Dewatering of coating colours in the film press

When coating with filmpress equipment, a premetered amount of coating colour is applied to the filmpress roll. This coating layer is then transferred to the paper in the filmpress nip. At the nip exit not ail of the coating colour is transferred to the paper but instead a thin film of coating colour is recirculated to the application unit Thus, the cohesive farce of the colour, or the adhesive force between the coating colour and the filmpress rollis expected to influence the film splitting location within the coating layer at the nip exit An experimental study was made in order to investigate some parameters which might be of importance on the development of the cohesive force within the coating colour and of the adhesive force at the boundary coating colour/roll cover surface. Of the investigated parameters the solids content of the colour at the time of splitting showed to be the most important one, the cover material the pigment type and the latex content of the colour having no or negligable effect This indicates that the state of dewatering of She coating colour in the filmpress nip could be of importance. A theoretical study based on the assumption of elastic roll cover deformation was therefore made in order to investigate what parameters might influence the dewatering rate in the filmpress nip. Of the parameters studied the application solids content, the dewatering behaviour of the colour, the machine speed as well as the linear load showed to be the most significant for the state of dewatering and thus for the location of film splitting.