ࡱ; v  !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuwxyz{|}~Root Entry FL`CompObjbWordDocumentObjectPoolmjNmjN  FMicrosoft Word 6.0 Document MSWordDocWord.Document.6;  Oh+'0 D h   @d (C:\MSOFFICE\WINWORD\TEMPLATE\NORMAL.DOT1WOOD COLOUR AND DISCOLOURATION DURING WEATHERING Any Body Any ܥe= e:7lllllll&YBBBBBBBBӧJTTlBBBBBBllBBBBBBlBlB8^llllBBBBWOOD COLOUR AND DISCOLOURATION DURING WEATHERING HOLZFARBE UND HOLZFARBEVER(NDERUNGEN DURCH VERWITTERUNG Wood exposed to the outdoors undergoes photooxidative degradation in the natural weathering process. UV light interacts with lignin to initiate discoloration and deterioration. The photodegradation of wood is a major drawback in the use of wood for outdoor applications. This problem can be minimised if the wood is coated with a fully opaque paint. Clear coating system have become popular due to thair ability to enhance the aesthetic properties of the wood. Most of them do not function effectively. In addition, many UV absorbers, quenchers, and hindered amine light stabilizers have also been used. The results of an investigation into the discoloration of a typical softwood ( fir wood , Abies alba Mill.) and tipical hardwood (oak wood, Quercus robur L.), both uncoated and coated with a clear film-forming water-based stain with various photostabilisers are discussed in this paper. In einem nat(rlichen Verwitterungsprozess ist das Holz au(en der photooxidative Degradation unterworfen. UV-Licht arbeitet zusammen mit Lignin und verursacht Diskolorieren und Zerfall. Die Photodegradation des Holzes ist das Haupthindernis bei Holznutzung im Au(enbau. Dieses Problem k(nnte minimisiert werden, wenn das Holz mit deckende Anstrichsysteme bedeckt ist. Beliebt geworden ist das farblose Beschichtungsysteme durch seine Eigenschaft, die (sthetischen Z(ge des Holzes hervorzuheben. Dazu werden viele UV-Absorber, Quencher, und Radikalf(nger auch benutzt. Diese Arbeit ist eine Diskussion (ber die Resultate der Forschung von Diskolorierung bei den typischen Nadelholzarten (Tanne, Abies alba Mill.) und den typischen Harth(lzern (Eiche, Quercus robur L.), bedeckt oder unbedeckt, mit filmbildenden Lasuren und verschiedenen Lichtschutzmittel. V.Jirou-Rajkovi, PhD., Prof.B.Ljuljka,PhD., A.Bogner, PhD. Faculty of Forestry,10000 Zagreb, Svetoimunska 25, Croatia 1.INTRODUCTION 1.1.Colour of wood Wood has always held a special place in our culture because of its many enchanting properties, one of which is the absorption and reflection of light, resulting in colour and gloss. Wood colour is a composite property whose value is relative to its basic chemical composition (5) The colour of wood varies with wood species. The surface of wood is not uniform like metal; it is composed of cells of various sizes. Various cell volumes and the difference of components cause delicate differences of colour even on the same wood surface. Wood is a complex and heterogeneous macromolecule, and the absorption of light is one of its fundamental properties (3). The principal constituents of wood are cellulose, hemicellulose, lignin, and a large family of polyphenolic substances (extractives). Cellulose and hemicelluloses do not absorb visible light. Native lignins that are isolated with minimum chemical or physical changes are pale yellow (7). In wood, it is assumed that lignin is incorporated into a cellulose matrix and absorbs wavelengths below 500 nm . Many woods absorb light beyond 500 nm due to the presence of phenolic substances such as flavonoids, stilbene, lignan, tannin, and quinone. Ordinary sapwood has a lighter color than heartwood. The transition of sapwood into heartwood is accompanied by the loss of its physiological activity and the formation of various organic substances with darker colour (7). Physical factors affecting wood colour are irradiating direction of light, moisture content and roughness of the surface (7). 1.2. Discolouration of wood surfaces The reaction of wood components to light, heat and chemicals will change color of wood. Discoloration occurs both indoors and outdoors. The pattern of wood colour change is very complex and depends on natural colour, extractives and the character and intensity of chemical changes of particular species (10). When wood is exposed outdoors above ground, a complex combination of chemical, mechanical and light energy factors contribute to what is described as weathering (4,2). The main factors believed responsible for natural weathering in wood include: solar radiation (in particular ultraviolet), moisture(rain, snow etc.), temeperature and atmospheric gases including oxygen as well as pollutant gases such as sulphur dioxide and nitrogen dioxide. Of these factors, the photon energy in solar radiation is thought to be most damaging component