Synergistic effects of organically modified montmorillonite on the flame-retardant and smoke suppression properties of transparent intumescent fire-retardant coatings

Abstract A series of novel montmorillonite polyphosphate (OPEA) flame retardants were successfully synthesized by chemical grafting of cyclic phosphate ester acid (PEA) with different contents of organically modified montmorillonite (OMMT). Their chemical structures were confirmed by Fourier transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectroscopy (1H NMR). Five types of transparent fire-retardant coatings applied to plywood boards were prepared by mixing melamine formaldehyde resin with PEA or OPEAs. The transparency analysis reveals that chemical grafting of PEA on the surface of OMMT imparts a high degree of transparency to the resulting fire-retardant coatings due to the uniform dispersion and completely exfoliated states of the MMT platelets in the coatings, as determined from X-ray diffraction patterns and transmission electron microscopy images. The fire protection tests show that the introduction of OMMT decreases the mass loss, char index, and flame spread rating of the coatings, concomitant with an increase in the intumescent factor. The cone calorimeter and smoke density tests reveal that incorporation of OMMT remarkably decreases the heat release and smoke production of the coatings, which is ascribed to a more thermally stable, compact and intumescent char layer formed during combustion, judging from digital photographs and scanning electron microscopy images. Further, the synergistic flame-retardant and smoke suppression effects of OMMT in the coatings depend on the content of OMMT, and excessive OMMT will decrease the synergistic effects. The thermo-gravimetric analysis shows that the thermal stability and residual weight of the coatings increase efficiently with increasing OMMT content. The FTIR and elemental analysis demonstrate that the incorporation of OMMT generates more phosphorus-rich cross-linked char and aromatic char in the condensed phase, thus effectively reducing the heat release, smoke production, and mass loss of the coatings.