New approaches in wood coating stabilization.

Abstract A major drawback in the use of wood materials for construction and decorative applications is its sensitivity to light. Numerous researchers have shown that the ultraviolet (UV) as well as fractions of the visible (VIS) spectrum of solar radiation cause most of the chemical modification and mechanical breakdown of exposed wood. To insure the long life of wood substrates, they are usually coated with various decorative and protective finishes. The protection effect depends strongly on the opacity, i.e., the degree of pigmentation of the coatings. However, the use of UV absorbers (UVA) and hindered amine light stabilizers (HALS) both in the coating system and also in a direct wood impregnation step can improve the light stability considerably. Here the knowledge of the light sensitivity of wood is crucial for an optimal photoprotection concept for coatings and the wood substrate. This paper highlights findings of a systematic investigation of the light sensitivity of pine wood (Pinus radiata). Furthermore, it describes the correlation between quantity and type of organic UVA and HALS as well as the lignin stabilizer concept and the degree of pigmentation of the coating. These findings are of great importance for an efficient and economic use of light stabilizers in film-forming wood coating systems for pale wood species and allow a cost-effective stabilization.

Keywords Wood, Wood coatings, Photooxidation of lignin, FTIR, UV absorber, HALS, Lignin stabilizer

Introduction

Wood remains a remarkable material for construction and decoration purposes. Displaying the elegance and appearance of the color, grain, and texture of wood supports the general trend toward natural products. Transparent or semitransparent coatings are designed to highlight and preserve the natural beauty of wood. High performance and durability of the coating systems is mandatory to maintain this look during the entire service lifetime of the end product. The popularity of natural wood as a design and construction material for outdoors would even be increased if significant improvements could be achieved in longer durability and reduced maintenance work. Color change may occur early in the life of even indoor applications, especially for pale wood species which tend to yellow and darken as a result of wood surface photooxidation. Under outdoor conditions the deleterious effect of wood weathering has been ascribed to a complex set of reactions induced by a number of factors like solar radiation (UV and VIS), water (dew, rain, snow, and moisture), temperature (night and day, seasons), and oxygen. These changes are caused by the photooxidation (indoor) of wood leading to wood surface discoloration, and subsequent photodegradation (outdoor) resulting in cracking and flaking of the wood coating film and finally erosion and graying of the wood.

The photooxidation of wood due to sunlight occurs fairly rapidly on the exposed wood surface. (1) To understand the unwished behavior of wood it is important to know about the wood biology and the related mechanism of photooxidation and degradation. Chemically wood is a complex biopolymer composed of structural polysaccharides, essentially cellulose, hemicelluloses, and lignin. Cellulose is a long, linear polymer based on repeating dimeric anhydro cellobiose units and acts as structural substance (2); i.e., the skeleton of wood and comprises about 40-50% of wood. Hemicellulose consists of a mixture of polysaccharides with short side chains and a much lower degree of polymerization than cellulose surrounding the cellulose fibers of plant cells and accounts for almost 20-30%. (3) Lignin (15-30%) is an amorphous aromatic 3D polymer based on repetitive phenyl propanol units and plays an essential role as strength provider. (4,5) Lignin contains chromophores with aromatic conjugated bond systems and carbonyl groups, and as such, its interaction with light (UV/VIS) in the presence of oxygen is the main cause of photooxidation of wood whereas the contributions of cellulose and hemicellulose are minimal. (6)