of the outdoor environment, initiating a wide variety of chemical changes at wood surfaces (8). The natural weathering process often results in discoloration, physical deterioration of the wood surface, and loss of paint-retaining properties (7). The change in colour is the first obvious indication of the chemical changes in wood during weathering. Wood also loses gloss and lightness (6). At the microscopic level, the chemical nature of the wood surface is changed, and the ultrastructure of wood cell walls is damaged (1). Although this weathering process develops primarily at the surface of wood, the susceptibility to photooxidative degradation significantly reduced wood(s aesthetic values and performance. A great deal of effort has been put into the development of protective systems for wood to prevent photodegradation during outdoor weathering. The traditional method of protecting wood against photodegradation is to apply a surface coating and there is much demand for clear or semi-transparent coatings which retain the natural appearance of wood. The susceptibility of clear coatings to degradation by the ultra violet (UV) wavelengths of solar radiation has long been recognised as a major cause of their early failure. In addition ultraviolet light can penetrate through them to initiate photochemical reactions at underlying wood surfaces resulting in discolouration and adhesion failure betwen the wood and coating system (5). Efforts have been made to extend the useful life of clear coatings on wood by the addition of organic UV absorbers. This paper is about the colour changes of fir and oak wood surfaces exposed to natural and artificial weathering, both uncoated and coated with a clear film-forming water-based stain with and without UV absorbers. The effectiveness of these coatings in preventing discolouration of wood was evaluated. 2. MATERIAL AND METHODS 2.1. Wood and coating materials Fir wood panels (density 404 kg/m3 at 12% M.C.) and oak wood panels (density 681 kg/m3 at 12% M.C.) with dimensions of 100 x 200 x 10 mm for natural exposure, and with dimensions of 75 x 100 x 5 mm for accelerated weathering. All the specimens were machined by fine planing, sanded (grit size 150) and conditioned before finishing. The moisture content of the specimens before application of the coating was 9%. The survey of the specimen types is presented in Table 1. Table 1. Sample codes and description of the treatments. Sample code Description of the treatment and material usedKBUntreated sample (control)LBImpregnating primer, applied by dipping Wood stain without a photostabiliser, air sprayedF1LB Impregnating primer, applied by dipping Wood stain with a photostabiliser F1 (TiO2), air sprayedF1LBImpregnating primer, applied by dipping Wood stain with a photostabiliser F2 (1% Tinuvine 1130 + 1 % HALS Tinuvine 292), air sprayedL2BImpregnating primer, applied by dipping Pigmented (semi-transparent) wood stain* All finishes used in this work are products of COLOR MEDVODE. Samples of the L type were finished with the top-coat, film forming, transparent water-based wood stain*. The binder of this stain is a modified acrylic co-polymer emulsion with 45 % solid content, pH value of 8.7 and with a minimum film forming temperature of 0C. The binder was combined with the polyurethane dispersion in order to improve the surface resistance, and to reduce the effect of "blocking" (mutual sticking of the products in the stack). This also should improve the weather resistance. The F1L samples were coated with the same stain which was modified with the 1.5 % of the ultra-fine colloid dispersion of the TiO2. The F2L samples were finished with the same stain enriched with 1% of the Tinuvine 1130 UV-absorber and 1 % of the HALS Tinuvine 292 (weight proportions in respect to the final formulation). L2 is the code which marks the specimens finished with the semitransparent EKOL DS 2040 stain. This stain contains pigments of iron oxide instead of photostabilisers. Before the stain application all the samples were impregnated with Ecol primer which contains an alkyd-acrylic copolymer dispersion as a binder. The primer was applied by dipping for 10 seconds. After that the samples were dried overnight and gently hand-sanded with a "scotch brite" sponge. The stain was air-sprayed with a 2.5 mm nozzle which gave a wet film thickness of 300 + 10 (m. After drying the stain for 24 hours at room temperature, the samples were additionally conditioned for a week at a temperature of 21 + 2C and relative humidity of 60 + 5%, and finally exposed. 