By Dr. Christian Schaller and Daniel Rogez,
Ciba Inc., Basel, Switzerland

A major drawback in the use of wood materials for construction and decorative applications is its sensitivity to light. Coatings are usually used to prevent deterioration of the wood surface. The protection effect depends greatly on the opacity of the coatings; low-opacity coatings, which transmit the most radiation, are less effective. However, optimised use of UV absorbers and HALS, both in the coating system and also in a direct wood impregnation step, can improve the light stability considerably.
The popularity of natural wood as an exterior design and construction material would be even greater if significant improvements could be achieved in extending its durability with reduced maintenance work. But under outdoor conditions, weathering causes deterioration of the surface, due to a complex set of reactions induced by such factors as solar radiation, water, temperature, and oxygen.

How Light Degrades Wood

Many researchers have shown that ultraviolet (UV) and elements of the visible (VIS) spectrum of solar radiation cause most of the chemical modification and mechanical breakdown of exposed wood. To understand these effects, it is important to understand wood biology and the related mechanisms of photo-oxidation and degradation.
Chemically, wood is a complex biopolymer composed of structural polysaccharides, essentially cellulose as a structural substance, hemicelluloses as a kind of glue, and lignin as strength provider. Lignin contains chromophores with aromatic conjugated bond systems and carbonyl groups, and this is why its interaction with light (UV/VIS) in the presence of oxygen is the main cause of wood photo-oxidation.
The lignin undergoes chemical modifications resulting in radical formation and finally decomposition, producing coloured and hydrophilic byproducts. The resulting surface property changes lead to discolouration and increased water sensitivity followed by hydrolysis, leaching of the chromophores, and finally erosion and greying of the wood. Today, the photo-oxidation of lignin is well understood, and at least four degradation pathways have been identified [1].
The key to increasing the service life of wood with film-forming finishes is the optimisation of light protection of the wood surface and the coating. Since the early 1970s, it has therefore been common practice to combine UV absorbers (UVA) with hindered amine light stabilizers (HALS).

The Chemistry of HALS and UVA

HALS are mainly derivatives of 2,2,6,6-tetramethylpiperidine and help to prevent surface defects such as gloss loss, chalking in pigmented systems, and cracking in clear coats. Figure 1 shows the chemical structures of some commercial UVA and HALS products.
The most important HALS compounds are difunctional piperidine derivatives, as shown in Figure 1. Here the substituent on the nitrogen atom determines the properties of the HALS due to basicity. For instance, HALS-1 with R = CH3 is basic (pKb ~ 5) and interferes with acid-catalysed and some air-drying alkyd systems. Therefore, the non-basic aminoether function N-OR—e.g., HALS-2 with R = OC8H17 (its pKb of ~ 10 denotes a much weaker base)—has gained wide acceptance in the paint industry, as it has the major advantage of non-interaction with biocides, acidic media, and the drying process of acid-catalysed or air-drying systems.

Figure 1: Chemical structure of some commercial UVA and HALS productsIn contrast to HALS, which trap radicals at the surface, UVA filter out the harmful wavelengths of the light spectrum before photochemical reactions can take place, and therefore reduce the rate of radical generation [2]. Today, UVA based on 2-(2-hydroxy-phenyl)-benzotriazole (BTZ) chemistry are considered to be the most important class for the stabilisation of clear coats. Commonly used UVA are BTZ-1 for solventborne and BTZ-2 for waterborne applications.
Some limitations of benzotriazole-based UVA have pushed the paint industry to adopt a new UVA class based on 2-hydroxyphenyl-s-triazine (HPT) chemistry [3]. This new class of UVA can be fine-tuned by selective choice of substituent, allowing tailor-made additives to be created for powder [4], high performance automotive, industrial, and wood coating applications [5].

For wood, a UVA based on substituted tris-resorcinol triazine (Figure 1, HPT-1) with exceptional photo-permanence and a more pronounced red-shifted absorption was introduced for UV curable systems [6], for high performance solventborne [7, 8], and, in encapsulated form, for waterborne wood coating applications [9, 10].

