Lactic acid-based compounds as green alternative solvents for cultural heritage: Application on canvas painting restoration

Abstract

In the restoration of canvas paintings, solvent mixtures derived from non-renewable sources are commonly employed (e.g. acetone, ethanol, and isooctane), which not only contribute to pollution but also pose potential health-risks to the operators. This research aims to introduce alternative biobased and eco-friendly solvents as possible replacements for common solvents used in artifact restoration. 5-methyl-1,3-dioxolane-4-one (LA-H,H), 2,2,5-trimethyl-1,3-dioxolane-4-one (LA-Me,Me), ethyl lactate (EL), and 2-ethylhexyl pelargonate (ARGO) were selected as biobased solvents candidates. They can be produced from biomass-derived synthons, including lactic acid (LA), which is obtained by carbohydrate fermentation, and pelargonic acid (PA), which is obtained by the oxidative cleavage of oleic acid and high-oleic vegetable oils. To rationalize their solvent properties, their Hansen parameters were used (dispersion δ_d, polarity δ_p, hydrogen-bonding δ_hb), enabling their placement within the Teas Triangle. Lactic acid-derived solvents, LA-H,H and LA-Me,Me, were prepared in multigram at lab-scale, and characterized using nuclear magnetic resonance spectroscopy (NMR) and gas chromatography-mass spectrometry (GC-MS). Their solvent abilities were tested by their application to ad-hoc prepared specimens with different film-forming substances. These tests were performed with both fresh and artificially-aged samples. Their evaporation behavior was studied through thermal gravimetric analysis (TGA) while their ready biodegradability was investigated according to OECD guideline 301F as a key property influencing environmental impact. These solvent mixtures were then employed for the removal of oxidized terpenoid varnish from a 17^th-century canvas painting, demonstrating their potential as green alternatives for restoration practices.