Dewatering of Coating Colors in the Film Press, 1996 Coating Conference Proceedings

When coating with film-press equipment, a premetered amount of coating color is applied to the film-press roll. This coating layer is then transferred to the paper in the filmpress nip. At the nip exit not all of the coating color is transferred to the paper but instead a thin film of coating color is recirculated to the application unit. Thus, the cohesive force of the color, or the adhesive force between the coating color and the film-press roll, is expected to influence the place where the film splitting takes place within the coating layer at the nip exit. An experimental study was carried out in order to examine some parameters which might be of importance for the development of the cohesive force within the coating color and of the adhesive force at the boundary coating color/roll cover surface. Of the parameters studied, the solids content of the color at the time of splitting proved to be the most important, the covering material, the pigment type and the latex content of the color had no or only negligable effect. This indicates that the state of dewatering of the coating color in the film-press nip could, indeed, be of importance. A theoretical study based on the assumption of elastic roll cover deformation and on the assumption of filtercake formation during coating color dewatering was, therefore, made in order to determine what parameters might influence the dewatering rate in the film-press nip. Of the parameters studied, the application solids content, the dewatering behavior of the color, the machine speed as well as the linear load proved to be the most significant for the state of dewatering and, thus, for the location of film splitting.