THE ROLE OF HEMICELLULOSES IN THE HORNIFICATION OF BLEACHED KRAFT PULPS

During recycling of chemical pulps, deterioration of pulp properties o ccurs due to hornification. This phenomenon is caused by irreversible structural changes taking place during drying. In this work, the role of pulp hemicelluloses in the hornification process of kraft pulps pro duced by different cooking and bleaching methods was investigated. Abo ut 25-45% of xylan or 30% of glucomannan were selectively removed from pulps by xylanase and mannanase treatments, respectively. Subsequentl y, the pulps were dried and the effects of drying on the fiber propert ies such as water retention value (WRV), fiber stiffness, pore size di stribution, sheet density and tensile strength were evaluated. Drying of the fibers resulted in hornification phenomenon which could be clea rly observed as deteriorated fiber properties. A decrease of WRV, shee t density, tensile strength and total pore volume as well as an increa se of fiber stiffness demonstrated the loss of swelling and bonding ca pacity. An extensive removal of pulp xylan or glucomannan had no negat ive effect on the properties of never-dried bleached kraft fibers. How ever, the changes caused by drying became even more significant after removal of xylan or glucomannan. Thus, the hemicelluloses located in t he fiber pores and in the interfibrillar spaces seem to hinder the hor nification of kraft pulps, suggesting that pulps with higher hemicellu lose contents may have a lower tendency to hornify during recycling.