Photodegradation and photostabilisation of wood – the state of the art

Polymer Degradation and Stability 88 (2005) 268e274

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Photodegradation and photostabilisationof wood e the state of the art

Beatrice Georgea,), Ed Suttieb, Andre Merlina, Xavier Deglisea

aLERMAB, UMR 1093 INRA/ENGREF/Universite Henri Poincare, Boulevard des Aiguillettes, BP239 54506 Vandoeuvre, FrancebBRE, Centre for Timber Technology & Construction, Watford WD25 9XX, UK

Received 10 June 2004; received in revised form 9 October 2004; accepted 23 October 2004

Available online 6 January 2005

Abstract

In this article we try to give an overview of the photodegradation of wood and the different ways available to stabilise thiscomplex substrate. The mechanisms of wood photodegradation have been investigated and it appeared that lignin is the keystructure because this component is able to absorb in the UV/visible region due to its chromophoric groups. Thus, some solutionscan be offered to protect wood against photodegradation. One of the easiest consists in applying finishes such as paints, coatings,

varnishes etc., but it is also possible to modify the substrate chemically (e.g. by acetylation) or to stabilise its colour by thermal andphotochemical pre-weathering. The use of UV absorbers, HALS, antioxidants and the recent development of new additives help toprevent the degradation of the coated wood system.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Wood; Photodegradation; Stabilisation; UV absorbers; HALS

1. Introduction

It has been known for a long time that wood exposedto solar radiation is subject to surface degradation eprimarily colour changes and mechanical breakdown[1]. Despite this behaviour and increasing pressures fromcompetitor materials such as aluminium and PVC, woodremains a well accepted material for construction anddecorative purpose such as furniture, parquetry, joinery,cladding and decking. To ensure its long term durabilitywood is usually coated with various decorative andprotective finishes such as opaque paints and semi-transparent stains as well as penetrating finishes or film-forming clear varnishes. Studies of the weathering oraccelerated ageing of coated wood systems have shownthat protection depends not only on the top coat

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doi:10.1016/j.polymdegradstab.2004.10.018

performance, but also on the substrate and particularlyon the wood/coating interface [2e4].

2. Degradation of uncoated wood

In an outdoor service environment, uncoated woodlike cladding, is subjected to two different kinds ofdegradation that vary as a function of time.

- In the short term, we observe mainly colour changes,darkening in the yellowered region of the CIElabsystem [5,6] for softwoods (Fig. 1) and for hardwoodslike oak (Quercus robur L. and Quercus Petraea), thecolour changes aremore complex (Fig. 2).We observean increase of the colour differences between sapwoodand heartwood. This result explains why it isimportant to eliminate sapwood from the timbersused for parquetry, for example.

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269B. George et al. / Polymer Degradation and Stability 88 (2005) 268e274

- In the long term, we initially observe a variation ofthe viscoelastic properties of wood under irradia-tion. They are characterized by a decrease of theglass transition temperature (Tg) of irradiated wood,as measured by thermomechanical analysis [7]. Afterseveral years of exposure, the mechanical break-down of the wood layers yields greater degradationin early wood than in late wood.

Fig. 1. Evolution of the chromatic coordinates of Vancouver fir, as

a function of the time of exposure, to a mercury vapour lamp giving

2 mW/cm2 at 360 nm [5].

3. Degradation of coated wood

Wood protected by a coating and exposed outdoors issubjected to moisture changes, due to the uptake andrelease of water, which induces tension within thespecimens, causing dimensional modifications and,depending on the coating characteristics, surface defectssuch as cracking and flaking. In order to carry out itsprotective function, a wood coating should be able tofollow the dimensional changes of the substrate withoutcracking and peeling. Up to now, degradation is mostlyrelated to the fracture of the wood/top coat interface thanto a decrease in the film cohesiveness. Degradation isreduced by adding stabilisers, UV absorbers, antioxidantagents, and Hindered Amine Light Stabilisers (HALS) tothe formulation resins as explained in Section 5.3.

