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

In this paper, a multifunctional castor oil based bio-mercaptan (abbrev. COSH) was synthesized by esterification between castor oil (abbrev. CO) and 3-mercaptopropionic acid (MPA). In addition, [(6-Oxido-6H-dibenz [c,e][1,2] oxaphosphorin-6-yl) methyl] butanedioic acid (DDP) and allyl glycidyl ether (AGE) were used to synthesize the organic phosphorus flame retardant containing a double bond of allyl ether DDP-AGE (abbrev. DA) via a ring-opening reaction. The synergistic flame-retardant UV-cured coating containing phosphorus (P), silicon (Si) and sulfur (S) was prepared on wood surface with varying molar content of COSH, 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 COSH, DA and TTC is 5: 3: 3, the UV-cured coated wood samples pass the vertical UL-94 V-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) of 16.99 MJ/m² is much lower than other samples and the total smoke production (TSP) is equivalent to that of uncoated wood sample; the final char residue rate can reach 14.41% during the cone calorimetry (CONE) tests. Moreover, the thermogravimetric analysis (TGA) demonstrates that COSH promotes the degradation of films at a lower temperature, while it reduces the maximum degradation rate and delays the decomposition process simultaneously. Besides, the scanning electron microscope (SEM) shows that 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 if the wood is coated following the molar ratio of COSH, DA and TTC of 5: 3: 3. Energy dispersive spectrometer (EDS) was used to compare the content of C, O, N, P, Si and S of the char layers formed by the coatings on the surface of coated wood samples after CONE tests, which addresses the synergistic flame retardance of P, S and P, Si.