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Considering interior applications, sunlight, both direct through open window and through window glass, and artificial lighting are the main sources of radiation possessing sufficient energy to trigger photodegradation processes in wood. LED lamps, which emit mostly visible light, are becoming the dominant artificial light source in various interiors. In the present study, photodegradation of thermally modified (TM) and unmodified (UM) ash (Fraxinus excelsior), aspen (Populus tremula), and pine (Pinus sylvestris) due to exposure to UV radiation and LED lamps was evaluated and compared by analysing wood discolouration (CIELAB colour space), changes in reflectance and FTIR spectra, and formation of water-soluble components. The results show that, apart from UV radiation, LED lamps may cause considerable photodegradation of both TM and UM wood resulting in visually perceptible colour change, alteration in chemical structure and formation of water-soluble components. Improved photo-stability was observed for TM wood exposed to UV radiation, while even more changes in FTIR spectra were detected for TM than UM wood in the experiment with LED lamps. Comparing TM and UM wood, the changes due to photodegradation were quite similar for TM wood of all species while significant differences were observed in the case of UM wood.

The use of TiO2 nanoparticles for photoprotection comprise a side effect due to the photocatalysis of the nanoparticles under UV radiation. In this work we studied how the suppression of TiO2 photocatalytic activity may affect the production of phenoxy radicals and the color of wood surfaces exposed to UV radiation. The experimental work considered the modification of TiO2 nanoparticles to reduce its photoactivity and the use electron paramagnetic resonance to test free radical production. Wood samples were treated with the different TiO2 nanoparticles and the radical production and color changes were evaluated after UV exposure. Experimental results showed that in wood samples exposed to UV radiation the use of TiO2 with suppressed photoactivity yielded increased amounts of phenoxy radicals, in comparison to samples treated with photoactive TiO2. Similar results were obtained in terms of color change, where samples treated with suppressed photoactivity TiO2 showed significantly higher color changes values, after 2000 h of UV exposure, than samples treated with photoactive TiO2. These results suggest that in wood surfaces, the photocatalytic effect of TiO2 may be crucial on the performance of the nanoparticles as photoprotective treatment.

The effect of acetylation on weathering resistance of a hybrid poplar (Pacific Albus®) was studied by colour change evaluation, mechanical tests, FT-IR spectroscopy and moisture absorption measurements. The acetylation process was performed without any pre-treatment and it was verified by means of weight percentage gain and FT-IR spectroscopy: the average weight percentage gain was 14.4 ±0.6% and the band areas at 1370 and 1740 cm-1 increased about 50% compared to unmodified wood. It was also demonstrated that acetylation process increased mechanical performance and reduced wood moisture absorption. Unmodified and acetylated wood samples were artificially weathered with a Xenon-arc lamp up to 400 h. The mechanical properties of both acetylated and untreated wood showed the same trend, but acetylated samples exhibited higher stiffness, strength and elongation at break. The outcomes demonstrated that acetylated samples were more stable to degradation mechanisms induced by artificial weathering.

The wavelength sensitivity of unprotected fir wood surfaces was assessed using a series of glass cut-off filters under which three 80 μm thick radial wood strips formed a composite surface layer of wood exposed to xenon light. Colour and microtensile changes in the different layers followed different patterns and intensities. During exposure, the colour and microtensile changes shifted ever deeper into the surface and further into the visible region of the spectrum. A relatively narrow waveband of most active wavelengths was causing the greatest proportion of the recorded photodegradation (360 – 435 nm), whereas visible light of wavelengths up to 515 nm also contributed significantly to surface damaging. This is an important finding, assuming that the current knowledge implicitly assumes that the actinic effect of light progressively increases with a reduction in wavelength. As a consequence, an efficient protection of wood from photodegradation would demand also screening longer wavelengths of UVA light and some violet and blue light wavebands of the visible part of the spectrum.

