A Durable Nano-SiO2-TiO2/Dodecyltrimethoxysilane Superhydrophobic Coating for Stone Protection

Water can trigger freeze-thaw cycles, acid rain corrosion, and microbial colonisation, all of which destroy stone. Water is one of the most influential factors in the destruction of outdoor stone heritage. Therefore, materials with excellent hydrophobic properties and durability are urgently required to effectively retard long-term stone weathering. In this study, two nanoparticles, TiO2 and SiO2, were used to modify dodecyltrimethoxysilane (DTMS), a waterproof coating commonly used for stone heritage protection, to fabricate nanocomposite superhydrophobic coatings. The micromorphology, water repellence (water contact angle and capillary water absorption), suitability to protect stone heritage (color change and water vapor permeability), and durability (thermal, light, and chemical stability) of DTMS and nanocomposite coatings were evaluated. The scanning electron microscope (SEM) images revealed that adding 0.5% (w/w) SiO2 produced nanoscale roughness on the sandstone surface, leading to superhydrophobicity. The results of ultraviolet -visible (UV-Vis) spectrophotometer showed that adding 0.01% TiO2 shielded more than 90% of UV light but accelerated the decrease in the contact angle under UVA irradiation. The addition of SiO2 was able to avoid the detrimental effect of TiO2 under UV light. The thermogravimetric analysis (TGA) results showed that both SiO2 and TiO2 nanoparticles improved the thermal stability of the coatings. In particular, the fabricated nanocomposite coating, SiO2 and TiO2 co-modified DTMS, had excellent water repellence, low color change and outstanding durability, and retained about 85% of the water vapor permeability of the stone, showing promise for stone protection.