*2.2. UV Irradiation*

Four fluorescent 100R's Lightech lamps of 100 W each and the spectrum 300–400 nm (90% of the radiation spectrum is a wavelength of 340–360 nm) were used for irradiating. The time of irradiation was 24 h. The source of radiation applied in this study imitated solar radiation and, in particular, the UVA component of solar radiation. This component causes the greatest changes in the appearance and structure of organic materials exposed in an outdoor environment. This is due to the fact that UVA radiation accounts for 90%–95% of the solar radiation reaching the Earth's surface [31].

#### *2.3. Artificial Weathering*

The artificial weathering method was based on the literature [10,32]. It took 30 h to complete one artificial weathering cycle. The first step of each cycle was soaking samples in water at 20 ◦C for 16 h. The conditions of the second step were 70 ◦C and 5%–10% RH for 8 h, and the third step was performed at 30 ◦C and 20%–25% RH with irradiation with UV rays (24 h). The tested wood species were subjected to one and four cycles of artificial weathering. That weathering method was previously used for tropical wood [33], but obtained results regarding the effect of long-term aging.

#### *2.4. Scanning Electron Microscopy SEM*

The microstructure of samples was examined using a scanning electron microscope HITACHI, model TM-3000 (Hitachi Ltd., Tokyo, Japan) with a digital image record. The samples were chosen randomly and put into the vacuum chamber. The photos at accelerating voltages equal to 5 kV were taken with 1000 magnification and the record was saved using SEM software (TM3000, Hitachi Ltd., Tokyo, Japan). The analysis was conducted at least in three repetitions for each sample.

#### *2.5. FTIR*

The spectra were recorded using an Agilent Cary 630 FTIR spectrometer (Agilent Technologies Inc., Santa Clara, CA, USA) equipped with a single bounce diamond ATR unit. Scans were performed in the range of 650–4000 cm−<sup>1</sup> with a resolution of 4 cm<sup>−</sup>1, and 64 scans for each sample. Background correction with 32 scans was performed after each measurement. The measurements were conducted in three repetitions.

#### *2.6. Statistical Analysis*

Statistical analysis of the test results was carried out using Statistica v. 10 software (TIBCO Software Inc., Palo Alto, CA, USA). Data were analysed and provided as the mean ± standard deviation, the scatter plot of results around the median, and minimum and maximum values. To compare and determine the significance of difference between data, *t*-test was used. Analysis of variance (ANOVA) was used to determine the influence of UV irradiation, full artificial weathering process and anatomical properties, such as the direction of measurements and cross-section (tangential or radial).

#### **3. Results and Discussion**

#### *3.1. Colour Changes*

The colour of wood exposed to external conditions can rapidly change in quite a short time [23]. Due to the photodegradation (photooxidation) of lignin and extractives, wood colour usually turns yellowish or brown. During UV radiation, the surface colour changed visibly. The deviation Δ*E*\* in all the wood species exceeds the value of 3 (Table 2), which is considered the limit for visibility to the naked eye [34]. The Figure 1 shows the clear contrast between wood surfaces when they were fresh and after different treatments. Distinctive differences in colour can be seen. In general, wood surfaces become darker and turn to shades of brown. The previous studies conducted on softwoods [35], and tropical wood species as well [36], confirmed that the most rapid changes in Δ*E*\* occurred during

