Oil-in-water microemulsions to solubilize acrylic copolymers: application in cultural heritage conservation

Acrylic copolymers have been widely used in the past for the surface coating of porous materials of artistic/architectonic interest. Information on the alteration of the physicochemical properties of the porous materials is quite scarce. In this study we showed that the surface area, the contact angle, the water vapour permeability, and the capillary rise profiles of aerial mortar specimens were strongly modified by coating the surface sample with these copolymers. Therefore, a crucial topic is the development of a suitable method to remove these copolymers from the surfaces of the work of art/architecture. Moreover, fluorescence spectra collected from these acrylates photochemically aged by UV radiation indicated an alteration of the chemical structure enhancing the difficulty in removing the coating layers. We succeeded in developing two different four-component oil-in-water (o/w) micro-emulsions where the oil phase was p-xylene: the first using Tween 80 as surfactant and 1,2-propandiol as cosurfactant and the second with sodium dodecyl sulfate and 1-pentanol. These o/w microemulsions were shown to be able to solubilize and remove acrylic copolymers from the surface of porous materials constituted of aerial mortar. The microemulsions were tested during the restoration of the wall paintings by Spinello Aretino in the Cappella Guasconi in the San Francesco church, Arezzo. Scanning electron microscopy images, Fourier transform IR spectra and energy-dispersive X-ray data indicated that the removal of the hydrophobic polymeric resins from the painted surface was very satisfying without any negative side effects.