photoprotection and photostabilisation of timber surfaces using … · 2008. 8. 14. · based on...
TRANSCRIPT
Photoprotection and Photostabilisation of Timber Surfaces Using Clear Coatings
S. Baur1, A. Easteal1, N. Edmonds1, D. Waddingham2, R. Jones2, R. Devendra2
1The University of Auckland
Department of Chemistry – Centre of Advanced Composite Materials 2Uroxsys Ltd.
Auckland, New Zealand
ABSTRACT A range of timber types was studied to determine timber-specific colour degradation and to develop appropriate protection strategies. Specific additives were incorporated into primer and top coats to provide a transparent coating system, while conferring protection against sunlight degradation. Treated timbers were exposed to different types of weathering, including artificial accelerated weathering (QUV), exposure to sunlight behind glass, and outdoor exposure in both Auckland and Allunga, Australia. Weathering performance was evaluated by measuring colour changes of treated timber surfaces.
INTRODUCTION
Outdoor weathering, particularly sunlight and rain cause serious damage to timber surfaces.1 Solar radiation generates free radicals that promote darkening, fading and bleaching, which subsequently lead to the decomposition of the lignin network.2,3 Additionally, dimensional stresses such as swelling, shrinking and surface erosion result in a complete break-up of the cell structure at the wood surface.4 Wooden products exposed to indoor light conditions also tend to yellow and darken as a result of lignin deterioration.2,5
Timber surfaces can be protected by the application of wood primers and coatings depending on coating properties, wood durability and climate. Adhesion of the coating to the wood surface is considered to be an important characteristic of the coated wood product.6 Therefore the wood-coating interface was investigated and it was found that penetration of the primer or coating into wood cell walls and lumens is essential to obtain an optimal anchorage of the coating.6,7,8 In addition a suitable coating system is required to protect the underlying wood cells from photochemical degradation and disintegration.9 Generally, opaque paints provide superior performance in comparison to clear coatings by preventing solar radiation to reach the timber surface. However, the application of an opaque material covers the wood surface and conceals the natural beauty of timber, such as the grain or texture.10,11 Organic UV absorbers are frequently added into clear varnish formulations to protect the varnish, and the substrate from the impact of UV radiation. However, UV absorbers are not permanent and degrade upon extended exposure periods depending on the coating matrix.12
Readily available wood coatings perform well towards weathering, but the aesthetic appearance of the coated wood product is merely satisfactory. Our studies concentrate on developing transparent coating systems that provide long term protection to wood surfaces, as well as preserve the natural timber appearance. Protection against aesthetic deterioration is endeavoured.
MATERIALS AND METHODS
Wood Samples
Pine (Pinus radiata), kwila (Intsia spp.), teak (Tectona grandis), blackwood (Acacia melanoxylon), beech (Nothofagus fusca), mahogany (Swietenia macrophylla), maple (Acer saccharum), oak (Quercus spp.), ash (Eucalyptus delegantensis & regnana) and brown walnut (Juglans nigra) were selected as substrates for the application of primer and coating systems. Pine and kwila are commonly used in outdoor situations in New Zealand and hence were investigated in detail.
Primers and Top Coats
Colour protection towards sunlight degradation could be achieved by incorporating protective additives in primer and top coat systems to minimise solar radiation from reaching the timber surface (Figure 1). Transparent UV absorbers, light-stable dyes and nano-pigments were chosen to provide clear and aesthetically satisfying primers and top coats. Pigment characteristics depend on refractive index, which describes the ability to scatter light, capability of UV-Vis absorption, and particle size along with state of dispersion.
Figure 1: Interaction of pigments with light 13
Coloured timbers were treated with primers containing dyes or transparent iron oxides. A controlled penetration primer containing zinc oxide was applied to light coloured timbers. Additionally zinc oxide, titanium dioxide and mica in combination with UV absorbers were dispersed in top coats and applied to various timbers.
