The essential scientific problems in wet pressing are concerned with water removal from the wet web, its runnability and the effect of pressing on the quality of the web and the paper produced from it. This paper briefly reviews the present understanding of the effect of wet pressing on the web and paper quality and discusses some questions concerning the runnability of the web through the press section. The main emphasis is placed on water removal. A short historical review of the development of our present understanding of wet pressing fundamentals is presented. The modelling of wet pressing is also discussed.
The water removal from the fibre cell wall starts at fairly low solids contents of the web, in the range of 20–25%. In modern press sections, the solids content of the web after pressing is about 45–50%. At this solids content, most of the water is in the fibre wall. Thus, when trying to enhance water removal further, it is necessary to understand the mechanisms and controlling factors in cell wall dewatering. Present scientific efforts should therefore be focused on finding as invariant and quantitative knowledge as possible on the behaviour of the cell wall under wet pressing conditions.
Recent research on cell wall dewatering is reviewed in the paper. Advanced measuring methods such as NMR, solute exclusion, WRV(CCV) and DSC techniques have produced new and to a certain extent invariant information on the cell wall structure and dewatering. As a result, a clearer picture of the differences in the behaviour of mechanical and chemical pulps, softwood and hardwood pulps and different types of fines material has emerged. The effect of hornification and beating has also been clarified. Further development of measuring techniques such as DSC-based thermoporosimetry is most likely to improve our understanding in this area, helping to make it more accurate and invariant.