Progress in Organic Coatings 48 (2003) 297309A new innovative stabilization method for theprotection of natural woodPascal Hayoz, Wolfgang Peter, Daniel RogezCiba Specialty Chemicals Inc., Schwarzwaldallee 215, CH-4002 Basel, SwitzerlandAbstractThe UV-light exposure of natural wood leads initially to a fast color change, and in the further stages to large chemical modicationsand mechanical breakdown of the wood surface layer. Even in diffuse light exposure, prevailing in indoor conditions, the discoloration ofwood is a serious esthetical drawback, for example, for furniture and parquetry.Progress has been made with the development of new stabilizers, which offer a signicant benet compared to the currently usedbenzotriazole UV-absorbers. Using these new stabilizers the wood photo-protection against discoloration and the durability of clear andtransparent pigmented coating systems can be signicantly improved. 2003 Published by Elsevier B.V.Keywords: Stabilizer; Wood; UV; Protection1. IntroductionWood is widely used as a natural raw material in construc-tion, in the furniture industry and for parquetry end uses. Itsphysical properties and its warm appearance distinguish itin many areas from competitive materials such as concrete,metals and plastics. Today especially pale wood species liker, pine, birch, beech and maple are in trend.Toprotectthesematerialsfromadversefactorssuchasvisible light, UV-light, oxygen, heat, humidityandwa-ter, biological attackandair pollutants, theyareusuallycoated with various protective and decorative nishes suchas paints, transparent stains and penetrating nishes orlmforming clear varnishes (Fig. 1). Transparent sys-tems whichallowthenatural features of thewood(e.g.color andtexture) toremainvisible, areattractinginter-est and the demand for themhas been increasing. Onthe one side the protective coatinghas tobe stabilizeditself fromyellowing, cracking, blistering, delaminationandlossofgloss, andontheothersidethediscolorationandchemical decompositionof the woodhas tobe di-minishedtokeepitsnatural appearanceandvalue. Evenin indoor applications under diffuse light conditions woodwill yellowanddarkenwithtime, if it is not protectedappropriately.Corresponding author.E-mailaddress: pascal.hayoz@cilasc.com (P. Hayoz).2. Composition and degradation of natural woodNatural woodisaquitecomplexmaterial comparedtosynthetic plastics or metals. Depending on the wood speciesits dry mass is composed mainly by three natural polymericstructures, whicharecellulose, ligninandhemicellulosessuchasglucomannan, glucuronoxylanandotherpolysac-charides. The rest (extractives) consists of extractable prod-ucts like terpenes, waxes, tannins, mineral salts etc., whichare usually present in the range 25% (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. r, pine, spruce, etc.)andbetween16and29%forhardwoods (e.g. birch, beech, maple, etc.).In contrast to cellulose and the hemicelluloses, which donot absorbdaylight, lignincontainsmanychromophores,which absorb especially in the UV region, and is thereforeeasilydecomposedbyphoto-oxidative processes. Chem-ically lignin is a randomcrosslinked product with anaveragemolecularmassofaround20,000containingdif-ferent C9phenylpropaneunitsoriginatingfromconiferyl,sinapinylandp-coumarylalcohols. Ligninisaverycom-plexnatural polymeranditscompositiondependsonthedifferent woodspecies. Onlyaveragechemical structurescan be drawn (Fig. 2). On the other hand, it is com-monlyacceptedthatthefollowingchemicalunitsplayanimportant role in the photo-oxidative