Sheet properties of high yield pulps related to different pressing and drying conditions - part I

The objective of this investigation was to evaluate how physical properties of laboratory sheets, made from different thermomechanical and chemithermomechanical pulps, depend on the pressure and temperature levels during pressing and drying. Sheet were formed from long fibre fractions, defined as the pulp retained on a 50 mesh wire in a Britt Dynamic Drainage Jar, and pressed and dried according to a number of different pressure and temperature settings. The results showed that there were large differences in the sheet properties depending on what type of method that were used for pressing and drying. High freeness pulps were more sensitive to the type of method used for pressing and drying than low freeness pulps. The results indicate that the difference in sheet properties between ISO and Rapid Köthen sheets might be explained by the fact that ISO sheets are only wet pressed, the fibres are then allowed to spring back when pressure is released whereas the fibres in Rapid Köthen sheets are locked in their positions as the sheets are pressed and dried till over dry.