Weathering Testing
A QUV accelerated weathering tester was used according to ASTM G154-00a, cycle 1. The arrangements for behind glass exposure and the outdoor exposure rack were based on ASTM G24-97 and ASTM D1006-01. Coloured filters were placed over
panels and used in outdoor exposure conditions to study colour changes caused by selected wavelength regions of visible light. Weathering performance of treated timbers was evaluated by measuring the colour of the wood surface using an X-Rite spectrophotometer. Five preset points on each treated panel were measured before and after predetermined periods of exposure. The colour difference �E was obtained using equation 1 and the mean �E value was calculated.
222 )()()( baLE ∆+∆+∆=∆
Equation 114
RESULTS AND DISCUSSION
Colour Degradation of Pine and Kwila
Colour degradation patterns of pine and kwila are demonstrated (Figure 2 and 3). Colour values �L (grey), �a (red), �b (yellow) and �E (purple) were plotted versus exposure time in months/hours. Colour changes of coated panels exposed to different weathering regimes were compared.
a) Behind glass exposure b) Outdoor exposure
-5
0
5
10
15
20
0.5 1 2 4 0.5 1 2 4 0.5 1 2 4
blank no additives UV absorber
-20
-10
0
10
20
0.5 1 2 4 0.5 1 2 4 6A 0.5 1 2 4 6A
blank no additives UV absorber
c) QUV exposure d) Exposure to sunlight through filters
-5
0
5
10
15
20
65 112 254 500 1000 65 112 254 500 1000 65 112 254 5001000
blank no additives UV absorber
-5
0
5
10
15
20
0.5 1 2 6A 0.5 1 2 6A 0.5 1 2 6A 0.5 1 2 6A
light yellow f. yellow filter orange filter red filter
Figure 2: �L, �a, �b and �E values versus exposure time for pine 6A stands for 6 months exposure in Allunga
Pine demonstrated strong yellowing and darkening effects, accompanied by a weaker reddening effect. Within each set of results, �L, �a and �b values increased continuously with exposure time. A decrease of colour values was observed for each weathering type in the order of blank – no additives – UV absorber (Figure 2a, b and c). An increase of colour values was observed comparing figure 2a) behind glass – b) outdoor – c) QUV exposure. After one month outdoor exposure, �L, �a and �b values of blank panels have decreased, this indicates colour fading. Subsequent mould growth was represented by strong darkening effects and a loss of red and yellow colour. In the case of UV absorber an initial fading effect was observed that slowly
inverted, suggesting that UV absorbers act as discolouration retarders. The light yellow filter appeared to promote similar effects as the UV absorber. The yellow filter caused bleaching represented by negative �L and �a values, whereas the orange and red filters promoted weak yellowing and reddening effects. Pine appeared to be a reasonably colour-stable timber for indoor situations bearing a top coat containing UV absorber. It is suggested that colour stability can also be achieved by applying a translucent light red top coat to simulate the red filter effect.
a) Behind glass exposure b) Outdoor exposure
-15-10
-505
101520
0.5 1 2 4 0.5 1 2 4 0.5 1 2 4
blank no additives UV absorber
-15-10-505
101520
0.5 1 2 4 0.5 1 2 4 6A 0.5 1 2 4 6A
blank no additives UV absorber
c) QUV exposure d) Exposure to sunlight through filters
-10
-5
0
5
10
15
20
65 112 254 500 1000 65 112 254 500 1000 65 112 254 500 1000
blank no additives UV absorber
-10
-5
0
5
10
15
20
0.5 1 2 6A 0.5 1 2 6A 0.5 1 2 6A 0.5 1 2 6A
light yellow f. yellow filter orange filter red filter
Figure 3: �L, �a, �b and �E values versus exposure time for kwila 6A stands for 6 months exposure in Allunga
Kwila represented strong darkening, high negative �b values and small changes in the red component. Colour fading was observed on blank panels with subsequent mould growth in outdoor conditions. UV absorbers had almost no effect on colour change, suggesting that the coloured components in kwila are not sensitive towards UV light. Kwila presented higher �E values for behind glass exposure (Figure 3a) than outdoor exposure (Figure 3b). Yellow filters showed a small reduction in the yellow attribute, whereas orange and red filters promoted a weak reddening effect. It is suggested that kwila is suitable for use in outdoor situations due to its insensitivity towards UV light. A translucent light red top coat can also provide reasonable colour stability.
