*2.6. Statistical Analysis*

Data were analysed using MS Excel (Microsoft, Redmond, WA, USA) and STATISTICA 13.2 (StatSoft, Palo Alto, CA, USA) using mean values, bar graphs and ANOVA for analysing the statistical significance of selected factors with significance level α = 0.05. Spearman rank correlation between NW after 12 months and AW after 6 weeks on the basis of ΔE\*, ΔL\*, ΔG\* and ΔW\* values of tested coatings was also analysed. The Spearman rank correlation coefficient was calculated by Equation (2):

$$Rho\_{\mathbb{S}} = 1 - \left[ \frac{6 \cdot \sum (\text{Rank Difference})^2}{n^3 - n} \right] \tag{2}$$

where *n* is number of items evaluated.

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

Results on coated samples exposed to artificial and natural weathering showed different behaviours. The type of coating system applied on oak wood samples has a statistically significant effect (*p* < 0.05) on the evaluated properties both during AW and NW (Table 3).


**Table 3.** Statistical evaluation of significance of selected factors.

 signifies *p* < 0.05 (statistically significant at significance level 0.05).

#### *3.1. Colour Change of the Samples*

\*

Total colour di fference ΔE\* calculated from measured colour parameters was the main indicator representing coating durability during weathering [18,22,30]. The specific colour parameters (L\*, a\*, b\*) describe the colour change more closely. During AW, the observed increase of values a\* and decrease of b\* values showed a tendency of the wood surface to turn reddish and become less yellow shade. Decreases of both a\* and b\* parameters were observed during NW, which indicates the opposite trend. But in most cases, ΔE\* was a ffected mainly by changes in lightness ( ΔL\*) as in the study of Oberhofnerová et al. [31]. Changes in lightness of di fferent coating during weathering is illustrated in Figure 1. There are obvious di fferences in lightness parameters based on the weathering type-decrease (negative value) of lightness during AW (except reference samples) indicating a tendency to turn into darker and increase during NW indicating lightening. This trend is caused by the di fferent weathering conditions and ratio of degradation and leaching of photodegraded extractives and lignins observed mainly on the transparent tested coatings. NW di ffering also in the presence of mould and dust and other pollution in exterior which infiltrate in the degraded surface of wood or coating [10]. These conditions are not simulated in laboratory testing [21,43]. But darkening of the surfaces caused by the action of pollutants and moulds was negligible during this period of NW and mainly leaching of darker oak extractives and changes in pigmented coatings caused increasing of L\* parameter.

Total colour di fference ( ΔE\*) of tested coatings, was closely linked to lightness changes. Control samples manifested the highest colour di fferences during both types of weathering. Total colour di fference values are characterized by a systematic increase during exposure [43,44], with higher changes during initial phases of weathering [28,43]. In this study, only coatings AC2 and AL1 followed this trend during AW (Figure 2). The lowest colour di fference was noticed for O1 during AW and O2 during NW, which is in accordance with lightness changes in Figure 1. Those were the only coatings able to protect the wood to the extent of ΔE\* < 3, which is considered as a low colour di fference that cannot be distinguished by a subjective observer [45]. These oil coatings di ffered from each other only by the type of pigments used (Table 1). The pigmented coatings (O1, O2, AL2, S1) were characterized by significantly lower colour changes than transparent ones.

**Figure 1.** *Cont.*

**Figure 1.** Lightness difference (ΔL\*) of tested coating systems during artificial and natural weathering (CS means control sample).

**Figure 2.** Total colour difference (ΔE\*) of tested coating systems during artificial and natural weathering (CS means control sample).

#### *3.2. Gloss Change of the Samples*

Except for small fluctuations, all coatings were characterized by decreased gloss values during both weathering methods (Figure 3). The reference samples, on the other hand, manifested increases in this property (with the decrease in the final phase of NW). The same findings regarding protected and unprotected weathered wood were found by Ghosh et al. [46]. The best results were noted for acrylate and AL1 coatings, while the highest changes were recorded for oil coatings.