Photooxidation of lignin refers to a process where this polymer undergoes chemical modifications such as bond cleavage, and hydrogen abstraction resulting in radical formation, formation of peroxides with oxygen and finally decomposition with production of colored and hydrophilic by-products, so called chromophores. These modifications induce surface property changes ending up with discoloration, increased water sensitivity followed by hydrolysis, leaching, and cracking. (7) The mechanisms of photooxidation of lignin are well understood and documented. (8,9) It is generally accepted that at least four reaction pathways are identified: (1) direct absorption of UV light by conjugated phenolic groups to form phenoxyl free-radicals, (2) abstraction of phenolic hydrogen as a result of aromatic carbonyl triplet excitation to produce a ketyl and phenoxyl free-radical, (10) (3) cleavage of nonphenolic phenacyl-[alpha]-O-arylethers to phenacyl phenoxyl free-radical pairs, (11) and (4) abstraction of benzylic hydrogen of the [alpha]-guaiacylglycerol-[beta]-arylether group to form ketyl free-radicals which then undergoes cleavage of the [beta]-O-4 arylether bond to produce an enol and phenoxyl free-radical. (12,13) The enol then tautomerizes to a ketone. Alkoxyl and peroxyl free-radicals produced from the reaction of oxygen and lignin free-radicals react with the phenoxyl free-radical formed to produce colored chromophores, e.g., quinoides, aromatic ketones, aldehydes, and acids as photodegradation products. (8,9)

The key for improving the service lifetime of wood coating systems with film forming clear finishes is the optimization of UV and VIS light protection of the wood surface itself in addition to the prevention of the alteration processes in the coating film. (14,15) This can be partly achieved by the use of certain transparent micronized pigments like titanium dioxides and iron oxides which reflect and/or absorb UV and VIS light but at the levels required to get sufficient UV protection these pigments tend to reduce film clarity and provide some degree of color. In contrary organic UVA do not have the drawback of reducing coating transparency and masking the wood grain and pattern. Two of the early classes of UVA employed commercially were 2-hydroxybenzophenones (BP) and oxalanilides (Ox), both characterized by poor spectral coverage and inferior photopermanence. Today, UVA based on 2-(2-hydroxyphenyl)-benzotriazoles (BTZ) chemistry with broader absorption profiles and better photostability are considered to be the most important class for the stabilization of clearcoats in a variety of end uses. (16) Still, some limitations of benzotriazole-based UVA (17) have, for certain applications, pushed the paint industry to adopt a new UVA class based on the 2-hydroxyphenyl-s-triazine (HPT) chemistry with a number of advantages. (18) For example, for wood applications, due to the particular sensitivity of wood, and especially lignin, to UV and also VIS light, it became necessary to develop a concept for optimal protection of wood substrates and wood coatings. First, a new UVA based on a substituted tris-resorcinol triazine derivative with improved photo-permanence, highest extinction, and providing broadest UV-A wave (so-called red-shifted) absorption was introduced for high-performance solventborne (19) and in encapsulated form for waterborne wood coating applications. (20,21) The transmission spectra of organic state-of-the-art BP-1, BTZ-1, and HPT-1 (1% in LOA, dry film thickness (DFT) = 20 [micro]m on glass) are shown in Fig. 1 (see "Material and methods" section). The 50% transmission wave numbers of the UVA types (BP-1: 362 nm; BTZ-1: 385 nm; HPT-1: 392 nm) clearly indicate the broadest UV absorption profile of HPT-1 but none of these UVA is designed to screen the VIS light over 400 nm as they have to remain colorless. Therefore a way had to be found to stop the effect of unscreened VIS light hitting the wood surface and causing lignin photooxidation. The means has been found by application of a lignin stabilizing pretreatment, which provides improved color stability in interior applications and long-term durability to transparent and semitransparent wood coatings in exterior applications. (22,23) The active ingredient is based on a special HALS derivative which traps the radicals formed at the wood surface by VIS light not screened by organic UVA (>400 nm) (24,25) and provides effective color stabilization. According to this new concept an optimized wood protection can be achieved, when the lignin stabilizer is applied as a dilute aqueous solution or in an already utilized primer formulation directly onto wood. (26) To obtain durable protection effects, the treatment has to be associated with selected UVA (for indoor applications) or UVA/HALS (for outdoors) in the same treatment (internal UV filter effect) or better in a subsequently applied topcoat (external filter effect). Besides the improved aesthetical aspects other benefits of the better wood surface protection can be extended retention of properties such as coating adhesion and resistance to failures as flaking and cracking and therefore resulting in higher durability and extended maintenance intervals. Especially for high-performance outdoor applications where long-lasting coatings are required, e.g. wooden window frames, this concept finally allows the use of non- or low-pigmented clear coatings which often failed in the past due to insufficient light protection of the wooden substrate. However, most of the paints used are semitransparent finishes, where the used pigments may act partly as VIS light protectors. This protection effect can be increased by combination with organic UVA for substrate protection and HALS for coating protection. It is well known that such combinations of organic and inorganic UV/VIS light screeners show good performance when used in certain ratios. The knowledge of how to use such combinations is still limited and has to be improved to obtain efficient and economic use of light stabilizers for film forming wood coating systems depending on the opacity of the coating, i.e., the degree of pigmentation.

