Effect of cooking and bleaching on the structure of xylan in conventional pine kraft pulp

Pine chips were cooked with a conventional kraft process, and the structure of the surface xylan as a function of the cooking time was analyzed by enzymatic peeling and ¹HNMR spectroscopy. A brownstock pulp with kappa no. 25.9 was
also bleached to about 80% ISO brightness with bleaching sequences OZEP, OPZEP, OPPP, and ODEDED. The structure of the kraft pulp xylan was different from the native wood xylan. During kraft pulping, the 4-O-methylglucuronic
acid (MeGlcA) sidegroups were converted to 4-deoxyhex-4-enuronic acid (HexA) groups almost completely, and as a consequence, only about 12% of the carboxylic groups of the accessible xylan were 4-O-methylglucuronic acid groups. During bleaching with ozone or chlorine dioxide, the hexenuronic acid sidegroups of pulp xylan were, however, degraded, whereas the MeGlcA and arabinose sidegroups were relatively stable toward the bleaching chemicals. Thus, both cooking and bleaching affect the carboxylic acid profile and content in kraft pulps.