Chemical characterization of the surface layers of unbleached pine and birch kraft pulp fibres

The content of wood polymers in different morphological parts of un-bleached pine and birch kraft pulp was studied using a mechanical peeling technique. The relative distribution of hemicelluloses and cellulose in the surface material of the pine kraft fibres was similar to that in the whole fibres. A somewhat higher concentration of xylan wa found in the primary fines (<100 mu m; ray-cell-enriched fraction) than in the rest of the pulp. Furthermore, the primary fines and the surface material obtained by peeling the fibres had a considerably higher lignin content than that in the corresponding pulp. the relative distribution of the wood polymers seems to be similar to that reported in the native wood, indicating that no extensive relocalization of xylan on the outer fibre surface of on the primary fines occurs during the kraft pulping of pine. The surface material of the birch kraft fibres was considerably richer in xylan than the remainder of the fibres. Furthermore, the primary fines obtained from the birch kraft pulp were highly enriched in xylan. the lignin content in the surface material and in the primary fines fraction was considerably higher than that in the corresponding pulp. In the case of birch it is obvious that all external surfaces are rich in xylan lignin. However, it is difficult to judge whether this is a reflection of the original wood composition or is due to reprecipitation of xylan and/or lignin on the fibre surfaces and primary fines during the cook.