In recent years there has been increased customerdemand for film-forming transparent systems forexterior use which keep the natural aspect of woodsuch as colour, grain and texture. However, suchtransparent surface finishes inherently have moredifficulty in providing long term performance. The mainreason for such premature failure of the coating is theUV light transparency of the top coats and the reactivityof the underlying wood, particularly lignin, to UV lightcausing degradation. More recent research has high-lighted the importance of the shorter visible wavelengthsof light in causing lignin degradation. Wavelengths upto 450 nm have shown changes in surface properties. Itis thought that the reaction results are the same as forUV light but they simply occur slowly [8].

Fig. 2. Colour changes for oak (sapwood and heartwood) under irradiation: (a) luminance; (b) chromatic coordinates [5].

270 B. George et al. / Polymer Degradation and Stability 88 (2005) 268e274

4. Mechanisms of wood photodegradation

In order to stabilise coated wood it is necessary tounderstand the mechanisms of degradation. Of the mainconstituents of wood, cellulose, hemicelluloses andlignin, only lignin absorbs relatively strongly in theUV/visible region (Fig. 3).

These spectra explain the degradation of wood notonly under UV light, but also under the shorterwavelengths of visible light. They are related to thechemical composition of wood.

The major component (40e50%) is cellulose whichis a high molecular weight linear polymer of 4 b-linkedD-glucose monomers, arranged in microfibrils whichgive the mechanical properties to wood. The hemi-celluloses (25e30% of the wood composition) areassociated with cellulose and are branched low-molec-ular weight polymers composed of several kinds of sugarmonomers e.g. pentose (arabinose and xylose) andhexose (galactose, glucose, and mannose) (Fig. 4).Finally lignin (15e30% of wood) has a phenolicstructure and acts as the binder between the microfibrilsin the cell walls of the tracheids. A model of lignin’spartial structure as given in Fig. 4, shows the twochromophoric groups A and B which absorb UV lightand undergo the different steps of the photodegradationmechanism.

In addition to the principal carbohydrate componentsof wood there are some minor constituents such as thewood extractives. They account for about 0.2e1.0% ofthe composition of wood and are organic compoundslike resins, tannins, polyphenols, waxes, fatty acid estersC16eC18, terpenes, cyclic diisoprenoids and stilbenes.In addition, between 0.2 and 1.0% of wood was found tobe inorganic ash (up to 5% in tropical woods),containing K, Ca, Mg-salts and cations of Fe, Mn and B.

Considering the chemical composition of wood,lignin is the key structure in wood photodegradation.In some cases, for tropical hardwoods, it is possible thatthe extractives play a role as antioxidants and radical

Fig. 3. Reflection spectra of wood (a), lignin (b) and cellulose (c) [M].

quenchers, but this has not been, up to now, clearlyreported.

The mechanism of lignin photodegradation is com-plex, with different pathways giving free phenoxyradicals [7e11] leading to chain cleavage and yellowing(Fig. 5).

One of the main phenoxy radicals is a long lifeguaiacoxy radical the structure of which is given inFig. 6 and which is observed by ESR [12e14]. Theseradicals undergo transformation into quinoid structureswhich are the origin of the yellowing of the surface ofthe wood [15,16].

5. The different ways of protecting wood

against photodegradation

When considering a protection strategy we must firstconsider the coated wood system and whether it ispossible to prevent the exposure of the wood to solarradiation. In exterior applications, we must not forgetthat the design of the building is a convenient and simpleway to achieve this, for instance for the protection ofcladding construction to include a significant roofoverhang can protect cladding significantly from weath-ering.

5.1. Finishing

For long term protection, wood substrates are usuallycoated with various decorative and protective finishes.

Paints, opaque pigmented coatings, act as veryeffective UV/visible light screen. However, the naturalaspect of wood is concealed beneath the coating. Wenormally get good protection of the wood, if we keep theTg lower than the temperature of use of the coated woodsystem, as the coating needs to be able to follow thesubstrate dimensional changes without cracking andpeeling.