Investigation of Correlation between Colour Change and Extractives Content
Figure 4: �E and wood extractives content of selected timber types
Extractives Content [%] 17 14.8 16.1 - 6.8 1.5 2.5
Classification colour stable non colour stable good protection with UVA
0
2
4
6
8
10
12
14
16
18
teak blackwood beech mahogany pine maple
dE of Selected Timbers
indoor blank indoor -UVAindoor +UVAoutdoor -UVAoutdoor +UVA
Figure 4 shows colour changes �E of selected timbers, that were coated (see legend) and exposed to indoor (blue) and outdoor (yellow) conditions. After exposure periods of two weeks, one month, two months and four months, which correspond to the first, second, third and fourth column, respectively, �E values were obtained and compared to the extractives content of each timber.
Teak and blackwood contain high concentrations of extractives and presented low �E values, and therefore were classified as colour stable timbers. However, beech and mahogany have low amounts of extractives and presented high �E values, and were non-colour stable. Light coloured timbers such as pine and maple with equally low amounts of extractives could be protected with a top coat containing UV absorbers. A more detailed study is required to investigate the relationship between �E values and extractives content.
Pigment Properties
UV-Vis absorption properties of zinc oxide, titanium dioxide and mica are illustrated (Figure 5). Zinc oxide presented the best absorption properties up to almost 400 nm, compared to titanium dioxide and mica.
Figure 5: UV-Vis reflectance spectra of ZnO Figure 6: Transmission electron micrograph of TiO2 and mica dispersed ZnO and TiO2 particles
Figure 6 illustrates the particle size and state of dispersion of zinc oxide (left), and titanium dioxide (right). Both pigments have primary particle sizes of about 20 to 50nm. However, only the left picture represented a well dispersed system with separated zinc oxide particles. Primary titanium dioxide particles were agglomerated. Therefore the dispersion needs to be optimised in order to obtain fully separated pigment particles.
Controlled Penetration Primer
A controlled penetration zinc oxide primer was applied to pine and teak. Figure 7 shows the penetration depth of the zinc oxide primer illustrating how deep the primer penetrated into the wood surface. It only penetrated into the first and second wood cell layers, which was desired. In addition the primer also penetrated into parenchyma rays.
U V -V is R e flecta nce G raph o f P ig m ents
w avelength λ [nm ]200 300 400 500 600 700 800
refle
ctan
ce [%
]
0
20
40
60
80
100
T iO 2ZnOm ica
Figure 7: Scanning electron micrograph and zinc mapping of pine and teak
Weathering Performance
Figure 8 shows colour changes �E of different treated timbers. After exposure periods of two weeks, one month, two months and four months, which correspond to the first, second, third and fourth column, respectively, �E values were obtained and compared. Low �E values represented satisfactory colour stability.
Figure 8: �E values of white and coloured timbers exposed outdoors with photos The right hand side of the photo shows the unexposed reference panel.
Oak represented relatively high �E values for all coating system. Ash demonstrated low �E values when titanium dioxide was present in the top coat. However, due to agglomerated pigment particles the top coat appeared milky. Therefore, the dispersion needs to be optimised to obtain a more transparent top coat. Both pine and maple represented low �E values when over coated with a top coat containing zinc oxide, titanium dioxide and mica in combination with UV absorbers. Teak demonstrated good colour stability when over coated with a top coat containing mica and UV absorber. Kwila represented low �E values with a system containing dyes in the primer and UV absorbers in the top coat. In contrast, brown walnut performed best using a dye primer and a top coat without UV absorber. Reasonable colour stability was achieved for pine, maple, teak, kwila and brown walnut. Oak, ash and beech could not be protected by any of these treatments. A detailed study is required to investigate the unexpected high �E values obtained for the zinc oxide primer.
