Thermal polymerization of lacquer sap and its effects on the properties of lacquer film

•A series of thermal polymerized lacquers were prepared from a dead lacquer.•The thermal polymerization was different from the laccase-catalyzed polymerization.•The thermal polymerization mainly occurred in the side chain of urushiol.•The addition of polymerized lacquer into the lacquer sap improved its properties.•The dead lacquer was efficiently utilized through its thermal polymerization. A series of thermal polymerized lacquers (TPL) were prepared from a dead lacquer at the temperature of 120–200°C under nitrogen atmosphere. The thermal polymerization of lacquer sap was analyzed by GPC, UV–Vis, FT-IR and NMR methods. The results showed that the higher the temperature, the faster the polymerization of urushiol monomer in the lacquer sap. Different from the laccase-catalyzed oxidative polymerization in air which formed the biphenyl structures, the thermal polymerization mainly occurred in the unsaturated side chain of urushiol and could involve the pericyclic reaction between the unsaturated carbon-carbon double bonds and the reactions of the side chains with the phenolic hydroxyl groups and the carbons in the aromatic ring. Addition of the thermal polymerized lacquer into a raw lacquer sap with high activity of laccase at a suitable mixing ratio promoted its drying in air, and the higher the extent of thermal polymerization of lacquer sap, the shorter the drying time of lacquer film. In addition, due to the combination of the thermal polymerization and the laccase-catalyzed polymerization, the blend lacquer films showed higher degree of crosslinking, smoother surfaces, and more uniform and compact structure. This resulted in significant improvement in the pencil hardness and gloss of lacquer film and slight enhancement of its flexibility.