Preparation and flame retardancy of castor oil based UV-cured flame retardant coating containing P/Si/S on wood surface

•Castor oil-modified bio-mercaptan is used to prepare UV-cured flame-retardant coatings.•The pencil hardness of castor oil-based coating has improved to H grade.•P, S and Si in the coating have a synergistic flame retardant effect.•The coated wood of F4 remains the wood tracheid basic skeleton after combustion.•Bio-mercaptan can reduce the maximum degradation rate of the coatings.