THE EFFECT OF SUPERHEATED STEAM DRYING ON THE PROPERTIES OF PAPER FROM CHEMICAL PULP

The effect of drying paper by direct contact with superheated steam wa s determined with respect to the physical, optical and chemical proper ties of the sheet. The results were compared with those for paper drie d by direct contact with hot air in a similar fashion. It was found th at the effects of superheated steam drying on the properties of paper made from bleached and unbleached kraft pulps beaten to different leve ls were smaller and in the opposite direction than those observed for mechanical pulp. Relative to air drying, steam drying of kraft paper h ad the effect of increasing sheet bulk by up to 6%, decreasing strengt h properties by up to 13% and improving optical properties by up to 10 %. It is proposed that, unlike mechanical pulp, kraft pulp fibres alre ady have a high bonding potential and derive no benefit from any therm al softening of the limited amount of lignin and hemicellulose polymer s. In fact, the higher sheet temperature during the constant drying ra te period of steam drying probably results in a thermally induced dryi ng stress relaxation leading to decreased strength properties.