Environmentally stable superhydrophobic coating via FAS-SiO2/resin hybridization for multi-barrier protection of outdoor sandstone heritage

The conservation of sandstone cultural relics has gained increasing attention due to their scarcity, historical and cultural value. However, they exhibit a vulnerability that must endure degradation caused by prolonged outdoor exposure, particularly the hazards caused by water infiltration. Herein, we have designed and developed a hydrophobic composite coating for safeguarding outdoor-exposed sandstone relics. The coating has integrated the silica nanoparticles modified by perfluorodecyl triethoxysilane (FAS-17) with fluorinated resin and polymethyl methacrylate (PMMA) to achieve waterproofing for outdoor-exposed sandstone. The protective effect of the coating has been systematically evaluated through water contact angle (WCA) measurements under various environmental tolerances such as acid rain resistance, mechanical bearing capacity, and adaptability for wide temperature range adaptability (-20 ℃ and 80 ℃). The tested results demonstrate that the coating on sandstone shows superhydrophobic ability and stability. For example, the coating exhibits exceptional hydrophobicity (WCA > 150°) and self-cleaning properties after various accelerated aging cycles such as under acidic rain, mechanical peeling, sand impact, high and low temperature environments. Importantly, the coating on sandstone can preserve the inherent permeability of sandstone substrates while providing robust interfacial adhesion. These outcomes highlight the potential of the FAS-SiO 2 based composite coating as a durable and compatible coating for mitigating outdoor environment-induced deterioration in sandstone cultural heritage conservation.