Superhydrophobic Nanocomposite of Paraloid B72 and Modified Calcium Carbonate Nanoparticles for Cultural Heritage Conservation

Superhydrophobic materials have clear potential for mitigating rain/humidity-induced damage to cultural heritage. In the present study, the wetting properties of Paraloid B72 were tailored to achieve superhydrophobicity by incorporating modified calcium carbonate (CaCO₃) nanoparticles (NPs). B72 is a well-established conservation product while CaCO₃ is chemically compatible with calcareous materials commonly found in cultural heritage buildings and objects. Initially, the wettabilities of CaCO₃ NPs, functionalised with caproic (C6), caprylic (C8), lauric (C12), myristic (C14), palmitic (C16), and stearic (C18) acid, were evaluated by measuring water contact angles (CAs) on NP pellets. For NPs with short hydrocarbon chains, CA increased with chain length, from 66.3° for CaCO₃-C6 to 118.0° for CaCO₃-C12 NPs. For NPs with longer chains, CA remained stable and around 118°. Based on these results, CaCO₃-C12 NPs were selected for further investigation and subjected to transmission electron microscopy analysis, which revealed chain-like agglomerates of aggregated nanocrystallites (5–10 nm) forming 40–150 nm polycrystalline NPs. Scanning transmission electron microscopy combined with elemental mapping revealed a homogeneous distribution of Ca, C, and O within the NPs. Next, CaCO₃-C12 NPs were dispersed in B72 solutions and sprayed onto limestone, which was employed as a model calcite-rich substrate. At optimal NP concentration, the resulting composite coating exhibited superhydrophobicity (CA > 150°), while it induced minimal colour alteration to limestone and effective resistance to capillary water absorption. The fluorine-free coating also demonstrated good durability against UV exposure, drop impact, salt attack, freeze–thaw cycles, tape peeling, drop pH variations, and thermal treatment.