The effect of controlled glucuronoxylan adsorption on drying-induced strength loss of bleached softwood pulp

This study investigates the effect of glucuronoxylan adsorption on pulp properties, with a focus on the influence on hornification and the location of the adsorbed xylan. A bleached never-dried softwood kraft pulp was treated at neutral conditions with birch glucuronoxylan solutions of various initial concentrations, yielding pulps with different amounts of adsorbed xylan (24-133 mg/g pulp). By comparing results of total and surface charge measurements of the modified pulps, it has been suggested that xylan adsorption occurs predominantly on internal fibre fibril surfaces after diffusion into the porous fibre wall. The treatments resulted in increased tensile strength and beatability, but the tensile strength at a given degrees SR, density and light scattering coefficient was more or less unaffected. This effect was seen both before and after drying of the modified pulp. However, the effect was more evident when the xylan treated pulp was subjected to a drying-rewetting cycle before beating and hand sheet preparation. Xylan adsorption increased apparent wet fibre flexibility, both for never-dried and once-dried fibres. The flexibility of modified once-dried fibres increased along with a significant decrease in the degree of hornification, defined as relative loss in WRV upon drying. An explanation for the data presented would be that modification of never-dried pulp fibres by xylan adsorption stabilises the cellulose fibrils, preventing aggregation upon drying (hornification). The swelling/softening properties of the fibre wall are thus retained to a high degree, which enhances beatability and allows for a larger contact area between the fibres of the once-dried pulp.