Mineral-based composition with deliquescent salt as flame retardant for melamine–urea–formaldehyde (MUF)-bonded wood composites

A flame-retardant composition (FRC) composed of a surface-treated calcium carbonate-based mineral, having high porosity and loaded with deliquescent calcium chloride, was assessed for its potential as a flame retardant. Two FRCs with 16% and 26% calcium chloride (dry solid) stored in the pore structure, respectively, were studied with respect to their ability to absorb and release free water, and their efficacy in melamine–urea–formaldehyde (MUF)-bonded wood composites was investigated. Water absorption capacity was determined by performing absorption tests at a temperature of 20 °C and relative humidity (RH) of 65% and 95%, and the water release behavior was studied by performing thermogravimetric analysis. The FRCs have the capacity to hold substantial amounts of water (up to 60 wt%), but still behave as a free-flowing powder. The influence of addition of 10 and 20 wt% FRC in wood composites on reaction to fire and strength properties was determined by measuring the self-extinguishing time after flame exposure and internal bond strength, respectively. These effects were evaluated by comparison with ground calcium carbonate (GCC) and commercially available nitrogen containing phosphorus-based fire retardant. Although the FRCs had a negative impact on internal bond strength, the results confirmed their flame-retardant potential and showed that 10–15% by weight of the flame retardant would be a good compromise, in terms of the trade-off between flame retardancy and mechanical properties. The synergistic effects of multiple flame retardancy reaction mechanisms due to the presence of inorganic minerals and a hygroscopic agent, CaCl2, are also discussed. The unique properties of the FRC, which allow to exploit the fire-retardant potential of CaCl2 while at the same time eliminating the risk associated with the emission of hydrogen chloride gas during combustion, are confirmed by the results of FTIR spectroscopic analyses of the flue gas.