DRYNESS AND PHYSICAL PROPERTIES OF PAPER WEBS IN LABORATORY-SCALE IMPULSE PROCESSING

Abstract

This investigation evaluates the changes in dryness and physical properties of impulse processed paper webs. A laboratory-scale platen press was used to process 60 g/m2 webs under a wide range of pressure impulses and temperatures (23–380°C). The heat transfer rate to the web was also varied by using two different platen materials (steel and copper). Both thermomechanical and bleached kraft pulps were tested, and the dryness, density, surface roughness, and Scott bond were measured for each trial. Results show that dewatering is enhanced with increased temperature, pressure, and nip residence time, while pressure pulse shape and platen material have little influence. An empirical correlation is given to relate sheet dryness to the key process variables. This correlation suggests that resulting dryness values are not unique; that is, different combinations of temperature, pressure, and nip residence time can be used to obtain a similar level of dryness. Physical reasoning was used to develop a second correlation that links the resulting dryness to the density of the completely dried web. Surface roughness decreased with increasing surface temperature, while Scott bond rose sharply under conditions of high temperature and long nip residence time.