Kraft Pulp Fibre Surfaces. Chemical Composition and Effect on Pulp Bleachability
The work presented in this thesis concerns studies of kraft fibre surfaces in respect of chemical composition and effect on pulp bleachability. Kraft fibre surfaces were studied by mechanical peeling using a disintegrator operated at 20.000-200.000 revolutions and a pulp consistency of 3% or 4.5%.
Surfaces of unbleached kraft pulp fibres of different origin and kappa numbers exhibited a high lignin content compared to the bulk of the fibres. The lignin content of pulp fractions enriched in the ray cells was also higher than that of corresponding fibres. In unbleached pine kraft pulp similar contents of xylan were found in the fibre surfaces and ray cells compared to the fibres, whereas these fractions exhibited a relatively high xylan content in birch kraft pulp. Results obtained for pine kraft pulp indicated no extensive reprecipitation of xylan on the pine fibre surfaces during the pulping process. Metal ion enrichment was observed in both surface material and ray cells of unbleached kraft fibres. The distribution patterns of the metal ions, Mn, Fe, Cu, Ca and Mg were similar to those found in wood. Furthermore, the content of Mn and Ca correlated well with the content of lignin and carboxy groups in the different pulp fractions.
In unbleached pine kraft fibres, xylanase acted relatively uniformly on all accessible surfaces, whereas the action of mannanase was more pronounced on the outermost fibre surface than in the inner fibre wall. In unbleached birch kraft fibres, attack on the fibre surface was more extensive compared to the inner parts of the fibres. The action of xylanase on pine and birch kraft fibres differed both in site and intensity, probably reflecting the different chemical composition of the two types of pulp fibres.
Fibre surface material and ray cells of both pine and birch kraft pulps affected the bleachability in an enzyme-aided bleaching with one stage of peroxide treatment. The effect of the enzymic treatment was enhanced when surface material and ray cells were removed prior to treatment and, following peroxide bleaching, a higher brightness level and lower kappa number were obtained compared to the reference pulp. However, removal of surface material and ray cells from a pulp manufactured according to an extended cooking procedure prior to bleaching in a TCF-sequence had no significant effect on the final brightness and kappa number. However, a higher viscosity and brightness stability, and a lower consumption of hydrogen peroxide, was recorded when surface material and ray cells were removed.