Measurements of Stress Development in Latex Coatings

Abstract

As a latex coating dries, volume shrinkage that occurs during film formation is constrained by the substrate, resulting in the development of a bi-axial tensile stress in the film. In situ measurements of stress development can be made using the cantilever beam bending technique. Modified walled cantilevers can be used to limit lateral drying and promote drying uniformity resulting in more accurate stress measurements. In this work, film formation and stress development were characterized for a series of acrylic latexes with varying glass transition temperature (T g ) of the latex polymer. Both the minimum film formation temperature (MFFT) and the maximum stress measured in the film were shown to increase with the polymer T g . Furthermore, the use of coalescing aids (both conventional and low-VOC) to enhance film formation was explored for a latex with an MFFT of 18 °C, and their role in stress development was characterized. All coalescing aids investigated were effective in lowering the MFFT and reducing the maximum stress developed in the coating. The maximum stress was greatly reduced as the concentration of the coalescing aid increased up to three weight percent, while only minor further reductions in the maximum stress were observed at higher concentrations.