Wet Pressing Research in 1989 – An Historical Perspective, Analysis and Commentary

Abstract

There is a large body of literature on wet pressing;almost all of it deals with water removal and much of it is empirical in nature. Though we have been forced to infer what happens inside a roll press nip by making observations from the outside, our qualitative and quantitative knowledge of the water removal process has improved greatly over the years. There have been some direct experimental measurements of several important variables inside the nip(applied pressure, fluid pressure, and mid nip roll separation), but only at the system boundaries (the roll surfaces). Direct data is still lacking on other important variables inside the nip such as localized pressure gradients, sheet thickness and sheet dryness, localized deformation, a good definition of the interfacial region, direct measurements of parameters in the thickness direction,and thermodynamic properties. So far, wet pressing models have had limited success in making a priori predictions of sheet water removal and have not begun to address paper properties, the next major thrust of wet pressing research. We have conjured up a mental picture of wet pressing which seems to fit well the observations made from the outside. It is quite possible that this picture is more inaccurate and incomplete than we imagine, but this state of affairs is actually exciting because it means much remains to be learned about the fundamental mechanisms of wet pressing. In this learning process, paper property development is expected to receive equal, if not greater, attention.