Multilayering of fine paper with 3-layer headbox and roll and blade gap former

The aim of this study was on the layering of fine paper with a 3-layer headbox and roll blade gap former. It consisted of methodological development and laboratory scale studies, as well as layering studies on the pilot paper machine scale. The potential of layered structures in papermaking and the phenomena affecting layer mixing were also studied. Filler distributions were characterized by two parameters – filler distribution shape and symmetry factors. The method developed made it possible to apply a statistical approach to experiments when studying the impact of typical wet end control parameters on the control of filler distributions and the contribution of filler distributions to paper quality parameters. A method by which the center layer fiber distribution may be characterized from the final paper is also presented. The method is based on the use of dyed fibers and a spectrofotometer. In addition, a laboratory device, a multilayer handsheet former for making layered structures, is presented. The process closely resembles a real continuous multilayer process by providing water phase interactions between layers during forming and two-sided dewatering characteristics. In a case study, retention chemical and filler layering with WFC base paper was performed by the multilayer handsheet former. Surface roughness decreased and brightness increased when the filler content in paper surfaces was increased. At constant filler content, paper strength increased as the filler concentration in the paper surfaces increased. The distinction between roll dewatering and blade dewatering in terms of layer mixing as well as differences in the mixing behavior of fibers and fillers in different dewatering phases was studied. Major fiber mixing takes place during the free jet and roll dewatering phase. In blade dewatering, fiber movements are related to formation improvements. The greatest mobility differences between fibers and fillers were found in the blade dewatering phase in roll and blade dewatering. Filler distributions were significantly affected during blade dewatering, with the mobility of the fibers remaining insignificant. The relationship between typical fine paper quality parameters and filler distributions was studied. Formation improvement and densification of the paper were achievable at the same running conditions. Low porosity was achieved by evenly distributed constituents (good formation) and a local rich filler concentration somewhere in the paper structure in the paper thickness direction. Oil absorption two-sidedness was minimized by symmetrical filler distributions. The combination of all these quality factors required the filler distribution to be symmetrical and as u- or turned u-shaped as possible. The use of additives layering as a part of paper quality control was also studied. Multilayering was found to be an effective tool for the control of filler distributions. On the other hand it was shown that, in order to get optimized impact of the multilayering on quality properties, the other process variables also need to be controlled.reviewe