Effect of press drying on sheet properties of high-temperature thermomechanical pulp (HTMP)

The objective of this study was to find out how much press drying improves the properties of high-temperature thermomechanical pulp (HTMP) sheets. It is well known that HTMP, which is also known as Asplund pulp, can be manufactured with low energy consumption (0.2-0.4 MWh/t). HTMP fibres have poor papermaking properties. They are coated by lignin, which makes them stiff and hydrophobic in nature and impairs their bonding ability. HTMP also has low brightness. It contains a high proportion of long fibres and a small amount of fines. Because of their poor bonding potential, HTMP fibres have not been used in papermaking, although some attempts have been made in this area, Perhaps the most interesting idea is to use press drying. However, there appears to be little experimental data on this. Experimental studies with spruce HTMP handsheets were performed at laboratory scale. The glass transition temperatures of HTMP at different moisture contents were determined with differential scanning calorimetry (DSC). A small amount of water (3-5 %) was found to reduce the glass transition temperature of HTMP dramatically. Press drying experiments were also made. Press drying was found to improve the bonding of HTMP fibres considerably. The tensile index of HTMP rose by more than 800 % when the sheets were press-dried (the tensile index level of normal dried sheets was low). The effect of the initial moisture content before press drying was also examined. The initial moisture content had a significant influence on sheet properties: the higher the moisture content in press drying, the higher the tensile strength and density.