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High performance clear-coat systems for wood used outdoors

University of British Columbia

Wood is one of the main materials of choice to reduce the environmental footprint of the building and construction sector. Wood is aesthetically pleasing and hence there is great demand for finishing systems that do not mask wood’s color and texture. Unfortunately, clear coatings used on wood outdoors fail within one to two years mainly because of photodegradation of the underlying wood substrate. The performance of clear coatings on wood can be enhanced by photostabilizing the wood before coating or by modifying the clear coating to improve its photostability and flexibility. However, these approaches have seldom been combined. In this thesis I hypothesized that pre-treatments that can photostabilize wood without adversely affecting coating performance could, when combined with state-of-the-art clear coatings, create coating systems that are a significant improvement on those that are currently available. I compared the ability of nine different chemical treatments to photostabilize wood. I examined the effect of the same wood pretreatments on coating properties (wettability and adhesion) and the permeability of three different coating types. The most effective photostabilizing pre-treatments were benzoyl chloride, vinyl benzoate and chromic acid, and a PF-resin containing a hindered amine light stabilizer. The solvent based polyurethane was more compatible than the water-based acrylic and alkyd coatings with treated wood surfaces. Wood treatments had no effect on coating adhesion, but coating adhesion was highest for the polyurethane. Subsequently, I examined the outdoor performance of seven different clear coatings on photostabilized wood substrates. Benzoyl chloride, vinyl benzoate, chromic acid and the PF-resin/HALS treatments effectively enhanced the clear coat performance during a year-long weathering trial in Australia. Clear coat performance was also improved by using coatings with double the amount of UVA/HALS additives. My findings confirm that clear coat performance on wood exposed outdoors depends on the combination of wood retreatment and coating type. I conclude that superior clear coat systems (pretreatment plus coating) for wood can be developed by combining very effective treatments that can photostabilize wood together with state-of-the-art photostable and flexible clear coatings.

Text

2012-04-30

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/33567

Forestry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/