Pigmented polymeric materials are widely used in the consumer market as surface coatings in the building and construction industry. The ability to preserve the original color and appearance of the coating, also known as “tint retention”, is a critical aspect of high-quality coatings. This presentation will investigate the root causes for the tint and gloss change in the exterior architectural coatings from both the analytical and modeling perspectives. The discussion will focus on the comparison between two coating formulations composed of polyacrylate-based binders and ≈15 v% TiO2 particles with and without an additive. Performance data will be shown to demonstrate the color and gloss change of the coating up to 2000 hours under accelerated weathering conditions. The control formulation showed significant gloss loss and color change within the first 1000 hours. The formulation with the additive was found to resist any major gloss or tint change up to 1500 hrs. Atomic force microscopy (AFM) and optical profilometry (OP) results will be presented as a function of the weathering time. The roughness values determined by the AFM from 30 x 30 µm areas could be well correlated to gloss based on a reflectivity model. Elemental analysis of the weathered coating by energy dispersive X-ray spectroscopy (EDS) and X-ray photon spectroscopy (XPS) revealed an increased concentration of TiO2 at the coating surface, suggesting preferential erosion of binder during weathering. The erosion rate estimated by EDS was found to be lower in the sample containing the coating additive. Scanning electron micrographs (SEM) of the coating films acquired at normal and a grazing angle will be shown. An increasing number of TiO2 particles could be seen protruding from the binder as the weathering time increased. Without the additive, TiO2 particles became densely packed at the surface, indicating severe binder loss near the surface. With the additive, the exposed TiO2 clusters were more isolated, suggesting that only the binder supporting the top layer of the TiO2 particles has degraded. Quantification of the TiO2 particles in the SEM cross-section also confirmed slower TiO2 densification in the additive-containing formulation. The accumulated results showed the root cause for the tint loss is the increase in surface roughness as a result of binder erosion. The coating additive reduces the overall binder erosion rate and delays the gloss and tint loss.

Binder erosion and tint retention in pigmented polymeric coatings

254th National Meeting (2017)

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