Multifunctional Metal-Oxide-Nanopowder-Polymer Composite Coatings for Stone Built Heritage Conservation ^†

Multifunctional composite coatings composed of metal oxide nanoparticles dispersed in polymer matrices are an innovative solution for stone heritage protection, specifically designed to be stable, durable, transparent (should not alter color), easy to apply, nontoxic, removable, hydrophobic, and permeable to allow stone transpiration. Sol-gel metal oxide nanopowders have photocatalytic and antimicrobial proprieties that act against environmental pollutants and biodegradation [1,2]. The protective coatings we propose in this work combine the hydrophobicity of sodium polyacrylate with the antimicrobial effect (even in the absence of light) and compatibility of MgO and TiO₂ eco-friendly nanopowders.

MgO (pure phase periclase) and TiO₂ (pure phase anatase) nanopowders were synthesized by the sol-gel route [3]. Aqueous dispersions of hydrophobically modified polymer (NaPAC₁₆, polyacrylic acid sodium salt) and MgO/TiO₂ nanopowders were deposited for testing on glass slides (through a layer-by-layer dip-coating technique) and stone fragments imitating mosaic stone from the fourth century AD Roman Mosaic Edifice, Constanta, Romania (through immersion).

The powder size and morphology were investigated by scanning electron microscopy (SEM), while the coating surface characteristics were evaluated by optical profilometry measurements. Photocatalytic activity measurements indicated highly efficient degradation of methyl orange dye under UV irradiation. Hydrophobic properties were confirmed by contact angle measurements, while the inhibition of microbial growth was shown by microbiology testing using Aspergillus niger cultures under illumination and in darkness conditions. Colorimetric tests indicated that the applied coatings do not induce perceivable changes in color hue and lightness.

Our study shows that MgO(TiO₂)/NaPAC₁₆ composite coatings are highly suitable for historic stone protection due to their demonstrated transparency, hydrophobicity, antimicrobial, and photocatalytic self-cleaning properties. A possible synergistic effect between the two oxide nanopowders is also suggested.