*3.4. Structure Changes*

Microstructure changes in the wood surface properties were characterized using SEM. Most of the previous conducted studies regarded softwoods. Figure 4 presents images of wood surface before weathering, after UV irradiation and after full ageing treatment. The untreated samples indicted cell walls and some easy to recognize damages caused by splitting after mechanical preparation.

As a result of weathering, some characteristic anatomical changes occurred. Both UV irradiation and full artificial weathering treatment caused the apparent erosion of wood structure elements. This also agreed with previous studies of wood surface deterioration after exposure to sunlight or UV irradiation [43]. A number of distinctive cracks formed in the specimens after exposure to UV light. Distortion and creasing of the cells wall occurred in the longitudinal direction and resulted in the delamination of the cell walls—degradation of S1 layer [44]. The deterioration of connections between cell elements were associated with the degradation of the middle lamella. At the same time, it confirmed the degradation of lignin. The SEM micrographs revealed much more degradation in wood specimens subjected to the full weathering process. Heavy damage to the pits within vessels was observed. Most of the cracks on the pits formed transverse to the cell axis, which resulted from microfibril orientation in the S3 layer of cell walls, produced from condensing compounds of degraded lignin and hemicelluloses. In all analysed specimens, thinning of parenchyma cell walls and their shrinkage or total degradation were observed. As seen in Figure 4, deterioration was present in parenchyma cells of wood rays. Similar observations were made by Mamo ˇnová and Reinprecht [44], who tested tropical wood species weathered in natural conditions. They stated that the highest incidence of micro-cracks after weathering is correlated with wood density.

The changes caused by UV irradiation were much smaller than those caused by full artificial weathering. Thus, this confirmed the influence of desorption tensions in structure changes that caused disruptions to wooden tissue subjected to cyclic humidity changes.

**Figure 4.** Comparison of microstructure exposed and non-exposed wood surface (from top: garapa, tatajuba, courbaril and massaranduba; from left: fresh wood, after 24 h of UV irradiation, after 4 cycles of artificial weathering); the described changes are marked with yellow arrows.

### *3.5. FTIR Analysis*

The FTIR spectra of woods before and after UV irradiation and full weathering (four cycles) are shown in Figure 5. The main changes were found at 2900, 1730, 1650, 1590, 1512, 1420, 1317, 1260, 1163, and 1030 cm−1. The garapa wood showed the greatest scope of changes (the highest differences in absorbance) and the smallest differences in absorbance showed massaranduba. The wood species in the studied group are characterized by the lowest and highest density. In the case of garapa and tatajuba wood, in general, the relative intensity of the bands increased more after UV treatment than after full artificial weathering. This can be explained by the fact that during the full weathering process, water-extractable compounds leached out and their accumulation on wood surfaces caused a negligible effect of weathering factors on wood. Nevertheless, the changes in the chemical structure of extractives during weathering cannot be determined by infrared spectroscopy, due to the significantly lower amount of lignin and polysaccharides [45–47]. According to the current knowledge [45–47], in the spectral range 1740–1720 cm−1, various overlapping stretching vibrations of bond C=O in carbonyl and carboxyl compounds can occur. The results showed that carbonyl groups, determined at 1730 and 1650 cm<sup>−</sup>1, increased more as an effect of full weathering process than after UV irradiation.

**Figure 5.** FTIR spectra of tested wood species before, after UV irradiation and after four full artificial weathering cycles: **a**—fresh wood, **b**—after UV irradiation, **c**—after four artificial weathering cycles.

The absorption near 1735 cm−<sup>1</sup> can be attributed to vibrations in bond C=O in xylan (hemicellulose), as well as in carboxylic acids (lignin oxidation products). The occurrence of oxidation products was confirmed by the changes in intensity band, reaching 1650 cm−1. These results are in accordance with the studies of other researchers, involving one or more different species and extended for longer periods of ageing [33,38,45,46]. This absorption was due to C=O carbonyl stretching in aromatic compounds, which can suggest the carbonyl compounds' formation, and the highest changes were observed in the case of garapa wood. The relative intensity of band at 1512 cm−<sup>1</sup> decreased after weathering process. That peak is due to the skeletal stretching vibration of C=C in the aromatic ring of lignin [46]. The absorption at 2900 cm−<sup>1</sup> was characteristic of alkane CH vibrations of methylene in cellulose, 1417 cm−1—CH2 bending crystallized, and amorphous cellulose and 1317 cm−1—CH2 wagging in crystallized cellulose [44,45]. Only in the case of massaranduba wood irradiated with UV did these peaks remain almost unchanged, which could be due to the high wood density and negligible effect of weathering factors on wood or their blocking with extractives. The peak at 1030 cm−<sup>1</sup> for the C–O stretching was linked to cellulose and wood extractives [45]. The notable decrease in the relative intensity of bands at 1260 and 1230 cm−<sup>1</sup> was observed after weathering. The absorbance in the range 1290–1200 cm−<sup>1</sup> is characteristic for stretching C–O vibrations in lignin (guaiacyl), hemicellulose (xylans) and the conjugation of the C–O of extractives [43]. After a full weathering process, in the case of garapa and tatajuba wood, a peak at 1260 cm−<sup>1</sup> was found, which might be associated with the

conjugation of the C–O of extractives. Tropical woods have a relatively high number of extractives in comparison to European wood species [37]. Wood, as a complex structure, is a material for which it is difficult to formulate general conclusions. Followed by Reinprecht et al. [46], differences observed between tested wood species and changes observed on its surfaces after UV irradiation and full weathering could be explained by chemical changes taking place individually for each of them during photo-oxidation processes in the lignin-polysaccharide system, as well as extractives contained in wood. Differences between results obtained after UV treatment and full artificial weathering process might be associated with the migration of extractives such as UV-protectable extractives [45] (water-soluble extractives accumulated on the wood surfaces after full weathering process) and with the fact that extractives photooxidation products can increase the delay photodegradation of lignin [48].

#### **4. Conclusions**

Based on the results of the research into the influence of selected ageing factors such as UV irradiation and complex artificial weathering methods on the colour stability, wettability and roughness changes in garapa, tatajuba, courbaril and massaranduba, it can be concluded that:


Thus, the effect of changes caused by aging factors is the result of wood structure and its density, but also chemical composition. The results show that the observed changes may affect the long-term durability of finishes applied over wood subjected to weathering factors for a short period before finishing.

**Author Contributions:** Conceptualization, A.J.; methodology, A.J. and P.B.; formal analysis, A.J. and P.B.; investigation, A.J., P.B., K.R. and M.N.; resources, A.J.; data curation, A.J., P.B., K.R. and M.N.; writing—original draft preparation, A.J.; writing—review and editing, A.J. and P.B.; visualization, A.J.; supervision, A.J., M.N. and P.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The APC was funded by the authors thank Warsaw University of Life Sciences—SGGW, Institute of Wood Sciences and Furniture for their financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.