de-polymerizationof lignin, which leads to the formation of brownish0300-9440/$ see front matter 2003 Published by Elsevier B.V.doi:10.1016/S0300-9440(03)00102-4298 P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309Fig. 1. Factors inuencing wood and coating.quinoide structures causing the darkening of irradiatedwood (Fig. 3) [2]. In outdoor weathering the degradedand de-polymerized lignin is washed out by rain from theunprotectedouterwoodsurfaces. Theremainingcellulosebers will thenappear as a woollysurface, whichwillturngrayishwithtimeduetocolonizationbybluestainfungi.Table 1Chemical composition of different wood species (% of dry wood weight)Species Common name Exta(%) Ligb(%) Celc(%) Glmd(%) Glxe(%) O.p.f(%)SoftwoodsAbiesbalsamea Balsam r 2.7 29.1 38.8 17.4 8.4 2.7Pseudotsugamenz. Douglas r 5.3 29.3 38.8 17.5 5.4 3.4Tsugacanadensis Eastern hemlock 3.4 30.5 37.7 18.5 6.5 2.9Juniperuscommunis Common juniper 3.2 32.1 33.0 16.4 10.7 3.2Pinusradiata Monterey pine 1.8 27.2 37.4 20.4 8.5 4.3Pinussylvestris Scots pine 3.5 27.7 40.0 16.0 8.9 3.6Piceaabies Norway spruce 1.7 27.4 41.7 16.3 8.6 3.4Piceaglauca White spruce 2.1 27.5 39.5 17.2 10.4 3.0Larixsibirica Siberian larch 1.8 26.8 41.4 14.1 6.8 8.7HardwoodsAcerrubrum Red maple 3.2 25.4 42.0 3.1 22.1 3.7Acersaccharum Sugar maple 2.5 25.2 40.7 3.7 23.6 3.5Fagussylvatica Common beech 1.2 24.8 39.4 1.3 27.8 4.2Betulaverrucosa Silver birch 3.2 22.0 41.0 2.3 27.5 2.6Betulapapyriferea Paper birch 2.6 21.4 39.4 1.4 29.7 3.4Alnusincana Gray alder 4.6 24.8 38.3 2.8 25.8 2.3Eucalyptuscamaldul. River red gum 2.8 31.3 45.0 3.1 14.1 2.0Eucalyptusglobules Blue gum 1.3 21.9 51.3 1.4 19.9 3.9Gmelinaarborea Yemane 4.6 26.1 47.3 3.2 15.4 2.5Acaciamollissima Black wattle 1.8 20.8 42.9 2.6 28.2 2.8Ochromalagopus Balsa 2.0 21.5 47.7 3.0 21.7 2.9aExtractives (extraction with CH2Cl2, followed by C2H5OH).bLignin.cCellulose.dGlucomannan.eGlucuronoxylan.fOther polysaccharides.3. State of the art stabilizationTo avoid such photo-oxidative degradation, wood has tobe protected. This is usually done by the application of oneormorecoatinglayers. Manytypesofcoatingsareusedincluding alkyd varnishes, 2P-PURsystems and acryliccoatings, which are either available as solvent-borne and/orP.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 299OOOOOHOHOHOOHOOOO HOHOHOOH OHOO OO HO HOOOHOOHOHOOH OO HO HOOOHOHO HOOOOOHOOOO HOHOHOFig. 2. Spruce lignin fragment.water-borne compositions, whereas acrylate binders aremainly used in UV-curable systems. It has been shown thattheUV-fractionofthesolarradiationiscausingmost ofthechemical modicationandmechanical breakdowninthe wood surface layer, thus explaining the origin of adhe-OOH ligninOOOligninHOHOOligninC

OOHHOligninOketyl radicalLigninR, RO,ROO. .h .R, RO,ROO. . .OOligninOHOligninOO

+ROROO..oxidation by-O-4 cleavageligninOOO ligninOOand other chromophoresSchmidt and Heitner, J. Wood Chem., Technol., 15, 223-245 (1995)Fig. 3. Photo-oxidation of lignin.sionfailuresofmost clearortransparent coatings[3]. Inthis sense UV-light has to be screened before it reaches thewood surface. This has lead to the development of differentUV-absorbers, mainlyofthebenzophenoneandbenzotri-azoletype, whichareincorporatedinthecoatinglayers.300 P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309Fig. 4. State of the art UV-absorbers and HALS.Fig. 5. Effect of UVA-2 and HALS-1 on color change of pine.For thecolor stabilizationof indoor clearcoats, typicallyorganic UV-absorbers are used in concentrations of 13%(calculated on binder), depending on the coating