Flame retardancy and smoke suppression properties of transparent intumescent fire-retardant coatings reinforced with layered double hydroxides

A series of novel layered double hydroxide (LDH)-modified phosphate esters (LPPBs) flame retardants were synthesized by the reaction of flexible phosphate ester (PPB) and LDHs with different mass ratio and then well characterized by Fourier transform infrared spectroscopy (FTIR) and H-1 nuclear magnetic resonance spectroscopy (H-1 NMR). The obtained LPPBs were mixed with melamine formaldehyde resin to produce transparent fire-retardant coatings applied on wood substrates. The coatings containing LDHs exhibit a high degree of transparency even at high LDH contents due to the well dispersed and completely exfoliated state of LDHs in the amino matrix, as determined from the results of X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images. The fire protection tests show that the values of weight loss, char index, and flame spread rating of the coatings are remarkably decreased with the introduction of LDHs concomitant with an increase in the intumescent factor. The cone calorimeter and smoke density tests show that the flame retardancy and smoke suppression properties of the coatings are significantly improved with the introduction of LDHs due to the formation of a more compact, continuous, and intumescent char during combustion. The results of thermogravimetric (TG) analysis show that the thermal stability and residual weight of the coatings are gradually increased with increasing LDH content. Char residue analysis shows that the introduction of LDHs into the coatings contributes to generating more phosphorus-rich crosslinking structures and aromatic structures in the condensed phase that produce a more compact and thermally stable char against the release of heat and smoke, thus exhibiting excellent flame retardancy and smoke suppression properties.