The role of hemicelluloses in the hornification of bleached kraft pulps

During recycling of chemical pulps, deterioration of pulp properties occurs 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 produced by different cooking and bleaching methods was investigated. About 25-45% of xylan or 30% of glucomannan were selectively removed from pulps by xylanase and mannanase treatments, respectively. Subsequently, the pulps were dried and the effects of drying on the fiber properties such as water retention value (WRV), fiber stiffness, pore size distribution, sheet density and tensile strength were evaluated. Drying of the fibers resulted in hornification phenomenon which could be clearly observed as deteriorated fiber properties. A decrease of WRV, sheet density, tensile strength and total pore volume as well as an increase of fiber stiffness demonstrated the loss of swelling and bonding capacity. An extensive removal of pulp xylan or glucomannan had no negative effect on the properties of never-dried bleached kraft fibers. However, the changes caused by drying became even more significant after removal of xylan or glucomannan. Thus, the hemicelluloses located in the fiber pores and in the interfibrillar spaces seem to hinder the hornification of kraft pulps, suggesting that pulps with higher hemicellulose contents may have a lower tendency to hornify during recycling.