EFFECT OF COOKING AND BLEACHING ON THE STRUCTURE OF XYLAN CONVENTIONAL PINE KRAFT PULP

Pine chips were cooked with a conventional kraft process, and the stru cture of the surface xylan as a function Of the cooking time was analy zed by enzymatic peeling and H-1-NMR spectroscopy. A brownstock pulp w ith kappa no. 25.9 was also bleached to about 80% ISO brightness with bleaching sequences OZEP OPZEP OPPP and ODEDED. The structure of the k raft pulp xylan was different from the native wood xylan. During kraft pulping, the 4-O-methylglucuronic acid (MeGlcA) sidegroups were conve rted to 4-deoxyhex-4-enuronic acid (HexA) groups almost completely, an d as a consequence, only about 12% of the carboxylic groups of the acc essible xylan were 4-O-methylglucuronic acid groups. During bleaching with ozone or chlorine dioxide, the hexenuronic acid sidegroups of pul p xylan were, however, degraded, whereas the MeGlcA and arabinose side groups were relatively stable toward the bleaching chemicals. Thus, bo th cooking and bleaching affect the carboxylic acid profile and conten t in kraft pulps.