Castor oil based UV-cured coatings using thiol-ene click reaction for thermal degradation with flame retardance

Abstract

In this paper, castor oil based UV-cured coatings are prepared via thiol-ene click reaction by varying molar content of a self-synthesized phosphorus-containing flame retardant (DA), a self-synthesized castor oil based-mercaptan (COSH) and 1,3,5-triethylene-1,3,5-trimethylcyclotriazane (TTC), which is used for the study of synergistic effectivities for flame retardant component, photopolymerization kinetics, and thermal degradation. The results demonstrate the synergistic effectivities between DA and TTC via effective quantification of Lewin’s parameter (ES). Under the molar ratio of DA, TTC and COSH of 3:3:5, the thermal stability of coatings is significantly better than others and the char yield can reach 14.54% at 800 °C under air condition despite achieving better flame retardant properties, and the conversion of thiol and vinyl groups can reach 70% or more. Further, scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) tests show that polyphosphoric acid formed through the decomposition of DA can promote SiO and SiC bond into compact and stable char layer timely to protect wooden substrates and delay burning, which indicates the synergistic fire retardant property of P and Si in the condensed phase. Moreover, there are nonflammable gases (CO2 and SO2) released from the coatings to dilute oxygen concentration during thermal degradation by the analysis of thermogravimetry/infrared spectroscopy (TG-FTIR) tests, which also leads to the generation of porous char layer on the surface of wooden substrates. Besides, the oxygen can be also consumed by the phosphorus-containing compounds generated by DA simultaneously, which shows the synergistic fire retardant property of S and P in the gas phase.