Comparison of Exterior Coatings Applied to Oak Wood as a Function of Natural and Artificial Weathering Exposure

Artificial weathering can significantly reduce the testing time needed for proving coating durability, nevertheless its reliability is still not thoroughly proven. In this study, eight different transparent and pigmented coating systems, namely oil, acrylate, alkyd and urethane alkyd were evaluated through natural and artificial weathering tests on oak samples by measuring colour, gloss and surface wettability and by macroscopic and microscopic evaluation. The oil coatings performed well in wood colour stability evaluations, while the best gloss and wettability change results were noted for acrylate coatings. Pigmented coatings were characterized by significantly lower colour changes than transparent ones. The gloss and wettability changes were more sensitive to coating disruption than to total colour changes of coated wood associated with chemical changes in wood. The findings in this work showed that values of gloss changes and surface wettability for all types of coatings exposed to artificial and natural weathering resulted in significant differences from each other. The data obtained by artificial weathering method provide basic results of coatings durability and, ideally, natural weathering should be performed at the same time to support the results from laboratory tests by exposing wood under real conditions.


Introduction
Oak (Quercus petraea L.) wood is often used for the exterior applications, mostly in construction of bridges, pergolas, balconies or garden furniture, where higher natural durability [1] is required. Oak contains a relatively high amount of phenol extractives, mainly vescalagin, castalagin, gallic and ellagic acids [2], creating problems in the field of surface treatment durability [3,4]. Tannins in oak wood also retard coating hardening [5]. The complex open vessel morphological structure of oak wood complicates the overall application of coatings. A photodegradation process of oak wood accompanied with significant discolouration and leaching of extractives from the surface takes place during the initial phases of outdoor exposure [6], more intensely in the heartwood zone [7], which leads to the need to protect oak wood by coatings to maintain its natural appearance.
Exterior wood coatings are used to improve the properties of substrate wood [8], reduce the effects of degradation factors [9][10][11] and prolong the service life of the material. The exterior coating generally protects against moisture uptake and related dimensional changes, protects against photochemical

Sample Preparation, Coatings and Weathering Process
Samples of oak (Quercus petraea L.) wood harvested in the Czech Republic having an average oven dry density of ρ 0 = 705 kg/m 3 [37] were used for the experiment. The samples were conditioned to 12 ± 2% moisture content. Test samples were prepared from the heartwood zone and they were visually sorted in order to minimalize the colour variability of the tested wood materials. The dimensions of the samples were 375 mm × 78 mm × 20 mm for natural weathering and 45 mm × 45 mm × 8 mm for the artificial weathering tests. Tangential surfaces were exposed to weathering in both cases. Two samples for NW and four samples for AW for each type of treatment were used.
Eight different transparent or pigmented coatings were applied to the samples based on the producer's recommendation. Their specification and application details are given in Table 1. One group of samples was left untreated as control samples to compare coatings performance on treated samples. The cross ends of samples were sealed with silicon to minimize additional water uptake. Acrylate thick-layer water-based stain with fungicides (5-chloro-2-methylisothiazol-3(2H)-one) Acrylate T 2 (40 µm)

O2
Thin-layer oil-based with dark micronized pigments (Fe 2 O 3 ) and fungicides (propiconazole < 1%) Oil-based coating with fungicides (propiconazole 0.5%) Oil T 3 (10 µm) S1 Thin-layer solvent-based stain-urethane alkyds with additives in white spirit with IPBC Urethane alkyd P 2 (25 µm) The natural weathering (NW) test was performed at Suchdol, Prague (50 • 07 49.68" N, 14 • 22 13.87" E) for 12 months. The climatic conditions during exposure are given in Table 2. The samples were exposed at a 45 • inclination, facing south, and placed approximately 1 m above the ground according to the procedure previously described [26]. The artificial weathering (AW) test was performed in UV-chamber QUV (Q-Lab, Cleveland, OH, USA) according to a modified EN 927-6 method [27]. The total time consisted of six cycles (1008 h) of weathering in the UV chamber and 36 h of temperature cycling. During the each weekly cycle of irradiation and spraying, the samples were transferred to a Discovery My DM340 conditioning chamber (ACS, Massa Martana, Italy) and exposed to three cycles each lasting 2 h of temperature changes from −25 to +80 • C (with 25% RH). The alternation of UV radiation, spray, and low temperature cycles, leading to a better imitation of the exterior conditions in Central and Northern Europe, was previously used by Van den Bulcke et al. and Pánek et al. [22,39].

Colour Change (∆E*) Test
Colour variations of the specimens were evaluated through natural and artificial weathering exposure of oak samples with 8 different coatings. The colour parameters L*a*b* [40] of the test Coatings 2019, 9, 864 4 of 15 specimens were measured after 0, 6 and 12 months of NW and after 0, 1, 3 and 6 weeks of AW using CM-600d Spectrophotometer (Konica Minolta, Osaka, Japan). For the observation of reflection, the specular component was included (SCI mode) at a 10 • angle and d/8 geometry with an illumination standard of D65 (corresponding to daylight in 6500 K). Six measurements of each tested sample were carried out for each weathering time. Colour changes evaluations were done in CIE L*a*b* colour space where L* is lightness from 0 (black) to 100 (white); a* is chromaticity coordinate + (red) or − (green); b* is chromaticity coordinate + (yellow) or − (blue).

