Development of Fully Green Castor Oil-Based Gels for Cultural Heritage Preservation

Polyurethanes and organogels are materials with many applications spanning from industrial sectors to the preservation of Cultural Heritage. They often show scarce "green" metrics due to the use of petrochemical resources for their synthesis. Herein we report the development of new, fully sustainable castor oil polyurethane gel platforms obtained for the first time employing a green isocyanate cross-linker and oligoester additives, with potential applications in the preservation of Cultural Heritage and beyond. The presence of the oligoester additives and the increase in isocyanate feed yield faster reactivity in pre-gels, affecting the size of nuclei formed during gelation. These variations affect the gels' mechanical properties and their swelling and release of organic solvents. The organogels were loaded with two green solvents to remove an aged, yellowed varnish from an early 20th century canvas painting. Their cleaning efficacy was optimized by playing on the isocyanate cross-linker feed, oligoester additive, and solvent polarity, and tuning the mechanic and release properties of the gels, leading to varnish removal in time-effective applications. These results open a new, versatile palette of sustainable castor oil-based organogels to use in the cleaning of works of art, potentially of interest also in different sectors where bio-based polyurethanes are increasingly required.