Development of flame-retardant waterborne alkyd resins for wood coating

Phosphorus-containing water-borne alkyd resins were developed as transparent flame-retardant wood coatings. The synthesis consisted of alcoholysis followed by esterification, incorporating the reactive flame-retardant polyethylene glycol phosphate ester (Exolit OP550). In addition, ammonium polyphosphate additives of different particle sizes (Exolit AP422 and Exolit AP423) were introduced into the reference alkyd resin. Chemical characterisation confirmed successful synthesis and showed that the reactive flame retardant influenced the esterification process and the resulting molar mass distribution. Fire performance evaluation by cone calorimetry demonstrated a significant reduction in peak heat release rate and an increase in char residue for the phosphorus-containing systems compared to the reference resin. The flame retardants also affected thermal stability and coating properties, including transparency, adhesion, gloss, hardness, and water sensitivity. Both reactive and additive phosphorus flame retardants improved fire performance compared to the reference resin. The additive systems showed a more pronounced reduction in total heat release under the tested conditions, while reactive systems enabled chemical incorporation into the polymer backbone while maintaining coating performance. Differences between the two ammonium polyphosphate grades highlight the influence of particle size on dispersion, residue formation, and overall fire behaviour, underlining the importance of formulation strategy in waterborne alkyd systems.