Facile dissolution of wood pulp in aqueous NaOH/urea solution by ball milling pretreatment

The softwood unbleached kraft pulp (UKP) with a lignin content of about 7 wt% was successfully dissolved in NaOH/urea/H2O solvent by a short time (1 h) ball milling pretreatment. Even though the milling time was only 20 min, the wood pulp fibers (width of around 30 μm and length of higher than 1 mm) of UKP had already become wood meals with a diameter of around 20 μm. After milling 1 h, the wood meals became more uniform and possessed a smaller average diameter of around 5 μm. Meanwhile, the crystallinity indices of the wood pulp decreased remarkably after the ball milling. Especially, the wood pulp became completely amorphous after 1 h ball milling. Furthermore, viscosity-average degree of polymerization (DPv) of the wood pulp decreased from 1300 to about 330. Moreover, the dissolved proportion of the wood pulp increased from 25.6% to 96.0%. The obtained wood pulp/NaOH/urea/H2O solution was transparent with yellow color. Moreover, after ball milling 60 min, the delignified thermomechanical pulp with a high lignin content of 13.5 wt% could also be dissolved and used to prepare the lignocellulose hydrogels. It may provide one way to study the structure and properties of the lignin-carbohydrate complex in wood pulp by dissolution of the milled wood pulp in NaOH/urea solvent and also promote the utilization of lignocellulose as novel high-performance materials.