A New Innovative Stabilization Method for the Protection of Natural Wood

Progress in Organic Coatings 48 (2003) 297–309A new innovative stabilization method for the protection of natural wood Pascal Hayoz∗ , Wolfgang Peter, Daniel Rogez Ciba Specialty Chemicals Inc., Schwarzwaldallee 215, CH-4002 Basel, Switzerland Abstract The UV-light exposure of natural wood leads initially to a fast color change, and in the further stages to large chemical modifications and mechanical breakdown of the wood surface layer. Even in diffuse light exposure, prevailing in indoor conditions, the discoloration of wood is a serious esthetical drawback, for example, for furniture and parquetry. Progress has been made with the development of new stabilizers, which offer a significant benefit compared to the currently used benzotriazole UV-absorbers. Using these new stabilizers the wood photo-protection against discoloration and the durability of clear and transparent pigmented coating systems can be significantly improved. © 2003 Published by Elsevier B.V. Keywords: Stabilizer; Wood; UV; Protection 1. Introduction 2. Composition and degradation of natural wood Wood is widely used as a natural raw material in construction, in the furniture industry and for parquetry end uses. Its physical properties and its warm appearance distinguish it in many areas from competitive materials such as concrete, metals and plastics. Today especially pale wood species like fir, pine, birch, beech and maple are in trend. To protect these materials from adverse factors such as visible light, UV-light, oxygen, heat, humidity and water, biological attack and air pollutants, they are usually coated with various protective and decorative finishes such as paints, transparent stains and penetrating finishes or film forming clear varnishes (Fig. 1). Transparent systems which allow the natural features of the wood (e.g. color and texture) to remain visible, are attracting interest and the demand for them has been increasing. On the one side the protective coating has to be stabilized itself from yellowing, cracking, blistering, delamination and loss of gloss, and on the other side the discoloration and chemical decomposition of the wood has to be diminished to keep its natural appearance and value. Even in indoor applications under diffuse light conditions wood will yellow and darken with time, if it is not protected appropriately. Natural wood is a quite complex material compared to synthetic plastics or metals. Depending on the wood species its dry mass is composed mainly by three natural polymeric structures, which are cellulose, lignin and hemicelluloses such as glucomannan, glucuronoxylan and other polysaccharides. The rest (extractives) consists of extractable products like terpenes, waxes, tannins, mineral salts etc., which are usually present in the range 2–5% (Table 1) [1]. Cellulose and the hemicelluloses build up the wood cells, whereas lignin acts mainly as binder between the wood cells. The lignin content varies between 27 and 37% for softwoods (e.g. fir, pine, spruce, etc.) and between 16 and 29% for hardwoods (e.g. birch, beech, maple, etc.). In contrast to cellulose and the hemicelluloses, which do not absorb daylight, lignin contains many chromophores, which absorb especially in the UV region, and is therefore easily decomposed by photo-oxidative processes. Chemically lignin is a random crosslinked product with an average molecular mass of around 20,000 containing different C9 phenylpropane units originating from coniferyl, sinapinyl and p-coumaryl alcohols. Lignin is a very complex natural polymer and its composition depends on the different wood species. Only average chemical structures can be drawn (Fig. 2). On the other hand, it is commonly accepted that the following chemical units play an important role in the photo-oxidative de-polymerization of lignin, which leads to the formation of brownish ∗ Corresponding author. E-mail address: [email protected] (P. Hayoz). 