Synthesis and application of novel magnesium phosphate ester flame retardants for transparent intumescent fire-retardant coatings applied on wood substrates

A series of magnesium phosphate ester (MPEA) flame retardants were synthesized by the salification of cyclic phosphate ester (PEA) with different contents of magnesium hydroxide (MH). The obtained PEA and MPEAs were well characterized and then thoroughly mixed with amino resin to produce transparent intumescent fire-retardant coatings applied on wood substrates. The optical transparency, flame retardancy and smoke suppression properties of the coatings were intensively investigated. The results show that the fire-retardant coatings containing MPEAs can keep a high degree of transparency even at high MH contents, and the fire protection performance, flame retardancy and smoke suppression properties of the coatings are significantly improved with introduction of MH due to the formation of a more compact, continuous and intumescent char during combustion. When the mass ratio of PEA/MH is 95:5, the obtained MPEA3 flame retardant exhibits the best synergistic flame-retardant and smoke suppression effects in the coatings. Remarkably, the flame spread rating, total heat release and total smoke release for the coating containing MPEA3 are 34.4%, 29.0% and 55.0% lower, respectively, than those of the coating containing PEA. TG analysis shows that the introduction of MH improves the thermal stability and residual weight of the coatings, and the coating containing MPEA3 shows the maximum residual weight of 34.4% at 800 degrees C. Char residue analysis shows that the introduction of MH positively contributes to enhance the quality and quantity of intumescent char via the formation of more phosphorus-rich crosslinking structures and aromatic structures in the condensed phase, and an excessive content of MH diminishes this positive char-forming effect as well as the synergistic flame-retardant and smoke suppression effects in the coatings.