thickness.For exterior applications, clear varnishes andtransparentpigmented stains require 15% UV-absorber (calculated onresin solids) or 13% UV-absorber/HALS (hindered aminelight stabilizer) blends to obtain the desired protection, e.g.UVA-1, UVA-2, HALS-1 and HALS-2 (Fig. 4). Some per-formancedataofsuchstateoftheartlightstabilizersaregiven in Fig. 5. To meet the increasingly higher quality re-quirements regarding the protection of the wood surface andthe retention of the initial natural wood color, new high per-NNNOOHO O HOHOOOO O O OC8H17iC8H17i C8H17i NNNOOHOH O HOHOOOO OC8H17iC8H17iNNNOOO O HOHOOOO O O OC8H17iC8H17i C8H17iOOC8H17ibis tris tetraFig. 6. UVA-3: a mixture containing new TRT-derivatives.formancestabilizersandatwo-componentprotectionsys-tem have been developed.4. Development of a new UV-absorber for woodprotectionInadditiontoUV-light intherange300350 nm, alsoUV-light in the region from 350 to 400 nm and even visiblelighthasaneffectonthediscolorationofwood.Depend-ing on the wood species, discoloration can be observed afterdaylightirradiationwithwavelengthsupto500 nm[3].Itis therefore necessary to develop new UV-screeners, whichhave a more pronounced red-shift in their absorption spectracompared to the currently used benzophenones (e.g. UVA-1)and benzotriazoles (e.g. UVA-2) which have mainly absorp-tions with max in the region of 325345 nm.Ideally such newUV-absorbers would absorb all UV-lightup to 400 nm, but without any absorption tailing into the vis-ible to avoid a yellow coloration of the wood substrates tobe protected. In addition they have to be very photo-stablefor efcient long-term protection. High compatibility of theUV-absorber in the different coating substrates is very im-portant, in order to prevent loss of performance by washingout or evaporation of the UV-absorbers. From an applicationpoint of view, liquid UV-absorbers are preferred.Basedonthisproductprole, manynewUV-absorberswere designed, synthesized and tested. Screening results re-vealed derivatives of the tris-resorcinol-1,3,5-triazine (TRT)P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 30100.10.20.30.40.50.60.70.80.9250 300 350 400 450 500nmODBisTrisTetraConcentration: 10mg / L ethylacetateFig. 7. Spectra of bis-, tris- and tetra-substituted TRT.class as the most promising candidates. A mixture of simi-lar products was nally chosen (new UVA-3) containing thefollowing products (Fig. 6).Eachof thethreeUV-absorber partshasitsownUV-spectrum (Fig. 7), whereas the bis- and tris-para-substitutedtris-resorcinol-triazines have a red-shiftedspectrumwitha maxaround 360 nmand the tetra-substituted com-poundwitha maxat about 350 nm(dependingonthesolvent). The tetra-substituted compound is in additionConcentration: 20mg / L toluene 00.20.40.60.811.2290 310 330 350 370 390wavelength (nm)absorbanceUVA1UVA2UVA3Fig. 8. Comparison of different UVA absorption spectra.to its UV-absorbance responsible for the high solubil-ityof this mixture. Incomparisonwiththestateof theart UV-absorbersUVA-1andUVA-2, thered-shift oftheabsorbance of the newUVA-3 is very pronounced. Anadditionalbenetisthemuchhigherabsorptionpergramcompared to the state of the art UV-absorbers (Fig. 8). Themixture of these three TRT-derivatives resulted in a viscousoil, which is soluble in almost any solvent from non-polarhexane to polar dimethylformamide, but not in water. Very302 P.Hayozetal. / ProgressinOrganicCoatings48(2003)2973090204060801000 500 1000 1500 2000UVA-340 exposure time (h)remaining UV absorber (%)UVA 3UVA 3 /UVA 22:1UVA 3 /UVA 21:1UVA 2UVA 1 Experiment done in an alkyd long oil coatingFig. 