Direct Wood Pretreatment Based on Special HALS

As both UV and visible light up to 500 nm causes lignin degradation [11], wood has to be protected from parts of the visible light spectrum. Therefore, a new two-step protection strategy was introduced, in which a lignin stabiliser is applied as a dilute pre-treatment solution or in an existing primer formulation directly onto wood and selected UVA (for indoor applications) or UVA/HALS (for outdoors) are used either in the same treatment stage (internal UV filter effect) or preferably in a subsequently applied top coat (external filter effect).
Such lignin-stabilising treatments, which improve colour stability in interior applications and long-term durability in low-opacity exterior wood coatings [12], are based on a special HALS. The HALS traps the radicals formed by visible light at the wood surface, which have not been screened by organic UVA (> 400 nm) [13, 14]. This approach finally allows the use of clear or low-pigmented coatings even in high-performance, high-durability outdoor applications—e.g., wooden window frames.
Most of the paints used today are semi-transparent finishes, where the pigments may act as partial VIS light protectors. This protection can be increased through a combination of organic UVA for substrate protection and HALS for coating protection as well as the lignin stabiliser. It is well known that such combinations of organic and inorganic UV/VIS light screeners show good performance in certain ratios.
The knowledge of how to use this two-step protection strategy is still limited and has to be improved to obtain efficient and economic use of light stabilisers for film-forming wood coating systems according to the shade of the coating, i.e., the degree of pigmentation. It is evident that clear, semi-transparent, and opaque coatings behave differently during weathering and therefore require different light stabiliser packages.

Experimental

The effectiveness of various light-protective treatments was evaluated using the following conditions:

• Wood Samples: Pine (pinus radiata) 200 mm x 90 mm x 10 mm with back- and end-grain sealing with an automotive 2K-PUR clear coat


• Paint System: Linseed long oil alkyd (LOA) with different amounts of transparent black, red, and yellow iron oxides plus a white pigmented system (2 x 120 g/m2 applied by brush)


• Stabilisers: HPT-1 as UVA, HALS-2 (decanedioic acid, bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-piperidinyl) ester) and a lignin stabiliser pre-treatment (2% aqueous solution; 1 x 80 g/m2 brush applied)



• Artificial Weathering: Atlas Ci65A Weather-Ometer®, Xenon light according to the Xe-WOM CAM 7 cycle for exterior applications (inner and outer borosilicate filter) according to DIN EN ISO 11341 A (0.35 W/m2/340 nm: 102 min light and 18 min light and spray)


• Natural Weathering: 45° south open rack exposure in Pfeffingen, Switzerland


• Colour Difference Measurement: Minolta “CM-3600d” device (gloss included) and calculation of L*, a*, b*, C*, h and DE* with “CGREC” software according to DIN 6174


• Gloss Measurement: 60° gloss with Byk/Gardner “Micro-Tri-Gloss” equipment according to DIN 67530


• Cracking: Visually according to DIN EN ISO 4628-4


• UV-VIS Spectroscopy: Integrated transmission spectra of 20 μm dry film thickness (DFT) on glass 200 nm to 800 nm with 1 nm resolution (Perkin Elmer “Lambda 900” UV/VIS/NIR spectrophotometer)

Figure 2: Integrated transmission spectra of the LOA paints tested
Figure 3: Colour deviation of pine with LOA with HPT-1/HALS-2 with and without lignin stabiliser

Artificial Weathering

The integrated transmission spectra (i.e., the light protection capacity) of different test coatings are shown in Figure 2. Based on the transmission value at 500 nm they can be classified as:

• clear: > 90% transmission (LOA-1)


• low-pigmented: 90%–60% (LOA-2, -3)


• medium-pigmented: 60%–30% (LOA-4, -5)


• dark-pigmented: 30%–0% (LOA-6, -7) and


• opaque-pigmented: 0% (LOA-8)

Organic light stabilisers—e.g., HPT-1 and HALS-2—and lignin stabiliser pre-treatments were compared. The colour deviation of pine with LOA stabilised with UVA/HALS (0.5% and 1.0% / 0.5% on total paint) with and without lignin stabiliser pre-treatment (2% in aqueous solution) after 1000 h Xe-WOM CAM 7 exposure is shown in Figure 3.

