Abstract & Overview
Computer simulations of the blade-over-roll coating process so far have been based mostly on Newtonian or generalized Newtonian models, which do not take into account the viscoelasticity of coating materials. Strong viscoelastic effects can influence both the flow characteristics in a blade nip and the coating film thickness. Because of limitations of the constitutive equation which describes coating materials, comparisons of simulation results with experiments had limited success. We have simulated the coating process with a fixed blade using the finite element method (FEM). Newtonian data are well reproduced by using a full two-dimensional model with realistic boundary conditions. For the case of viscoelastic fluids, i.e., polymer solutions with either a constant viscosity (Boger fluids) or a shear-thinning one, we used an integral constitutive equation of the K-BKZ type. Key characteristics of the K-BKZ model are that it accounts for a spectrum of multiple relaxation parameters and a fading memory. Numerical results accurately predict the coating film thicknesses from experiments reported in the literature. For a given geometry and speed, the film thickness decreases with increasing viscoelastic behavior. This is particularly evident when comparing the film thicknesses of the Newtonian and Boger fluids. Results show that the K-BKZ viscoelastic model is adequate to capture memory phenomena associated with the strain history of homogeneous polymeric coating materials, and can have application to simulating paper coating dispersions.