László Tolvaj

Artificial Ageing of Wood Monitored by DRIFT Spectroscopy and CIE L*a*b* Color Measurements 1. Effect of UV Light

The effect of UV irradiation up to 200hrs was monitored on five wood species (Picea abies, Pinus sylvestris, Larix decidua, Populus euramericana, Robinia pseudoacacia) by spectro colorimetry and diffuse reflectance infrared Fourier transform (DRIFT) spectrometry. The CIE L * a * b * color calculations exemplified the rapid change of total color difference (ΔE * ) in the first 25 to 50hrs of irradiation. All samples exhibited yellowing, i.e. a pronounced steep shift to positive Δa * values in the Δa * , Δb * coordinate system. Robinia p. showed an additional shift to redness, i.e. towards positive Δb * values. The colored compounds developed could be removed by water extraction only to a moderate extent. Thus it was concluded that the yellow color was mainly due to oligomeric chromophores, arisen probably from leuco chromophores of the lignin moiety of wood. In agreement with the literature, DRIFT measurements revealed the preferred degradation of aromatic structures and the formation of nonconjugated carbonyl and carboxyl groups. The DRIFT spectra indicate dehydration, oxidation to carboxyl groups and the formation of lactones or other esters. It was demonstrated by spectral subtraction that the increasing carbonyl bands consisted in the case of softwoods of two sub-bands with equal intensity at 1763 and 1710 cm -1 . The formation carbonyl bands of the hardwoods obeyed different rules. The formation of conjugated carbonyl groups below 1700 cm -1 was observed only to a limited extent. The carboxyl group containing photodegradation products could easily be removed by water extraction. This was interpreted that the oxidized compounds did not necessarily contribute to the formation of colored compounds. The plots of DRIFT band intensities, derived from baseline corrected and normalized spectra, vs. the irradiation time revealed pronounced intensity changes in the first 50 hrs of irradiation. The coherent intensity profiles were beveling above 100hrs of irradiation.

Light source dependence of the photodegradation of wood

The aim of this study was to determine whether artificial ultraviolet (UV) light sources are able to imitate properly the photodegradation of wood caused by sunlight. In this study, wood specimens were irradiated with a xenon lamp and a mercury lamp. The xenon light simulated sunlight only in the case of long-term irradiation. The photoinduced yellowing of wood was faster and greater in the case of short-term exposure to xenon light than that caused by sunlight. The number of UV light-generated carbonyl groups absorbing infrared light around 1700 cm−1 showed good correlation with photoinduced yellowing. On the other hand, mercury light did not simulate sunlight. However, the mercury lamp, as a strong UV light emitter, can be applied to determine the valid limits of the Kubelka–Munk (K-M) equation. Our results show that the K-M equation cannot be applied to determine the absorption properties of the sample if the values of the K-M units exceed 50.

Thermal degradation of wood during photodegradation

In this study, wood samples were exposed to light irradiations (direct sunlight, xenon lamp, mercury vapour lamp) and thermal treatments were carried out in dry- and in humid conditions at 90°C. One part of the samples was covered by an aluminium plate during light irradiation. The samples under the aluminium plate also suffered considerable chemical changes, monitored by infrared technique and colour measurement. The sunlight produced greater colour change under the aluminium plate than the artificial light sources. During light irradiation, the carbonyl band having two maximum at 1700 and 1,746 cm(-1) increased and the peak of the aromatic skeletal vibration arising from lignin (1,510 cm(-1)) decreased together with the guaiacyl vibrations at 1,275 cm(-1). There was absorption decrease at 1,174 cm(-1) because of the ether band splitting. Under the covered surface only the ether band at 1,174 cm(-1) decreased and one carbonyl band increased with a maximum at 1,715 cm(-1). Degradation of lignin was negligible for the covered surface. Colour change generated by thermal degradation was much greater in humid condition than in dry condition.

Photodegradation of wood at elevated temperature: Colour change

The purpose of this investigation was to evaluate the effect of elevated temperature on the photodegradation of solid wood. The work presented here, dealt with the changes of colour during the degradation process. Wood samples were irradiated by mercury vapour lamp at 80°C and at 30°C to screen out the effect of thermal decomposition during photodegradation. Results demonstrated that the same light irradiation resulted in considerably greater redness increase at 80°C than at 30°C. Pine samples showed 57% higher redness change at 80°C than at 30°C during the 200h light exposure. Corresponding data for spruce, ash and poplar were 33%, 40% and 15%, respectively. Results indicated that the extractive content has an important role in thermal decomposition during photodegradation. The elevated environmental temperature did not affect yellowness.

