Alkaline Pre-Impregnation and Inter-Stage Washing for Enhanced Soda-AQ Pulping of Oil Palm EFB

Abstract Alkaline pre-impregnation prior to soda-anthraquinone pulping of oil palm empty fruit bunches enables conversion into highly delignified, high-strength cellulosic matrices. A 23 factorial design was established to evaluate the main and interaction effects of pre-impregnation variables─temperature, time, and alkali charge─against conventional soda-anthraquinone pulping. While pulp yield was invariant, pulp and paper properties were strongly governed by distinct main and two-way interaction effects. Crucially, temperature dominated the pulp viscosity, burst, and tearing indices. Leveraging this insight, an inter-stage washing step was integrated while fixing temperature at a lower level to preserve fiber integrity, with the remaining conditions anchored by the factorial model. This integrated sequence significantly outperformed conventional and unwashed two-stage alternatives by removing dissolved lignin, while maximizing the retention of structural hemicelluloses to promote fiber bonding, yielding a lower kappa number (10.4) alongside elevated tensile (22.68 N·m/g), burst (4.40 kPa·m2/g), and tearing (8.36 mN·m2/g) indices. These findings provide an efficient, low-temperature framework for agricultural residue valorization into high-strength bio-based materials.