Synergistic action of zinc borate or magnesium hydroxide with phosphorus were demonstrated in novel PIN FR coatings applied to 4mm plywood board (up to 75×75 cm, tunnel, cabinet tests, cone calorimeter and smoke density tests). A cyclic phosphate ester was reacted with zinc borate or with magnesium hydroxide, then incorporated into an amino resin, then brush applied to the plywood, with several applications to result in a 0.4 mm coating. Different levels of minerals were tested (0 – 7% by mass). The addition of zinc borate at 7% halved the flame spread rating (FSR) of the plywood, nearly halved the total heat release and reduced the peak smoke production rate and total smoke release to around one third. Magnesium hydroxide showed optimal performance at 5%, reducing total heat release and peak smoke production by around 30% and reducing total smoke release by just around 55% Analysis suggests that these effects were due to formation of a more compact and intumescent char layer, in particular by increasing phosphorus cross-linking. Coating opacity is somewhat increased by the zinc borate, but it remains transparent.
“Synergistic flame-retardant and smoke suppression effects of zinc borate in transparent intumescent fire-retardant coatings applied on wood substrates”, L. Yan et al., Journal of Thermal Analysis and Calorimetry 2018 https://doi.org/10.1007/s10973-018-7819-1
“Synthesis and application of novel magnesium phosphate ester flame retardants for transparent intumescent fire-retardant coatings applied on wood substrates”, L. Yan et al., Progress in Organic Coatings 129 (2019) 327–337 https://doi.org/10.1016/j.porgcoat.2019.01.013