Therefore, softwood cellulose pulp fibers are pre-treated by swelling in various solvents, including a selection of ionic liquids (IL), deep-eutectic solvents (DES), and natural deep eutectic solvents (NADES), studying the influences of different swelling media and conditions concerning the variations in fiber morphology and microstructure. In a second step, the tendency for fibrillation under homogenization in small-scale lab conditions or large-scale twin-screw extrusion is investigated. The swelling kinetics highly depends on substituting the alkyl chain of imidazolium chloride (IL) to provide homogeneous swelling favorable for fibrillation. The selection of optimum parameters (temperature, time, substrate concentration, water, stirring) leads to controlled swelling and unfolding of the individual elementary fibrils without complete dissolution. Potassium carbonate/glycerol (DES) swelling requires longer treatment times. It induces almost no variations in cellulose structure while processing conditions in choline chloride/urea or choline chloride/glycol (NADES) are better controllable compared to ionic liquids with a critical balance between fiber disintegration, dissolution, and fluent homogenization.