Surface Characterization of Softwood and Hardwood Kraft Pulp Fibers from Different Stages in a Bleaching Sequence

Pulp samples from a softwood kraft pulp line and a hardwood kraft pulp line were used to study how the chemical surface composition of different kraft pulp fibers are modified as the fibers are processed in the fiber line. The pulps were bleached according to the following sequence: OQ(OP)(ZQ)(OP). Three different surface analyzing techniques were used and compared to each other in order to determine the content of lignin and extractives on the fiber surfaces. The three techniques studied were electron spectroscopy for chemical analysis (ESCA), ESCA in combination with mercurization of the lignin, and mechanical peeling of the outer part of the fiber surface followed by standard chemical analyzing techniques. All analyzing techniques show that the surface coverage by lignin and extractives is higher compared with that of the bulk of the fibers. Standard ESCA, that uses both the theoretical and the measured values of the O/C ratio and C1 carbon content, in most cases gives the highest values on the surface coverage by lignin on both softwood and hardwood kraft pulp fibers, followed by the mercurization and mechanical peeling techniques. More extractives are detected on all kraft pulp fiber surfaces when standard ESCA is used as compared to the mechanical peeling technique. The obtained results indicate that the mercurization technique in combination with ESCA analysis is a good alternative for surface analysis of kraft pulp fibers. However, further investigations are needed to support this recommendation. Standard ESCA indicates that the surface lignin on both unbleached softwood and hardwood kraft pulp fibers has lower reactivity in an oxygen delignification stage as compared to the bulk lignin. In opposite, the mercurization technique and the mechanical peeling technique indicate similar reactivity of bulk and surface lignin towards oxygen. Furthermore, all ESCA techniques show that the decrease in surface coverage by lignin and total lignin content of both softwood and hardwood pulp fibers is low in the last hydrogen peroxide bleaching stage. Oxygen delignification is most efficient in removing surface extractives on both softwood and hardwood pulps and adsorption of surface extractives occurs after the ozone bleaching stage on all pulp samples.