0300-9440/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/S0300-9440(03)00102-4 6 3.7 5.5 2.2 27.2 2.4 25.8 2. d Glucomannan.8 21. Tsuga canadensis Juniperus communis Pinus radiata Pinus sylvestris Picea abies Picea glauca Larix sibirica Balsam fir Douglas fir Eastern hemlock Common juniper Monterey pine Scots pine Norway spruce White spruce Siberian larch 2. followed by C2 H5 OH). 2P-PUR systems and acrylic coatings. which will turn grayish with time due to colonization by blue stain fungi. 3.5 1.1 27.6 3.9 a Extractives (extraction with CH2 Cl2 .9 15. 1.3 3.0 25.7 33.7 3.8 31.5 41.4 5.1 1.2 14.4 28.5 4.3 1.2 3.7 3.3 4.5 26.5 16.3 2.5 1.4 41.6 1.7 8.8 1. quinoide structures causing the darkening of irradiated wood (Fig. 3) [2].1 19.0 41.0 8.7 3.4 6.7 39.0 22.2 21.4 17. wood has to be protected.9 2.1 23.5 29.f (%) Softwoods Abies balsamea Pseudotsuga menz.4 2. State of the art stabilization To avoid such photo-oxidative degradation.5 8.5 18.6 3. This is usually done by the application of one or more coating layers.4 3.3 47.6 2.4 17.4 27.7 1.9 47.1 8.0 16.4 2.3 21.8 14.0 21.8 38.298 P.4 3. Lignin.4 2. e Glucuronoxylan.2 2.6 10.1 20.1 29.2 2.4 6.0 37.4 24.9 8. which are either available as solvent-borne and/or Table 1 Chemical composition of different wood species (% of dry wood weight) Species Common name Exta (%) Ligb (%) Celc (%) Glmd (%) Glxe (%) O. Eucalyptus globules Gmelina arborea Acacia mollissima Ochroma lagopus Red maple Sugar maple Common beech Silver birch Paper birch Gray alder River red gum Blue gum Yemane Black wattle Balsa 3.2 1. c Cellulose.4 20.3 30.3 45.4 40.p.8 27.2 2.3 3.0 51.1 3.7 25. Hayoz et al.1 1.8 38.9 3.8 2. Factors influencing wood and coating.9 26.5 32.8 3.8 22. b .0 3. In outdoor weathering the degraded and de-polymerized lignin is washed out by rain from the unprotected outer wood surfaces.4 38.6 4.7 39.2 4. Many types of coatings are used including alkyd varnishes.7 2.5 10.4 3.8 29.7 27.8 37. The remaining cellulose fibers will then appear as a woolly surface.8 2.0 40.2 24.3 17.8 3.5 42.7 3.7 Hardwoods Acer rubrum Acer saccharum Fagus sylvatica Betula verrucosa Betula papyriferea Alnus incana Eucalyptus camaldul. f Other polysaccharides.0 39.4 16. / Progress in Organic Coatings 48 (2003) 297–309 Fig.3 2.3 42.6 27. 223-245 (1995) Fig. In this sense UV-light has to be screened before it reaches the wood surface.. ketyl radical O lignin lignin O O O O and other chromophores Schmidt and Heitner. ROO. ROO lignin O sion failures of most clear or transparent coatings [3]. ROO OH lignin lignin O C O O O β-O-4 cleavage O + O lignin O O O lignin O lignin oxidation by O RO . 15. 2. This has lead to the development of different UV-absorbers. R . . J.. . water-borne compositions. / Progress in Organic Coatings 48 (2003) 297–309 299 OH O OH O OH OH OH O O OH OH OH O O O HO OH O HO O O O HO OH OH O O O HO O OH O O O HO O HO O O O O HO OH OH O O OH O O OH OH Fig. Technol. RO . Spruce lignin fragment. 3. whereas acrylate binders are mainly used in UV-curable systems. which are incorporated in the coating layers. Photo-oxidation of lignin. RO . thus explaining the origin of adhe- . R . h Lignin OH HO H OH HO O . mainly of the benzophenone and benzotriazole type. . Wood Chem. Hayoz et al. It has been shown that the UV-fraction of the solar radiation is causing most of the chemical modification and mechanical breakdown in the wood surface layer. . .P. 300 P. To meet the increasingly higher quality requirements regarding the protection of the wood surface and the retention of the initial natural wood color. In addition they have to be very photo-stable for efficient long-term protection. 