Eco-friendly nanocomposite products based on BPA-free epoxy–silica hybrid materials for stone conservation

Epoxy–silica hybrids (i.e., as such, enriched with TiO2 and cerium-doped TiO2 nanoparticles), based on a bisphenol (BPA)-free cycloaliphatic precursor, were designed for potential applications as stone conservation multifunctional materials. To this aim, nanoparticles were specifically prepared and their suitability for incorporation into the hybrids was assessed by means of X-ray diffraction (XRD), BET porosimetry, and photocatalytic activity measurements. On the other hand, the organic-inorganic materials, both undoped and doped with nanoparticles, were sol-gel synthesized in methanol starting from 1,4-cycloexanediglycidyether (CHDM-DGE) and 1,8-diaminooctane (DAO), in the presence of silica-forming additives, such as 3-glycidyloxypropyltrimethoxysilane (GPTMS), tetraethyl orthosilicate (TEOS), and isobuthyl (trimethoxy) silane (iBuTMS). A multianalytical methodology was employed for material characterization. The homogeneity, the extent of the epoxy cleavage, and the absence of partially hydrolyzed Si (OR)3 groups in the cross-linked hybrid networks were established thanks to a combination of scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS), and imaging Raman analysis. Moreover, the thermal, hydrophobic, and chromatic properties of the hybrids were investigated by thermogravimetry/differential thermal analysis (TG-DTA), differential scanning calorimetry (DSC), contact angle, and color measurements. Finally, the antimicrobial characterization was studied using a sowing strain of Gram-positive bacteria. The results showed that, among all, one of the epoxy–silica hybrid formulations features some of the main prerequisites to be fulfilled for a successful stone conservation treatment.