New insight into antioxidant capacity of kraft lignin in chemical pulp refining and thermal durability of refined pulp sheets

摘要

Cellulosic fibers are inherently thermally sensitive and their modification to withstand thermal degradation is essential for various applications, such as food packaging. The existing protection methods are mainly based on barrier coatings. Herein we demonstrate the possibility to improve the thermal durability of pulp sheets through the antioxidant properties of kraft lignin. Thermal treatments were conducted at a high temperature (225 °C) in the presence of water vapor (volume fractions 1 and 75%), mimicking food-preparation conditions. Two different concepts were chosen to study the antioxidant capacity of lignin in chemical pulp. The first approach exploited softwood kraft pulps with different residual lignin contents (i.e. unbleached, oxygen delignified and ECF bleached pulps). Our results demonstrated that the tensile strengths of lignin containing pulps were remarkably higher than that of the bleached pulp after water vapor enhanced thermal degradation, indicating the stabilizing effect of lignin, which acts as a hydroxyl radical scavenger. The objective of the second approach was to elaborate the lignin assisted intensive refining of pulp. Lignin impregnated pulp was produced by suspending bleached kraft pulp in a caustic solution of alkali lignin (pH 13.0) with subsequent precipitation induced by lowering the pH to 6.5. The presence of lignin on the pulp fiber surface was confirmed using scanning electron microscope imaging. The thermal durability of the lignin impregnated pulp sheets was assessed via tensile strength measurements. Our results indicate that non-covalently bound lignin protects fibers against strength loss of paper during intensive refining, which is presumably initiated, in part, by the mechanoradicals formed during mechanical treatment. The antioxidant capacity of low-cost lignin could be utilized to inhibit the adverse effects of radical formation in pulp bleaching, refining and drying to allow for controlled pulp quality, in order to enhance the durability of paper-based food packages.