2.2. Accelerated weathering Accelerated weathering was performed in the QUV weathering tester. A 24-hour cycle consisted of 8 hours of UV- radiation with the UVA-340 lamps at 60 C, followed by 4 hours of condensation at 50 C; the exchange of 8 hours of radiation and 4 hours of condensation was then repeated. The samples were withdrawn after 1 week, 2,weeks, 4 weeks, 6 weeks, 8 weeks, 10 weeks,12 weeks and 16 weeks of accelerated weathering. 2.2. Natural weathering Natural weathering was performed on the roof of the building. The specimens were fully exposed at 45 facing south for six months between 01.02.1997 and 01.08.1997. The colour measurements were made at the end of each month. 2.3. Colour measurements Colour measurements were made with spectrophotometer DC 3890 manufactured by DATACOLOR with measuring geometry d/8(. The colour of wood was measured with an illuminant D light source (D 65, 6500 K) with a measuring aperture 27 mm, and standard observer 10(. Three measurements always on same places on same specimen were made to obtain an average value. The CIE color defference system was used. 3. RESULTS AND DISCUSSION  Fig. 1. Change in colour during natural weathering of fir-wood  Fig. 2. Change in colour during accelerated weathering of fir-wood  Fig. 3. Change in colour during natural weathering of oak-wood  Fig. 4. Change in colour during accelerated weathering of oak-wood Results of the measurements done can be seen in Figures 1, 2, 3, and 4. It is obvious that fir and oak wood both finished and unfinished change colour readily when exposed either to outdoor weathering conditions or indoor artificial weathering. The change in colour, as illustrated by colour difference ((E) on oak wood samples is less than on fir wood samples. Fir wood samples showed less discolouration during natural weathering while discolouration of oak wood samples was less during artificial weathering. This could be related to different activation spectra of fir and oak. The results of colour measurements during natural weathering differ from results obtained during artificial weathering and are not comparable. It should be recognized that under outdoor exposure conditions, in addition to sunlight, other factors such as moisture and microorganism growth are also involved on the wood surface. The roughening and checking of the exposed surface normally is faster due to the leaching of low molecular weight degradation products of lignin by water. This does not ocur in the artificial weathering (6). The results obtained here indicated that used various stabilised clear film forming water-based stains are not efective in preventing wood discolouration during weathering. The used photostabilisers appeared to have a small beneficial effect in reducing discolouration rates, especially on oak wood samples. Of all used stains the treatment with the semi-transparent pigmented stain proved as the only efficient 4. CONCLUSIONS Results of colour measurements on oak and fir wood specimens during natural weathering are not comparable with the results obtained during artificial weathering. Semi-transparent pigmented stain proved to be good protective agents against photodegradation of wood. 5. REFERENCES 1. Chang, S.-T.; Hon, N.-S.;Feist, W.(1982): Photodegradation and photoprotection of wood surfaces. Wood and Fiber 14(2):104-117. 2. Feist, W.C.; Hon, D.N.-S.(1984): Chemistry of weathering and protection. In: Rowell, R. (Ed): The chemistry of solid wood. Advances in chemistry series 207. Proceedings of 185th meeting of the American Chemical Society: March 20-25 1983, Seattle, Washington, DC. American Chemical Society, Washington. Chapter 11: 401-455. 3. Hon, D.N.-S.,Ifju, G. (1978): Measuring penetration of light into wood by detection of photoinduced free radicals, Wood Science, 11 (2). 4. Hon, D.N.-S. (1981): Photochemical degradation of lignocellulosic materials. In: Grassie, N., ed.: Developments in polymer degradation-3. Chapter 8: 229-281. Barking and London: Aplied Scince Publishers. 5. Hon, D.N.-S.,Chang, S.T.,Feist, W.C. (1985): Protection of wood surfaces against photodegradation, J.Appl.Pol.Sci.30,1429-1448. 6. Hon, D.N.-S.; Feist, W.C.(1986): Weathering characteristics of hardwoood surfaces. Wood Sci. Technol. 20: 169-183. 7. Hon, D.N.-S., Minemura, N. (1991): Color and discoloration. In: Wood and cellulosic chemistry, Ed. Hon, Shiraishi, Marcel Dekker. Inc, New York and Basel.. 8. Horn, B. A., Qiu, J., Owen, N.L., Feist, W.C. (1992): FT-IR studies of weathering effects in western red cedar and southern pine. In: Chemical modification of lignocellulosics, FRI Bull. 176. 1992 November 7-8; Rotorua, New Zealand . New Zealand Forest Research Institute: 67-76. 9. Minemura, N., Umehara, K. (1979): Color improvement of wood (I), Rept. Hokkaido For. Prod. Res.Inst., 68L92. 10. Sandermann, W., Schlumbom, F. (1962): (nderung von Farbwert und Farbempfindung an Holzoberfl(chen. 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