High-yield hydrogen peroxide treatment of refined softwood in recyclable deep eutectic solvent to produce moisture-tolerant wood nanofiber films

In this study, a novel hydrogen peroxide treatment of refined softwood (thermomechanical pulp) in urea-LiCl-based deep eutectic solvent with negligible material loss (<2 %) was studied as a pre-treatment in the production of wood nanofibers (WNF). A short treatment time (2 min) was shown to slightly alter the structure of wood by increasing the oxygen content at the surface of the fibers and increasing the fine fiber fraction content due to the mild oxidation of surface of fibers potentially via hydrogen peroxide mediated formation of lithium hypochlorite. The mild alteration of the fiber structure resulted in the formation of a WNF film with 33 % higher tensile index compared to films obtained from WNF produced without hydrogen peroxide treatment. Furthermore, deep eutectic solvent recyclability was successfully demonstrated for five consequent treatments. In addition, hot pressing of the WNF films was investigated to improve the water tolerance (wet tensile strength) of the films. By studying various pressing conditions (time, temperature, and pressure), the wet tensile strength of films was increased from 1.0 kNm/kg to almost 20 kNm/kg with only ten minutes pressing time. The increase in wet tensile strength was effectively correlated to a decrease in the water uptake of the films during submersion in water. Simultaneously to the increased water tolerance, the dry tensile properties of the films decreased, but always remained above 40 kNm/kg. • Hydrogen peroxide-treatment of refined wood in deep eutectic solvent was studied. • Short treatment induced minor changes in surface properties of wood fibers. • Wood nanofiber films from treated sample exhibited improved tensile strength. • Almost 20-fold increase in wet tensile strength was obtained by hot pressing. • Correlation between wet tensile strength and water absorption of films was noted.