Paper properties of TMP and CTMP handsheets from spruce, as affected by the solids content from pressing

This paper discusses how the properties of dry handsheets from two different mechanical pulps and one chemithermomechanical pulp and their respective long fibre fractions depend on the level of dryness to which the sheets are pressed at high temperature. Sheets were pressed and dried to different dryness levels at 400 kPa and 93 degrees C in a modified Rapid Kothen dryer. Prior to testing of the physical properties, the sheets were dried without pressure at, room temperature to full dryness and kept in a conditioning room. This was performed with the aim of evaluating the consolidation and development of final strength properties with respect to dryness after a press section in a paper machine.The most interesting finding from these trials was that the increase in density and strength were far from linear with increased press dryness, especially for the long fibre fractions when looking at the Z-strength development. There was a distinct inflection of the curve when the sheets reached a press dryness level of some 50%. The most important dryness interval for internal strength development was found between 50 and 80%.The fact that most paper and board machines press the sheet to only some 50% dryness, before feeding it into the drying section, implies that much of the inherent strength potential of mechanical pulps may be unexploited. Further research on pressing to higher dryness levels will in the future be continued at FSCN at Mid Sweden University.