Surface filling of uncoated wood free paper ; Päällystämättömän hienopaperin pinnan sulkeminen

The target of this thesis was to study the advantages, disadvantages and limitations of the surface filling method and yield and analyze scientific information for helping better to understand and develop the surface filling method. Thesis also aimed to running a successful commercial WFU paper machine trial and developing a continuous surface filling method to the WFU paper machine. The greatest benefits of surface filling were to reduce the porosity, increase the ash content and balance the two-sidedness of the paper sheet. Surface filling also improved the printing properties of the paper sheet. In the preliminary laboratory tests was seen that the addition of the ground calcium carbonate pigment did not have any remarkable effect on the viscosity of the pigment/starch solution with the starch solid content under 15%. The solid content of the surface size could be increased by adding the pigments to the surface size. In the pilot trial were tested four different pigment products and two surface size starch solid contents. Porosity could been decreased at maximum 75 % with the 1.2 µm mean particle size pigment with the 1.125 g/m 2 pigment amount per side and with the 15 % starch solid content. The ash content of the sheet increased over 3 % units and compensation in the two-sidedness was seen, as the filler distribution in the Z-direction was much even after the surface filling. When the pigment mean particle size increased the dusting tendency increased also because the bigger pigment particles filled the pores rapidly and settled closer to the paper surface where pigments loose easier in rubbing. In the mill trial the best combination taking in the account of all the examined paper properties was the pigment with a mean particle size of 0.7 µm. The starch solid content should let rise near to the 12 % and the suitable pigment amount would be 1.125 g/m 2 pigment per side. Then the dry solid content of pigment is in the ratio of 3:4 to the surface starch dry solid content. According to the measurements with ...