Polyurethane/acrylic coatings for wood made hydrophobic by melamine addition

Hydrophobic polyurethane/acrylic hybrid dispersions were obtained for exterior wood coatings. Thus, polyurethane was synthesised using as solvent methyl methacrylate, and after phase inversion provoked by water addition, the acrylic monomer radically polymerised giving rise to the polyurethane/acrylic hybrid dispersion. Melamine was added as a hydrophobic and fire retardant additive by two different approaches: in the phase inversion (A) and the beginning of the synthesis (B). FTIR-ATR, 1H NMR, and molar mass measurements (AF4/MALS/RI) showed that in the B method, the melamine reacted with isocyanate. However, the addition of melamine in the phase inversion provoked a reduction of the molecular weight of the acrylic part because of the interaction with the radical initiator. According to TGA, Pyrolysis Combustion Flow Calorimetry (PCFC), and water permeation experiments, melamine induced the formation of a carbonaceous char when heating and reduced the water vapour transmission rate of the hybrid films. Moreover, wood application tests showed that the incorporation of melamine improved the transparency, adhesion, and water resistance of the coatings but the employed amount was not enough to produce significant changes in the fire behaviour of the coatings measured in the cone calorimetry.