9. Photo-permanence of different UVAs.little organic solvent (1020 wt.%) was nally necessary tocreate a liquid and easy to handle UV-absorber compositionwith a viscosity of about 2 Pa s.5. Application data and performance tests for UVA-3A photo-permanence study was made in a solvent-bornelong oil alkyd coating under dry conditions with UV-340 nmFig. 10. UVA de-activation mechanism for HPTs.exposure(uorescent light tubes) for upto2000 h. Thisrevealed a very high photostability compared to ben-zophenone and benzotriazole UV-absorbers. As wellblends of UVA-3 with UVA-2 in a 2:1 and 1:1 ra-tio showed better photostability than UVA-1 or UVA-2alone(Fig. 9). Theenergy-dissipationmechanism, whichleadstothishighphotostabilityseemstobeacyclicex-citedstateintramolecularproton-transfer(ESIPT)mecha-nisminanalogytothat ofotherhydroxy-phenyl-triazinesP.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 3030 5 10 15 20 25 30 35 40 45without stabilizer0.5 % UVA 31 % UVA 31.5 % UVA 32 % UVA 34 % UVA 31 % UVA 22 % UVA 23 % UVA 24 % UVA 21 % UVA 12 % UVA 13 % UVA 14 % UVA 1delta Gloss 60 after 2800 h QUVA for woodDifferent amounts UV absorber on solid content; 3 coats, DFT: ~80 mFig. 11. Accelerated weathering of a long oil alkyd system on r.(HPTs), which have been studied extensively (Fig. 10)[46].Inanacceleratedweatheringexperiment, differentcon-centrationsfrom0.5upto4%ofUV-absorberswereap-pliedinalongoilalkydsysteminthreecoatsonr(drylmthickness(DFT)ca.80 m)andexposed2800 htoacycle of 5 h UV-340 nm light exposure at 58C and 1 h ofrainat 22C. Deltaglossat 60hadbeenmeasuredandshowedremarkableglossretentionforUVA-3, evenwithverylowconcentrations(Fig. 11). Inanotherexperiment,2 % UV absorber on solid content; 3 coats, DFT: ~80 m051015202530350 500 1000 1500 2000QUV wood exposure (h)E*unstabilizedUVA 3UVA 1UVA 2*****crackingFig. 12. Accelerated weathering of long oil alkyd system on r wood.E was measured for 2% UV-absorber in a long oil alkydsystemon r with the same weathering spray cycle describedabove, until rst cracks appeared. UVA-3 gave a somewhatlonger protection against cracking than the state of the artUV-absorbers and as well a better protection against discol-oration (Fig. 12). For indoor application simulation, UVA-1,UVA-2 and UVA-3 were applied in concentrations of 1 and2% in a 2P-PUR system on r (three coats, total DFT ca.100 m) and exposed over 3000 h to UV-340 nm. Whereasthe E value of the unstabilized sample rose very rapidly,304 P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309Fig. 13. UVA exposure of 2P-PUR system on r.theE increase of the stabilized samples was retarded intheorderUVA-3 >UVA-2 >UVA-1(Fig. 13). Asim-ilar picturewasobtainedduringUV-340 nmexposureofalongoil alkydsystemonr(fourcoats, total DFTca.120 m) withaUV-absorber concentrationof 2%. AfterFig. 14. UVA exposure of long oil alkyd system on r.05101520unstabilized2% UVA 22% UVA 3cure speed (m/min)050100150200pendulum hardness (s)cure speedpendulum hardness3% Hydroxyketone PI; 2xHg 80W/cm, 5m/min, 50 mFig. 15. UV absorber inuence on through cure and cure speed in an epoxy acrylate.an exposure of over 4000 h, UVA-3 showed an even morepronounced color retention than the UVA-1 and UVA-2 intheprevious experiment (Fig. 14). It canbestated, thatUVA-3 is a clear improvement to the state of the art in theseapplications.The questionthencame up, if this newUV-absorberhad an inuence on the curing speed and pendulum hard-ness of UV-curing coatings systems. In a rst experiment,2%of