**Figure 3.** Gloss difference (ΔG\*) of tested coating systems during artificial and natural weathering (CS means control sample).

#### *3.3. Surface Wettability of the Samples*

The contact angle, which indicates the wettability by water on the exposed surfaces of coated wood, is an important indicator of the rate of weathering [22,24]. Surface wetting changes (Figure 4) indicated the overall impairment of the protective function of the coating systems against water [24,39]. During AW, the most stable values were noted for acrylate coating systems and O3. The rest of the samples were characterized by decreased surface wettability (the most in the case of reference samples and AL2). During NW, the wettability decreased for all samples, but the smallest changes were also recorded for acrylate and O3 coating.

**Figure 4.** Surface wettability difference (ΔW\*) of the tested coating systems during artificial and natural weathering (CS means control sample).

The Spearman rank correlation of properties after 12 months of NW in comparison with 6 weeks of AW measured both the strength and direction of the relationship between the ranks of data (Table 4).


**Table 4.** Spearman rank correlation between AW and NW.

Note: \* means statically significant at 95% level (*p* < 0.05); R = 1 is a perfect positive correlation; R = −1 is a perfect negative correlation; R = 0 is no correlation.

Based on the results of Spearman rank correlation, strong statistically significant relationships between results from AW and NW were only found with gloss changes of all coatings and surface wettability changes and gloss changes of transparent coatings (*p* < 0.05). Further evaluation revealed the remaining results from AW and NW, including total colour difference were statistically insignificant (*p* > 0.05) and very poorly correlated with each other.

#### *3.4. Macroscopic and Microscopic Evaluation of the Samples*

The coating performance of the samples during NW was also evaluated visually in accordance with other studies [18,20]. The visual evaluation confirmed that weathering causes colour changes and surface degradation both in natural and laboratory conditions [23,24]. Visual inspection (Figure 5 for AW and Figure 6 for NW) confirmed the exact previously measured values–darkening of coated samples during AW and lightening of samples during NW (Figure 1) and associated total colour (ΔE\*) and gloss changes (Figures 2 and 3).

**Figure 5.** Visual evaluation of tested coatings before and after 6 weeks of AW exposure.

**Figure 6.** Visual evaluation of tested coatings before and after 12 months of NW exposure.

Confocal laser scanning microscopy was employed to assess degradation of selected coatings. Figure 7 illustrates the degradation of oil coating (O1) and acrylate coating (AC1) after 6 weeks of AW or 12 months of NW. Lower colour changes were noted for oil coatings (Figure 2), but also a more obvious disruption and degradation of the coating surface (Figure 7) which are more connected with the higher gloss and surface wettability changes. In the line with this, the acrylate coatings were

characterized by lower gloss and surface wettability changes associated with the lower degree of coating degradation.

**Figure 7.** Confocal microscopy of oil (O1) and acrylate (AC1) coating after 6 weeks of AW and 12 months of NW.

3D images of samples roughness profile for selected tested coatings with higher gloss change (AL2 and O3) are shown in Figure 8. It is possible to see increasing of roughness of the surfaces after 6 weeks of accelerated weathering. These images (Figure 8) also confirm that decreasing of gloss is influenced mainly by decomposition of coating film (O3) or its top layer (AL2) (see also Figure 5).

Native oak wood has higher natural durability and lower colour changes in comparison with other hardwood species during weathering [47], but to find durable coatings systems suitable for its specific chemical and morphological structure is desirable. The findings of this study confirmed the effect of polymer base on the overall performance of coatings [12,48]. It is clear that effect of surface protection against weathering was demonstrated by the difference between uncoated reference and coated samples (Figures 1–6). Generally, the oil coatings (O1–O3) performed well in the colour analyses, the acrylate coatings (AC1–AC2) reached the best results in the gloss and wettability evaluation. These properties are more likely connected with coating degradation and disruption than with chemical changes in wood [34,39]. Acrylate and oil coatings reached the best performance on larch wood also in study of Šim ˚unková et al. [15]. In opposite, in the study of Sivrikaya et al. [48], the better performance against atmospheric conditions on oak wood was recorded for alkyd coatings compared with other tested coatings.

