A stylus profilometer has been used to evaluate the effect of wet pressing on the collapse of individual fibres from mechanical, chemimechanical and chemical pulps. The chemimechanical and chemical pulp fibres begin to collapse at low pressures and approach complete collapse at high pressures, while the mechanical pulp fibres do not exceed 80% collapse to 5000 kPa. The degree of collapse of southern pine TMP at a given pressure is about the same as that of northeastern spruce/balsam TMP. Since the thick-walled Southern pine fibres are less flexible, it is concluded that transverse collapsibility and flexibility are two independent fibre properties.
On the basis of modelling results and the difference between fibre thickness-measurement from networks pressed in contact with smooth and with rough surfaces, it is suggested that wet pressure transferred locally at fibre contacts within a sheet leads to local collapse forces higher- than expected from nominal wet pressure vlues. Fibre contacts are initially present in the unpressed sheet.On pressing, free fibre segments will be deflected into contact with other segments above or below them, producing additional fibre crossings as pressing progresses . The effective pressing pressure will be highest at the initial fibre contacts, decreasing to zero at contacts just formed at the end of pressing. Because of the demonstrated effect of wet pressure on fibre collapse, it is the thickness at the initial fibre contacts or those formed early in the pressing process that is important in determining sheet density.
Use of the more appropriate fibre thicknesses, substantially improves the prediction of sheet density by the Interactive Multiplanar Model of sheet structure for a range of pulp types.