UBC Theses and Dissertations
Plasma modification of wood to improve the performance of clear coatings Haase Masek, Jonathan George
Wood is susceptible to photodegradation, particularly by ultraviolet (UV) light which triggers photochemical reactions in wood up to 100 µm from the surface. Painting may protect the wood from sunlight, however, consumers prefer clear-coatings because they reveal wood’s grain. Unfortunately, clear finishes on wood used outdoors fail within 1-2 years because sunlight transmitted through the clear-coating degrades the underlying wood. I hypothesized that the performance of clear-coatings on wood used outdoors would be improved if the clear-coating could penetrate deeply into the wood and bond with sub-surface layers of wood that are less affected by UV light. Glow discharge plasma derived from water was used to etch and open up flow paths at the surface of the refractory wood species, black spruce. Image analysis was used to assess the penetration of two polyurethane clear-coatings into wood, and the adhesion (pull-off) of clear-coatings to unweathered wood was measured. A second experiment examined the changes in color, gloss and integrity of clear-coats on plasma treated and coated specimens subjected to artificial accelerated weathering. Plasma treatment of black spruce for 20 min significantly increased the penetration of clear coatings into sub-surface layers of wood (50-75 µm). However, the adhesion of the polyurethane clear coatings on wood depended much more on coating type (adhesion of the water-borne was better than that of the oil-borne coating). Plasma pre-treatments decreased film failure of coatings on flat-sawn specimens exposed to artificial weathering, but increased the surface yellowing of coated wood specimens. The oil-borne polyurethane performed better in terms of three performance criteria (redness, yellowness and gloss), while the water-borne was superior in terms of changes in lightness. I conclude that plasma treatment can increase the penetration of coatings into wood if the plasma can etch the wood and open up important flow-paths. However, plasma treatments have no positive effect on the adhesion of polyurethane coatings to black spruce wood and adhesion is influenced more by the type of coating. Finally, plasma treatments can have positive and negative effects on the exterior performance of coatings on wood depending on the criteria used to assess performance.
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