Investigation of the change in the DRIFT spectra of light-irradiated wood with heat treatment

Materials and methods This study examined heartwood of Japanese cedar (Cryptomeria japonica D.Don). Specimens were irradiated with artificial sunlight from a xenon lamp at 180 Wm in the range of 300–400 nm for up to 10 hours at 63 C (black panel) and 50%RH in a commercial chamber (SX-75: Suga Test Instruments). After irradiation, the wood was treated with humid heat at 90 C and 90%RH for up to 72 hours. As a control, unirradiated specimens were treated at the same temperature and humidity condition. The IR measurements were performed using a JASCO FTIR spectrometerl-8900 equipped with a diffuse reflectance unit (JASCO: DR-81). The resolution was 4cm and 64 scans were averaged. After a three point baseline correction at 3800, 1900, and 850 cm the spectra were normalized to the band between 1352 and 1406 cm. The spectrum of the untreated sample was subtracted from the spectrum of the irradiated and heat-treated ones to obtain the difference spectra. Results Figure 1 shows the difference DRIFT spectra of Japanese cedar after irradiation, heating of the irradiated one, and heating alone. Tolvaj and Faix (1995) revealed an intensity increment in the carbonyl region in two distinct wavenumber ranges, at 1763 (CO stretching in unconjugated ketones, carboxyl group, and lactones) and 1700–1710 cm (CO stretching of various functional groups originated from unconjugated structure) after UV irradiation. In case of light-irradiated Japanese cedar, these intensive bands overlapped, whereas after the light-irradiated wood was heat treated, the intensity of the carbonyl groups decreased and two distinct wavenumber ranges were observed at 1767 and 1705 cm. On the other hand, measuring the heated specimen without light-irradiation, the change in carbonyl groups was hardly observed. Therefore, it is assumed that only the amount of carbonyl groups produced by light-irradiation decreases with heat treatment. It is well-known that the intensity of the aromatic skeletal vibration around 1510 cm decreases (Tolvaj and Faix 1995, Barta et al. 1999, Kataoka and Kiguchi 2001, Ohkoshi and Saijo 2001). However, it is stable against heat treatment because it hardly changes after the light-irradiated wood was heated. The 1271 cm band that shows CO stretching and guaiacyl ring vibration, decreased with light-irradiation. However, after the irradiated wood was heated, it hardly changed. Tolvaj and Faix (1995) reported that the photodegradation leads to hydrophilic substances with low molecular weight containing carboxylic acids which can easily be removed by water. Furthermore, it is assumed that they easily degrade by heat treatment. Detailed results, which included the wide range of wavenumber and the relationship between the intensity of DRIFT spectra and remarkable change in the color of light-irradiated wood with heat treatment that Mitsui et al. (2001) previously reported, will be reported soon.

Photodegradation of wood at elevated temperature: Infrared spectroscopic study

The purpose of this investigation was to evaluate the effect of elevated temperature on the photodegradation of solid wood. The work presented here, deals with the changes of infrared spectrum generated by the photodegradation process. Wood samples were irradiated with a mercury vapour lamp. The photodegradation behaviours of conifers and deciduous species were studied at elevated (80 °C) and at ambient (30 °C) temperatures. The infrared data were analysed using the difference spectrum method. The properly calculated difference spectrum gave much more information about the chemical changes than the visual comparison of the absorption spectra measured before and after the irradiation. The results showed considerably greater degradation at 80 °C than at 30 °C. The difference spectra revealed the absorption increase of tree different types of carbonyl groups. Remarkable differences were found between the photodegradation behaviours of softwoods and hardwoods. Poplar belongs anatomically to the hardwoods but its photodegradation properties were between that of hardwoods and softwoods.