The photodegradation of wood is essentially a surface phenomenon and even though it has very small – if any - effect on the mechanical properties of wood, it has a great influence on its aesthetical properties and maintenance requirements. Wood protected with clear or lightly pigmented coatings in general has poor surface stability under outdoor weather exposure. Studies of wood weathering phenomena are numerous, and principle breakdown processes during exposures are generally known. However, results obtained using different methods are incoherent and dependent on the particularities of the testing method applied. In order to better understand the processes which happen on and under the wood surface during weathering as well as to identify the most influential wavebands of light causing photodegradation of wood, several weathering tests (artificial and natural) were performed. Batches of thin strips of fir wood, assembled in packs of matching three strips (thus forming a 240 m thick surface layer), were exposed to irradiation behind a set of cut-on glass filters that transmitted selected regions of the UV and visible spectrum. Weathering regimes included different light sources (xenon light, UV fluorescent light and natural sunlight) and various levels of climatic conditions (temperature and relative humidity of air). Disassembling the pack of three strips after withdrawals from exposure enabled monitoring of the relevant changes in each particular strip i.e. in the depth of the wood surface during weathering. After weathering the strips were suscepted to measurements of colour, microtensile strength and chemical changes using FTIR spectroscopy. Microtensile testing was executed at zero span of grips (reflecting cellulose changes) and at 10 mm span (reflecting properties of the lignin-rich matrix). Studies of changes of colour, mechanical integrity and chemical composition of wood were shown to follow different patterns of order of appearance and intensity. Depth profiles of those changes develop in time and are specific to observed properties. Studies of activation spectra of wood indicated relatively narrow waveband of most active wavelengths in causing greatest proportion of all recorded damages (360 – 435 nm). Visible light of wavelengths up to 510 nm was shown to contribute significantly to surface damaging. With the duration of exposure the colour changes shift ever more into the depth and into the visible region of the spectrum. Strength and chemical changes are more regularly depth-profiled than colour changes. The amount of strength loss is evident below 80 μm, and the changes in deeper layers exhibit gradual reduction. The mechanical disintegration at prolonged exposures can also be attributed to the wavelengths in the visible region, but to a very small extent. The whole spectrum of wavelengths employed in this work actively contributes to degradation of wood surface, particularly to discoloration. In the spectral region below 335 nm the contribution of wavebands to the overall colour changes is very small. This is an important finding assuming that the former habitual knowledge implicitely included that the actinic effect of light progressively increased with reduction in wavelength. Shorter wavelengths (295 to 317 nm) seem to contribute to both zero and finite span strength losses during the complete exposure. However, since there is very little fluctuation of strength values below 360 nm, this region is obviously not predominantly influential for mechanical properties. The spectral region that contributes the most to strength changes is defined by inclusion of light between 360 and 435 nm. Longer wavelengths appear to contribute to strength losses of top strips, but to a little extent and only in final stages of exposure. All the methods employed in studies of wood photodegradation have their limitations and give different indications as to the development of degrading processes. Only the comparisons of different methods give a complete image of the process. Colour and strength develop their changes in different chronological orders and different depth profiles. If exposed to full spectrum of sunlight top strips rapidly change colour at the beginning of exposure, while the underlaying strips develop discoloration later in the process. Strength changes, on contrary, start with some delay in the process but subsequently increase in continuous manner during exposure. The high emission of QUV lamps in most active wavelength bands causes intensive acceleration of development of damage of top surface layers. However, the low emission in visible spectrum of the QUV lamps resulted in small changes in deeper layers (middle and bottom strips). This will render a Xenotest generally better, though slower and more expensive accelerating weathering machine than QUV. Efficient protection of wood from light would demand screening not only UV light, but also some violet and blue light wavebands. The UV protection must be efficient over prolonged periods of time in order to secure long-lasting stability of wood surface.

RESUMO Os objetivos do trabalho foram avaliar o processo de fotodecomposição da madeira de cinco espécies tropicais e analisar o efeito de proteção de dois produtos de acabamento de madeiras quando submetidos ao intemperismo artificial. Foram empregadas as madeiras de ipê-roxo (Tabebuia impeginosa), itaúba (Mezilaurus itauba), maçaranduba (Manilkara huberi), tatajuba (Bagassa guianensis) e tauari (Couratari sp.). Amostras destas madeiras foram submetidas a ciclos de radiação ultravioleta (UV) e água até perfazer um total de 2.000 horas de irradiação e 400 horas de lixiviação em água. A mudança da cor natural das madeiras, indicativo do processo de fotodegradação, e o grau de proteção de um verniz poliuretânico transparente e de um “stain” semitransparente foram monitorados por espectrocolorimetria. As espécies mais resistentes foram o ipê-roxo e tauari, seguidas pela maçaranduba, itaúba e tatajuba respectivamente. O verniz forneceu proteção por tempo maior ao ipê-roxo e à maçaranduba, enquanto o “stain” foi mais eficiente para a itaúba e tatajuba. Ambos os produtos tiveram desempenho similar para a madeira de tauari. ABSTRACT This work aimed at evaluating the photodecomposition process of five tropical wood species and the protection effect of two wood finishing products under artificial weathering. Woods of Tabebuia impeginosa, Mezilaurus itauba, Manilkara huberi, Bagassa guianensis and Couratari sp. were employed. Samples were submitted to cycles of 2,000 hours of ultraviolet radiation (UV) and 400 hours of water leaching. Changes in the natural color of wood, indicative of the photodegradation process, and the protection degree of a transparent poliuretanic varnish and a semitransparent stain were monitored by spectrocolorimetry. The most resistant wood species were Tabebuia impetiginosa and Couratari sp. followed by Manilkara huberi, Mezilaurus itauba and Bagassa guianensis, respectively. The varnish protected Tabebuia impetiginosa and Manilkara huberi for a long period, while the stain seemed to be more efficient for Mezilaurus itauba and Bagassa guianensis. Both products presented similar performance for Couratari sp.