Surface lignin on kraft wood pulp fibres.

摘要

A systematic study of the surface lignin of kraft wood fibres is reported in this thesis. The existing X-ray photoelectron spectroscopy (XPS) methodologies for surface lignin analysis were critically evaluated, and a modified XPS methodology was established. A novel technique has been developed for imaging lignin distribution on the fibre surfaces and across the fibre walls, using fluorescence confocal laser scanning microscopy (CLSM). The origins of fibre surface lignin were investigated, and the mechanism of lignin adsorption on the fibres in pulping and pulp washing was studied.; It was found with both XPS and CLSM that lignin was highly concentrated on the surfaces of various kraft pulp fibres which were produced in lab or in commercial pulping. CLSM revealed that lignin was concentrated not only on the exterior surfaces of fibres, but also on the fibre lumen surfaces and the borders of fibre pits.; In conventional kraft pulping, middle lamella was found to be completely removed, so the influence of middle lamella remaining on the fibre surface lignin is negligible. Lignin adsorption occurred under both pulping and pulp washing conditions, and it was identified to be the primary cause for high lignin concentration on the fibre surfaces after kraft pulping.; Under conventional kraft pulping conditions, the concentration of the adsorbed lignin on the fibre surfaces was as high as 16–18%, appearing to be a 2 nm thick monolayer on the fibre surface. Lignin adsorption in the bulk fibres was 0.8–1.0% of the fibre mass, which accounts for 20–30% of residual lignin in unbleached kraft pulp. When the calcium ions in the system exceed a certain concentration threshold, they cause lignin coagulation and, in turn, lignin precipitation on the fibres.; The findings obtained from this work are crucially important for developing technical measures for preventing lignin adsorption on the fibres in pulping and pulp washing. It also sheds light on the potential impact of surface lignin on bleaching, papermaking, and paper properties.