RHEOMETRY OF PAINTS WITH REGARD TO ROLL COATING PROCESS

The rheology of industrial paints is investigated with regard to roll coating (coil-coating) process. Within the nip between rotating rolls, the paint experiences high deformation rates over a very short time. Just after that, the liquid film has to level the defects generated between the rolls, and this occurs at low shear rates. Transient and steady-state rheological measurements are performed using shear and extensional rheometers with regard to the time and strain scales of the process. Paints are nearly Newtonian at high shear rates but frequently behave as thixotropic yield-stress fluids. The duration of the shearing between the rolls is found to be long enough to provide a total structural breakdown. Therefore, the steady-state viscosity is relevant for the determination of the film thickness which is deposited onto the web. Conversely, the structure recovery under low shear rates determines the rate of leveling and hence the surface quality of the finish. It is shown that the rate of recovery can vary considerably from a paint to one another. The thixotropy of the paints also affects the extensional measurements, but the results indicate the probable absence of actual extensional properties, even at high extension rates. © 1995, The Society of Rheology. All rights reserved.