Three market hardwood pulps; one acacia of Indonesian origin (Acacia mangium) and two eucalyptuses of Iberian and Brazilian origin were refined in an industrial conical refiner. In addition an 80/20 mixture of acacia and Scandinavian softwood was refined using two different fillings. Fibre properties were evaluated using image analysis and microscopy and the drainage characteristics and handsheet properties were related to the differences in fibre properties. The main distinguishing difference between the acacia and the eucalyptus was that the acacia had thinner fibre walls at about the same fibre perimeter. This resulted in a lower coarseness and a higher degree of fibre collapse. The acacia fibres also had fewer defects. The refining response was slower for the acacia than for the eucalypt and the difference was of the same magnitude as the difference in coarseness. WRV was related to the fibre bonding in the sheet as it correlated with the Z-strength measured as Scott Bond and with the calculated material density (derived from the theoretical relationship between bending stiffness, tensile stiffness and density). WRV together with some measurement of fibre defects explained the differences in tensile strength and tensile stiffness observed. The degree of fibre collapse together with the level of bonding developed by refining was responsible for the differences in structural sheet properties, apparent density, surface roughness and air permeability. The acacia containing furnishes had higher apparent density at the same WRV. The light scattering ability was shown to be controlled by the inverse of wall thickness. Together with WRV as the measure of bonding all the differences observed in light scattering coefficient could be explained with a simple linear regression model. All the results indicate that pulps containing fibres with thin fibre walls like for the acacia pulp are interesting fibre sources for high-quality printing papers. The relationships between fibre properties and sheet properties revealed in this study may lay the ground for a further development of new fibre sources.
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