4.3. new high per- O O C8H17i O O O OH N C8H17i O O O O OH OH O C8H17i O O N bis C8H17i O N HO O O OH O OH N N N HO O O C8H17i O C8H17i O O O tris Fig. depending on the coating thickness. e. UVA-1. 4). 4. For the color stabilization of indoor clearcoats. From an application point of view. UVA-3: a mixture containing new TRT-derivatives. discoloration can be observed after daylight irradiation with wavelengths up to 500 nm [3]. Development of a new UV-absorber for wood protection In addition to UV-light in the range 300–350 nm. Depending on the wood species. HALS-1 and HALS-2 (Fig. It is therefore necessary to develop new UV-screeners.g. UVA-2. Screening results revealed derivatives of the tris-resorcinol-1. Some performance data of such state of the art light stabilizers are given in Fig. 5.g. but without any absorption tailing into the visible to avoid a yellow coloration of the wood substrates to be protected. typically organic UV-absorbers are used in concentrations of 1–3% (calculated on binder). For exterior applications. which have a more pronounced red-shift in their absorption spectra compared to the currently used benzophenones (e. 6. Based on this product profile. liquid UV-absorbers are preferred. UVA-1) and benzotriazoles (e. Effect of UVA-2 and HALS-1 on color change of pine. also UV-light in the region from 350 to 400 nm and even visible light has an effect on the discoloration of wood. Ideally such new UV-absorbers would absorb all UV-light up to 400 nm. Hayoz et al. State of the art UV-absorbers and HALS. High compatibility of the UV-absorber in the different coating substrates is very important. / Progress in Organic Coatings 48 (2003) 297–309 Fig. 5. synthesized and tested.g. clear varnishes and transparent pigmented stains require 1–5% UV-absorber (calculated on resin solids) or 1–3% UV-absorber/HALS (hindered amine light stabilizer) blends to obtain the desired protection. formance stabilizers and a two-component protection system have been developed. UVA-2) which have mainly absorptions with λmax in the region of 325–345 nm. in order to prevent loss of performance by washing out or evaporation of the UV-absorbers.5-triazine (TRT) Fig. OH N N N HO tetra C8H17i O O O O C8H17i C8H17i O . many new UV-absorbers were designed. Spectra of bis-. The mixture of these three TRT-derivatives resulted in a viscous oil. which is soluble in almost any solvent from non-polar hexane to polar dimethylformamide.3 0.1 0 250 300 350 400 450 500 nm Fig.9 0.and tetra-substituted TRT. Very Concentration: 20mg / L toluene absorbance 1. A mixture of similar products was finally chosen (new UVA-3) containing the following products (Fig. but not in water. 6). Comparison of different UVA absorption spectra. whereas the bis. 8. Each of the three UV-absorber parts has its own UVspectrum (Fig.P.8 UVA 2 UVA 3 0.6 0.and tris-para-substituted tris-resorcinol-triazines have a red-shifted spectrum with a λmax around 360 nm and the tetra-substituted compound with a λmax at about 350 nm (depending on the solvent). the red-shift of the absorbance of the new UVA-3 is very pronounced. 7.2 0 290 310 330 350 370 wavelength (nm) Fig. tris.2 0.4 0. An additional benefit is the much higher absorption per gram compared to the state of the art UV-absorbers (Fig. class as the most promising candidates. 8). 7). 390 . In comparison with the state of the art UV-absorbers UVA-1 and UVA-2.7 Bis 0.6 Tetra 0.8 0. The tetra-substituted compound is in addition to its UV-absorbance responsible for the high solubility of this mixture.2 1 UVA 1 0.4 0. / Progress in Organic Coatings 48 (2003) 297–309 301 Concentration: 10mg / L ethylacetate 0. Hayoz et al.5 Tris OD 0. 