Changes in surface ultrastructure of Norway spruce fibres during kraft pulping - visualisation by field emission-SEM

Changes occurring in the surface ultrastructure of spruce fibres during a conventional kraft cook were studied by means of field emission scanning electron microscopy (FE-SEM). Unwashed and washed wood chips or pulps were sampled at different stages of delignification and prepared for FE-SEM by cryofixation followed by freeze drying. Results showed that cell. wall porosity increased as the cook proceeded. This was visualised by a progressive delineation of the cellulose macrofibril ((MF) network as encrusting materials (i.e. lignin/hemicellulose) were solubilized. However, compact macrofibrillar surface regions were also observed adjacent to porous regions suggesting a heterogeneous distribution and removal of wood chemical components during kraft cooking. The cell wall layers observed on pulp fibre surfaces after kraft pulping were the primary wall and/or the S1 secondary wall layer. Unwashed fibre surfaces were covered by amorphous black liquor residues which increased with cooling time and rendered underlying fibre surface visualisation impossible. The mean MF width measured on three fibre surfaces after 19, 158 and 366 mins of the kraft cook were ca 16, Ig and 20 nm respectively.