Fluid absorption during forward roll coating of porous webs

Forward roll coating is a common process to deposit thin liquid films onto a continuous web. When the web is porous, some amount of the fluid is forced into the web in the nip. This removal of fluid, along with the deformation of the backing material, influences transfer in the nip as well as operational issues such as misting and coating defects. While much has been reported on forward roll coating for non-porous webs, little has been done when the web is porous.A laboratory roll coating device is used to characterize the pressure profile, the rubber deformation, and the film thickness as the fluid is in contact with a porous web. A pressure transducer is used to record the pressure profile in the nip. The film thickness on the steel roll surface and the gap between the rolls are measured with capacitance probes. Silicone oils, with three different viscosities, are used as test fluids. Three different papers are used in these tests. A model is proposed to describe the pressure profile, rubber deformation, and absorption in a forward roll coating device. The differential equations are solved to describe the nip behavior. A simplified model is also proposed, using an average nip pressure and Darcy's law, to predict penetration in the nip. The proposed models compare well with the experimental results and predict the dependence on viscosity, nip load, and paper permeability. The experimental results with low viscosity fluids show some speed dependence that is not captured by the model, but the high viscosity fluid behavior agrees well with model predictions.