Adapting UVA Use to the Opacity

In the clear-, low- and medium-pigmented finishes, the use of UVA alone reduces the colour deviation on pale wood species to a certain extent, with better results for a higher UVA concentration. The visible light screening of the pigment evidently also improves performance. In all cases, the lignin stabiliser system has the best performance.
The dark LOA-6/7 and white opaque LOA-8 show lowest colour deviation of the non-stabilised paints, and neither the use of UVA nor the use of lignin stabiliser shows any significant benefit in terms of colour retention.
Hence, UVA use can be significantly reduced or eliminated in dark and opaque paints. As an initial recommendation, 1.0% for clears, 0.5% for low to medium opacity, and no UVA for dark or opaque coatings are suggested.

Dark and Opaque Systems Require HALS

However, it is well known that colour retention is not the main problem for dark and opaque pigmented systems. Earlier studies showed that these tend to suffer more from chalking and then complete erosion of the paint and the wooden surface, whereas clear- and low-pigmented systems primarily show cracking. In this case, higher amounts of HALS are necessary to protect the binder against photo-oxidation and avoid surface defects such as gloss loss, chalking, and subsequent flaking [15].

Figure 4: Pine with LOA (A: no additive; B: 1% HPT-1 + 0.5% HALS-2; C: 2% lignin stabiliser pre-treatment/1% HPT-1 + 0.5% HALS-2) after 1 year of natural exposure
Figure 5: Effect of the HALS-2 concentration on 60° gloss of LOA paints after 1 year of natural exposure

Direct Lignin Stabilisation Yields Best Performance

The effect of the lignin stabiliser on colour and coating protection of pine with a clear UVA containing top coat after 1 year of outdoor exposure is shown in Figure 4. Here LOA-1 was applied without (A), with 1% HPT-1 and 0.5% HALS-2 on total paint (B), and the second coating was applied over 2% aqueous solution of the lignin stabiliser (C). System (A) shows gloss loss, cracking, and erosion with greying of the wood surface. The use of HPT-1 and HALS-2 obviously improves the coating properties, but some colour deviation with darkening and yellowing can be seen.
The lignin stabiliser further improves the colour retention to almost retain the initial colour of the wood. In general, this approach shows excellent results in colour retention as well as mechanical properties of the paint film for all common pale wood species like pine, fir, and ash.

Natural Weathering Tests

The effect of HALS on the 60° gloss of LOA paints with different degrees of pigmentation after 12 months 45° south exposure in Pfeffingen, Switzerland, is shown in Figure 5. The initial 60° gloss was around 30 for all paints. After exposure, paints without HALS had a 60° gloss of around 15. The use of UVA shows only slight improvement for all the paints.

HALS Are Always Beneficial

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.

There Is No Single Optimal Light Stabiliser Package

These results show the value of light stabilisers in retaining the mechanical and aesthetic properties of outdoor wood coatings. Unfortunately, a single light stabiliser package is not equally suitable for all shades from clear to opaque pigmented systems. But it is possible to give rough guidelines based on the transmission profile of the pigmented paints. In general, the concentration of UVA can be reduced and the concentration of HALS should be increased with increasing pigmentation.
For clear paints, higher amounts of an optimum UVA (0.5% to 1% on total paint) have to be used to protect the substrate, with HALS (0.5%) added to protect the coating from surface defects. In addition, a solution of 1% to 2% lignin stabiliser, preferably used in aqueous solution or an existing primer formulation, helps to retain aesthetic and mechanical properties.
For light- and medium-pigmented systems, the amount of UVA can be reduced to 0.5%, as the pigment itself protects against light, but the amount of HALS should be increased (0.5% to 1.0%). The lignin stabiliser again improves results. In dark- and opaque-pigmented systems, the UVA and lignin stabiliser add no real benefit to the overall performance of the system as long as sufficient amounts of HALS (1% to 2%) are used to avoid surface defects.
This has to be seen as a basic general guideline for starting point formulations. The benefits may differ according to the quality of the binder system, the solids content of the paint, the quantity and type of the pigments used, the final application, and the local weather conditions. Ultimately, it is always the real experiment that tells us the final answer.

Results at a Glance

» The degradation of wood surfaces on exposure to light is mainly due to breakdown of the lignin component, which discolours and becomes more hydrophilic.

» Hindered amine light stabilisers (HALS) protect the coating surface by blocking radical reactions initiated by light, while UV absorbers prevent UV light from reaching the substrate.

» A wood pretreatment system that uses a special HALS has been developed to protect lignin from photo degradation due to VIS light. It is shown to have considerable benefits when using low-opacity coatings.

