**1. Introduction**

During the external exposure of wood there are some key procedures which should be followed to reduce the effects of weathering. The natural weathering was defined as a process of irreversible changes to the appearance and properties of wood effected by the long-term impact of outdoor factors, such as the solar radiation, air and oxygen contained within it, and changes in the temperature and humidity (no direct influence of biotic factors should be assumed) [1–7]. This complex phenomenon is caused by solar radiation, hydrolysis, and the leaching of wood components [8–10]. The harsh outside environment makes it necessary to consider wood durability, in accordance with EN 350-2 [11]. Next, a proper construction solution is required, such as proper usage conditions. Moreover, surface treatments can be used to prolong the service life of wood [12,13].

Despite the high durability of many tropical wood species against biological factors such as fungi or insects, it is always recommended to protect wood surface during outside exposure due to weathering. On the other hand, one of the most important factors in securing sustainable development is utilization of renewable natural materials, which undoubtedly include wood [14]. In order to reduce the environmental burden, the surfaces of wooden elements are not treated with any painting and varnishing products. In addition to the traditional interior design elements, the use of non-treated wood is expanding even further to external use. According to contemporary trends, it is recommended to use untreated wood and allowing it turn grey upon exposure to weather under aboveground conditions over using non-durable wood with applied surface coatings. Many tropical species are considered to be the most durable wood. However, the use of tropical wood has several negative consequences including illegal deforestation. Hence, impacting significant climate changes. Pronounced through net carbon emissions, deforestation leads to a global warming [15]. However, as wood of tropical species is utilized in Europe, knowledge of their weathering characteristic is important to optimize their performance. Materials should be recognizable in terms of their surface properties. The wettability of wood is one of the most significant parameters influencing the gluing as well as the coating processes [16–19]. One of the most critical factors for extending the durability of painted wood is freshness of the wood surface and only a fresh, high-energy surface guarantees optimum adhesion conditions. The loss of coating ability and glueability due to the increasing age of a wood surface was studied over time by several researchers [20–22], who arrived at the conclusion that the changes caused by the weathering are the effect of the migration of wood extractives to wood surfaces after their preparation, which causes a decrease in wood surfaces' wettability.

It is also convenient to determine the direction and degree of colour changes in individual tropical wood species caused by external factors. The aesthetic function of wooden products used externally can be extended by selecting the most resistant wood species in terms of its surface properties stability. Some knowledge about changes occurring in tropical wood species was gained, but mainly during colour testing wood subjected to different weathering treatments [23,24]. This paper is a part of an extensive study determining the influence of artificial weathering on the surface properties of several species of wood from tropical and subtropical zones. The main aim of the presented research is to compare the influence of selected ageing factors such as ultraviolet radiation and complex artificial weathering methods on the colour, wettability and surface roughness changes in garapa (*Apuleia leiocarpa* (Vogel) J.F. Macbr.), tatajuba (*Bagassa guianensis* Aubl.), courbaril (*Hymenea courbaril* L.) and massaranduba (*Manilkara bidendata* (A. DC.) A. Chev.) wood species, as these are popular for external usage in European countries.

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

The wood species used in this study are presented in Table 1. All test materials were heartwood, because the heartwood of tropical species is more commercially usable than sapwood. All species came from South America (Brazil) and wood was acquired from DLH Poland (Warsaw, Poland). The material was identified using macroscopic techniques and was deciduous diffuse-porous in all cases. Characterization of the tested wood species was supplemented with density determination, performed in accordance with the ASTM D2395 standard [25].

Wood samples were prepared for investigation and analyses by using standardized methods. To avoid differences in the tested properties caused by differences in wood anatomy, identical samples of each wood species were collected from one log. Each one was sawn to produce planks approximately 4 cm thick. The obtained planks were air-dried in a room with relative humidity up to 50% and a temperature of 21 ◦C for approximately 6 months before testing. Then, the defect-free planks were sized into samples for the tests. Forty samples of each wood species were prepared, each with a radial and tangential cross-section of 10 <sup>×</sup> 10 mm<sup>2</sup> and a length of 70 mm. Following Gardner [26] and Liptakova et al. [27], the radial-oriented or tangential-oriented surface of the wood block was planned. The aim was to make wood showing the roughness caused by the cellular structure of wood and only a negligibly small roughness caused by cutting. Moreover, the wood surface is chemically heterogeneous, and therefore does not comply with the requirements of the physicochemical theory of contact angle in a strict sense.


#### **Table 1.** Wood species used in tests.

\* means and standard deviations in parentheses.

#### *2.1. Properties Measurements*

The parameters of the colour of unmodified and modified wood were measured on the basis of the mathematical CIE (International Commission on Illumination known as the Commission Internationale de l'Elcairage) *L\*C\*h* colour space models. The parameter *L\** represents lightness. The parameters *C\** and *h* describe the saturation (colour intensity) and hue angle, respectively. The total colour change Δ*E*\* was determined in accordance with ISO 7724-3 [28]. The 3NH NH300 spectrophotometer made by X-Rite Europe GmbH (Regensdorf, Switzerland) was used to examine the colour parameters. The sensor head was 8 mm in diameter. Measurements were made using a D65 illuminate.

Surface properties can be characterized by the water contact angle (wettability). A single measurement of the water contact angle provides information on several important parameters, such as the surface free energy, contact angle and wetting coefficient or work of adhesion. To predict interactions with wetting materials such as lacquers or adhesives, the surface properties are characterized. The lower the contact angle value (θ), which is a measure of the wetting impact of the substrate by solution, the better the wettability of the material. As soon as the samples were placed in the contact angle measuring apparatus, measurements were started. The contact angle is defined as the angle between the solid surface and the tangent, drawn on the drop-surface, passing through the three-point liquid-solid atmosphere [29]. Using a contact angle analyzer, Haas Phoenix 300 Goniometer (Surface Electro Optics, Suwon City, South Korea), equipped with microscopic lenses, a digital camera connected to a computer with software—image analysis system (Image XP, Surface Electro Optics, version 5.8, Suwon City, South Korea) that provides an image of the drop on the examined wooden surfaces, the contact angles of the expanding droplets (advancing angels) were determined. To test the wettability, a re-distilled water was used as a liquid. All contact angles were measured along the grain direction followed by Gindl et al. [16]. Based on the research of Liptáková et al. [27], the measurements of the contact angle were taken 30 s after each drop of reference liquid.

The roughness was evaluated in accordance with the requirements of ISO 4287 [30]. As part of the conducted research, the arithmetic mean deviation of the assessed profile (*R*a) was measured. The surface roughness was tested using the Surftest SJ-210 Series 178-Portable Surface Roughness Tester (Mitutoyo Corporation, Takatsu-ku, Japan). The parameter *R*<sup>a</sup> was measured in parallel and perpendicular to the grain direction.

Properties were measured on a fresh wood surface after UV irradiation and complex artificial weathering treatment. Before and after the treatments, each of the samples were conditioned in a climatic chamber with a temperature of 21 ◦C and relative humidity of about 50%. Measurements were done for each tested variant 30 times. In case of roughness and wettability testing, measurements were made both on the tangential and radial cross-section.
