Alkyd artists’ paints: Do pigments affect the stability of the resin? A TG and DSC study on fast-drying oil colours C. Duce a , L. Bernazzani a , E. Bramanti b , A. Spepi a , M.P. Colombini a , M.R. Tiné a, * a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy b National Research Council of Italy, C.N.R., Istituto di Chimica dei Composti Organo Metallici-ICCOM-UOS Pisa, Area di Ricerca, Via G. Moruzzi 1, 56124 Pisa, Italy article info Article history: Received 10 January 2014 Received in revised form 17 March 2014 Accepted 25 March 2014 Available online 3 April 2014 Keywords: Alkyd paints Pigments Natural ageing Acetic acid artificial ageing TG DSC abstract We studied ten alkyd artists’ paints from the Griffin Alkyd, “fast drying oil colours” series (Winsor & Newton) in order to evaluate the effect of pigmentebinder interactions on the stability of the alkyd resin. The literature on alkyd paints has focused above all on the characterization of the paints and their ageing processes using various techniques, however, to the best of our knowledge, research on pigment-resin interaction is still lacking. We selected four alkyd colours (red, blue, yellow and green) in two formulations based on inorganic or organic pigments together with titanium white and black formulated with carbonized bones and we applied a combined differential scanning calorimetric (DSC) and thermogravimetric (TG) approach. The pure pigments and the whole paint replicas were analyzed, and the effect of natural ageing on the paints was monitored for eight months. Artificial ageing in an acetic acid atmosphere was also studied. We found that organic and metallorganic pigments interacted the strongest with the alkyd resin, while the inorganic pigments appear to act more as dispersants. The chemical drying of the paint on the other hand, via auto-oxidation, is almost independent of the pigment and only depends on the alkyd content. The paint is fully dried after eight months. Paint samples from two hyper-realistic artworks, “Salto di qualità”, 2003, and “Senza nome”, 2007, by the Italian painter Patrizia Zara were investigated by DSC in order to evaluate the effects of several years of natural ageing on alkyd fast-drying oil colours. At the end of chemical drying, the alkyd colours seemed very stable. Artificial acid ageing visibly damaged the paint replicas, but appeared to act mostly on the pigment rather than on the resin network. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Since they were synthesized by Kienle in the mid 20th century [1], alkyd resins have been extensively used as binders for paints [2,3]. By exploiting the versatility of their formulation, they have also been used to produce a very broad range of coating materials for different uses, such as varnishes, anticorrosive coatings, acid curing coatings, and so on [2]. Alkyd paints have been used in contemporary art by a number of famous artists including Jackson Pollock, Pablo Picasso, Frank Stella and Roy Lichtenstein [3]. Alkyd paints are polyesters modified by the addition of oils or fatty acids, and mixed with solvents, pigments, fillers and driers. The oil content gives the paint its visual properties, while driers, generally metal soaps or coordination compounds, shorten the duration of the drying [4e6]. Many studies have investigated the effects of both the oil and the drier on the resin properties in order to optimize the paint formulation, to reduce dark yellowing, and to improve the film properties [2,4,5,7,8]. More recently, attention has been focused on finding alkyd-based systems with a zero volatile organic compound (VOC) level, especially alkyd emulsions and high solid alkyds for environmentally-friendly coatings [2,9e12]. The success of alkyd paints in art is due to the final visual appearance which is very close to traditional oil paints, and to the higher speed of drying. Several companies produce the alkyd resins used in contemporary art, such as Winsor & Newton, Ferrario, Da Vinci Paint Co. and Kremer, and a number of works have already been published on the characterization and the ageing of alkyd paints using various techniques [3,13e22]. Research on alkyd art- ists’ paints has focused on two main objectives: studying the mechanism of the resin drying process and ageing [14e22], and characterizing the various alkyd resins in order to identify them in artworks [3,13,20e22]. * Corresponding author. Dipartimento di Chimica e Chimica Industriale, Uni- versità di Pisa, Via Risorgimento 35, 56127 Pisa, Italy. Tel.: þ39 (0) 50 2219268; fax: þ39 (0) 50 2219260. E-mail address: mariarosaria.tine@unipi.it (M.R. Tiné). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab http://dx.doi.org/10.1016/j.polymdegradstab.2014.03.035 0141-3910/Ó 2014 Elsevier Ltd. All rights reserved. Polymer Degradation and Stability 105 (2014) 48e58