Tailored-Made Fluorinated Methacrylic Coatings for Marble Protection

INTRODUCTION The degradation of cultural heritages and buildings is a natural and irreversible decay process; the water permeation into the stone is considered the main responsible of its degradation1,2, especially for porous marbles, such as Calcite and Dolomite3, largely used in ancient and contemporary art. Thus, the application of hydrophobic coatings to stone materials is an effective way to preserve stone artefacts and protect cultural heritages and constructions from decay, limiting the water penetration into the porous substrates. During the last decades, the most satisfactory commercial acrylic resin, such as Paraloid B72® (PB72®) due to its good water repellence and optically clear appearance4. Despite the common use of photo stabilizing agents with PB72®, the presence in the acrylic monomer of a hydrogen atom in alpha position to carbonyl group that is capable to start photo-chemical degradation reactions leads to severe durability issues. In order to overcome the above-mentioned limits of PB72®, the aim of the present work was to synthesize a new polymer coating as stone protective with satisfactory water repellent properties and improved durability, thanks to the combined use of fluorinated and methacrylic monomers and without the use of any photo stabilizers agents. EXPERIMENTAL A new kind of a fluorine-based methacrylic monomer, 1H,1H,2H,2H-Perfluoro-octyl-methacrylate (POMA), was synthesized via esterification reaction using methacryloyl chloride and 1H,1H,2H,2H-Perfluoro-1-octanol. A new type of polymer protective was prepared via free radical polymerization between POMA and methyl methacrylate (MMA_POMA), thus overcoming the poor durability and propensity to microbial attack of acrylic protectives. As references, PB72® and fluorinated PB72® (fPB72®) resins (obtained thanks to the introduction of POMA in PB72® polymeric chain) were synthesized. RESULTS AND DISCUSSION The properties of PB72®, fPB72® and MMA_POMA in terms of macromolecular structure, molecular weights, thermal features and water repellency were determined; furthermore, the long-term behavior of these polymeric protective agents was estimated by means of accelerated aging tests exploiting UV radiations. Their behavior over time was checked via Size Exclusion Chromatography (SEC), Fourier Transform Infrared (FT-IR) spectroscopy and biodegradation assay. Lastly, MMA_POMA resin was applied on two marble substrates (Calcite and Dolomite) and its wetting properties were successfully assessed and compared to the same features displayed by the references synthesized. Combining the chemical and biological data measured before and after the aging test, it was possible to conclude that a polymer protective prepared starting from methacrylic and fluorinated monomers is an efficient way to obtain a resin with satisfactory water repellent properties and enhanced durability than acrylic based polymers (Figure 1). Furthermore, the combined use of acrylic and fluorinated monomers promotes the fungal degradation of polymeric coating CONCLUSION The use of polymeric protectives is very advantageous in fields when the hydrophobic properties and high chemical and biological stability are requested. Within this context, the use of polymer resins bearing methacrylic and fluorinated monomers along the polymeric chain can be a way to create tailor-made water repellent materials with enhanced durability, without the addition of any stabilizing agent.