the first stages of UV radiation. The tested wood species are rich in extractives [37]. The rapid change in the first hours of exposure could have been caused by the reaction of extractives contained within the wood to UV radiation. Pandey [38] compared the behaviour of unextracted and extractive free wood of *Acacia auriculaeformis*. The unextracted wood surface showed a rapid colour change at the initial period of exposure, which decreased upon prolonged exposure. Tests on wood (coniferous, deciduous) from moderate climates during weathering in natural external conditions did not show a wide range of changes, which can be explained by the relatively low content of extractives. The maximum value of Δ*E*\* was for spruce—34.1 after 12 months of weathering. In the study here and in the case of courbaril wood, Δ*E*\* = 51.21 was caused only after 24 h of UV radiation. Soaking wood samples in water meant that the wood dyes contained within were washed out, and accumulated on the top layers of wood, which caused wood darkening (lower values of *L*\*)—Table 3. The full artificial weathering process gave higher colour changes and also a response to changes in the appearance of wood surfaces in natural external conditions. In all cases of the tested wood species, wood became darker both after UV irradiation and after full weathering treatment. Conducting a higher number of artificial weathering cycles caused higher changes.


**Table 2.** Overall colour change (Δ*E*\*) during exposure.



In the case of wood species of a lighter colour—garapa and tatajuba—*C*\* and *h* values followed a similar trend: they increased after UV irradiation and decreased during the full artificial weathering process. In the case of courbaril and massaranduba wood (both of darker colour), *C*\* value was lower after each treatment. The different behaviours of individual wood species regarding chromatic parameters probably could have been caused by the presence of specific types of extractives in the wood. While studying the influence of extractives on discoloration, Pandey [38] found that this relation is, to a great extent, dependent on the nature and chemical composition. Both Δ*C*\* and Δ*h* are determined mainly by the changes in the chromophore groups in extractives and change through the lignin degradation and later leaching. Thus, it can be assumed that a higher proportion of extractives within one wood species can cause a darker colour of the wood. The test results of tropical wood discoloration caused by simulated sunlight [23] confirmed that lighter samples manifest a larger colour change due to sunlight. In the research here, the results were different. The scope of change was similar for both garapa, tatajuba of light colour and massaranduba of dark colour. Massive colour changes (total colour change Δ*E***\****)* showed courbaril wood, known for its high extractives content [37].

According to the above, the conclusion can be frawn that wood, as a complex structure, differing between species, requires a unique approach for each individual species. Especially in the case of tropical wood, which, despite many reports, remains little known. With a large number of different wood types, the formulation of dependencies is only indicative. Due to the huge variability of properties and structure details [39], deep knowledge is necessary, especially in the context of proper wood application.

**Figure 1.** Visual comparison of contrast among the exposed and non-exposed surface (from left: garapa, tatajuba, courbaril and massaranduba; from top: fresh wood, after 24 h of UV irradiation, after 1 cycle of artificial weathering, after 4 cycles of artificial weathering).

#### *3.2. Roughness Changes*

The results of the surface roughness *R*<sup>a</sup> of tested wood species are shown on Figures 2 and 3. Tests were performed on both the tangential and radial wood section, parallel and perpendicular to the grain. The wood roughness is a complex phenomenon because wood is an anisotropic and heterogeneous material, and several factors such as anatomical differences and the machining properties should be considered in evaluating the surface roughness of wood [40]. The roughness of the tested wood species demonstrated variation, depending on the wood section and the measurement direction (parallel or perpendicular to the grain). As can be seen from Figures 2 and 3, mostly radial sections showed higher roughness. All tested wood species are characterized with interlocked fibres, which causes variable fibre orientation. As a result, they are cut in various ways on the radial section of the wood. In general, *R*a<sup>⊥</sup> values (measured perpendicular to the wood fibres) were twice as high as those measured parallel to the grain (*R*aII). Wood, as a non-homogeneous material, shows differences in its properties depending on the direction. The roughness perpendicular to the fibres is mostly caused by irregularities in the structural element sizes, such as the vessels' diameter.

**Figure 2.** Roughness (*R*aII) along the fibres of tested wood species: "O"—fresh wood, "UV"—after 24 h of UV irradiation, "1 c"—after one cycle of artificial weathering, "4 c"—after four cycles of artificial weathering.

**Figure 3.** Roughness (*R*a⊥) perpendicular to the fibres of tested wood species: "O"—fresh wood, "UV"—after 24 h of UV irradiation, "1 c"—after one cycle of artificial weathering, "4 c"—after four cycles of artificial weathering.

