Oxygen delignification chemistry and its impact on pulp fibers

Two southern pine kraft pulps with kappa numbers of 30.0 (SW1-0) and 48.0 (SW2-0) were oxygen delignified by 30-60% by varying the reaction temperature (78-110degreesC) and charge of sodium hydroxide (1.6-4.4%). O-bleachability was found to be correlated to the incoming kappa number and charge of sodium hydroxide employed. In general, a lower charge of caustic and a higher brownstock kappa number improved pulp bleachability. The residual lignin in the brownstocks and O-delignified kraft pulp samples was isolated and characterized by C-13 and (PNMR)-P-31. C-13 NMR analysis of the residual lignin samples indicated that the post-oxygen delignified pulps were content of beta-O-aryl structures was increased by 23-36% depending on the extent of oxygen delignification. The post-oxygen delignified pulps were also shown to have increased substituted aryl carbons. (PNMR)-P-31 indicated that the relative content of condensed phenolic units increased by 9-20% after the oxygen delignification, depending on the severity of the O-stage. This observation was probably due to the accumulation or formation of 5,5-biphenyl structures in the process. The physical strength properties of brownstock and post-oxygen delignified pulps were assessed in terms of zero-span strength, tensile strength, tear strength, and burst strength. Oxygen delignification led to a slight increase in the curls and kinks of the pulp fibers. The O-stage was shown to cause a 4.8-15.6% decrease in zero-span strength. In contrast, oxygen delignification increased tensile strength. This result could be explained as the improvement of fiber bonding after the oxygen bleaching.