Ability of Transparent Coatings to Retard Leaching of Borates in a Weather-Ometer® Test

By P.J. Morris, J. Wang, J.K Ingram

The general purpose of the project is to develop decay-resistant glulam and LVL using combined borate treatment and effective transparent coating systems. To choose the coating with potentially the best performance on borate-treated composites and potentially the highest capability of preventing borate leaching during outdoor weathering, 15 types of commercial transparent coatings were selected and tested on borate-treated solid wood samples using arti­ficial weathering.

Ponderosa pine sapwood coupons, double-length and with a central 10 mm section for prior analysis, were pressure-treated with boron, coated as per manufacturers rec­ommendations, then cut into two daughter pieces plus the analysis sample-then the cut ends were re-coated.

After the weathering the samples were evaluated for coating deteriora­tion and a piece cut from the center of the exposed samples and the unexposed analysis sam­ple were analyzed for borate. Losses for two or three coats of water-based transparent coatings ranged from 5% to 92%.

What was most surprising was that the reference opaque paint product, an acrylic primer plus two acrylic topcoats, lost 60% even though the surface looked perfect in terms of physical deterioration.