Controlled production of spruce cellulose gels using an environmentally “green” system

Stable spruce cellulose suspensions were generated in NaOH/urea aqueous solutions and used to make thermally induced gels with various swelling ratios and compressive strengths. Wood cellulose cannot be easily dissolved in water or any common organic solvent due to its high molecular weight, which largely limits its applications. Spruce cellulose was hydrolyzed by diluted sulfuric acid of various concentrations and hydrolysis times. The dissolution of these partially degraded samples was investigated in a NaOH/urea aqueous solution system considered environmentally “green.” The effects of acid hydrolysis on the structure and properties of subsequent thermally induced gels were examined using scanning electron microscopy, swelling and re-swelling experiments, and mechanical testing. The molecular weight of spruce cellulose was significantly reduced by acid hydrolysis, whereas its crystallinity slightly increased because of the removal of amorphous regions. All samples could be partially dissolved in the NaOH/urea aqueous solution and formed stable suspensions. Hydrolyzed cellulose samples with lower molecular weight exhibited a higher solubility. Rheological experiments showed these cellulose suspensions could form gels easily upon heating. A porous network structure was observed in which dissolved cellulose was physically crosslinked upon heating and then regenerated to form a three-dimensional network, where the dispersed swollen cellulose fibers filled spaces to reinforce the structure. The swelling behavior and mechanical properties of these ‘matrix-filler’ gels could be controlled by varying the mild acid hydrolysis conditions, which adjusts their degree of solubility. This research provides several opportunities for manufacturing wood cellulose based materials.