[FIGURE 1 OMITTED]

This paper highlights a systematic investigation of the effect of light at different wavelengths on pine (Pinus radiata) in the area of 300-500 nm. The knowledge of the light sensitivity of pine is crucial for an optimized use of light stabilizers. Based on these findings a new concept for wood coatings and wood substrate photoprotection is introduced. Here the latest developments of the use of simple additives that can prevent (UVA) and/or control (HALS) the photooxidative reactions for pale wood species and their consequences in film-forming systems are discussed.

Material and methods

For the light sensitivity experiment, standard pine veneers (P. radiata) (50 mm x 50 mm x 2 mm) were used. The light source for this study was Xenon light for interior applications according to Xe-WOM CAM 0 cycle (outer filter "sodalime" (window glass)/inner filter Boro S) in accordance with DIN EN ISO 11341 C (0.35 W/[m.sup.2]/340 nm: permanent light, black panel temperature (BPT) (50 [+ or -] 2)[degrees]C, relative humidity (RH) (50 [+ or -] 5)%) by using an Atlas Weather-Ometer Ci-65 A. For the artificial and natural weathering studies, the samples were made of standard pine panels (P. radiata) (200 mm x 90 mm x 10 mm) with back and end grain sealing with an automotive 2K-PUR clearcoat. The light source for the artificial weathering study was Xenon light according to the Xe-WOM CAM 7 cycle for exterior applications (outer filter boro silicate/inner filter boro silicate) in accordance with DIN EN ISO 11341 A (0.35 W/[m.sup.2]/340 nm: 102 min light, BPT (60 [+ or -] 2)[degrees]C, RH (35 [+ or -] 5)%; 18 min light and spray, BPT (35 [+ or -] 2)[degrees]C, RH (95 [+ or -] 5)%) by using an Atlas Weather-Ometer Ci-65 A. The natural weathering was a 45[degrees] south open rack exposure in Pfeffingen, Switzerland (E 7[degrees]35'0" N 47[degrees]25'0"; elevation 400 m; climatological data 2006 (http://www.klimabasel.ch): average ambient temperature: 10.9[degrees]C. annual mean rainfall: 1055 mm, annual mean total radiant exposure: 3150 Wh/[m.sup.2]).

The paint used is a commercially available linseed oil based Long Oil Alkyd (LOA) paint for exterior wood coatings with different degrees of pigmentation based on black, red, and yellow transparent iron oxides (TIO) as well as white pigments. The used UVA and HALS types are: BP-1: methanone, 2-hydroxy-4-(octyloxy)-phenyl-; BTZ-1: benzene propanoic acid, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-, C7-9-branched and linear alkyl esters; HPT-1: 1-isoctyloxycarbonyl ethylated 2,4,6 tris(2,4-hydroxyphenyl)-1,3,5 triazine derivatives; HALS: decanedioic acid, bis(2,2,6,6-tetramethyl-l-(octyloxy)-4-piperidinyl) ester. The lignin stabilizer pretreatment was performed with a 2% aqueous solution of 4-hydroxy-2,2,6,6-tetramethylpiperidinoxyl applied by brush (1 x 80 g/[m.sup.2]). To simulate the light influence at different wavelengths, long-pass filters (N-WG 305, GG 385, GG 400, GG 435, GG 475, GG 495 from Schott, Germany) were used with different transmission profiles; the figure in the filter name reflects the 50% transmission wavelength. Integrated transmission spectra were recorded on paint films (DFT = 20 [micro]m over glass) between 200 and 800 nm with 1 nm resolution by using a Perkin-Elmer UV/VIS/NIR spectrophotometer Lambda 900. The transmission spectra of the used long-pass filters and the LOA paint stabilized with 1% UVA on total paint are shown in Fig. 1.

