How metal-polymers interactions can affect the aesthetic features of protective coatings for artistic objects: the case of chitosan-based films

When new protective coatings for cultural heritage materials are developed, these need to be carefully validated on reference materials before the application on real artworks. Both protective properties and aesthetic requirements have to be satisfied when dealing with materials to be used on artistic objects. An easy and time-saving method used to evaluate such features is to submit reference samples to accelerated ageing treatments and to compare through time the behaviour of both coated and uncoated surfaces. However, accelerated degradation tests, which duration is usually no longer than a few months, are not suitable to monitor the long-term coatings behaviour. Interactions between the substrate and the protective film, as well as chemical or physical degradation of this latter, can indeed occur after a period of time longer than the one investigated by the accelerated ageing treatments. Studying long-term effects that can potentially compromise the protective and aesthetic properties of the coatings is thus mandatory when new conservative materials are developed. In this work, we propose an example of how short and long-lasting ageing tests might induce different treatment responses leading to an equivocal evaluation of the coating protective and aesthetic requirements. Chitosan-based coatings have been selected as green and eco-friendly materials for the protection of modern silver artifacts. They have been developed during the European Union’s project NANORESTART [1] and previously tested on copper-based alloys with promising results [2, 3]. Here, two different coatings formulations have been applied on sterling silver reference substrates and subjected to distinct ageing tests. These have been developed to simulate both a short-time exposure to a highly polluted environment and a long-time conservation in a real-like storage box. Optical microscopy (OM), electron scanning microscopy coupled with an energy dispersive spectrometer (FE-SEM-EDS) and UV-vis spectroscopy have been used to investigate the coatings’ physico-chemical properties, both before and after the tests. Differences in treatments’ response are related to the occurrence of short or long-term interactions between the metal substrate, the chitosan-based coatings and the environmental aggressive species. [1] H2020 NANORESTART project, “Nanomaterials for the restoration of works of art”, Grant Agreement n. 646063, http://www.nanorestart.eu [2] C. Giuliani, et al., Progress in Organic Coatings. 2018, 122, 138-146. [3] M. Salzano de Luna, et al., Angew. Chem. Int. 2018, 57, 1-6.