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

Abstract

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 1H nuclear magnetic resonance (1H 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 MPPE0 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.