The use of waterborne paints in contemporary murals: Comparing the stability of vinyl, acrylic and styrene-acrylic formulations to outdoor weathering conditions

This paper reports on the photochemical degradation of waterborne paints, products which have been widely used in the creation of outdoor contemporary murals. For this purpose, 15 artist-grade formulations, including three binders (poly(n-butyl acrylate-co-methyl methacrylate), poly(vinyl acetate-co-vinyl versatate), poly(n-butyl methacrylate-co-styrene-co-2-ethylexyl acrylate)) and five pigments (titanium white, artificial ultramarine blue, mars yellow, quinacridone red, phthalocyanine green) were selected and brushed on cement lime mortar supports, simulating the stratified structure typically used by artists. Samples were exposed to UVA-340 lamps (295 < λ < 370 nm) for 800 h, and changes in paint layer properties were monitored using Fourier transform infrared spectroscopy, colorimetric measurements, light and scanning electron microscopy. Thanks to this multi-analytical approach it was possible to study the relationship between chemical degradation experienced by polymer binders, changes in paint surface morphology and discoloration. The results confirmed photo-oxidative reactions undergone by the binding media, with acrylic and styrene-acrylic samples being more stable than the vinyl ones. The influence of pigments and extenders on the stability of polymers was also evaluated and it was observed that, regardless of binder typology, samples containing artificial ultramarine blue underwent the most evident degradation.