Bio-based BPA-free epoxy resins for stone conservation: a review of renewable precursors, curing strategies and application potential

The development of safer and more sustainable materials for stone conservation is increasingly important, as conventional treatments often prioritize short-term performance, low cost, and ease of application over long-term compatibility and environmental impact. In this context, bio-based bisphenol A (BPA)-free epoxy resins have emerged as promising alternatives to petroleum-derived systems because their thermal, mechanical, adhesive, and hydrophobic properties tailored through precursor design and curing strategy. For consolidation and protection of decayed lithic substrates, however, these materials must ensure not only penetration and mechanical recovery, but also compatibility with fragile pore network of the stone. This review examines recent advances in renewable epoxy precursors derived from natural feedstocks and waste-based resources, with particular emphasis on their potential relevance for stone conservation. The main families of bio-based epoxy monomers are discussed together with the hardeners used to obtain the corresponding networks, highlighting the properties most relevant for lithic substrates, including rigidity, adhesion, hydrophobicity, thermal behavior, and aesthetic impact. Consideration is also given to the transition from conventional irreversible thermosets to covalent adaptable networks, including selected bio-based vitrimeric systems, as potentially useful strategies for improving retreatability. Overall, bio-based BPA-free epoxy systems offer significant opportunities for the development of more sustainable conservation materials. However, their suitability still depends on a careful balance between environmental benefit, processability, compatibility with the substrate, aesthetic acceptability, and long-term performance under outdoor exposure.