According to the ANOVA results (at the 0.05 confidence level), the surface roughness varied significantly depending on the examined factor (species, section, used methods of weathering) (Table 4). However, the most important influencing factors were wood section and kind of treatments used (45% and 46%, respectively).


**Table 4.** Statistical evaluation of the factors influencing wood surface roughness.

\*—significant at the 0.05 level.

Generally, the surface roughness values increased due to UV irradiation when compared with the surface roughness values of the control wood samples. The differences were statistically significant at the 0.05 confidence level in most of the cases. UV treatment initiates surface oxidation (increase in the acid/base or polar component), which leads to the introduction of functional (carboxyl) groups [15]. *R*<sup>a</sup> values increased after UV irradiation at a similar level as after one cycle of artificial weathering and significantly higher after a further full weathering treatment. Exposure to UV light and water caused fewer cracks. Soaking wood in water and then drying caused the raising of wood fibres. Initial roughness was not decisive in the scope of changes.

### *3.3. Wettability Changes*

The effects of UV irradiation on the changes in the chemistry of wood surfaces were verified using contact angles with distilled water. Results are given in Table 5. The wettability decreased (contact angle increased) for all investigated surfaces with treatment by UV radiation and the full artificial weathering process. The contact angle (θ) measurements with distilled water showed the same trend for the radial and tangential surfaces of all tested wood species. In contrast to changes in surface roughness, changes in wettability caused by exposure to UV light were not statistically significant. In general, for most of all tested wood species, UV irradiation for 24 h caused an increase in surface contact angle of 8%–15% (decrease in wettability). The inclusion of water in the weathering process resulted in a much wider range of changes. Currently, one full weathering cycle caused a change in the wettability of the surface of up to 51% (for massaranduba wood). Further ageing resulted in minor changes. Carrying out four weathering cycles resulted in slightly larger wettability changes. This can be explained by the fact that leaching of extractives with water and its accumulation on the wood surface effect decreased the degree of surface hydrophilicity [40,41]. Thus, the decrease in wetting of a wood surface is related to the chemical changes after outdoor exposure. Leaching of the extractives from the surface of weathered wood reduces water repellence, while the degradation of lignin results in a more hydrophilic surface [40]. Garapa wood has shown a different nature of change. UV exposure caused a slight decrease in the contact angle (increase in wettability). This reverse direction of change can be explained by the fact that garapa wood has the lowest density in the studied group. Thus, UV penetration of structure was more possible. The degradation of hydrophobic lignin and allowing cellulose to become more abundant on the wood surface increased the degree of surface hydrophilicity [41].


**Table 5.** Contact angles (θ) tested wood species before and after used weathering treatments (means and standard deviations in parentheses).

UV light can destruct pits, which enable coatings and adhesives to penetrate deeper into the wood surface, and therefore enhance mechanical anchoring. Each of the chemical components of wood (i.e., lignin, cellulose, hemi-cellulose or extractives) is sensitive to UV radiation with a consequential deterioration effect. Of these chemical constituents, lignin, because of its strong ultraviolet absorbing characteristic due to the phenolic nature of its molecular architecture, appears to be oxidized and degraded very rapidly by UV light [42]. This information is important in the context that, in the wood working industry, UV irradiation is usually used for the curing of coating systems and a UV source mostly exists in the production line, so it seems feasible to integrate UV-modification into the finishing process. Contrary to other activation methods, UV treatment is suitable for an online manufacturing process [16].

According to ANOVA results (at the 0.05 confidence level), the surface wettability varied significantly depending on the used methods of weathering (Table 6). It was the most important influencing factor (76%). Wood section and wood species did not have a significant influence.


**Table 6.** Statistical evaluation of the factors influencing wood surface wettability.

\*—significant at the 0.05 level, NS—not significant.
