Eliška Oberhofnerová

The adhesion of coatings to wood is important for their long-term performance. In this study, the adhesion strength of water-based acrylate coatings used for wooden windows after exposure to artificial weathering (AW) and temperature cycling (TC) was investigated. The analysis of the adhesion quality of coatings was performed via a pull-off test and failure characteristics. The 3-layered and 4-layered white and brown acrylate dispersions from six different producers were compared and the effect of coating thickness on adhesion strength was investigated. The adhesion strength values proved to be very variable. After AW, the adhesion strength and its variability increased for all the samples. TC had no statistically significant effect on the adhesion strength values. White coating systems were initially characterized by lower adhesion strength, but after AW and TC, they reached higher adhesion strength values than brown ones. The overall highest adhesion after AW and TC was recorded for the coatings based on alkyd-acrylate hybrid basis (Producer 3), while the lowest adhesion variability after AW was measured for one type of tested acrylate coating (Producer 4). The effect of different layering on adhesion strength was not demonstrated in this study.

A top hydrophobic layer can increase the durability of exterior coatings applied on wood. Two hydrophobic topcoats - synthetics and waterborne acrylate resin with wax additives, were tested as top layers on twenty-four different coating systems applied on oak wood in this experiment. Artificial accelerated weathering lasted for six weeks. Changes of color, gloss, surface wetting were evaluated, and microscopic analyses of coated surfaces were done during weathering. The results have shown that a top hydrophobic layer increases the durability of tested coating systems in most cases. However, the effectiveness of the two tested hydrophobic topcoats turned out to be different depending on the specific coating systems used.

Top hydrophobic layer can increase durability of exterior coatings on wood. Two hydrophobic topcoats—synthetics and water-based acrylate resin with wax additives were researched as top layer on twenty-four different coating systems applied on oak wood in this experiment. Artificial accelerated weathering lasted six weeks. Changes of color, gloss, surface wetting were evaluated, and microscopic analyses of coated surfaces were done. The results have shown that top hydrophobic layer increases durability of tested coating systems in most cases. However, the effectiveness of the two researched hydrophobic topcoats was different depending on the specific tested coating systems.

This paper compares the resistance of 20 commercial transparent and semi-transparent coatings applied to European and Siberian larch during artificial weathering in Xenotest. The change in gloss, colour, contact angle of wetting, resistance to Aspergillus niger and Penicillium brevicompactum moulds was evaluated, and visual changes at the top surface of treated wood species were measured. Overall, the durability of coatings on European larch was higher than that on Siberian larch. The most durable of the tested coatings was a thin-film, i.e., semi-transparent oil-based film containing TiO₂ pigment and propiconazole fungicide. Of the transparent coatings, the most stable was a thick acrylic coating. Conversely, penetrating transparent oil systems had low colour stability and overall lifespan. Artificial weathering of all of the coatings resulted in a marked decrease in their resistance to moulds.

Reducing discoloration of wood due to photodegradation caused by ultraviolet (UV) and visible (VIS) radiation enhances its aesthetical value and prolongs the overall service life of protective coatings. In this study, the efficiency of pretreatments with different active ingredients to reduce degradation and stabilize the color of Siberian (Larix sibirica Ledeb.) and European larch (Larix decidua Mill) wood was investigated. UV absorbers (UVA), hindered amine light stabilizers (HALS) and zinc oxide nanoparticles were used in twenty different pretreatments. The ability to protect wood surface against radiation was evaluated via color and gloss change measurements during artificial ageing. The efficiency of tested color-stabilizing pretreatments differed for Siberian and European larch and not all of them reduced discoloration. The most effective pretreatments were based on a combination of UVA and HALS in a synergistic effect. Overall, the best efficiency from tested variants for larch wood generally was observed for combination of Eversorb 80 on benzotriazole basis + Eversorb 93 on a piperidinyl basis. The pretreatments did not significantly affect the gloss values. The results revealed convenient variants of stabilizers for Siberian and European larch wood and confirmed different compatibility between specific wood species and color stabilizers.