ATR spectra were recorded between 4000 and 850 [cm.sup.-1] with 1 [cm.sup.-1] resolution and 64 scans by using a Bruker IFS55 spectrometer. As measuring size of the absorption change the relative intensity (RI) was introduced. The relative intensity RI is defined as the intensity of the absorption after light exposure I divided by initial intensity [I.sub.0] (I/[I.sub.0]); i.e., RI = 1: no change, RI > 1: increase in absorption due to structure formation, and RI < 1: decrease in absorption due to disappearance of structures. For RI a standard deviation of [+ or -]5% is assumed. The coloristics are measured with a Minolta CM-3600d device (gloss included) and calculation of L*, a*, b*, C*, h, and [DELTA]E* with CGREC software according to DIN 6174 with a standard deviation on wood of [+ or -]1 [DELTA]E* units. The gloss evaluation is performed at 60[degrees] with a Byk/Gardner Micro-Tri-Gloss equipment according to DIN 67530 with a standard deviation on wood of [+ or -]2 gloss units. The cracking and chalking evaluation is performed visually according DIN EN ISO 4628-4.

Results

Light sensitivity experiment

The chemical changes occurring on pine covered with different long-pass filter types during Xenon light exposure are recorded by FTIR ATRP spectroscopy. The assignments of bands in the infrared spectrum of pine are summarized in Table 1.

The change of intensity of absorption peaks at specific wave numbers of the characteristic C=O stretching vibration of the carboxyl groups of hemicellulose and chromophores (1730 [cm.sup.-1]), the C=C stretching vibration in aromatic ring of lignin (1450, 1505 [cm.sup.-1]), and the OH in-plane deformation/CH bending vibration of cellulose and hemicellulose (1370, 1330 [cm.sup.-1]) provide indication about the chemical changes on the wooden surface during light exposure. The absorption region of the C=O stretching vibration of the carboxyl groups (1850-1550 [cm.sup.-1]) and the C=C stretching vibrations of lignin (1530-1440 [cm.sup.-1]) during Xe-WOM CAM 0 exposure are shown in Fig. 2.

The absorption of the carboxyl C=O stretching vibration at 1730 [cm.sup.-1] increases during Xe-WOM CAM 0 exposure (see Fig. 2A). In contrary, the absorption of the lignin C=C stretching vibration at 1510 [cm.sup.-1] decrease (see Fig. 2B). The absorption of the OH in-plane deformation/CH bending stretching vibration of the cellulosic and hemicellulosic materials stays more or less constant during exposure (not shown here). This first result fits to the theory and the mentioned lignin degradation mechanism where mainly lignin breaks down during light exposure under formation of chromophores containing carboxylic structures. The chromophore formation occurs very fast (1) and can be related to the formation of quinoides, aromatic ketones, and aldehydes. (8,9)

[FIGURE 2 OMITTED]

Figure 3 shows the relative intensity RI of the absorption of the chromophore C=O stretching vibration at 1730 [cm.sup.-1] ([RI.sub.1730]), the absorption of the lignin C=C stretching vibration at 1510 [cm.sup.-1] ([RI.sub.1510]), and the absorption of the cellulose and hemicellulose OH in-plane deformation/CH bending vibration at 1330 [cm.sup.-1] ([RI.sub.1330]) during Xe-WOM CAM 0 exposure. Besides this, the color deviation [DELTA]E* and the chroma deviation [DELTA]C* of the pine samples during Xe-WOM CAM 0 are given.

