Role of xylan in softwood kraft pulp fiber sheets and fiber foams

Pulp fibers are paramount in paper products. However, they have lately also seen emerging use in fiber foams. Xylans, that are integral in pulp fibers are known to contribute to the strength of paper. However, if what is known about strength of paper, applies to strength of fiber foams, has not been elucidated. In this contribution we investigated the role played by xylan hand sheets and foams. We removed enzymatically 1% and added by adsorption 3% xylan on fibers corresponding to approximately a decrease of a tenth and an increase of a third of the total xylan content. The mechanical properties increased for sheets made using the xylan enriched pulp, however, the decrease in mechanical properties for the enzyme treated pulp was more pronounced. For networks with lower fiber-fiber contacts, that is, in foams, the mechanical properties were not as much influenced by the removal of xylan. The adsorption of additional xylan, on the other hand, increased the compressive strength of the fiber networks significantly. We propose that the improvement of mechanical properties for the sheets results from a combination of increased number of fiber-fiber bonds and higher sheet density, while the deterioration in mechanical properties of the enzyme treated fibers is caused by the opposite effect. In foams, the effect of density should not play such a significant role and hence the enzymatic treatment did not affect the mechanical properties as much. These findings suggest that xylan is a key component in strengthening the internal bonds in sheets and in foam although the underlying mechanisms may be different.