Preparation of cellulose fibers with high cellulose content from hardwood bleached kraft pulp by aqueous NaOH treatment

Commercial hardwood bleached kraft pulp (HBKP) for printing/writing grades was soaked in 5–20% aqueous NaOH to prepare wood cellulose materials with high cellulose purity. The mass recovery ratios decreased to 78% with 20% NaOH treatment under the conditions of cellulose mercerization, and the original cellulose I crystal structure was mostly converted to cellulose II. Although the α-cellulose content of the 20% NaOH-treated HBKP was ~ 100%, neutral sugar composition analysis showed that it contained ~ 5% xylose and ~ 1% mannose. The glucose content increased from 83% in the original HBKP to 94% in the 20% NaOH-treated HBKP. However, complete removal of xylose units could not be achieved even with 20% NaOH treatment, as approximately ~ 30% of the xylan molecules remained, likely because of strong physical interactions with cellulose molecules. The mass-averaged degree of polymerization (DPw) decreased from 1990 to 1264 with 20% NaOH treatment, indicating partial depolymerization of high-molar-mass cellulose during NaOH treatment. The NaOH-treated HBKPs possessed almost the same average fiber length, fiber width, and fines content. Therefore, cellulose II-rich fibers with 94% glucose content were prepared from HBKP using NaOH treatment. These NaOH-treated HBKPs may serve as alternative resources of cotton linters for producing sheet materials for medical or electronic applications.