Figure 3 clearly shows the increase in [RI.sub.1730] related to chromophore formation while lignin structures are destroyed (decrease in [RI.sub.1510]) and a--within the error margin--constant [RI.sub.1330] of cellulose and hemicellulose. Changes are very fast within the first 100 h of exposure, followed by flattening out in a plateau value. This can be seen as a total conversion of light accessible lignin into chromophores and other byproducts resulting in a kind of preservation layer on the wooden surface. A comparison with the coloristic results of the exposed pine samples shows an excellent correlation between chromophore formation and discoloration of the wooden surface. The changes in the chroma C* and [DELTA]E* follow the same pattern as the relative intensity of the chromophores [RI.sub.1730]. Based on these results it can be stated that the color deviation of pine during light exposure is related to the formation of carboxylic chromophores as a result of the lignin degradation. This allows tracking the lignin degradation in a simple way by measurement of the color deviation; i.e., to follow microscopic changes by macroscopic evaluation. In a next step the influence of the light wavelengths on the lignin degradation was determined. Figure 4 shows the relative intensity of the chromophore [RI.sub.1730] of pine covered with long-pass filters of different wavelengths during Xe-WOM CAM 0 exposure.

[FIGURE 3 OMITTED]

The relative intensity [RI.sub.1730] clearly indicates a correlation between lignin degradation with chromophore formation and the wavelength of the exposed light, i.e. the used long-pass filter. With decreasing filter wavelengths [RI.sub.1730] decreases and only by using the 495 nm cut-off filter the chromophore formation can be completely suppressed. This result indicates that pine as a pale wood species is sensitive to light up to 500 nm and therefore needs UV and VIS light protection. The 385 nm cut-off filter corresponds to an organic UVA of the BTZ class and verifies the findings that pine can only be partially protected by the sole use of a BTZ UVA. Other experiences show that UVA with 50% light cut-off spectra in the 380-400 nm range only reduce pine wood discoloration in terms of [DELTA]E* increase by some 30% to 50% which corresponds well to the degree of reduction of [RI.sub.1730] observed here (Fig. 4). The recently introduced lignin stabilizer concept is the only way to overcome this problem. To verify the experience based findings that lignin stabilization improves the color stability, the filter experiment was repeated in the presence of the lignin stabilizer pretreatment. Figure 5 shows the relative intensity [RI.sub.1730] of pine covered with different long-pass filters (see Fig. 1) as well as LOA coated glass with BP-1 and HPT-1 in comparison with and without lignin stabilizer pretreatment after 230 h Xenon exposure.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

As already seen in Fig. 4, [RI.sub.1730] decreases with increasing filter wavelength. A complete inhibition of chromophore formation is given by using the 495 m cut-off filter. If the lignin stabilizer pretreatment is used without filter or at 305 nm no to only small effects on reduced chromophore formation can be seen. First improvement can be seen by combining the lignin stabilizer treatment with BP-1 (~362 nm). A first significant improvement of [RI.sub.1730] to almost no chromophore formation is obtained when combined with a filter of 385 nm (~BTZ-1) or 392 nm (~HPT-1). These results show that a remarkable color protection effect is obtained with the combination of the lignin stabilizing pretreatment and the UVA with the broadest absorption. Without any UVA the pretreatment has practically no effect, with poor UVA the protection effect appears to be much weaker. This experiment clearly confirms the already known behavior that the lignin stabilizer has to be combined with a UVA of the BTZ or even better the HPT class to obtain effective color stabilization.

Based on these results it can be stated that in principal two possible long-term stabilization approaches for pale wood species exist: the use of UV/VIS screeners with transmission profiles acting like the 495 nm filter (meaning pigmented systems) or a clear coating system combining a UV screener with 385 nm transmission profile preferably used in a topcoat in association to the lignin stabilizing pretreatment. The 495 nm filter types correspond to the transmission profile of transparent iron oxides with the drawback of coloration of the coating resulting in a semitransparent coating. If nonpigmented transparent clear coatings are required only the approach of combining organic UVA of the BTZ or HPT class with the lignin stabilizer guarantees good color retention and therefore good long-term durability. (28)

