Measuring the “tack” of waterborne adhesives

Abstract

Recent "tack" experiments on Newtonian liquids have shown that the force versus gap measurements obtained by pulling apart two surfaces separated by a thin liquid film result from a complex and dynamic balance between the viscous force resisting the separation and the spring force exerted by the compliant load cell of the instrument [Tirumkudulu et al. Phys. Fluids 15(6), 1588-1605 (2003)] . Here, we present similar experiments with waterborne adhesives that are colloidal dispersions of soft polymer spheres dispersed in water. By employing a simple power law to describe the complex rheology of the waterborne adhesives and a lubrication approximation for the viscous force, we predict pull-off forces in close agreement with those observed experimentally. However, experiments with high particle concentration adhesive and/or large separation rates cause cavitation in the gap, resulting in forces that are lower than predicted. Finally, a simple analysis for adhesives of two different rheologies shows that the tack test may result in contradictory conclusion, since the forces are sensitive to small variations in the instrument settings.