TiO2–SiO2–PDMS nano-composite hydrophobic coating with self-cleaning properties for marble protection

This research work presents the design of a transparent-hydrophobic hybrid semi-crystalline SiO2–TiO2coating and its application in the field of monument conservation. The SiO2–TiO2 coating derived from amixture of tetraethoxysilane (TEOS) and titanium-tetra-isopropoxide (TTIP) incorporating an organosilane,the hydroxyl-terminated polydimethylsiloxane (PDMS). The complete hydrolysis of TEOS wasachieved by oxalic acid, which also controlled the gel drying. XRD, DTA–TG, FT-IR and SEM analyses evaluatedthe designed coating, which is transparent, crack-free with a crystallite size of 5 nm. The evolutionof the hydrolysis of TEOS, as well as the copolymerization of TEOS, TiO2 and PDMS as a function of timewere assessed by FT-IR analysis. The effectiveness of TiO2–SiO2–PDMS nano-composite as hydrophobiccoating was investigated by capillary water absorption and static contact angle measurements. The SEManalysis of the treated sample revealed a homogeneous crack-free coating on marble, while FT-IR provedthat condensation and copolymerization reactions took place on the marble surface. The removal of bothmethylene blue stains and biofilm on treated samples support the self-cleaning properties of the designedcoating. The total color differences and the decrease of the water vapor permeability range within acceptablelimits. After treatment, a decrease in the water capillary coefficient and an increase in the contactangle were observed. The innovation of this synthesis pertains to the application of a hydrophobic transparentnano-composite based on SiO2–TiO2 with self-cleaning properties, without modifying the colorof the marble surface and the water vapor permeability.