0
2
4
6
8
10
12
14
16
18
Oak Ash Pine Maple
dE of White Timbers - Outdoor Exposure
Teak Kwila Beech Brown Walnut
dE of Coloured Timbers - Outdoor Exposuredyedye, UVATIOTIO, UVA
no additivesUVAZnO, UVATiO2, UVAmica, UVAZnO primerZnO primer, UVA
CONCLUSIONS
Timber types show specific responses to different weathering regimes. Pine is a reasonably colour-stable timber for indoor situations bearing a top coat containing UV absorbers. Kwila appears to be insensitive towards UV light and is recommended for use in outdoor situations. Colour stability can also be achieved by applying a translucent light red top coat to simulate the red filter effect. A relationship between colour change and wood extractives content can be assumed. Timber-specific treatments are needed to provide the necessary protection. The majority of timber types can be reasonably well protected against colour change. All coating systems give satisfactory protection, except the top coat containing titanium dioxide whose particle size needs to be optimised. Contrary to our expectations, the zinc oxide primer did not perform well in extended outdoor exposure.
Acknowledgement The authors are grateful to Technology New Zealand for financial support. -------------------------------------------------------------------------------------------------------Correspondence to [email protected] References 1. Evans, P. D., Michell, A. J. & Schmalzl, K. J. Studies of the degradation and
protection of wood surfaces. Wood Science and Technology 26, 151-163 (1992).
2. Feist, W. C. & Hon, D. N.-S. in The chemistry of solid wood (eds. Society, A. C. & Cellulose, P. a. T. D.) 401-451 (Rowell, Roger M., Washington, D.C., 1984).
3. Hon, D. N.-S., Ifju, G. & Feist, W. C. Characteristics of free radicals in wood. Wood and Fiber 12, 121-130 (1980).
4. Chang, S.-T., Hon, D. N.-S. & Feist, W. C. Photodegradation and photoprotection of wood surfaces. Wood and Fiber 14, 104-117 (1982).
5. Ellison, R. The effects of daylight. www.buildingconservation.com 6. Van den Bulcke, J., Rijckaert, V., Van Acker, J. & Stevens, M. Quantitative
measurement of the penetration of water-borne coatings in wood with confocal laser microscopy and image analysis. Holz als Roh- und Werkstoff 61, 304–310 (2003).
7. Dawson, B., Gallagher, S. & Singh, A. in Microscopic view of wood and coating interaction, and coating performance on wood (Rotorua, New Zealand, 2003).
8. Hill, C. A. S., Mastery Farahani, M. R. & Hale, M. D. C. The use of organo alkoxysilane coupling agents for wood preservation. Holzforschung 58, 316-325 (2004).
9. Jirous-Rajkovic, V., Bogner, A. & Radovan, D. The efficiency of various treatments in protecting wood surfaces against weathering. Surface Coatings International Part B: Coatings Transactions 87, 15-19 (2004).
10. Rogez, D. Color stabilization of wood and durability improvement of wood coatings: a new UV light-protection concept for indoor and outdoor applications. PCI Magazine (2002).
11. Hon, D. N.-S., Chang, S.-T. & Feist, W. C. Protection of wood surfaces against photooxidation. Journal of Applied Polymer Science 30, 1429-1448 (1985).
12. Pickett, J. E. UV absorber permanence and coating lifetimes. Journal of Testing and Evaluation 32 (2004).
13. Brill, T. B. Light, its interaction with art and antiquities (Plenum Press, New York, 1980).
14. Brock, T., Groteklaes, M. & Mischke, P. Lehrbuch der Lacktechnologie (ed. Zoll, U.) (Vincentz (Coatings Compendien), Hannover, 1998).
15. Fengel, D. & Wegener, G. Wood chemistry, ultrastructure, reactions (Walter de Gruyter & Co, Berlin, 1984).