Barta, Edit

Joóbné Preklet, Edina

Papp, György

Tolvaj, László

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Dependence on light sources of the artificial photodegradation of woodTolvaj, L., E. Preklet, E. Barta, G. PappUniversity of West Hungary Institute of PhysicsSopron, HungaryThe photodegradation in wood caused by sunshine is a complicated process. The intensity andwavelength content of sunlight depends on plenty of uncontrolled parameters. In addition theseparameters are changing not only during a year but even within a day. This is the reason thatartificial light sources are often used to clarify the mechanism of wood photodegradation. Someof these light sources can imitate the contents of sunlight, while the others emit only a part (orsome parts) of the spectra emitted by the Sun.The emission spectrum of the Sun is shown in Fig.1. The Ozone layer of the Earth filters someparts of the emitted light (black area). An other part is scattered by the air. The energydistribution which can reach the surface of the Earth is presented by the dotted line.Fig. 1. The emission spectrum of the Sun. (The black area is filtered by the Ozone layer.) Thedoted line represents the light intensity distribution on the surface of the Earth.For photodegradation the ultra violet (UV) part (?<380nm) is the most effective wavelengthregion. Under normal condition only the "A" part of the UV light can go through the air. If theOzone layer is thinner even missing the "B" and "C" parts also are able to reach the outdoorwooden materials. During artificial simulation of the sunlight the UV "B" part also need to takeinto consideration. The spectrum of the Xenon lamp is similar to the spectrum of the Sun. Theonly exception is that the spectrum of Xenon lamp does not contain UV "B" and "C" parts. So theXenon lamp can be recommended if the imitation of normal sunlight without UV "B" part isneeded.The spectrum of Mercury vapour lamp is presented in Fig. 2. This spectrum contains only somanarrow peaks. Most of peaks are in UV region covering all of parts (A;B;C). For the investigationof the UV photodegradation the Mercury lamp is recommended.Fig. 2. The emission spectrum of Mercury vapour lampThe chemical changes of wood caused by photodegradation are mostly monitored by infra red(IR) spectroscopy and by colour measurement. The colour data are usually given in CIELABcolour co-ordinate system. Fig. 3. presents the colour changes of black locust (Robiniapseudoacacia) caused by Xenon lamp, Mercury vapour lamp, heat treatment at 900C under 100% relative air humidity and under total dry condition, respectively. The total duration of alltreatments were 200 hours. (The a* values represent the red colour and the b* values the yellowcolour part of the samples.) The natural black locust wood has yellow colour represented by highb* value and low a* value. These are the starting points on the left side. The right end representthe effect of 200 hours treatment. Both of light treatment changed the colour of the samplestowards red, considerably. The yellow content slightly increased. The Mercury lamp madecontinuos yellowing in contrast to the xenon lamp where the yellowing started after 40 hours ofexposure.During irradiation by Xenon lamp the half part of the sample was cowered by a metal plate toprevent the light irradiation. The colour of the covered area after 200 hours of treatment isrepresented by the "? " marked dot. This colour change towards red was created by the heatdegradation. This thermal degradation was simulated with thermal treatment at 900C under dryand humid conditions. (The colour change under natural conditions is between these twochanges.) The thermal treatment changed the colour towards red in both cases and decreased theyellow content with rising air humidity. It can conclude, Xenon lamp makes thermal degradationtoo which must be taken into consideration during the test.1520253035403 6 9 12 15a* colour co-ordinateb* colour co-ordinateXenonNonirrMercuryWet 90°CDry 90°CFig. 3. Colour changes of black locust wood samples caused by different irradiations.The most useful analytical tool for chemical analysis of wood photodegradation is the IRspectroscopy. Regarding the wood is an excellent light absorber and the photodegradation is asurface phenomenon only the diffuse reflectance Fourier transform (DRIFT) technique can besuccessfully applied. Fig. 4 represents a typical DRIFT spectrum of Scots pine (Pinus silvestris).These intensities include both of absorption and scattering. The scattering does not sowwavelength dependence in the examined region, so the shape of the DRIFT spectrum is similar tothe absorption spectrum. The DRIFT spectrum is usually presented as the absorption spectrum.All of peaks represent the IR light absorption of different chemical groups. The IR spectrum ofthe wood is rather combined because of the extremely complicated chemical structure of thewood material. The 1000-1800 cm-1 range is individual for all wood species. It is usually calledas fingerprint domain.The band assignment is given in Table 1.Fig. 4.DRIFT spectrum of Scots pine (earlywood part of sapwood)Table 1. Band assignment of IR spectrum of Scots pine in wavenumbers (cm-1)3468 OH stretching 1427 C-H deformation (asymm.)2929 CH,CH2 stretching (asymm.) 