**Figure 8.** 3D images of surfaces before (left) and after 6 weeks of artificial accelerated weathering (right). Figures were created using Confocal laser scanning microscope with 108-fold magnification. Size of analysed area is 2560 μm × 2560 μm.

The total colour changes (ΔE\*) were the most connected with the change of lightness parameter ΔL\* (Figures 1 and 2) as in the other studies [43,49]. The lowest colour changes were observed for oil coatings O1 and O2 (thin-layer oil-based with micronized pigments TiO2 and Fe2O3). The positive effect of white TiO2 pigments on photodegradation was already discussed in the work of Moya et al. [30]. The thickness of the coating system is a criterion affecting its service life [23] but for tested coatings in this work, the pigment content was the more important factor (Table 1, Figures 5 and 6). The pigmented coatings generally provided more effective protection and reached the lower colour changes during NW than transparent ones as in the study of Sivrikaya et al. [48].

In the gloss and surface wettability evaluation of the samples, the performance of coating differed in comparison with colour analysis. All the tested samples except control samples were noted for loss of gloss during NW and AW. The best results were observed for acrylate AC1-AC2 and AL1 coatings. Although the role of gloss change is still discussed - according to Pánek et al. [39], the gloss change is more sensitive to the coating degradation than to total colour difference (Figure 8). Merlatti et al. [34] stated that loss of gloss should not be systematically correlated to the advance in chemical degradation during weathering. The lowest change of surface wettability was recorded for acrylate coating systems AC1-AC2 and oil coating O3 both after AW and NW. The rest of the samples were characterized by decrease of surface wettability, this was the most significant in the case of control and AL2 samples.

The comparison of both weathering methods only by evaluating colour difference would be insufficient, as stated in other previously done studies [23,28,30]. The combination of different testing parameters of coating systems and visual evaluation gives a better idea about the durability of coatings [49,50], despite that the total colour difference still remains the main indicator of coating degradation. Coatings defoliated after AW were in most cases highly degraded after 12 months of NW (Figure 5 versus Figure 6). Based on this, AW can be recommended as the first step for selection of nondurable coatings mainly on woods with specific chemical or morphological structure. The non-linear correlations were performed to compare the strength of the relationship between the total colour differences, gloss and surface after NW and AW of transparent and pigmented coatings as in the study of Pánek and Reinprecht [51]. The results were highly varied, and, in the most cases, without any statistical significance. Comparisons of colour changes mainly showed weak correlation for tested

oak, as for black locust and spruce wood in the work Pánek and Reinprecht [52]. The strong correlation was found only for the gloss changes of all coatings, in agreemen<sup>t</sup> with the work Q-Lab [53], and surface wettability and gloss change of transparent coatings separately. Both methods revealed the certain durability among the tested coating systems and come to the greater agreemen<sup>t</sup> than in the case of unprotected wood weathering. These inconsistent results confirm the previously stated di fficulty to mathematically correlate data from outdoor and laboratory conditions [22,30]. The significant impact of climatic and local environmental conditions at the testing site is still one of the dominant factors preventing the accurate prediction of real weathering in the exterior via artificial accelerated weathering [36,54]. Even there is an e ffort to simulate outdoor conditions in UV chamber by setting the parameters of weathering, the more accurate correlation for prediction changes of coated wood via artificial weathering in laboratory has proven to be di fficult. Despite the obvious advantages of artificial weathering, the results provided by this method still lack reliability of natural weathering and they should always be carefully interpreted and in the best scenario accompanied by natural weathering tests to verify the performance of coatings in an end-use environment [11].