9).302 P. little organic solvent (10–20 wt. As well blends of UVA-3 with UVA-2 in a 2:1 and 1:1 ratio showed better photostability than UVA-1 or UVA-2 alone (Fig. Photo-permanence of different UVAs. Hayoz et al. . Application data and performance tests for UVA-3 A photo-permanence study was made in a solvent-borne long oil alkyd coating under dry conditions with UV-340 nm exposure (fluorescent light tubes) for up to 2000 h. UVA de-activation mechanism for HPTs. 5. The energy-dissipation mechanism. This revealed a very high photostability compared to benzophenone and benzotriazole UV-absorbers.%) was finally necessary to create a liquid and easy to handle UV-absorber composition with a viscosity of about 2 Pa s. which leads to this high photostability seems to be a cyclic excited state intramolecular proton-transfer (ESIPT) mechanism in analogy to that of other hydroxy-phenyl-triazines Fig. 9. / Progress in Organic Coatings 48 (2003) 297–309 Experiment done in an alkyd long oil coating remaining UV absorber (%) 100 UVA 3 80 UVA 3 /UVA 2 2:1 UVA 3 /UVA 2 1:1 UVA 2 60 40 20 UVA 1 0 0 500 1000 1500 2000 UVA-340 exposure time (h) Fig. 10. 10) [4–6].5 % UVA 3 1 % UVA 3 0. / Progress in Organic Coatings 48 (2003) 297–309 303 Different amounts UV absorber on solid content.5 up to 4% of UV-absorbers were applied in a long oil alkyd system in three coats on fir (dry film thickness (DFT) ca. which have been studied extensively (Fig. until first cracks appeared. Hayoz et al. UVA-1. 100 ␮m) and exposed over 3000 h to UV-340 nm. Accelerated weathering of a long oil alkyd system on fir. 3 coats. 12. Delta gloss at 60◦ had been measured and showed remarkable gloss retention for UVA-3. Accelerated weathering of long oil alkyd system on fir wood. 11. DFT:µm ~80 35 * ** 30 E* 25 * unstabilized UVA 1 20 UVA 2 15 UVA 3 10 5 *cracking 0 0 500 1000 1500 2000 QUV wood exposure (h) Fig. different concentrations from 0. total DFT ca. . E∗ was measured for 2% UV-absorber in a long oil alkyd system on fir with the same weathering spray cycle described above.5 % UVA 3 without stabilizer 0 5 10 15 20 25 30 35 40 45 delta Gloss 60˚ after 2800 h QUVA for wood Fig. In an accelerated weathering experiment. 11). DFT: ~80 µm 4 % UVA 1 3 % UVA 1 2 % UVA 1 1 % UVA 1 4 % UVA 2 3 % UVA 2 2 % UVA 2 1 % UVA 2 4 % UVA 3 2 % UVA 3 1. (HPTs). 80 ␮m) and exposed 2800 h to a cycle of 5 h UV-340 nm light exposure at 58 ◦ C and 1 h of rain at 22 ◦ C.P. even with very low concentrations (Fig. 2 % UV absorber on solid content. For indoor application simulation. 3 coats. 12). UVA-2 and UVA-3 were applied in concentrations of 1 and 2% in a 2P-PUR system on fir (three coats. In another experiment. UVA-3 gave a somewhat longer protection against cracking than the state of the art UV-absorbers and as well a better protection against discoloration (Fig. Whereas the E∗ value of the unstabilized sample rose very rapidly. 14. 13. 120 ␮m) with a UV-absorber concentration of 2%. the E∗ increase of the stabilized samples was retarded in the order UVA-3 > UVA-2 > UVA-1 (Fig. total DFT ca. In both cases. It can be stated. 14). 5m/min. The question then came up. the cure speed was only slightly affected compared to an unstabilized sample. . Hayoz et al. that UVA-3 is a clear improvement to the state of the art in these applications. UV absorber influence on through cure and cure speed in an epoxy acrylate. UVA exposure of 2P-PUR system on fir. a somewhat better performance (E∗ ) was found with the UVA-3 stabilized samples. In a 600 h UV-340 nm exposure experiment of UV-cured epoxy-acrylate and polyurethane-acrylate systems containing 3% of a hydroxyketone/acyl-phosphine-oxide mixture as photoinitiator and 2% of the UV-absorbers UVA-2 and UVA-3.304 P. 15. After an exposure of over 4000 h. A similar picture was obtained during UV-340 nm exposure of a long oil alkyd system on fir (four coats. / Progress in Organic Coatings 48 (2003) 297–309 Fig. Also the pendulum hardness was in every case comparable (Fig. In a first experiment. so it was shown. UVA-3 showed an even more pronounced color retention than the UVA-1 and UVA-2 in the previous experiment (Fig. if this new UV-absorber had an influence on the curing speed and pendulum hardness of UV-curing coatings systems. that the Fig. 200 15 150 10 100 5 50 cure speed pendulum hardness (s) cure speed (m/min) 3% Hydroxyketone PI. 2xHg 80W/cm. 13). 