Solvent-free thermal polymerization of cardanol and urushiol and its application in raw lacquer sap

Utilizing renewable resources to prepare environmentally friendly coatings is an important direction for sustainable development. Here, two renewable monomers, urushiol and cardanol, were subjected to solvent-free thermal polymerization under a nitrogen atmosphere. This thermal polymerization mainly involved the substitution of urushiol’s aromatic protons, the Diels-Alder cycloaddition of conjugated double bonds, and minor ether formation reaction. The activity of cardanol was lower than that of urushiol, and the substitution of its aromatic protons was slower, which made it difficult for cardanol to be completely converted. The modification of raw lacquer sap (RLS) from Rhus vernicifera tree by thermal polymerization product accelerated the drying of its film. This was the result of the thermal polymerization product containing urushiol units participating in laccase-catalyzed oxidative polymerization. However, the residual cardanol in the thermal polymerized product was difficult to be catalyzed by laccase for oxidative polymerization, resulting in a decrease in the cross-linking degree and pencil hardness of the lacquer film. By extending the drying time, the residual monomers in the lacquer film could continue to polymerize through slow autoxidation, thereby improving these properties of the lacquer film. In addition, the color of the films was lighter than that of RLS film, which was conducive to the formulation of colored paint. Their flexibility and chemical resistance were basically the same as those of RLS film. This method of using cheap and easily available cardanol modified RLS, which is environmentally friendly, is expected to have practical applications.