The aim of this study was to investigate the effects of xylan removal on characteristics of bleached birch kraft pulps and on the behavior of such modified pulps in a papermaking process, particularly from the chemical retention point of view. The two methods used to reduce the pulp xylan content to an intermediate level between paper-grade kraft pulps and dissolving pulps were alkaline extraction and enzymatic treatment with endo-xylanases. The resulting changes in pulp properties were evaluated and, for the xylanase-treated pulps, the changes in pulp filtrate properties were also monitored. The wet end retention behavior was studied in the presence of typical papermaking additives in both laboratory and pilot-scale environments, making it possible to assess the effects of applied pulp treatments on colloidal charge interactions taking place in the wet end section of a papermaking process.

The effect of xylan removal on fiber characteristics depended on the pulp treatment applied. Alkaline extraction of dried pulp resulted in a 25-45 wt% reduction in the pulp xylan content together with altered fiber dimensions, increased water retention value (WRV), accessibility to high molecular weight (HMW) dye, and a lower total charge of the pulps, particularly of the fines fraction. The apparent surface charge of fines was lower in the alkali-extracted pulp than in the unextracted pulp but there were no differences in the surface charges of the corresponding long fiber fractions. The xylanase treatment of never-dried birch pulp reduced the pulp xylan content by up to 24 wt% depending on the xylanase dose and dosing strategy, resulting in pulps with slightly modified fiber dimensions, lower WRV and lower drainage resistance. In addition, the xylanase-treated pulps showed an increased accessibility for HMW dye together with ballooned fiber sections, confirming a loosened fiber wall structure. The negative zeta potential of the pulps obtained from streaming potential measurements decreased as a result of the xylanase treatment with the simultaneous release of negatively charged xylan-based substances into the pulp filtrates. The alkaline extraction increased the nanoscale surface roughness of fibers whereas the xylanase treatment had the opposite effect.

The removal of xylan from dried pulp by alkaline extraction was found to be beneficial for the retention of filler, starch and alkyl ketene dimer (AKD), which was attributed in particular to the reduced charge content of fines. A similar behavior was observed with the xylanase-treated pulps after washing, i.e. in the absence of dissolved xylan-based residues, but the presence of large amounts of these residues interfered with the retention system and led to significant retention losses. Therefore, the obtained results emphasize the importance of re-evaluating the retention system and strategy when using pulps with a lower xylan content for papermaking.