Effects of polyethylene glycol borate on the flame retardancy and smoke suppression properties of transparent fire-retardant coatings applied on wood substrates

A series of novel phosphorus and boron-containing flame retardants (PPEAs) were synthesized via the esterification of cyclic phosphate ester (PEA) and polyethylene glycol borate (PEG-BA) with different mass ratio, and then well characterized by Fourier transform infrared (FTIR) spectroscopy and H-1 nuclear magnetic resonance (H-1 NMR) spectroscopy. The obtained PEA and PPEAs were applied in amino resin to produce transparent fire-retardant coatings applied on wood substrates. The results of transparency analysis and fire protection tests show that the fire-retardant coatings containing PEG-BA exhibit a high degree of transparency even at a high PEG-BA content and supply excellent fire protection for the wood substrates simultaneously. Cone calorimeter and smoke density tests show that the introduction of PEG-BA exerts excellent synergistic effect on reducing the heat release and smoke production of the coatings, and an excessive content of PEG-BA will diminish this synergistic effect. Especially, when the mass ratio of PEA/PEG-BA is 80:20, the obtained MPPE3 coating exhibits 51.5% reduction in peak heat release rate, 48.1% reduction in total heat release and 59.7% reduction in smoke density rating compared to those of the MPPEO without PEG-BA. Thermo-gravimetric (TG) analysis shows that the introduction of PEG-BA improves the residual weight and high-temperature stability of the coatings. In addition, the same addition of PEG-BA exerts better synergistic effects on the fire performance and char formation of the fire-retardant coating than those of polyethylene glycol (PEG). The superior flame retardancy and smoke suppression properties of the coatings containing PEG-BA can be ascribed to the formation of more phosphorus and boron-containing crosslinking structures and aromatic structures in the condensed phase that produce a more thermally stable, compact and intumescent char against fire during burning.