Light stabilization of transparent and semitransparent coatings

After the effectiveness of the lignin stabilizer concept for pine could be confirmed with model long-pass filters it was of interest to evaluate this concept in state-of-the-art wood coatings. Therefore a series of non-pigmented clear transparent, semitransparent, and pigmented coatings were investigated in the presence of organic light stabilizers in order to check their influence on color protection. As already seen the degree of pigmentation, i.e., the transmission profile of the pigments acting as VIS light screener, have severe influence on coloristic of the wood during exposure. Therefore the knowledge about the correlation between quantity and type of organic UVA and HALS as well as the lignin stabilizer concept in presence of inorganic pigments is an important factor for efficient and economic use of light stabilizers. Semitransparent LOA with different degrees of pigmentation based on TIO and also a white pigmented paint were used to examine the influence of the light stabilizers; i.e., UVA and HALS, and the lignin stabilizer concept on the overall performance of the system. The integrated transmission spectra of the used LOA paints are shown in Fig. 6. The LOA can be classified into clear; i.e., nonpigmented, (LOA-1), low (LOA-2, -3), medium (LOA-4, -5), and dark (LOA-6, -7) pigmented. Thereby the clear LOA-1 allows full transmission of light, i.e., no filter and LOA-1 with UVA corresponds to the 385 nm filter. The low to medium pigmented LOA correspond to filters between 400 and 475 nm and the dark pigmented LOA can be seen as filters >475 nm.

[FIGURE 6 OMITTED]

The improved performance of the lignin stabilizer concept on color and coating protection for nonpigmented clear systems is already known. The effect of the lignin stabilization concept on pine after 18 months 45[degrees] south exposure is shown in Fig. 7. Here the clear LOA-1 (wet: 2 x 120 g/[m.sup.2] by brush) without additives (LOA-1 A), with 1% HPT-1 and 0.5% HALS on total paint (LOA-1B) and with 1% HPT-1 and 0.5% HALS applied over a 2% aqueous lignin stabilizer pretreatment (1 x 80 g/[m.sup.2] by brush) (LOA-1C) were used.

[FIGURE 7 OMITTED]

After 6 months exposure a clear trend in color deviation can be seen. LOA-1A shows severe discoloration with loss of gloss. The use of HPT-1 and HALS (LOA-1B) obviously improves the coating properties (no gloss loss) but to a certain extent color deviation can be seen. The lignin stabilizer concept (LOA-1C) further improves the color retention to retain almost the initial color of the wood. After 12 months exposure LOA-1A exhibits loss of gloss, with starting delamination and cracking, whereas LOA-1B and LOA-1C show only slight changes in color. After 18 months LOA-1A shows severe erosion with graying of the wood surface, whereas for LOA-1B only slightly starting erosion can be seen. LOA-1C still shows no surface defects and excellent color retention. In general a combination of lignin stabilizing pretreatment and UVA/HALS in a nonpigmented paint shows excellent results in color retention of the substrate as well as retaining the mechanical properties of the paint film itself with good gloss retention and no cracking for all common pale wood species like pine, fir, and ash.

In a next step the lignin stabilizer concept should be evaluated in semitransparent pigmented LOA. The color deviation of pine with LOA (wet: 2 x 120 g/[m.sup.2] by brush) stabilized with 0.5% and 1.0% UVA and 0.5% HALS on total paint with and without lignin stabilizer pretreatment (2% in aqueous solution, 1 x 80 g/[m.sup.2] by brush) after 1000 h Xe-WOM CAM 7 exposure is shown in Fig. 8.

