Abstract
Structural organisations of a range of softwood kraft and other pulps are described in terms of their response to pulp drying and refining. Fibre widths, thicknesses, wall thicknesses, and wall areas, as well as cross-sectional shapes can be very different depending on whether pulps are dried and/or refined. Fibres can also respond to refining in different ways depending on whether the fibres have thin or thick walls.
Actual cross-sectional wall areas (including void space associated with delaminations) of undried kraft fibres normally remain unchanged with refining whereas those of dried and rewetted fibres increase. For refined fibres, delaminated walls are envisaged as consisting of several concentrically oriented lamellae aggregates or coarse lamellae (containing wall substance plus water) interdispersed with void space filled predominantly with water. Thus, effects of pulp refining could be to make the structural organisation of the coarse lamellae but not the walls of undried kraft fibres more dense, and the walls of dried and rewetted fibres less dense. Furthermore, the thicknesses of uncollapsed, undried fibres normally decrease with pulp refining while those of collapsed, dried and rewetted fibres increase. Finally, cross-sectional shapes and dimensions of dried and rewetted, thick- and thin-walled fibres are modified in different ways and at different rates.
Wall organisations of kraft, soda-AQ and neutral sulphite-AQ (NSAQ) fibres are very different. Kraft fibre diameters (widths and thicknesses), wall thicknesses, wall areas and hemicellulose contents are substantially lower than those of NSAQ and soda-AQ fibres when compared at either the same lignin content or pulp yield. Effects of refining are to cause kraft fibre diameters to be decreased further, and walls to delaminate and expand into fibre lumens. The unique properties of kraft fibres are explained by a tightening and contraction of the fibrillar and lamella wall organisations.
Observed changes in fibre dimensions and behaviours are discussed in terms of fibre wall structural organisations, compositions and behaviours .
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