| Setting is used to describe the thickening of a fluid layer after contact with a substrate as in gluing, painting, coating and printing operations. Unique experimental and theoretical techniques are proposed here to quantify and predict setting.; Polyvinyl alcohol (PVA) solutions and coatings at different concentrations are applied as a thin film to the bottom plate of a mechanical tester. The substrate is pressed against the bottom plate for a given waiting time and separated at constant velocity. The tensile force and gap are recorded; the integration of the force-gap information is used to calculate a separation energy which quantifies setting. The effect of different separation velocities, concentrations, substrates, and film thickness are reported. For PVA experiments, cellophane showed the highest separation energy followed by porous plastic, metal plates and paper at all waiting times. For the coating, as polymer concentration increases, the viscosity of the coating increases, but the force to separate the plates decreases; the polymer must slow the rate of water loss from the coating layer.; A model based on fundamental transport laws, is proposed to describe setting. The two dimensional, unsteady state, mass transfer and fluid flow equations are solved in the lubrication limit. Concentration profiles are calculated as a function of time and position; these are used to calculate viscosity, the velocity flow field and the pressure distribution within the fluid layer. The pressure profile is integrated at the moving boundary to calculate pull forces. Dehydration causes a gradient in viscosities near the substrate and a stagnant fluid layer during flow. Pull forces increase with increases in diffusion coefficient, increase in waiting times, and decreases in initial film thicknesses. If the ratio of substrate to fluid Peclet number is less than one, diffusion through the fluid layer is controlling and viscosity gradients are predicted. Model predictions are within experimental accuracies for various film thickness, pull speeds, and diffusion coefficients when cellophane is the substrate. |
