Different qualities of microfibrillated cellulose (MFC) were blended with thermomechanical pulp (TMP) and ground calcium carbonate (GCC) filler. The addition of MFC reduced the drainage of the pulp suspension but improved strength properties. Wet pressing experiments showed that optimal use of MFC and filler could enhance the strength and optical properties without reducing the solids content after wet pressing. Field-emission scanning electron microscopy (FESEM) revealed that MFC adsorbed onto and contributed to the bonding of the filler particles and fibres. The MFC binds the filler-MFC-fines aggregates to the fibre network and partially filled the pore network. As a result, MFC addition increased the air resistance and internal bonding of the sheet.

This study shows that wet-pressing TMP and DIP with a shoe press pulse may yield similar afterpress solids, provided that an adequate shoe pulse length with similar pressure profile is applied. A wet web with more porous structure in the sheet dewatering (felt) layer seems to contribute to the increased dewatering during wet pressing. In addition, a shoe press pulse with high peak pressure at the end yields higher solids content after wet-pressing and higher bulk compared to a pulse with a peak pressure in the beginning. The increased dewatering during wet-pressing implies a reduction of steam consumption in the dryer.

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 easi ly 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 I 0 11m 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 (4 1 %) after wet pressing when the total amount of filler in the sheets remains constant, 1 3 .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 (3 6% at 4.4 MPa) and DIP pulps (40% at 4 . 9 MPa).