Phytate-based transparent and waterproof intumescent flame-retardant coating for protection of wood

Phytic acid-based phosphate esters (PAPEGs) were prepared through the reaction between phytic acid (PA) and different proportions of polyethylene glycol 200 (PEG-200). Intumescent flame-retardant coating (IFRC) with PAPEG3, PAPEG6, PAPEG9 and PAPEG12 were obtained via thermal curing of PAPEGs and melamine formaldehyde resin (MFR), respectively. The backside temperature of steel with MFR/PAPEGs coating was much lower than that of MFR/PA coating. MFR/PAPEG6 coating showed the best flame retardancy, and the char thickness reached 48 mm from initial thickness of 0.4 mm, and the expansion ratio was 120 times. Cone calorimeter confirmed that the incorporation of PAPEG6 could minimize the heat release rates and total heat release of materials during combustion. Moreover, the MFR/PAPEGs coating possessed both excellent transparency and water resistance. The transmittance of MFR/PAPEG6 and MFR/PAPEG9 was above 87% at 600 nm, and transmittance of MFR/PAPEG12 even reached 90%. The proposed flame-retardant mechanism was that MFR/PAPEGs produced phosphorus-containing acids to dehydrate and carbonize PEG, leading to the formation of graphitized chars. Meanwhile, the nitrogen-containing intermediates simultaneously released noncombustible gases to expand the char and interrupt the combustion reaction.