The aim of this work was to evaluate the colour stability of wood treated with transparent coatings usable for outdoor wooden buildings and to monitor their defects due to aging. In addition to a suitable treatment, the colour durability is also conditioned by the type of wood used and partly also by the surface treatment.
In this case, black locust tree (Robinia pseudoacacia) and Norway spruce (Picea abies) were selected for testing. The wood samples were sanded on part of the surface with coarse sandpaper and on part with fine sandpaper. Three types of colourless protective coatings with different compositions were applied. The first was * a coating with a synthetic resin oil, the second ** a semi-matt oil based on natural vegetable oils with UV protection and finally a water repellent coating with UV protective components. The first two coatings were applied to the sample in two layers, half of which were treated with water-repellent as a topcoat.
The samples, mounted in metal stands, were exposed to weather aging outdoors in the city of Zvolen in Slovakia for 6, 12, 24 and 36 months. The city, located at an altitude of 290 m, is characterized by frequent fog, smog and, thanks to its continental climate, especially large temperature differences, when temperatures rise to 35 ° C in summer and fall to -25 ° C in winter. At the end of the test periods, the treated samples were compared with the untreated samples and statistics of the effectiveness of the coatings used were compiled.
The results showed that transparent paints, especially those with a higher amount of UV stabilizers, can quite successfully prevent changes, or at least delay them. As for surface treatment, the different roughness of the treated samples did not always have a clear effect on their colour fastness, but they were usually higher on a finer surface. In the same way, the paint was damaged less on a fine surface and over a longer period of time.
In general, it turned out that the colour of treated and untreated locust wood was more stable than that of spruce. The better behaviour of transparent coatings on locust wood compared to spruce was already visible after 12 months of aging. Of the coatings used, the semi-matt oil based on natural vegetable oils with UV protection had the best results against weather conditions. However, the top, water-repellent layer has also proven successful. On the one hand, it had a positive effect on the stability of coatings and at the same time reduced the negative effects of higher roughness of wooden surfaces.
The conclusions of the research point to the great importance of using suitable coatings to prolong life, but also to maintain the aesthetic function of wooden buildings and structures. Black locust tree, as the hardiest tree in Central Europe, has great potential for these buildings and structures in the future. On the other hand, in comparison with Norway spruce, cheaper to build and more traditional wood, it has more disadvantages, which include less dimensional stability, higher density and greater susceptibility to cracking.
* PerlColor, ** Osmo UV-Protection Oil 420, *** AquaStop
Ing. Miloš Pánek, Ph.D.
He is a graduate of the Technical University in the field of Wood Engineering in Zvolen, where he then obtained a doctorate for his work "Improving Impregnability". Since 2014, he has been working as an assistant professor at the Department of Wooden Products and Structures, Faculty of Forestry and Wood Sciences, CULS. The subject of his research is the surface treatment of wood and the improvement of its protection.
Prepared by: Jakub Málek