[FIGURE 8 OMITTED]

As already seen the color deviation of pale wood species like pine can be reduced to approximately 50% by using a clear nonpigmented paint with the appropriate amount of UVA, here 1% HPT-1. The use of the lignin stabilizer concept further improves the color stability to retain almost initial color. For low or medium pigmented LOA without organic light stabilizers, the color deviation after weathering is reduced compared to a clear LOA due to the VIS light screening effect of the used TIO. The color retention can be further improved by the use of an UVA and again the lignin stabilizer concept exhibits the best performance. However, the difference between using 0.5% HPT-1 and 1.0% HPT-1 is less pronounced compared with LOA-1: here, the use of 0.5% HPT-1 is sufficient to get a reasonable color retention. The dark and white pigmented systems show lowest color deviation and neither the use of UVA nor the use of lignin stabilizer show any significant benefit in terms of color retention. However, it is well known that color retention is not the main problem for dark and opaque pigmented systems. Earlier studies show that these types of coatings tend to suffer more from chalking and then complete erosion of the paint and the wooden surface whereas clear and low pigmented systems show primarily cracking. Therefore the use of UVA can be significantly reduced in dark and opaque paints but higher amounts of HALS are necessary to protect binders against photooxidation and avoid surface defects like gloss loss, chalking, and flaking. (29) The effect of HALS on the 60[degrees] gloss of LOA paints with different degrees of pigmentation after 12 months 45[degrees] south exposure outdoor exposure is shown in Fig. 9. The initial 60[degrees] gloss was around 30 for all paints. After exposure, paints without HALS had a 60[degrees] gloss of around 15. The use of UVA shows almost no improvement for all paints. The use of HALS is beneficial, regardless of whether UVA is used. For the low and medium pigmented paints (LOA-3, LOA-5) the higher levels of HALS have little advantage over the 0.5% addition. For the dark LOA-6 and the opaque LOA-8, obvious improvements in performance are only obtained by using 1.0% or even 2.0% HALS. These results indicate that the use of higher amounts of HALS is necessary with increasing pigmentation in the paint.

Conclusion

The outcome of the filter study clearly showed that UV and VIS light up to 500 nm cause lignin degradation with chromophore formation and discoloration of pine. This indicates that pale wood species cannot be properly protected by the single use of organic UVA. Here the additional use of the lignin stabilizer concept for clear systems or the use of VIS light screeners, i.e., colored transparent pigments, are shown to provide improvements depending on the shade of the paint, i.e., the degree of pigmentation.

[FIGURE 9 OMITTED]

Unfortunately it is not efficient to use one simple light stabilizer approach for all shades from clear to semitransparent to opaque pigmented systems. However, based on the results of this study it is possible to give a rough guideline for an efficient and economic use of light stabilizers to improve the long-term performance of wood coatings based on the transmission spectra of the used paints.

* It can be said that for clear nonpigmented systems higher amounts of an optimal UVA (1% to 1.5% on total paint) must be used in order to screen the harmful UV light and protect essentially the substrate, with additionally HALS (0.5%) to prevent the formation of coating surface defects. In addition the lignin stabilizer concept (1% to 2% in aqueous solution) helps to keep wood color in its almost initial condition, to improve aesthetics and mechanical properties retention.

* For light and medium pigmented systems a reduced amount of UVA (0.5%) is sufficient due to the fact that the pigments themselves act as UV/VIS screeners and an increased amount of HALS (0.5% to 1.0%). The lignin stabilizer concept again adds further benefit to the overall performance.

* For dark and opaque pigmented systems the use of UVA and lignin stabilizer adds no real improvement to the overall performance of the system as long as sufficient amounts of HALS (1% to 2%) are used to avoid chalking and other unwished surface defects.

Nevertheless, this has to be seen as a basic general guideline for starting point formulations. The improvements due to this approach of wood coating stabilization can differ according to quality of the binder system, the quantity and type of the used pigments, the quantity and type of light stabilizers, the final application, and the local weather conditions. At the end, the real experiment tells us how to properly stabilize wood.

