Wood Surface Finishing with Transparent Lacquers Intended for Indoor Use, and the Colour Resistance of These Surfaces during Accelerated Aging

Abstract

This work evaluates the effects of accelerated aging on the discolouration of surface-treated spruce wood and oak wood coated with solvent-based polyurethane lacquers, and surface-treated spruce wood coated with water-based transparent coating systems. All concerned coating materials were intended for indoor use. It was also explored how the colour stability of spruce wood and oak wood surfaces treated with solvent-based polyurethane lacquers was affected by wood surface layer modifications with pigment or stain mordants applied before these lacquers. Another issue studied was how the lignin stabilizer admixed into the primer and pigments admixed into the top coating layers affected the stability of water-based coating systems on spruce. The experimental results showed that the accelerated aging process with a simulation of indoor conditions induced significant discolouration of wood surfaces coated with solvent-based polyurethane lacquers and water-based coating systems. There were also confirmed significant impacts of all the studied factors (wood species, lacquer/coating system type, lacquer modification, wood pre-treatment with pigment and stain mordants). The spruce wood surfaces coated with solvent-based polyurethane lacquers were less stable (ΔE = 10–19, dependent on the lacquer type) than the oak surfaces treated in the same ways (ΔE = 4–11). There were also confirmed significant impacts of the particular surface treatment on the colour stability as well as significant impacts of wood surface pre-treatment with pigment and stain mordants (ΔE = 4–17—for spruce wood, and ΔE = 5.5–13—for oak wood). In the case of water-based lacquers, the ΔE values ranged between 3 and 11 (according to the coating system type). The results show that an appropriate UV absorbent combined with an appropriate lignin stabilizer and pigment mordant may enable attaining the required colour stability for a given surface treatment applied on a given wood species.

Effect of Two Different Ageing Exposures on the Colour Stability of Transparent Polyurethane Finishing

“…In our previous study [ 30 ], discolouration of a finishing system based on polyacrylic, and aldehyde resin resulted mainly from the photo yellowing of underlying untreated and HTT-treated wood due to sunlight and increased with irradiation time. Furthermore, the results of the study [ 46 ] showed that an accelerated ageing process with simulation of indoor conditions induced significant discolouration of surface-treated wood, both coated with solvent-based polyurethane and water-based lacquers. The spruce wood surfaces coated with solvent-based polyurethane lacquers were less stable than the oak surfaces treated in the same ways.…”

Light-Induced Colour Changes in Wood Surfaces in Indoor Conditions Determined by an Artificial Accelerated Test: Influence of Wood Species and Coating Materials

“…This would mean a common and interconnected influence of the two factors for each wood species and type of coating. A significant impact of wood species and type of lacquer/coating system, alongside pre-treatments of wood surfaces and modification of coatings, on the light-induced colour changes in wood surfaces was previously reported [59]. The interaction of the coating material with the wood substrate at the interface level, consisting in wetting and penetration, depending on both the structural features and surface properties of the substrate and the coating material, might be part of this significant twoway influence on the UV-induced colour changes determined in this research.…”

“…For instance, an increased protective effect of a 2 k polyurethane topcoat (based on hydroxyl-bearing polyacrylate and aliphatic poly-isocyanate) compared to an acrylic topcoat was reported for Radiata pine boards primed with a moisture curing isocyanate [58]. Colour change values ∆E varied in the range of approximately 2-10 units after 700 h light exposure (accelerated aging test simulating natural light passing through window glass) for spruce samples coated with four different three-layer coating systems (primer, middle layer, top layer) with waterborne coatings based on acryl/alkyd or polyurethane resins and their mixture [59].…”

11th Hardwood Conference Proceedings

“…It is important to point out that the topics of light-resistant clear coatings for indoor applications has been addressed less (Kúdela J. et al 2023) but it becomes increasingly important (Silverman C. 2016), as the market request for furniture and other wooden interior design elements with natural aspect is increasing. The influence of wood species (Oltean L. et al 2007), type of the clear coating materials (Aloui F. et al 2007(Aloui F. et al , Özgenç Ö. et al 2020, time and exposure / testing conditions (Kropat M. et al 2020), (Kanbayashi T. et al 2024) on the light induced colour changes of wood surfaces were addressed by worldwide research. Ultraviolet (UV) radiation from the natural light or artificial sources was recognised as the most aggressive ageing factor causing both colour and surface chemistry changes, but short wavelengths components of the visible light, in the violet (403nm) and blue range (434-496) may also contribute to the overall colour changes ), (Živković V. et al 2014).…”