Abstract
Artificial weathering can significantly reduce the testing time needed for proving coating durability, nevertheless its reliability is still not thoroughly proven. In this study, eight different transparent and pigmented coating systems, namely oil, acrylate, alkyd and urethane alkyd were evaluated through natural and artificial weathering tests on oak samples by measuring colour, gloss and surface wettability and by macroscopic and microscopic evaluation. The oil coatings performed well in wood colour stability evaluations, while the best gloss and wettability change results were noted for acrylate coatings. Pigmented coatings were characterized by significantly lower colour changes than transparent ones. The gloss and wettability changes were more sensitive to coating disruption than to total colour changes of coated wood associated with chemical changes in wood. The findings in this work showed that values of gloss changes and surface wettability for all types of coatings exposed to artificial and natural weathering resulted in significant differences from each other. The data obtained by artificial weathering method provide basic results of coatings durability and, ideally, natural weathering should be performed at the same time to support the results from laboratory tests by exposing wood under real conditions.
Highlights
Oak (Quercus petraea L.) wood is often used for the exterior applications, mostly in construction of bridges, pergolas, balconies or garden furniture, where higher natural durability [1] is required.Oak contains a relatively high amount of phenol extractives, mainly vescalagin, castalagin, gallic and ellagic acids [2], creating problems in the field of surface treatment durability [3,4]
A photodegradation process of oak wood accompanied with significant discolouration and leaching of extractives from the surface takes place during the initial phases of outdoor exposure [6], more intensely in the heartwood zone [7], which leads to the need to protect oak wood by coatings to maintain its natural appearance
The type of coating system applied on oak wood samples has a statistically significant effect (p < 0.05)
Summary
Oak (Quercus petraea L.) wood is often used for the exterior applications, mostly in construction of bridges, pergolas, balconies or garden furniture, where higher natural durability [1] is required. Oak contains a relatively high amount of phenol extractives, mainly vescalagin, castalagin, gallic and ellagic acids [2], creating problems in the field of surface treatment durability [3,4]. Tannins in oak wood retard coating hardening [5]. The complex open vessel morphological structure of oak wood complicates the overall application of coatings. A photodegradation process of oak wood accompanied with significant discolouration and leaching of extractives from the surface takes place during the initial phases of outdoor exposure [6], more intensely in the heartwood zone [7], which leads to the need to protect oak wood by coatings to maintain its natural appearance. The exterior coating generally protects against moisture uptake and related dimensional changes, protects against photochemical
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