UVA-2andUVA-3wereappliedwithamixtureof 3%of hydroxyketone andacyl-phosphine-oxide pho-toinitiatorinanepoxyacrylateandcuredwithamercurylamp. Inbothcases, thecurespeedwasonlyslightlyaf-fectedcomparedtoanunstabilizedsample. Alsothepen-dulumhardnesswasineverycasecomparable(Fig. 15).Ina600 hUV-340 nmexposureexperiment ofUV-curedepoxy-acrylate and polyurethane-acrylate systems contain-ing3%ofahydroxyketone/acyl-phosphine-oxidemixtureas photoinitiator and 2% of the UV-absorbers UVA-2 andUVA-3,asomewhatbetterperformance(E)wasfoundwith the UVA-3 stabilized samples, so it was shown, that theP.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 3053% Hydroxyketone PI; 2xHg 80W/cm, 5m/min, 50 m051015202530unstabilized2% UVA 2 2% UVA 3E* variationEP-ACPU-ACFig. 16. UV absorber inuence on color stabilization of r wood after 600 h UVA exposure.newUV-absorberhadnonegativeinuenceonUV-curedsystems (Fig. 16).Overall these application data showan important improve-ment of the wood protection against discoloration comparedto the state of the art. As it was mentioned earlier, completescreening of UV-light up to 400 nm would give potentiallyevenbetter results against thediscolorationof irradiatedwood. Because of the more or less gaussian curve shape of allUV-absorption bands, such a full screening of the UV-lightwould be only possible, if at the same time a considerableamount of visible light would be absorbed, leading to strongyellow coloration of the protected wood. In this sense thedevelopment of a new stabilizer working by a different pro-tection mechanism than light absorption is required.6. Development of a new pre-treatment for woodprotectionAsitwasshownabove,ligninleadstocoloreddecom-positionproductsduetoaphoto-oxidativeradicalmecha-nism, when irradiated with light. Since not all wavelengthsofthedaylightelectromagneticspectrumcanbescreenedout asdiscussed, anattempt wasmadetocontrol there-maining radicals by bringing different HALS radical scav-engers into very close contact with lignin in the outer woodsurface. The radicals should then be converted into more orless harmless and colorless molecules by these radical scav-engers (Fig. 17).Different HALS in the amino form or their nitroxyl-aminoform were screened by applying them as a 2% solution inwater or ethanol directly onto r wood. After this procedure,the panels were coated with three layers of a clear varnishcontaining 2% of a commercial benzotriazole UV-screener(based on solid content) and the panels were then ex-posedunderdryconditionstoUV-340 nmlight for500 hto see if there was a stabilization effect. Indeed this HALSExcitedMoleculesFreeRadicalsYellowProductsUVlight O2UVAbsorbersRadicalScavengersLigninHarmless HeatHarmlessColorlessProductsFig. 17. Inhibiting the yellowing of wood (lignin).pre-treatment showed in one case a very good color reten-tion, measured by thebvalue (Fig. 18). The compoundHALS-3inquestionwasthenitroxyl formofthesimple4-hydroxy-2,2,6,6-tetramethylpiperidine (Fig. 18).It may be postulated that the observed color reten-tionistheresult of aradical scavengingmechanismac-cording to the modied Denisov cycle [7,8], where alignin-radical combines withHALS-3togivethecorre-sponding NOR-product. This NOR-product will react withaperoxy-radical tofreeagainHALS-3andat thesametimeproduceamoreorlessharmlessketoneandalcoholfunctional species(Fig. 19). Theexact mechanismisnotknown up to date.7. Application data and performance tests for HALS-3From the screening experiments it became obvious, thatthe solvent hada signicant importance for the perfor-mance of the different tested HALS. HALS-3 was ap-pliedindifferent solventsasa2%solutiononr wood,andthepanel coatedwiththreecoats of aUV-absorber(based on solid content). Water and interestingly a306 P.Hayozetal. / ProgressinOrganicCoatings48(2003)2973092 % HALS;3 layer top coat with 2 % bzt-UV-absorber (on solid content) on fir; 500 hoursUVA-340024681012no pretreatmentHALS 3in ethanolHALS 3in waterNO

