Influence of nano-silica on the flame retardancy and smoke suppression properties of transparent intumescent fire-retardant coatings

A cyclic phosphate ester acid (PEA) was synthesized by the reaction of phosphoric acid (PA), pentaerythritol (PER) and n-butyl alcohol (BA). Then nano-silica was successfully introduced into the structure of PEA to obtain SPEAs with different nano-silica contents. The chemical structures of PEA and SPEAs were confirmed by Fourier transform infrared spectroscopy (FTIR) and H-1 nuclear magnetic resonance spectroscopy (H-1 NMR). A series of fire retardant coatings coated on the plywood boards were prepared by mixing of melamine formaldehyde resin with PEA and SPEAs. The transparency analysis indicates that the incorporation of nano-silica into the coatings through chemical grafting method acquires high degree of transparency due to the homogeneous dispersion of each element in the coatings, judging by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The influence of nano-silica on the thermal stability, flame retardancy and smoke suppression properties of coatings were investigated in details by cabinet method test, tunnel method test, cone calorimeter test, smoke density test, thermo-gravimetric analysis (TG), FTIR, SEM and EDS. The results show that the values of weight loss, char index, flame spread rating, heat release rate, total heat release, smoke production rate, total smoke release and specific optical density of the coatings are remarkably decreased with the introduction of nano-silica, which is due to the formation of a compact and intumescent char layer during combustion. The TG results demonstrate that the incorporation of nano-silica increases the thermal stability and residual weight of the coatings. The char residue analysis reveals that the nano-silica enhances the char-forming ability, intumescence and antioxidation property of the coatings due to the synergistic effect existed between phosphorus and silicon, leading to a stronger shielding effect in condensed phase. Based on these facts, the introduction of nano-silica could promote the formation of a more compact and intumescent char layer and then effectively reduces the heat and smoke release, thereby enhancing the flame retardancy and smoke suppression properties of coatings.