50 µm 20 pendulum hardness 0 0 unstabilized 2% UVA 2 2% UVA 3 Fig. UVA exposure of long oil alkyd system on fir. 2% of UVA-2 and UVA-3 were applied with a mixture of 3% of hydroxyketone and acyl-phosphine-oxide photoinitiator in an epoxy acrylate and cured with a mercury lamp. 15). 17. 7. This NOR-product will react with a peroxy-radical to free again HALS-3 and at the same time produce a more or less harmless ketone and alcohol functional species (Fig.6. 19). leading to strong yellow coloration of the protected wood. Hayoz et al. 5m/min. new UV-absorber had no negative influence on UV-cured systems (Fig. Since not all wavelengths of the daylight electromagnetic spectrum can be screened out as discussed. Inhibiting the yellowing of wood (lignin). In this sense the development of a new stabilizer working by a different protection mechanism than light absorption is required. The radicals should then be converted into more or less harmless and colorless molecules by these radical scavengers (Fig. The compound HALS-3 in question was the nitroxyl form of the simple 4-hydroxy-2. such a full screening of the UV-light would be only possible. Indeed this HALS UV Absorbers UV light Lignin Harmless Heat Radical Scavengers Excited Molecules Free Radicals O2 Yellow Products Harmless Colorless Products Fig. 16). if at the same time a considerable amount of visible light would be absorbed. It may be postulated that the observed color retention is the result of a radical scavenging mechanism according to the modified Denisov cycle [7. 2xHg 80W/cm. As it was mentioned earlier. complete screening of UV-light up to 400 nm would give potentially even better results against the discoloration of irradiated wood. HALS-3 was applied in different solvents as a 2% solution on fir wood. Water and interestingly a . UV absorber influence on color stabilization of fir wood after 600 h UVA exposure. measured by the b∗ value (Fig. 16. 50 µm 30 EP-AC 25 E* variation PU-AC 20 15 10 5 0 unstabilized 2% UVA 2 2% UVA 3 Fig. 18). an attempt was made to control the remaining radicals by bringing different HALS radical scavengers into very close contact with lignin in the outer wood surface. After this procedure. and the panel coated with three coats of a UV-absorber (based on solid content).6-tetramethylpiperidine (Fig. where a lignin-radical combines with HALS-3 to give the corresponding NOR-product. The exact mechanism is not known up to date.8]. Development of a new pre-treatment for wood protection As it was shown above. that the solvent had a significant importance for the performance of the different tested HALS.P.2. pre-treatment showed in one case a very good color retention. 6. Overall these application data show an important improvement of the wood protection against discoloration compared to the state of the art. 17). Because of the more or less gaussian curve shape of all UV-absorption bands. Different HALS in the amino form or their nitroxyl-amino form were screened by applying them as a 2% solution in water or ethanol directly onto fir wood. / Progress in Organic Coatings 48 (2003) 297–309 305 3% Hydroxyketone PI. when irradiated with light. 