Mid Sweden University

The present study evaluates effects of moisture on sheet properties made different Bauer-McNett fractions from TMP, both individually and in combinations. It is well known that the coarse, stiff, long fibre fractions cause surface roughness in dry sheets. However, this study indicates that these fibres remain relatively unchanged when moisture diffuses into the sheet structure, i.e., the degree of decollapse is low. The fibres from the middle fraction rise easily, causing the greatest change in surface roughness when moisture is added to the sheet. Sheets including both coarse fibres and fibres from the middle fractions display the highest surface roughness values at a high moisture content. The trials also indicate that fines contribute to increased surface roughness: when the degree of bonding in the sheet structure is increased, there is a risk that the whole structure or the flocks, instead of the individual fibres, may swell.

Effects on fibre and sheet properties of preheating to temperature well above the softening temperature of lignin in refining of spruce TMP reject were evaluated. This study indicates that an improved surface smoothness can be attained on TMP-based wood-containing papers through high-temperature reject refining. Under these conditions the degree of irreversible fibre collapse increased. Results from tests of the fibre-wall, fibre surface characteristics, pulp yield and sheet brightness show only marginal differences at high temperatures compared to standard reject refining. Experiments in which laboratory sheets were moistened showed that a high degree of irreversible fibre collapse correlates with surface smoothness.

Effects of preheating temperature on energy consumption,fibre collapse and pulp quality in refining of spruce TMP reject material were studied. At preheating to temperatures well above the softening temperature of lignin thick-walled fibres were collapsed to a higher degree. Shives and fibres in the BMN +10 fraction were efficiently reduced. An increase in sheet density was obtained. Energy consumption was reduced, at a certain freeness level. This study indicates that an improved surface smoothness can be achieved as result of high temperature reject refining.