THE INFLUENCE OF ORGANIC SOLVENTS ON THE MECHANICAL PROPERTIES OF ALKYD AND OIL PAINT

Organic solvents are often used to assist in the removal of varnishes and overpaints, however their effect on the properties of paint films has not yet been well established. The effects of organic solvents on the mechanical properties of alkyd and oil paint are here examined. Alkyd paints were very popular in the 1950s, when artists like Jackson Pollock stated: “New needs need new techniques”. The solvent response of household paints based on oil-modified polyester resin is compared with that of artists’ alkyd paints and oil paints. Swell tests on paint strips suggest that all paint systems respond to the same solvents, but alkyd paints are less sensitive. Leached components were characterised by FTIR. As expected, more polar solvents tend to extract more polar paint components. To detect the changes in the mechanical properties, stress-strain measurements were performed before and after solvent treatment of the different pigmented paint strips. These clearly show that alkyd and oil paints become more stiff and brittle due to treatment with different solvents. Although the extraction of polar and softening components has an influence on the mechanical properties of paint films, as widely reported, the results shown here demonstrate that it does not seem to be the only factor responsible for the mechanical changes. The swelling power of solvents and the extraction of low molecular components are just two factors. Other properties like evaporation rate have an important influence on the solvent-induced degradation of alkyd and oil paint films. Moreover, our results suggest a more complex degradation mechanism of the paint film when exposed to solvents, not simply explained by the extraction or swelling phenomena. The results reported here will help the conservator to estimate the potential harm of organic solvent-based conservation treatments like the removal of varnishes or overpaints. USE OF ALKYD PAINTS IN ART Alkyd resins are oil-modified polyester resins developed in the late 1920s [Küchenmeister 1978]. The first commercial alkyd resin-based paint was produced under the name "Glyptal" in 1926 in the United States by General Electric [Derrik et al. 2005]. They have been widely used in the European surface coatings industry since the 1950s. Since then, alkyd resins have replaced the drying oils in many industrial applications because of their shorter drying time and higher resistance properties [Schilling et al. 2004]. At the end of the 1950s, the first development phase was concluded and alkyd resins had, in terms of volume, become the most important raw material for synthetic coating resins. As a result of their popularity as household paints, these products naturally also became accessible to artists. Pablo Picasso was one of the first artists to experiment with synthetic paints. Cappitelli et al. [2005] has shown that Picasso was using such paints back in 1932 (Fig. 1). Fig. 1: Pablo Picasso: "Nude Woman in a Red Armchair", 1932. Alkyd resin-based paint on canvas. © 2008, ProLitteris, Zürich / Tate, London 1 9th International Conference on NDT of Art, Jerusalem Israel, 25-30 May 2008 For more papers of this publication click: www.ndt. net/search/docs.php3? MainSource=65