Superhydrophobic and superamphiphobic materials for the conservation of natural stone: An overview

In the last two decades superhydrophobic (contact angle of water drop, WCA > 150°) and superamphiphobic (WCA > 150° & contact angle of oil drop, OCA > 150°) materials have attracted considerable attention due to their numerous applications. These non-wettable materials have the potential to open new avenues for the protection, conservation and preservation of stone-built cultural heritage which is exposed to the atmospheric conditions and it is therefore deteriorated by the direct and indirect effects of rainwater. In this review, the recent progress achieved on methods and materials, designed to achieve extreme non-wetting properties on natural stone, is summarized. To enlarge the impact of the report, materials which induced nearly extreme non-wetting properties on stone and corresponded to WCA > 140° and OCA > 140° are included. Two strategies are identified. The most popular approach is to treat stone surfaces with dispersions of engineering nanoparticles in polymer solutions or preferably in polymer precursor solutions, composed of silanes and siloxanes. With the second strategy carefully designed sol-gel processes have been developed that lead to the production of nanostructured materials without using nanoparticles. Finally, the effects of the suggested conservation materials on other stone properties, such as capillary water absorption, vapour permeability and colour are briefly reviewed.