A multi-analytical approach for precise identification of alkyd spray paints and for a better understanding of their ageing behaviour in graffiti and urban artworks

Contemporary graffiti and murals became an artistic expression worthy of attention worldwide as well as their conservation. The application of spray paints with increasing technical performance mirrors the variety of the chemical composition of these increasingly common painting materials. An insight into the formulation and the ageing process is extremely important to predict and prevent the possible decay phenomena observed on outdoor urban artworks. This paper aims to investigate the relationship between the macroscopic outcomes and the chemical changes occurring on spray paints after the ageing by a multi-analytical approach. A systematic artificial ageing procedure was performed on multiple sets of concrete mock-ups painted with three different alkyd-based spray paints and in three different light-sensitive colours. The colour variation was assessed by colorimetric measurements, the changes in the paint cross section were observed by scanning electron microscopy, Fourier transform infrared spectroscopy (FTIR) was applied both to characterize the spray paints and to detect the chemical changes of the organic compounds during the ageing. A significant and innovative contribution to this research is the application of the evolved gas analysis-mass spectrometry (EGA-MS) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with tetramethylammonium hydroxide in double-shot mode. These two analytical methodologies allowed an accurate classification of the different alkyd-based binders, the identification of plasticizers, organic pigments and other components often hard to spot in complex formulations, and an analysis of their fate following ageing.