Stone sustainable protection and preservation using a zein-based hydrophobic coating

In the present work, the efficiency of an ecofriendly 5% (w/v) solution of zein in DMSO was evaluated as protective material for stone. The solution was sprayed on 5 x 5 x 1 cm slabs of Serena stone, a sandstone widely used in Florentine architecture, in order to create a hydrophobic coating. The obtained coating turned out to be hydrophobic, showing WCA of around 120 degrees, while a reduction of 8.4% in water uptake was observed during 15 days. SEM and profilometer analyses stated that the zein coating application resulted in the formation of a compact film on the stone's surface with a continuous structure characterized by micro-scale roughness. XPS analysis confirmed the presence of the thin conformal layer over the stone. A mechanism of hydrophobic surface formation, in which both the roughness and the solvent's evaporation style are believed to play a significant role, was finally proposed. Accelerated aging tests indicated a good resistance to aging in terms of hydrophobicity and color variations, where the overall color parameter of Delta E < 2.0 was maintained, indicating invisible color changes to the naked eye. This sustainable approach can be easily scaled and may represent a valuable alternative to the most common synthetic treatments used nowadays in stone protection.