Accelerated UV ageing studies of acrylic, alkyd, and polyvinyl acetate paints
The stability of two types of acrylic binding media, alkyd and polyvinyl acetate (PVAc), four widely used synthetic binders in modern and contemporary art, to UV light also including the UV-B range (315–280 nm, middle UV) for simulating sunlight outdoor conditions was studied and compared by double-shot and single-shot Py-GC/MS, FTIR-ATR, and colour measurements. Thermally assisted hydrolysis and methylation (THM-GC/MS) analyses were used for the alkyd. Additionally, the influence of inorganic pigments on the photo-oxidative stability of the binding media was also considered. For this purpose, the binders in their pure form as well as mixed with eight different inorganic pigments (titanium white—anatase and rutile, cadmium yellow, cadmium red, hydrated chromium oxide green, ultramarine blue, raw umber Cyprus, and ivory black) were exposed to the accelerating artificial UV ageing for different periods of time and analysed before and after UV exposure. After UV ageing, the double-shot Py-GC/MS detected the effect of photo-oxidative processes of the binders in a much more detailed way than by the single shot. For instance, photo-oxidation of the acrylics resulted in a production of oligomers even during the thermal desorption step of double-shot Py-GC/MS and in a decrease of the EA and nBA main monomers in the pyrolysis second step. Contrarily, the aged alkyd samples were mostly characterised by a decrease of unsaturated fatty acids and especially by the increase of free ortho-phtalic acid detected by double-shot Py-GC/MS and THM-GC/MS, which is also reflected in the FTIR-ATR results. On the other hand, an increase of free acetic acid was observed for the aged polyvinyl acetate by double-shot Py-GC/MS. Colour measurements recorded a greater sensitivity of the alkyd paints, shown by to the bigger shift of L*, a*, and b* coordinates and change of E* values. Of additional interest was the higher sensitivity to the UV light for synthetic binding media in combination with pigments.
Volume 124, January 2016, Pages 949–961