Innovative strategies for confining apolar compounds in water based soft matters: versatile and eco-compatible hydrogel dispersions for conservation of Stone materials

Calcareous stone materials have long been employed in the construction of buildings, monuments, and sculptures, with a well-known susceptibility to various forms of degradation. These degradation processes are multifaceted, deriving from the combined effects of physical, chemical, mechanical, and biological factors, occurring concurrently over extended periods. Prior to preserving these stone materials, it’s imperative to remove from surfaces any unwanted substances especially when potential threats for conservation. The challenge lies in removing these harmful materials without inadvertently triggering secondary degradation mechanisms. Numerous studies have highlighted the efficacy of PVA/Borax-based polymers in cleaning cultural heritage materials. Their viscoelastic properties facilitate easy application and removal, making them environmentally friendly and reducing the risk of operator toxicity. The proposed research focuses on developing versatile soft materials through the cross-linking reaction between Polyvinyl alcohol (PVA) and Borax. These materials have been designed to confine organic a-polar substances, such as essential oils useful for biofilm treatment, and biodegradable a-polar solvents, such as the esthers of fatty acids, proven to be effective in removing insoluble aged resins by the means of dewetting processes. Three types of PVAs have been used, with varying molecular weights and degrees of hydrolysis, to synthesize three types of soft matters, maintaining a weight ratio of PVA:B = 4.0:0.5. Each soft matter contained an aliquot of Origanum vulgare essential oil and methyl hexanoate, individually confined. The cleaning efficacy of these soft matters have been evaluated on calcareous stone surfaces; in particular the essential oil-enriched soft matters have been tested on bio-deteriorated surfaces and the Methyl hexanoate-enriched soft matters have been tested on artificially photodegraded surfaces, previously treated with Paraloid B72. Flow sweep experiments were conducted on all soft materials to assess dynamic viscosity, while frequency sweep experiments were performed to evaluate mechanical properties. Structural and chemical investigations were carried out on all formulations using high-field +H Nuclear Magnetic Resonance spectroscopy and relaxometry, FT-IR ATR. To further evaluate the cleaning efficacy and residue removal from treated surfaces, of proposed soft matters, portable NMR single-sided spectrometer, SEM EDS, FORS, and FT-IR ATR techniques have been used.