Colour changes and associated wood degradation in exterior and interior applications influenced by ultraviolet (UV) and visible radiation (VIS) decreases the aesthetic value of the products and shortens the overall life of transparent coatings. The aim of the paper is to achieve colour stabilization of oak, larch, Douglas fir and spruce heartwood via surface treatment with UV stabilizers, hindered amine light stabilizers (HALS), nanoparticles TiO₂, ZnO, and mixtures thereof, during exposure to UV and VIS radiation. Colour changes were evaluated during accelerated artificial ageing testing in Xenotest. The distinctly individual character of colour changes in surface treatments due to the underlying wood species was confirmed. A synergistic effect was found when using a combination of active substances compared to substances used individually. The mixture of benzotriazoles with HALS (Tinuvin 5151) in combination with TiO₂ and ZnO nanoparticles was confirmed as one of the most effective treatments for colour stabilization of wood due to UV and VIS spectrums.

The efficiency of surface pretreatment with natural essential oils relative to the weathering performance of Norway spruce wood was examined. This study investigated the combination of this pretreatment increasing biological resistance and oil-based coating with protection against ultraviolet light and hydrophobic topcoat with silicon nanoparticles during natural and artificial weathering. The coating systems were exposed to 24 months of natural weathering in climatic conditions of Central Europe and 2,016 hours of artificial weathering. The synergistic effect of a coating system based on safflower with essential oils and commercial oil-based coating was the most efficient. The application of one layer of an oil-based coating in a wet state of surface pretreatment exhibited results comparable to the application of two layers in a dry state. For all coating systems, increasing changes of colour, roughness, and surface wettability were observed, which differed according to the weathering method. Weathering performance of transparent coating systems was evaluated during both weathering tests by exact measurements, laser scanning microscopy and visual evaluation as well. Total colour difference did not prove to be a sufficient evaluation criterion to indicate coating performance during weathering.

The purpose of this study was to evaluate the impact of glue line thickness on joint strength properties while using two types of PVAc adhesive (i.e., one-component Rhenocell 3W, 4B Plus, resistivity class D3, and two-component Protovil VP244/2C, resistivity class D4) belonging to the group of thermoplastic adhesives intended for nonconstruction joints. Joint strength testing was carried out using the transverse tensile test for estimation of the joint strength method. It was proven that if specimens were conditioned for 7 days at 20°C ± 2°C and a relative air humidity of 65 ± 5 percent, when testing one-component Rhenocell 3W, 4B Plus (D3) adhesive, increasing thickness of the glue line results in falling strength of the bonded joint. When testing the two-component Protovil VP244/2C (D4) adhesive, increasing thickness of the glue line always results in increasing joint strength, even under conditions when water is directly affecting the joint or under increased temperatures.

As a result of contemporary ecological and sustainable movements, a number of constructions from wood, as the only renewable building material, is still increasing. In accordance with these trends, wood elements are often left untreated. The presented study is aimed to characterize and compare a surface degradation of untreated wood caused by weathering. Weathering characteristics of softwoods and hardwoods (spruce, pine, Douglas fir, larch, oak, black locust, maple, poplar and alder) during twelve months of exposure in the climatic conditions of Central Europe were determined. All the wood species were characterized by colour and gloss changes and increasing roughness. The lowest discoloration was observed for oak and the highest one for spruce wood, respectively. The lowest changes of roughness were observed for alder, the highest for spruce wood. The depth of colour changes was relatively similar among the softwoods. Regarding hardwoods, it varied more. The values of total colour changes had a significant correlation with the depth values of these colour changes. Maple, alder and poplar samples were characterized by the presence of mould and blue-stain fungi. Douglas fir wood was found to be the most liable for the formation of cracks.

Determination of Correlation between Destructive and Nondestructive Test Methods Applied on Modified Wood Exposed to Natural Weathering