Compressibility and disruptive shear stress in Norway spruce and Scots pine thermo-mechanical pulps was studied using shear and compression equipment. The volume fractions of the pulps as a function of the compressive pressure was examined in a steam environment at temperatures of up to 170 degrees Celsius, and the disruptive shear stress of a compressed pulp pad was determined as a function of compressive pressure and the volume fraction of the pulp. The experiments were carried out using sawmill pine, first thinning pine and spruce TMP pulps with different freeness values. Additional experiments were also performed using sawmill pine TMP pulp from which the fine material had been removed. The results showed that there are no differences in compressibility between TMP pulps of different freeness values and obtained from different wood species, but the pulps were more compressible at high temperatures than at room temperature. The shearing experiments showed that fines work as a lubricant in shearing, reducing the disruptive shear stress in the pulp pad. This disruptive shear stress also decreased as the temperature of the pulp increased. No differences were observed between the wood species, although the pulps differed in their extractive content. This indicates that extractives do not significantly reduce fibre-to-fibre friction in the refiner and thus tend to increase energy consumption in refining.
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