» The most effective means of protecting wood from light damage depends on the opacity of the coating.

» For opaque and dark pigmented coatings, HALS addition must be increased to protect the coating itself from light damage, while for clear-, low- and medium-pigmented systems, high UV absorber levels with lower HALS and lignin stabiliser pre-treatment are more effective.

Acknowledgments

The authors would like to thank Nadja Brugger and Raphael Meyer, from the wood coatings laboratory at Ciba Technical Service, for their contributions to this work. This article was first published in the European Coatings Journal (Vincentz Verlag) in December 2006.

References

[1]	G. J. Leary, Journal of Pulp and Paper Science 20 (1994) 6
[2]	A. Valet, Light Stabilizers for Paints, C. R. Vincentz Verlag Hannover 1997
[3]	A. Stährfeld, A. Braig, “Advanced UV Protection of Automotive Clear Coatings,” XXVI FATIPEC Congress, Dresden, Germany 2002
[4]	S. Zeren, “UV Stabilization of Powder Clear Coats,” XXVI FATIPEC Congress, Dresden, Germany 2002
[5]	C. Schaller, D. Rogez, A. Braig, Hydroxyphenyl-s-triazines: “Advanced Multi-Purpose UV-Absorbers for Coatings,” XXVIII FATIPEC Congress, Budapest, Hungary 2006
[6]	D. Rogez, P. Hayoz, W. Peter, “UV-Curing and Light Stabilization of Parquet Coatings,” European Coatings Conference on Parquet Coatings II, Berlin, Germany 2002
[7]	P. Hayoz, W. Peter, D. Rogez, Asia Pacific Coatings Journal (2003) 16 (2) 14
[8]	P. Hayoz, W. Peter, D. Rogez, Asia Pacific Coatings Journal (2003) 16 (3) 28
[9]	W. Peter, C. Schellenberg, D. Rogez, C. Schaller, G. Kalscheur, “New Concepts for Improved Durability of Waterborne Wood Coatings,” Coating Wood and Wood Composites Conference, Charlotte, NC, USA 2005
[10]	W. Peter, C. Schaller, C. Schellenberg, D. Rogez, “A New Concept of Aqueous Additive Preparations for Waterborne Coatings,” Waterborne and High Solids Coatings Conference, Brussels, Belgium 2006
[11]	C. Schaller, D. Rogez, W. Peter, “New Approaches for Wood Coating Stabilization,” 5th International Wood Coatings Congress, Prague, Czech Republic 2006
[12]	D. Rogez, “Color Stabilization of Wood and Durability Improvement of Wood Coatings,” 2nd Wood Coatings Congress, The Hague, Netherlands 2000
[13]	P. Hayoz, W. Peter, D. Rogez, “Improved Photo-Protection of Wood and Durability Improvement of Wood Coatings,” 3rd Wood Coatings Congress, The Hague, Netherlands 2002
[14]	P. Hayoz, W. Peter, D. Rogez, Farbe + Lack 109(7) (2003) 26
[15]	C. Schaller, D. Rogez, “New Concepts for Light Stabilization of Wood,” 18th SLF Congress, Elsinore, Denmark 2006

The Authors

Dr. Christian Schaller studied chemistry at the University of Stuttgart, Germany, and received his PhD in macromolecular chemistry from the Research Institute of Pigments and Coatings, Stuttgart, in 2001. After a post-doc year at the institute involving several industry projects, in 2002 he joined Technical Service, Business Line Coatings, Ciba Inc. In 2003 he took over the responsibility of Application Head for Coating Protection and Wood Coatings. Since 2007 he has been Application Head for Coating Protection, Wood and Architectural Coatings.
Dr. Schaller may be reached at Ciba Inc., CH-4002 Basel, Switzerland; +41 61 6366812 (phone); +41 61 6365116 (fax); or christian.schaller@cibasc.com.
Dr. Daniel Rogez, who has a PhD in Polymer Chemistry from the University of Haute Alsace, France, joined the former Ciba-Geigy company in 1979. Working initially in the paints and inks additives development laboratories, he specialised in antioxidants, light stabilisers and photoinitiators for the European market. He is currently Global Marketing Manager for Additives and Colorants for Decorative and Industrial Paints at Ciba Inc., Basel, dealing primarily with the wood coatings market