18). lignin leads to colored decomposition products due to a photo-oxidative radical mechanism. Application data and performance tests for HALS-3 From the screening experiments it became obvious. the panels were coated with three layers of a clear varnish containing 2% of a commercial benzotriazole UV-screener (based on solid content) and the panels were then exposed under dry conditions to UV-340 nm light for 500 h to see if there was a stabilization effect. 2. The samples were exposed during 1500 h to UV-340 nm and the b∗ value and E∗ value were measured. the measurement of b∗ value and E∗ value showed that the concentration of 1–2% of HALS-3 was ideal and that higher concentrations gave no significant improvement of color retention. Postulated mechanism (modified Denisov cycle) for radial scavenging with HALS-3. which concentration would be necessary for the best protection with HALS-3. To reach maximum protection against wood discoloration and decomposition. UVA-2 . the pre-treatment with a primer containing HALS-3 and a top-coat containing the new UVA-3 were combined and tested (Fig. UVA-1. Screening of HALS derivatives. The question was then. 18. 4 and 5% of HALS-3 in water as primer and a three layer top-coat containing 2% of a commercial UV-absorber. the protection against discoloration was more pronounced than for the darker species such as nut and cherry wood (Fig. 22). but the protection effect against the unstabilized sample was very clear. The new two-component system for wood protection Fig. The same trend was seen during the screening experiment. 19. whether this pre-treatment with HALS-3 was working for other wood species too. 500 hoursUVA-340 6 4 2 0 OH N O HALS-3 Fig. where the less polar HALS showed a lower performance. As a general result it was found that on pale wood species like ash or pine. 8. Hayoz et al. A concentration ladder experiment was made taking 0. polyethyleneglycol-derivative gave the best performance (measured by b∗ value). After 500 h UVA-340 exposure. / Progress in Organic Coatings 48 (2003) 297–309 8 A no 10 H 12 pr et r LS eat m H 3 in en A LS et t 3 han in o w l at er 2 % HALS. It seemed to be very important that the polar HALS-3 could be easily transported into deeper layers of the polar wood surface (Fig. 20). 1.306 P. and it was interesting to see. and the least polar solvent butylacetate gave the worst performance after UV-340 nm exposure. The result was that already one layer gave very good protection against discoloration compared to the unstabilized sample (Fig.3 layer top coat with 2 % bzt-UV-absorber (on solid content) on fir. Different wood samples were pre-treated with a 2% water solution of HALS-3 and then coated with three layers of a top-coat containing 2% of a commercial UV-absorber. In a similar experiment it was tested whether if the application of one or more layers of primer containing HALS-3 was important. 3. 23). All these experiments were so far performed on fir wood. 21). 21. 2 % HALS-3.P. Application of HALS-3 on different wood species. . / Progress in Organic Coatings 48 (2003) 297–309 307 3 layer top coat with 2 % bzt-UV-absorber (on solid content) 10 no pretreatment 8 water Delta b* 6 4 ethanol 2 butylacetate 0 100 300 diethylenglycolmonoethylether 500 -2 UVA-340 exposure time (h) Fig.3 layer top coat with 2 % bzt-UV-absorber(on solid content) 1500 hoursUVA-340 exposure 25 20 Delta b* no HALS-3 15 Delta b* with HALS-3 10 DE* no HALS-3 5 DE* with HALS-3 cherry nut pine oregon pine beech maple ash 0 oak Delta b*/E* Delta b*-/E* (500 h) 3 layer top coat with 2 % bzt-UV-absorber(on solid content) 500 hours UVA-340 lamps Fig. 20. Hayoz et al. Concentration dependent performance of HALS-3. 16 12 14 10 12 Delta b* (500 h) 14 8 6 4 2 10 8 6 4 2 0 -2 0 -4 -2 0 layer 0% 1% 2% 3% 4% 1 layer 2 layers 3 layers 5% HALS-3concentration in water Delta b* Delta E* Fig. Solvent dependent performance of HALS 3. 