a Department of Chemical Engineering, Norwegian University of Science and Technology, 7491 Trondheim, Norway
b Paper and Fibre Research Institute, 7491 Trondheim, Norway
The dewatering ability and wet sheet structure after pressing was studied using a dynamic wet pressing simulator in combination with electron microscopy and image analysis. Deinked pulp (DIP) that contains 5% ash dewaters more easily than thermomechanical pulp (TMP) when pressed with single roll pulse. The in-plane moisture flow (crushing) in DIP samples started at higher peak pressure and higher solids content compared to TMP samples. The TMP sample showed higher springback after wet pressing compared to DIP samples. When both TMP and DIP samples were designed to have a higher amount of small pores at the dewatering side surface layers, the samples with coarser TMP pulp could achieve similar dryness as DIP samples when pressed with higher nip pressure. In addition, the DIP samples with higher number of small pores in the outermost 10 Î¼m thick surface layer in the dewatering side resulted in lower dryness after pressing with a single roll pulse. The effect of filler distribution in paper zdirection on dryness and sheet structure after wet pressing using an 8 milliseconds roll pulse was also studied. In this study the distribution of filler does not affect the maximum achievable dryness (41%) after wet pressing when the total amount of filler in the sheets remains constant, 13.5%. In addition, the samples with more filler in the dewatering layer dewater more easily and yield the maximum achievable dryness (40%) after wet pressing at lower nip pressure (2.5 MPa) when compared to samples made from TMP (36% at 4.4 MPa) and DIP pulps (40% at 4.9 MPa).
Nordic Pulp and Paper Research Journal 27(4):790-797.