Preventing water-based opaque coatings from yellowing

Coating system comprising a polar primer and a nonpolar topcoat showed remarkable performance preventing the migration of the extractives and the discolouration of the painted surface.

Water-based opaque coatings used on pine wood show undesired discolouration related to the presence of knots, which have an outstanding high concentration of extractives. These compounds, of both polar and nonpolar nature, migrate through the polymeric structure resulting in a yellow-brown spot over the knot.

A water-based coating system comprising a polar primer and a nonpolar topcoat proved to have remarkable performance preventing the migration of the extractives and the discolouration of the painted surface. The knot bleeding resistance was tested following EN 927–7. The colour of the coating over the knot was measured using CIELab coordinates, and the resulting colour difference in comparison with the surrounding paint was in all cases less than the accepted limit for discolouration of opaque coatings. The need to have the combined system to successfully block the whole spectrum of extractives was proved by the evaluation of the individual coatings as well as the utilisation of polar dispersions as topcoats. The system presented in the new study constitutes a solution to the discolouration of water-based coatings over knotted pine wood.

The study has been published in Journal of Coatings Technology and Research, Volume 20, issue 2, March 2023.