Color changes in acetylated wood by the combined treatment of light and heat

This study investigated the change in color of acetylated wood by the combined treatment of light and heat. The color of acetylated wood was stable against light, however, heat treatment after light-irradiation made it change greater. Furthermore, the acetylated wood discolored greater than unacetylated one by light-irradiation after heat treatment. These results show that the acetylated wood is not stable when it is exposed under special condition. ZusammenfassungIn dieser Arbeit wurde die Farbänderung von acetyliertem Holz bei einer kombinierten Behandlung mit Licht und Wärme untersucht. Die Farbe von acetyliertem Holz war zwar lichtstabil, eine auf die Lichtbestrahlung folgende Wärmebehandlung verursachte jedoch eine grössere Farbveränderung. Darüber hinaus verfärbte sich acetyliertes Holz stärker als unacetyliertes bei Lichtbestrahlung nach einer Wärmebehandlung. Diese Ergebnisse zeigen, dass acetyliertes Holz unter besonderen Bedingungen nicht farbstabil bleibt.

Photodegradation of Timber of Three Hardwood Species Caused by Different Light Sources

Abstract - In this study, resistance of black locust, beech and poplar wood to photodegradation was tested, applying sunlight, a xenon lamp and a mercury vapour lamp. The irradiation time was 200 hours for sunlight and the xenon light and 20 hours for the mercury light. The changes were monitored by colour measurements and infrared spectroscopy. The colour change of black locust was more intensive at the beginning of the irradiation than that of the beech and poplar. The degradation of aromatic structure of lignin (absorbing at 1510 and 1596 cm-1) in black locust was minor compared to the same changes of beech and poplar during the first 10 hours. The mercury lamp induced more intensive changes both in colour and in infrared spectrum than the other two light sources. The results show that the high extractive content of black locust absorbs a considerable amount of light radiation protecting the main chemical components of wood. Kivonat - Három lombos fafaj faanyaga fotodegradációs tulajdonságainak összehasonlítása különböző fényforrások alkalmazása esetén. Akác, bükk és nyár faanyagok fotodegradációval szembeni ellenálló képességét vizsgáltuk napsugárzás, xenon lámpás és higanygőz lámpás besugárzás esetén. A kezelési idő 200 óra volt a napsugárzásos és a xenon lámpás besugárzásnál, és 20 óra a higanygőz lámpás besugárzásnál. A változásokat színméréssel és az infravörös spektrum felvételével követtük nyomon. A kezelés kezdetén az akác színváltozása sokkal intenzívebb volt, mint a bükké és a nyáré. Az első 10 órában a lignin aromás gyűrűjének degradációja (abszorpciós helyei: 1510 és 1596 cm- 1) akác esetében sokkal kisebb volt, mint bükk és nyár esetében. A higanygőz lámpás kezelés sokkal intenzívebb változást produkált (a színváltozásban és az infravörös spektrumban is), mint a másik két fényforrás. Az eredmények azt mutatják, hogy az akác magas extraktanyag tartalma elnyeli a fénysugarak jelentős részét, ezzel megvédve a faanyag fő kémiai összetevőit.

Wood degradation caused by UV-laser of 248 nm wavelength

Methods and materials Krypton fluoride (KrF) excimer laser of 248 nm wavelength is used to study the degradation of wood. All of the samples were irradiated by 5000 pulses selected from the pulse train of the laser operated with 10 Hz repetition frequency. The pulses had 15 ns duration and 20 mJ energy. We studied the effect of UV-laser irradiation on eight wood species by IR spectroscopic methods. The samples were chosen from four kinds of soft wood (Scotch pine, spruce, larch, Douglas-fir) and from four kinds of hard wood (Black locust, beech, ash, silver maple). Within these groups 4-4 samples were chosen so that the different issues (heartwood, sapwood, eartywood, latewood) represented all species. Diffuse reflected infrared spectra were recorded before and after irradiation using a Bio-Rad Digilab FTS-65A/896 FTIR-spectrometer. The spectra were baseline corrected using a linear algorithm and were normalized to the band between 1352 cm -~ and t406 cm-k The spectrum of the untreated sample was subtracted from the spectrum of the irradiated one, to obtain the difference spectrum.

Measurement of photodegradation-caused roughness of wood using a new optical method

The aim of this study was to clarify the intensity of the surface roughening of wood caused by light radiation using a fast optical method. The samples were irradiated by mercury lamp and the roughness change was monitored traditionally using a perthometer. The infrared (IR) diffuse reflectance spectrum was measured and the baseline shift was found to be a proper parameter to monitor the roughening effect of photodegradation. Linear correlation was found between the traditionally measured roughness and the baseline shift. This newly developed optical method is able to detect the degradation difference between earlywood and latewood. Some of the samples were immersed in distilled water for a day after an all light irradiation period of two days. This new baseline shift method was able to visualise and determine the small change in roughness caused by the leaching effect of water.