1373 C-H deformation (symm.)2905 CH,CH2 stretching (symm.) 1319 C-H deformation,CH2 wagging1740 CO stretching in unconjugatedketone, acetyl, carboxyl groups1275 Caryl-O, guaiacyl ring breathing withCO stretching1662 H2O,CO stretch. in conjugated syst 1170 C-O-C stretching (asymm.)1598 aromatic skeletal breathing 1132 C-O-C stretch. (symm.), arom.C-H1510 aromatic skeletal breathing 1003 Calkyl-O1457 C-H deformation (asymm.) 898 C-H deformation of celluloseDuring the photodegradation of wood same chemical structures are destroyed and others arenewly created presented by absorption decreases and increases, respectively. Fig. 5 shows theabsorption changes of Scots pine (earlywood part of sapwood) during 200 hours of UVirradiation by Mercury vapour lamp. The most important part of IR spectrum is zoomed here. Theirradiation was occasionally interrupted to determine the changes. During the irradiation thenumber of aromatic ring units of lignin decreased (absorption decrease at 1510 cm-1) andFig. 5. Absorption spectra of Scots pine (earlywood part of sapwood) after 0; 10; 25; 50; 100;200 hours of UV irradiationthe number of different carbonyl groups being in unconjugated position (absorbing around 1749cm-1) increased consequently. Comparing the spectra it can conclude one band decreased and oneother increased. Calculating the difference spectrum (irradiated minus bulk) can be realised notonly one peak increased but two close to each other (Fig.6.) at 1763 cm-1 and 1715 cm-1.Fig. 6. Difference spectra of Scots pine (earlywood part of sapwood) after 5; 10; 17; 25; 50; 70;100; 200 hours of UV irradiationThe difference spectrum presents only that peaks which have chanced during the treatment. It cangive more detailed information than the visual comparison of the absorption spectra. This is themain advantage of the difference spectrum method, if there are plenty of peaks covering eachother.The effectivity of the difference spectrum method is well represented in Fig. 7 where the effect of200 hours UV treatment is presented together with the effect of 24 hours of storage of irradiatedsamples under distilled water. The water leached out a part of the degradation products (negativepeak at 1760 cm-1). The curve in the middle represents the degradation products remaining in thewood after water leaching. Here is well presented the water leaching removes partly the differentcarbonyl groups (created during the photodegradation) but in different quantity.Fig. 7. Difference spectra of Scots pine (earlywood part of sapwood) after 200 hours of UVirradiation (top curve),  followed by 24 hours of water leaching (negative peak at 1760cm-1), degradation products remaining in the wood after leaching (curve in the middle)The difference spectrum method gives the possibility to present the differences between the UVphotodegradation properties of different wood species. Fig. 8. represents the difference spectra oflarch (Larix decidua), Scots pine, spruce (Picea abies), poplar (Populus euram I-214) and blacklocust wood species (earlywood part of sapwood) after 200 hours of UV irradiation. Thefingerprint region is zoomed below to show the differences. The spectra of conifers are quitesimilar. The spectra of hardwoods present three main differences comparing to the softwoods. InC-O-C linkage region (1100-1200 cm-1) the absorption increase and decrease are morepronounced at hardwoods than softwoods. In the absorption region of unconjugated carbonylgroups the two main absorption increases are closer to each other at poplar wood than softwoods.In the case of black locust the absorption increases of theunconjugated carbonyl groups are smaller comparing to all of examined wood species and thepeak at 1715 cm-1 is completely missing. There is  small shoulder at 1700 cm-1 what can be seenat all cases. This shoulder becomes visible at black locust wood because of the missingneighbouring peak.Fig. 8. Difference spectra of larch (L.d.), Scots pine (P.s.), spruce (P.a.), poplar (P.e.) and blacklocust (R.p.) wood species (earlywood part of sapwood) after 200 hours of UV irradiationIn focus of interest are recently the preliminary results of an ongoing project where the laserlight-wood interactions are studied. The laser beam contains exactly one wavelength and theintensity of the beam can be measured exactly. The applied wavelengths were between 581 nmand 193 nm (this latest wavelength in not part of the sunlight which can reach the surface of theEarth.Table 2 presents the physical parameters of the applied lasers. The wavelengths cover all of UVregions and represent all three parts (A; B; C) of it. The 581 nm is in the visible light regionrepresenting yellow colour. The total irradiation energy was set to 100 Joule in all cases. Theonly exemption the ArF laser was (emitting at 193 nm). In that case the surface of wood samplesstarted to evaporate after 15 Joule of irradiation energy so the irradiation had to be interruptedhere.Table 2. Data of the applied lasersLaser type Wavelength(nm)Impulse energy(mJoule)Impulse time(ns)Impulse frequency(Hz)?Energy(Joule)ArF 193 10 20 10 15KrF 248,5 20 15 10 100XeCl 308 20 20 10 100Nitrogen 337 2,8 - 30 100Rodamin 581 4 - 10 100The equal energy irradiation and the exact wavelength give the opportunity to examine thewavelength dependence of wood photodegradation. Fig. 9. represents the difference IR spectracaused by laser irradiation at 248; 308 and 337 nm in the case of Scots pine (earlywood part ofheartwood). The main chemical changes at 308 nm and 337 nm wavelengths of irradiation aresimilar to the changes caused by Mercury lamp. Differences can be seen in 1100-1200 cm-1region. This is the absorption of the C-O-C linkage.The irradiation at 248 nm was the most effective among the applied wavelengths. There werenew absorption decreases around 1539; 1465 and 1396 cm-1 These changes have never beenmentioned before. Beside the well known absorption increase in 1700-1800 cm-1 region(absorption of the unconjugated carbonyl groups) there was an intensive wide absorption increasearound 1600 cm-1 which is the absorption region of the conjugated carbonyl groups.1800 1600 1400 1200 1000-0,2-0,10,00,10,2Relative unitsWavenumbers cm-1 248 nm 308 nm 337 nmFig. 9. Difference spectra of Scots pine (earlywood part of heartwood) after 100 Joule laserirradiation at 248; 308 and 337 nmThe irradiation at 193 nm and at 581 nm hardly gave any changes (Fig.10). It can be because ofthe applied law energy, or this type of photons du not make changes in lignin molecules. Thisproblems need further investigation. These wavelengths could be the barriers of the effective areafor photodegradation.1800 1600 1400 1200 1000-0,2-0,10,00,10,2Relative unitsWavenumbers cm-1 193 nm 581 nmFig. 10. Difference spectra of Scots pine (earlywood part of heartwood) after laser irradiation at193 and 851 nmThe exact determination of irradiation energy and the difference spectrum method give thepossibility to determine the degradation nature of the different wood tissues. These data of Scotspine are presented in Fig. 11-12. The earlywood part is more sensitive against photodegradationthan the latewood, and the sapwood is also more sensitive than the heartwood. The degradationproperties of earlywood parts are similar, but there are differences between latewood part ofsapwood and heartwood. The main differences are in 1100-1200 cm-1 region and around 1750cm-1. These are the absorptions of the ether bonds and the unconjugated carbonyl groups,respectively. The change in ether bond system is similar to the changes caused by Mercury lampat hardwoods (Fig. 8.).Summarizing the results, we might conclude that the UV-laser not only causes the changes in thedifference spectra of the wood samples that are also produced by the traditional UV-radiators, butalso produces new decrease bands (1398, 1465, 1535 cm-1) that have never been mentionedbefore. Further studies are necessary to explore the chemical changes taking place in wood as aresult of UV-light that was produced by laser.1800 1600 1400 1200 1000-0,3-0,2-0,10,00,10,20,3Relative unitsWavenumbers  cm - 1 PHE PSEFig. 11. Difference spectra of earlywood (E) parts of Scots pine (heartwood (H) and sapwood(S)) irradiated by 248 nm wavelength1800 1 6 0 0 1400 1 2 0 0 1000-0,3-0,2-0,10,00,10,20,3Relative unitsW a v e n u m b e r s  ( c m - 1) P S L P H LFig. 12. Difference spectra of latewood (L) parts of Scots pine (heartwood (H) and sapwood (S))irradiated by 248 nm wavelength

Dependence on light sources of the artificial photodegradation of wood

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