22. . Effect of new UVA-3 and HALS-3 on color change of fir wood. / Progress in Organic Coatings 48 (2003) 297–309 Fig. respectively. Finally. 24. Two-component protection against wood discoloration.5% of UVA-3. 23. the same trend was observed after 1200 h exposure with UV-340 nm confirming the almost total protection of wood against discoloration with the new two-component system HALS-3/UVA-3 (Fig.25 and 0. 25). and UVA-1 and UVA-3 in 2% concentration after HALS-3 pre-treatment. with a 2% concentration in a long oil alkyd top-coat without pre-treatment. After over 2000 h of exposure time it was found that the sample protected by the combination of UVA-3 and HALS-3 showed almost no discoloration at all. E∗ values were measured after 200 h Q-UVA 340 exposure. Again the pale wood species ash and maple showed a distinct inhibition of yellowing. and then coated with two layers of a water-borne acrylate containing each 0.308 P. During the exposure with UV-340 nm the yellowness-index was measured. long oil alkyd top coat upon UVA 340 light exposure UVA in topcoat/ HALS-3 in primer 110 Yellowness index unstabilized 2% UVA-1 90 2% UVA-2 2% UVA-3 70 2%UVA-1/2%HALS-3 2%UVA-3 /2%HALS-3 50 0 500 1000 1500 2000 exposure time (hours) Fig. the four different wood species ash. 26). maple. 24). whereas the samples without pre-treatment showed significant yellowing (Fig. and UVA-3 were applied on a fir wood panel. due to the new stabilizers (Fig. Hayoz et al. cherry and nut were pre-treated with a 1% primer solution containing HALS-3. In a similar experiment with a solvent-borne 2P-PUR top-coat. . Both new products. 0. 15 (2) (1995) 223. Phys. Cordola. Effect of UVA-3 and HALS-3 on color change of different wood species.A. C. [2] J. Otterstedt. / Progress in Organic Coatings 48 (2003) 297–309 309 solventborne2P PUR coat upon UVA 340 light exposure UVA in top coat / HALS-3 in primer Yellowness index 100 unstabilized 2% UVA-1 80 2% UVA-2 60 2% UVA-3 2%UVA-3 /2%HALS-3 40 0 400 800 1200 exposure time (hours) Fig. Wood Chemistry. which was designed to meet these requirements. Academic Press. Proceedings of the International Symposium on Degradation of Polymers. Hayoz et al. et al. 1974. Denisov. J.-E.25 % UVA-3in mid coat (wb acrylate). Miller. [5] H. Wood Chem. which provides significant improvement in color stabilization of wood when used in impregnating pre-treatments with subsequent application of a clear topcoat including UVA-3 as UV-screener. 27 (1990) 65. 99 (1995) 10097. J. M. Derbyshire. Phys. 1993. Technol. Conclusion It has been shown that effective wood substrate color protection increases with the use of more photo-stable and more red-shifted UV-screeners. a hindered amine nitroxyl light stabilizer has been found. 39 (1981) 341. [4] J. 58 (1973) 5716. Heitner. In these cases almost total protection against discoloration was achieved. 2nd ed. References [1] E. [8] P. Polym. J. Werkst. R. Schmidt. Stab. 1 % HALS-3 inwbprimer 0. UVA-3. 26. Degrad. Klemchuk.E. Gande. showed superior performance to that of standard products. E. Effect of new UVA-3 and HALS-3 on color change of fir wood.A. A 104 (2000) 8296. [7] E. 9. 25. Even if the experiments were mainly focused on indoor applications.A.A. p. . In addition. [6] H. p. J. the combination of these new stabilizers are being tested in outdoor applications as well (accelerated weathering and outdoor testing). Brussels. Sjöström. Fundamentals and Applications. 137.P. Holz Rohw. Kramer. Chem. Phys. Chem..5 % UVA-3in top coat (wb acrylate) (amount is referred to formulation) 30 maple Delta E 25 20 ash cherry 15 tnu 10 5 0 unstabilized stabilized 200 hours UVA 340 exposure Fig.E. Kramer. et al. San Diego. especially in combination as a two-component protection system show very good application results on various natural light shade wood species.. Chem. 249. [3] H.