OHHALS-3Fig. 18. Screening of HALS derivatives.polyethyleneglycol-derivative gave the best performance(measured byb value), and the least polar solvent buty-lacetate gave the worst performance after UV-340 nm expo-sure. It seemed to be very important that the polar HALS-3could be easily transported into deeper layers of the polarwood surface (Fig. 20). The same trend was seen during thescreening experiment, where the less polar HALS showeda lower performance.Thequestionwasthen, whichconcentrationwouldbenecessary for the best protection with HALS-3. A concen-trationladder experiment was madetaking0, 1, 2, 3, 4and5%ofHALS-3inwaterasprimerandathreelayerFig. 19. Postulated mechanism (modied Denisov cycle) for radial scav-enging with HALS-3.top-coat containing 2% of a commercial UV-absorber. Af-ter 500 h UVA-340 exposure, the measurement of b valueandEvalue showed that the concentration of 12% ofHALS-3 was ideal and that higher concentrations gave nosignicant improvement of color retention, but the protec-tioneffectagainsttheunstabilizedsamplewasveryclear.In a similar experiment it was tested whether if the appli-cation of one or more layers of primer containing HALS-3was important. The result was that already one layer gavevery good protection against discoloration compared to theunstabilizedsample(Fig.21).Alltheseexperimentsweresofar performedonr wood, andit was interestingtosee, whether this pre-treatment with HALS-3 was workingforotherwoodspeciestoo. Differentwoodsampleswerepre-treated with a 2% water solution of HALS-3 and thencoatedwiththreelayersofatop-coatcontaining2%ofacommercial UV-absorber. The samples were exposed dur-ing 1500 h to UV-340 nm and the b value and E valuewere measured. As a general result it was found that on palewoodspecieslikeashorpine, theprotectionagainst dis-coloration was more pronounced than for the darker speciessuch as nut and cherry wood (Fig. 22).8. The new two-component system for woodprotectionTo reach maximumprotection against wood discol-oration and decomposition, the pre-treatment with a primercontaining HALS-3 and a top-coat containing the newUVA-3 were combined and tested (Fig. 23). UVA-1, UVA-2P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 3073 layer top coat with 2 % bzt-UV-absorber (on solid content)-20246810100 300 500UVA-340 exposure time (h)Delta b*no pretreatmentwaterethanolbutylacetatediethylenglycolmono-ethyletherFig. 20. Solvent dependent performance of HALS 3.3 layer top coat with 2 % bzt-UV-absorber(on solid content)500 hours UVA-340 lamps-202468101214160 layer 1 layer 2 layers 3 layersDelta b* (500 h)-4-2024681012140% 1% 2% 3% 4% 5%HALS-3concentration in waterDelta b*-/E* (500 h)Delta b* Delta E*Fig. 21. Concentration dependent performance of HALS-3.2 % HALS-3;3 layer top coat with 2 % bzt-UV-absorber(on solid content) 1500 hoursUVA-340 exposure0510152025oakashmaplebeechoregonpinepinenutcherryDelta b*/E*Delta b* no HALS-3Delta b* with HALS-3DE* no HALS-3DE* with HALS-3Fig. 22. Application of HALS-3 on different wood species.308 P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309Fig. 23. Two-component protection