Study on degradation of phosphorus and nitrogen composite UV-cured flame retardant coating on wood surface

A novel wood flame retardant coating has been obtained using acrylate oligomers and functional monomers containing elements phosphorus, nitrogen and sulfur as raw materials and applied on the wood surface based on thiol-ene click reaction. The UL-94, limiting oxygen index (LOI) test and thermal gravimetric analysis/infrared spectroscopy (TGA-FTIR) were used to examine the flammability and pyrolysis property of the coatings, respectively. The experimental data showed that the coating has better fire resistance on wood surface when the molar ratio of phosphorus and nitrogen monomer was 2: 4. The volatile products generated in the degradation processes of the coating were confirmed by TGA-FTIR, which are carboxylic acid, ammonia (NH3), carbon dioxide (CO2) and acid anhydride, with a temperature of maximum release rate corresponds to 99.80 °C, 170.96 °C, 346.75 °C and 358.80 °C, respectively. In addition, the coating released NH3 and CO2 as nonflammable gases during the degradation process, which diluted the oxygen concentration during combustion and was favorable to generate the intumescent char layer. The TGA curves of the coating showed three degradation, corresponding to the water loss and the C-S bond breaking of crosslinking networks, the degradation of phosphate and acrylates, the decomposition of unstable char layers, the main chain in poly N, N-dimethylacrylamide (PDMAA) and the formation of poly (phosphoricacid), respectively. The char yield of the coating reaches to 27.16% at 600 °C under nitrogen atmosphere, showing excellent flame retardant properties.