Preparation And Properties Of UV-cured Wood Flame Retardant Coating Based On Thiol-ene Click Reaction

In order to solve the problems of poor compatibility between the flame retardant component and the coating substrate in the traditional wood flame retardant coating and the resulting significantly reduced mechanical properties and flame retardancy,and alleviate the pressure of resources,energy and environment brought about by the rapid depletion of fossil raw materials simultaneously,and realize the green and efficient use of renewable resources.Based on the thiol-ene click reaction,the P/S/Si-containing castor oil-based and P/N-containing UV-cured flame retardant coatings were prepared on the wood surface.The physical properties,flame retardant properties and thermal degradation properties of wood flame retardant coatings were characterized to reveal the flame retardant mechanism of UV-cured wood flame retardant coatings.The main research contents are as follows:（1）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 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 degradation of the coating produced volatile products such as carboxylic acid,NH₃,CO₂ and anhydride,and the released non-flammable gases such as NH₃ and CO₂can dilute the oxygen concentration.The char yield of the coating reaches to 27.2%at600 ^oC under nitrogen atmosphere,showing excellent flame retardant properties.（2）The synergistic flame-retardant UV-cured coating was prepared on wood surface with varying molar ratio of synthetic castor oil-based thiol（CO-SH）,synthetic reactive flame retardant（DA）and 2,4,6-trimethyl-2,4,6-trivinylcyclotrisilazane（TTC）via the thiol-ene click reaction.The results show that when the molar ratio of CO-SH,DA and TTC is 5:3:3,the UV-cured coated wood samples pass the vertical UL-94V-0 rating;the limiting oxygen index（LOI）value can reach 27.52%;the second peak of heat release rate（P²HRR）and the second smoke production rate peak（P²SPR）of the coated wood samples can be effectively suppressed,while the total heat release（THR）is only 16.99 MJ/m² during the cone calorimetry（CONE）tests.Besides,the coated wood internal tracheid basic skeleton remains basically intact and the char layer tends to exhibit a more compact morphology after the CONE tests.（3）Through thermal degradation of castor oil-based UV-cured wood flame retardant coatings under air conditions,it shows that when the molar ratio of DA,TTC and CO-SH is 3:3:5,the thermal stability is the best;polyphosphoric acid formed through the decomposition of DA can promote Si─O and Si─C 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.Oxygen concentration can be diluted by the nonflammable gases（CO₂ and SO₂）released from the coatings during degradation,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.