The role of refining intensity and specific energy in refining of softwood kraft fibre fractions was studied. Several paper properties can be improved by selective refining of fractions. The tensile strength-dewatering resistance relationship benefits from low-intensity refining of the long-fibre fraction. The specific energy input determines the increase in fibre swelling which contributes to a higher sheet density and improved tensile strength. The apparent density-roughness relationship benefits from mild refining of the short-fibre fraction. Refining intensity has a strong effect on the magnitude of the gap between bar surfaces, on fibre shortening, and on the coarseness of fibres with high cell wall thickness. For the short-fibre fraction, which appeared to floccu-
late less, the maximum intensity causing “pad collapse” and more severe fibre shortening was lower than for the long-fibre fraction and feed pulp. The fraction-specific intensity and gap behaviour are believed to relate to the compressibility of flocs under the stress applied by bar surfaces – a phenomenon discussed in recent studies concerning the forces acting on fibre flocs.