**1. Introduction**

The glossiness of any surface is used to evaluate the quality of a finished product as a result of reflection due to the incident light from different directions [1,2]. High gloss surfaces are in demand in the furniture industry, but matte gloss still has its importance in a specific solid wood furniture market. A reflection structure image of high-gloss composite products was developed as an alternative method to describe the visual human perception of gloss [3]. However, recent studies have shown off the comfortable feelings given by both the coated and uncoated surfaces of wood products [4,5]. A model based on the value generation of wooden furniture has been validated by these qualitative, innovative, and ecological products. These products are used as reliable and environmentally friendly furniture [5,6].

The glossiness and color, apart from the visible wood texture, represent important aesthetic properties that influence the choice of any furniture customer. Several factors including wood species, surface roughness, chemical composition of the varnish, coating system, number of layers, angle of incident light, and the direction of gloss measurement influence the gloss quality of the finished wood product [7–10]. The anisotropic texture of the wood makes the reflection from a surface a complicated process. The variation of different structure patterns can be obtained from the most common lengthwise measurements on radial and tangential directions [11]. The diameters of vessels and tracheids in the earlywood and latewood are not the same because of the annual growth rate.

The roughness of the two areas of an annual ring differs under the same machining conditions and some irregular reflections of the properties appear [11]. The wood grain

**Citation:** Salca, E.-A.; Krystofiak, T.; Lis, B.; Hiziroglu, S. Glossiness Evaluation of Coated Wood Surfaces as Function of Varnish Type and Exposure to Different Conditions. *Coatings* **2021**, *11*, 558. https:// doi.org/10.3390/coatings11050558

Academic Editor: Marko Petric

Received: 19 April 2021 Accepted: 6 May 2021 Published: 9 May 2021

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may raise, twist, and lift during machining, with effects during the finishing step [12,13]. The surface quality influences the glossiness of the wood [8,14]. A certain correlation may exist between the surface gloss and its roughness. Such a correlation is valid when the dominant effect influences the reflection originating from the surface structure of the unit.

Different resin clear coatings could alter the reflection properties depending on the coating thickness [11]. Transparent coatings preserve the natural color of wood and improve its glossiness. However, they reduce stability over time when compared to that of pigmented coatings, which are less affected by the sunlight [15–17]. However, they still maintain their popularity. Different levels of gloss can be achieved as a function of varnish type and its application system [6,9,10]. Alternative ecological products including waterbased and UV varnish are widely used due to their low emissions, rapid application, and good gloss retention. In the case of water-borne coatings, the curing time and penetration are longer because of their lower water absorption [18].

Volatile organic compounds (VOC) regulations are applied to interior coatings as well, in order to reach abrasion and chemical resistance [19]. VOC emissions from indoor materials and finishing products are still current subjects of concern among studies on indoor air quality [20,21]. The acceptable total VOC level in the air for human health ranges from 0.3 to 0.5 mg/m3 concentration [20].

The cost of the UV equipment could be considered high for small companies [10]. In terms of gloss, a UV varnish applied by a roller system produces surfaces with a higher gloss than when applied by spraying. The gloss of water-based, nitrocellulose, and polyurethane varnish has already been studied by several authors [7,22]. Successive layers of coating and polishing could also contribute to a higher gloss. They are applied on furniture parts depending on the surface visibility and exposure during use, and therefore different values of gloss are recorded [10,23]. For beech wood, the gloss level increased by 5−20 gloss units with the increase in varnish layers of the water-based product compared to polyurethane [9].

The direction of viewing or measurement influences the glossiness of a wood surface [11]. The reflection properties of a surface depend on the angle of incident light. There are three standardized measuring angles: 20◦, 60◦, and 85◦. Generally, the 60◦ geometry is recommended for wooden surfaces, but it provides limited information.

Comparative measurements on the same surface using different measuring angles and gloss correlations between the angles may help to better evaluate the wood surface [8]. The scattering of the incident light strongly depends on the direction of gloss measurement relative to the wood grain; parallel gloss is higher than perpendicular gloss [10,11].

Wood exposed to the outdoors suffers due to the photo-degradation of the lignin and, to some extent, the hemicelluloses [24]. Changes in color and gloss, and also cracks that occur on the surface during weathering, limit the wood's utilization [25]. These changes appeared after a few hours of accelerated exposure or after a few days of natural exposure [26,27]. The lignin content in softwoods ranges from 25−35%, and consists mostly of guaiacyl units, while hardwoods have a lignin content of 15−28%, constituted mainly by guaiacyl and syringyl units [28]. It was shown that hardwoods underwent faster degradation than softwoods. The syringyl structure in hardwoods degraded faster than the guaiacyl structures in softwoods [29]. The natural, accelerated, and simulated weathering tests have proved very useful in the wood protection industry [30].

When used indoors, wooden products are subjected to less intensive UV radiation than when used outdoors [23]. Slow degradation of the coating layer by changes in the surface gloss and color, apart from some fine cracks, are noticed [31]. In a previous study, it was proven that the most relevant color changes of coated beech were generated during the first 100 h of artificial aging [23]. Retarding effects on the surface photo-degradation could be obtained with protective agents against UV radiation. Panek et al. [17] showed that the gloss of exposed surfaces decreased with the exposure time to radiation.

The coating resistance to different chemicals has been previously studied for wooden flooring. They are the most exposed elements of an interior, along with some horizontal visible furniture parts [32–36]. To evaluate the chemical resistance of coatings applied to lignocellulosic materials, a rating scale, included in the standards and test procedures, is commonly used [34]. Oils enhance the natural wood appearance, but they produce limited quality in terms of resistance to various chemicals [35]. No major differences in the surface resistance to cold liquids such as coffee, ethanol, red wine, water, or paraffin oil have been observed for oak parquet covered with different coatings [34]. It was found that the resistance to cold liquids depends on the properties of the topcoat used [34].

Currently, there is limited information about the glossiness of varnished alder wood. Therefore, the objective of this study was to evaluate the glossiness of varnished alder wood surfaces when using two varnish types and after their exposure to various test conditions including the dry heat test and artificial aging. The resistance to different cold liquids of the coated surfaces was also tested. The results of this study can provide a better understanding of the properties of the coatings of such wood species, and show its potential for furniture manufacturing.

#### **2. Materials and Methods**

In this study, planed specimens of black alder (*Alnus glutinosa* L.) wood, supplied by a local sawmill in Buzau County, Romania, were used. Black alder wood is a native species in Romania, less-utilized but known mostly for its good workability, properties, and pleasant appearance. The average basic density of the samples was 520 ± 10 kg/m3, and their moisture content was 8 ± 1%. A total of 29 samples were used for the experiment and they were grouped as presented in Table 1. The samples were conditioned for 7 days in a room with a temperature of 20 ± 2 ◦C and relative humidity of 50% ± 5% before any tests were carried out.


**Table 1.** Experimental design.

#### *2.1. Surface Preparation of the Samples*

The specimens (Figure 1) were subjected to parallel sanding by employing a portable sander (FESTOOL ETS 125, FESTOOL GmbH, Wendlingen, Germany) shown in Figure 2 with the technical data displayed in Table 2. Two types of sandpaper with aluminum oxide grains (FESTOOL Rubin 2) of 100 and 150 grit size were used.

**Figure 1.** Wood samples.

**Figure 2.** Festool ETS 125 sander.

**Table 2.** Technical characteristics of the FESTOOL portable sander ETS 125.

