Improvement of Paper Strength by Increasing the Xylan Content

Extracted xylan from beech dissolving pulp and eucalyptus kraft pulp was precipitated on unrefined, bleached, once-dried softwood kraft pulp and sulfite pulp. The temperature, pH, ionic strength, xylan concentration, pulp consistency, and the dwell time were analyzed regarding their influence on the adsorption of xylan. Furthermore, handsheets were made to investigate the impact of xylan on the tensile strength and the tearing resistance of the paper. The swelling behavior of the fibers was of interest as well as the determination of the total and surface charge of the pulp. The xylan content of the fibers could be significantly increased. The temperature, xylan concentration and ionic strength showed a large influence on precipitation. No significant change in the attachment of xylan between neutral and low alkaline level of the pH could be noticed. A higher pulp consistency, including a sufficient mixing during adsorption, is favorable. Xylan shows a large impact on the tensile strength of the softwood handsheets. The tensile index of the handsheets made of the softwood pulp and refined at 3.000 revolutions with a PFI mill could be increased by up to 38% compared to the reference sample. The xylan-modified samples also showed by trend a higher stretch at break of about 0.5%. This could be the result of a higher surface area and total charge of the fibers. Thus, enhanced swelling is caused leading to softer and more flexible fibers. These effects provoke a larger area in molecular contact. On the other hand, the trend of the tear index of the softwood handsheets with an increased amount of xylan declines stronger after a longer time period of refining.