against wood discoloration.and UVA-3 were applied on a r wood panel, with a2%concentration in a long oil alkyd top-coat withoutpre-treatment, andUVA-1andUVA-3in2%concentra-tion after HALS-3 pre-treatment. During the exposure withUV-340 nm the yellowness-index was measured. After over2000 h of exposure time it was found that the sample pro-tected by the combination of UVA-3 and HALS-3 showedalmost no discoloration at all, whereas the samples withoutpre-treatmentshowedsignicantyellowing(Fig.24).Inasimilar experiment with a solvent-borne 2P-PUR top-coat,thesametrendwasobservedafter 1200 hexposurewith2%UVA-3 /2%HALS-3unstabilized2% UVA-12%UVA-32%UVA-1/2%HALS-3long oil alkyd top coat upon UVA 340 light exposureUVA in topcoat/ HALS-3 in primer2% UVA-25070901100 500 1000 1500 2000exposure time (hours)Yellowness indexFig. 24. Effect of new UVA-3 and HALS-3 on color change of r wood.UV-340 nm conrming the almost total protection of woodagainst discoloration with the newtwo-component sys-temHALS-3/UVA-3(Fig. 25). Finally, thefour differentwoodspeciesash,maple,cherryandnutwerepre-treatedwitha1%primersolutioncontainingHALS-3, andthencoated with two layers of a water-borne acrylate con-tainingeach0.25and0.5%ofUVA-3, respectively. Evaluesweremeasuredafter 200 hQ-UVA340exposure.Again the pale wood species ash and maple showed a dis-tinct inhibitionof yellowing, due tothe newstabilizers(Fig. 26).P.Hayozetal. / ProgressinOrganicCoatings48(2003)297309 3092%UVA-3 /2%HALS-3unstabilized2% UVA-12% UVA-32% UVA-2solventborne2P PUR coat upon UVA 340 light exposureUVA in top coat / HALS-3 in primer4060801000 400 800 1200exposure time (hours)Yellowness indexFig. 25. Effect of new UVA-3 and HALS-3 on color change of r wood.051015202530Delta Eunstabilized stabilizedashmaplecherrytnu1 % HALS-3 inwbprimer 0.25 % UVA-3in mid coat (wb acrylate), 0.5 % UVA-3in top coat (wb acrylate)200 hours UVA 340 exposure(amount is referred to formulation)Fig. 26. Effect of UVA-3 and HALS-3 on color change of different wood species.9. ConclusionIt has been shown that effective wood substrate color pro-tection increases with the use of more photo-stable and morered-shiftedUV-screeners. UVA-3, whichwasdesignedtomeettheserequirements, showedsuperiorperformancetothat of standard products. In addition, a hindered amine ni-troxyl light stabilizer has been found, which provides signif-icant improvement in color stabilization of wood when usedin impregnating pre-treatments with subsequent applicationofacleartopcoatincludingUVA-3asUV-screener. Bothnewproducts, especially in combination as a two-componentprotection system show very good application results on var-ious natural light shade wood species. In these cases almosttotal protection against discoloration was achieved.Even if the experiments were mainly focused on indoorapplications, thecombinationofthesenewstabilizersarebeingtestedinoutdoor applications as well (acceleratedweathering and outdoor testing).References[1] E. Sjstrm, WoodChemistry, FundamentalsandApplications, 2nded., Academic Press, San Diego, 1993, p. 249.[2] J.A. Schmidt, C. Heitner, J. WoodChem. Technol. 15(2) (1995)223.[3] H. Derbyshire, R. Miller, Holz Rohw. Werkst. 39 (1981) 341.[4] J.-E.A. Otterstedt, J. Phys. Chem. 58 (1973) 5716.[5] H.E.A. Kramer, et al., J. Phys. Chem. 99 (1995) 10097.[6] H.E.A. Kramer, et al., J. Phys. Chem. A 104 (2000) 8296.[7] E. Denisov, Proceedings of the International Symposium onDegradation of Polymers, Brussels, 1974, p. 137.[8] P. Klemchuk, M. Gande, E. Cordola, Polym. Degrad. Stab. 27 (1990)65.