Nanolime-derived inorganic cross-linker for strong and water-resistant polymers in wall painting conservation

Paraloid B72 (B72) is widely used in cultural heritage conservation but suffers from poor consolidation performances under high relative humidity (RH). Here, we developed a strength and water-resistance enhanced polymeric hybrid material by mixing nanolime, B72 and polyvinyl butyral in the presence of ethyl acetate. Our results showed that both the tensile strength and elongation at break of the hybrid film are improved ∼2 times compared to that of B72 alone. Importantly, after 30 days of exposure to 95 % RH, the consolidation strength for wall painting decreased by just 6 % in the case of the hybrid material versus 36 % for B72. The proposed reason for this effect consists in the Ca(CH₃COO)₂ cross-linker through the reaction of nanolime and ethyl acetate, which tightly linked B72 and polyvinyl butyral chains by the formation of an ionic bond between Ca²⁺ and carbonyl groups, thereby enhancing the mechanical strength and the water resistance of the hybrid film. In addition, the hybrid material meets the requirements of conservation materials, such as limited color change, kinetic stability, and adequate penetration depth. Our study opens a new, promising way for improving the mechanical performance and durability under high RH of organic polymers used in the conservation of cultural heritage.