Gloss Change (∆G*) Test
The gloss of the different coatings before and during weathering tests was measured using MG268-F2 glossmeter (KSJ, Quanzhou, China) on the basis of [41]. Six measurements at a 60 • angle per sample after 0, 6 and 12 months of NW and after 0, 1, 3, and 6 weeks of AW were carried out to evaluate gloss changes of the samples.

Surface Wettability Change (∆W*) Test
The sessile drop method with static contact angle measurement was performed using a Krüss DSA 30E goniometer (Krüss, Hamburg, Germany) with the methodology used in previous studies [42,43]. Twenty measurements were taken for each sample, with distilled water drops with a dosing volume of 5 µL. The contact angle values were determined after 5 s of drop deposition on surface of the sample before weathering and after 0, 6 and 12 months of NW and after 0, 1, 3 and 6 weeks of AW.

Macroscopic and Microscopic Evaluation
Tested surfaces of the samples were regularly macroscopically evaluated using a Canon 2520 MFP scanner with 300 DPI resolution (Canon, Tokyo, Japan). Creations of cracks, defoliation of coating systems were visually analysed. Microscopic structural changes of coatings and surface of the samples, creation of ruptures, 3D-images of surface profiles were also studied employing confocal laser scanning microscope Lext Ols 4100 (Olympus, Tokyo, Japan) with 108-fold magnification.

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 ∆E*, ∆L*, ∆G* and ∆W* values of tested coatings was also analysed. The Spearman rank correlation coefficient was calculated by Equation (2): where n is number of items evaluated.

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).

Colour Change of the Samples
Total colour difference ∆E* 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, ∆E* was affected mainly by changes in lightness (∆L*) as in the study of Oberhofnerová et al. [31]. Changes in lightness of different coating during weathering is illustrated in Figure 1. There are obvious differences 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 different weathering conditions and ratio of degradation and leaching of photodegraded extractives and lignins observed mainly on the transparent tested coatings. NW differing 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 difference (∆E*) of tested coatings, was closely linked to lightness changes. Control samples manifested the highest colour differences during both types of weathering. Total colour difference 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 difference 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 ∆E* < 3, which is considered as a low colour difference that cannot be distinguished by a subjective observer [45]. These oil coatings differed 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.

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).

Colour Change of the Samples
Total colour difference 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 affected mainly by changes in lightness (ΔL*) as in the study of Oberhofnerová et al. [31]. Changes in lightness of different coating during weathering is illustrated in Figure 1. There are obvious differences 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 different weathering conditions and ratio of degradation and leaching of photodegraded extractives and lignins observed mainly on the transparent tested coatings. NW differing 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 difference (ΔE*) of tested coatings, was closely linked to lightness changes. Control samples manifested the highest colour differences during both types of weathering. Total colour difference 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 difference 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 ∆E* < 3, which is considered as a low colour difference that cannot be distinguished by a subjective observer [45]. These oil coatings differed 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.   Total colour difference (ΔE*) of tested coatings, was closely linked to lightness changes. Control samples manifested the highest colour differences during both types of weathering. Total colour difference 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 difference 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 ∆E* < 3, which is considered as a low colour difference that cannot be distinguished by a subjective observer [45]. These oil coatings differed 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.

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

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.

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.

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.

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. 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.  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). 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 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). 0.53 0.14 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.

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 (∆E*) and gloss changes (Figures 2 and 3).  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.  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. 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.  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 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 Šimůnková 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.   The total colour changes (∆E*) were the most connected with the change of lightness parameter ∆L* (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 TiO 2 and Fe 2 O 3 ). The positive effect of white TiO 2 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 agreement 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 agreement than in the case of unprotected wood weathering. These inconsistent results confirm the previously stated difficulty 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 effort 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 difficult. 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].

Conclusions
Eight different transparent and pigmented coating systems were applied on oak samples and tested using artificial and natural weathering. Total colour difference was mostly related to the lightness parameter change. Evaluation of discolouration with other criteria such as gloss, surface wettability and visual and microscopic evaluation more accurate predicted service life of coatings. Pigmented coatings had significantly lower colour changes than transparent ones, for both artificial and natural weathering. Oil coatings were more colour stable, acrylate coatings achieved the best results of gloss and wettability changes. The gloss and surface wettability changes better copy degradation and disruption of coated wood in comparison with total colour changes. Even there were some visually observed similarities in the test results of AW and NW exposed samples, this was not confirmed statistically. The Spearman rank correlation showed strong statistically significant relationship between results after artificial and natural weathering only for the gloss changes of all coatings and surface wettability and gloss changes of transparent coatings separately.