Ultrastructure of the cell wall of unbeaten Norway spruce pulp fibre surfaces

Laboratory kraft pulps were produced according to three industrially relevant pulping processes: isothermal continuous cooking, rapid displacement heating technique and 2-step polysulphide pulping. The pulps were further oxygen delignified and bleached according to three different sequences or chlorite bleached. A pre-hydrolysed kraft pulp was also included in the series. All pulps were prepared from the same spruce raw material. Pulp samples for electron microscopy were prepared by a rapid-freezing-deep-etching technique and thereafter the surface ultrastructure of fibre replicas characterized by transmission electron microscopy and an automatic computer approach based on image analysis. Results have shown the secondary cell wall S, and S, layers in all pulps to have a wide range of cellulose fibril sizes despite having mean fibril aggregate sizes that are not significantly different. This is confirmed by observations on electron micrographs from the various pulp types and individual layers. All pulp S, and S, layers show a distinct aggregation at ca 18-20 nm consistent with the aggregation of ca 4-5 fibrils in width. Such fibrillar structures are believed to be present in native cellulose. The results indicate that fibre cell wall fibrillar structure and cellulose fibril aggregation, especially aggregates wider than 20 nm, are the major factors affecting the tensile strength of unbeaten pulp fibres.