Xylan reactions in kraft cooking and their influence on paper sheet properties

Abstract: Xylan is the main hemicellulose in birch, eucalyptus, and most other hardwood species. During kraft pulping a series of chemical reactions and physical processes involving xylan take place. The processes studied in this thesis are the following: dissolution, degradation, redeposition onto the fibres, side group conversion, and cleavage of side groups off the xylan back bone. The side group in native xylan consists of methylglucuronic acid, which is mainly converted into hexenuronic acid during kraft cooking. Hexenuronic acid affect the pulp in terms of increased brightness reversion and deteriorated bleachability. The kinetics of the side group cleavage and conversion reactions were analysed using various analytical tools. The study revealed that the most commonly used methods for methylglucuronic acid measurements are not as accurate as has been claimed in the literature. A modification of two of the methods was suggested and evaluated.A common practice used to minimise the hexenuronic acid content involves use of a high cooking temperature. The kinetic study showed that the degree of substitution of pulp xylan is only slightly affected by temperature, and that the observed effects are likely to be more associated with the xylan content of the pulp than with the hexenuronic acid content of the xylan. For the dissolved xylan, however, the degree of substitution showed a high temperature dependency and moreover it was always higher than in the pulp.Xylan itself is known to have the capacity to increase the strength of the manufactured pulp. This knowledge was applied by exchanging cooking liquors between birch kraft cooks, in which a high amount of xylan is dissolved, and spruce cooks, which contain very small amounts of native xylan. This seems like an attractive alternative for softwood kraft cooking, since both the tensile strength and stiffness increased significantly. The magnitude of the strength increase was correlated with the molecular weight of the added xylan and with the increased surface charge of the fibres.