The effect of alkaline pre-treatment on cellulose pulp fiber dissolution

• Alkali and thermal treatments reduced swelling and induced cellulose hornification. • Alkali pretreatment above 10% NaOH severely inhibited kraft pulp dissolution. • Combined PLM–DSC method enabled kinetic and thermal analysis of dissolution. • Urea and ZnO additives improved dissolution onset and reaction completeness. • Oven-dried pulp was hornified but had surprising dissolution reactivity. This study investigates the dissolution kinetics of hydrolyzed softwood kraft pulp in several NaOH-based solvents using polarized light microscopy and differential scanning calorimetry (DSC). The target is a deeper understanding of how hornification, induced by either alkali exposure or drying, inhibits fiber dissolution. Polarized light microscopy measured the dissolution process by tracking the reduction in sample luminance, while DSC measured the energy involved in dissolution. Both techniques employed a dynamic temperature sweep, where the temperature was decreased at a constant rate over the dissolution event. Alkali pretreatment in 10% or greater NaOH solution severely diminished the fiber dissolution in all solvents. Oven drying at 150°C reduced the sample swelling compared to room temperature-drying, which is consistent with earlier hornification studies. However, oven drying did not reduce the dissolution parameters.