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17. Stahrfeld, A, Braig, A, "Advanced UV Protection of Automotive Clear Coatings." XXVI FATIPEC Congress, Dresden, Germany, 2002

18. Schaller, C, Rogez, D, Braig, A, "Hydroxyphenyl-s-Tria-zines: Advanced Multi-Purpose UV-Absorbers for Coatings." XXVIII FATIPEC Congress, Budapest, Hungary, 2006

19. Hayoz, P, Peter, W, Rogez, D, "Wood Protection Part II: Wood Protection with the Introduction of a Lignin Stabiliser." Asia Pacific Coat. J., 16 (3) 28 (2003)

20. Peter, W, Schellenberg, C, Rogez, D, Schaller, C, Kalscheur, G, "New Concepts for Improved Durability of Waterborne Wood Coatings." Coating Wood and Wood Composites Conference, Charlotte, NC, USA, 2005

21. Peter, W, Schaller, C, Schellenberg, C, Rogez, D, "A New Concept of Aqueous Additive Preparations for Waterborne Coatings." Waterborne and High Solids Coatings Conference, Brussels, Belgium, 2006

22. Rogez, D, "Color Stabilization of Wood and Durability Improvement of Wood Coatings." 2nd Wood Coatings Congress, The Hague, Netherlands, 2000

23. Hayoz, P, Peter, W, Rogez, D, Proceedings of the Athens Conference on Coatings Science and Technology, vol. 28, p. 67 (2002)

24. Hayoz, P, Peter, W, Rogez, D, "Improved Photoprotection of Wood and Durability Improvement of Wood Coatings." 3nd Wood Coatings Congress, The Hague, Netherlands, 2002

25. Hayoz, P, Peter, W, Rogez, D, "Verbesserter Lichtschutz fur Holzsubstrate: Farbstabilisierung bei naturbelassenen und gebeizten Holzern fur den Innenbereich." Farbe und Lack, 109 (7) 26 (2003)

26. Beaton, CR, Argyropoulos, DS, "Photostabilizing Milled Wood Lignin with Benzotriazoles and Hindered Nitroxide." Photochem. Photobiol., 73 (6) 605-610 (2001)

27. Evans, PD, Michell, AJ, Schmalzl, KJ, "Studies of the Degradation and Protection of Wood Surfaces." Wood Sci. Technol., 26 151 (1992)

28. Rogez, D, "Color Stabilization of Wood and Durability Improvement of Wood Coatings: A New UV-Light-Protection Concept for Indoor and Outdoor Appplications." Paint and Coating Industries Magazine (www.pcimag.com), March 2002

29. Schaller, C, Rogez, D, New Concepts for Light Stabilization of Wood." 18th SLF Congress, Elsinore, Denmark, 2006

[c] FSCT and OCCA 2007

Presented at the 5th International Woodcoatings Congress: Enhancing Service Life, organized by the Paint Research Association (PRA), in Prague. Czech Republic, October 17-18, 2006.

C. Schaller ([mailing address]), D. Rogez

Ciba Specialty Chemicals Inc., Klybeckstr. 141, CH-4002 Basel, Switzerland

e-mail: [e-mail]

Table 1: Assignments of bands in the infrared spectrum of pine (27)

Freq. ([cm.sup.-1])  Assignments

3350                 H-O-H stretching vibration of absorbed water in
                       carbohydrates
1730                 C=O stretching vibration of carboxyl groups of
                       hemicellulose and chromophores
1640                 H-O-H stretching vibration of absorbed water in
                       carbohydrates
1600                 C=C stretching vibration in aromatic ring of lignin
1505                 C=C stretching vibration in aromatic ring of lignin
1450                 C=C stretching vibration in aromatic ring of lignin
                       and [CH.sub.2] vibration in cellulose
1425                 [CH.sub.2] scissors vibration in cellulose and C=C
                       stretching vibration in aromatic ring of lignin
1370                 CH bending vibration in cellulose and hemicellulose
1330                 OH in-plane deformation vibration in cellulose and
                       hemicellulose
1315                 [CH.sub.2] wagging vibration in cellulose
1260                 C-O stretching vibration in lignin and
                       hemicellulose
 895                 Anomeric carbon group frequency in cellulose and
                       hemicellulose
 870                 CH out-of-plane bending vibration in lignin
 810                 Vibration of mannan in hemicellulose and CH out-of-
                       plane in lignin
 775                 Vibration of galactan in hemicellulose