Semi-transparent and clear systems which keep thenatural look of wood such as colour, grain and textureare increasingly demanded by customer. However, thechallenge for such transparent systems is to providea satisfactory long term performance for exteriorapplications. This still requires further improvementsand is a challenge to their increased application in themarket. The main reason for such coatings failingprematurely is the UV/visible light transparency of thetop coat and the sensitivity of the underlying woodcomponents, due to the photo-oxidation of lignin. Theradiation degrades the lignin and delaminates thecellulosic structure of wood. Coating adhesion thenfails on the loosened substrate. Enhanced colourprotection for indoor applications and improved dura-bility of exterior wood coatings are therefore necessary,


Fig. 4. Model of the structure of softwood lignin showing two chromophoric groups (A and B) [7].

these being related to a better screening of radiation andbetter lignin stabilisation.

5.2. Chemical modification

Prevention of photodegradation by wood modifica-tion offers protection of wood either with or withouta coating and seems therefore an attractive option forexterior wood use. Among the different possibilities[5,17], the chemical modification of the cell wall by OHsubstitution, by acetylation, seems to be the best way.This reaction corresponds to the acetylation of lignin,were hydroxyl groups in the aliphatic and the aromaticpart of the lignin can be substituted by acetyl groups asproposed in Fig. 7. By modifying the phenolic hydroxylgroup with acetyl groups the light induced formation ofphenoxy radicals can be prevented [17,18]. This resultsin a decrease in the susceptibility of the wood tophotodegradation. In addition acetylated wood isdimensionally stable under high humidity conditions.

5.3. Surface treatments with additives

The use of additives aimed at screening radiationand/or controlling the photo-oxidative reactions ofwood, and therefore of lignin (and of the coating)significantly retards the occurrence of defects in thecoated wood system [19].

For effective protection we should have:

- Stabilisation of the wood surface with radicaltrapping additives. These can be antioxidants, butwood itself, and some of its phenolic extractives havealready better antioxidant capacities than syntheticantioxidants. This means that in some cases, forclear softwoods for example, it might be interestingto protect wood by natural extractives coming fromdurable woods [5].

- UV absorbers to protect the wood from theUV radiation. They act by selective absorptionof UV radiation between 290 and 400 nm preventingthe photo-activation of UV sensitive groups in the


Fig. 5. Mechanism of free radical formation from lignin degradation [12,13].

coating binder and in the surface layer of theunderlying wood. UV absorbers therefore protectthe deeper coating layers and the wood/coatinginterfaces. The main effects are protection againstcoating and substrate discolouration, against blister-ing, flaking and peeling and even against the loss ofgloss and cracking of the top coats. Due to thesensitivity of lignin to the longer wavelengths of theUV region and also to visible light, the best protectioneffects of wood surfaces are expected from a UVabsorber absorbing in the 340e400 nm range.

- HALS, in the primer, helps to inhibit the photo-oxidation of lignin and binders by trapping the free

Fig. 6. Guaiacoxy radical from irradiated softwood lignin [17].

radicals. They maintain the mechanical properties ofthe wood surface and contribute to better retentionof coating/substrate adhesion. HALS are effectiveagainst loss of coating surface properties sucha gloss, micro-cracking of clear coats and surfaceerosion.

Progress has been made with the development of newadditives for protection of coated wood understandingthe need to protect the top coat and the underlyingwood. A new HALS derivative has been identified byCiba [19] which provides, when used in primers,excellent colour stabilisation of wood for both indoorand exterior applications, with a far superior perfor-mance to existing commercial HALS. A useful pro-tection strategy may therefore be to impregnate thewood surface with a solution of HALS which will inhibitor at least reduce the lignin sensitivity to photo-oxidation, before applying the transparent top coatcontaining the UV absorber.

The top coat layer prevents the migration andevaporation of the additives. It is also possible to graftthe additives, by covalent bonds with the OH functions,onto the wood surface, by adding to them an iso-cyanateor an epoxy group [20,21].


H3C

O

OH++

O

O O

CH3H3C

R

OH

OCH3OH

HC

C

C OH

R

R

R

O

O

H3C

O

CH3OHC

OCH3

R

R

R

RC

C

O

CH3O

3 3

Fig. 7. Mechanism of lignin acetylation.

5.4. Colour stabilisation by thermaland photochemical pre-weathering

In many applications, we are interested in achievinga stable colour, rather than necessarily maintaining theinitial colour for example, for furniture, decorativepanels and flooring. For these applications it is possibleto perform two kinds of colour stabilisations, bythermal treatment or by photochemical treatment.

The thermal treatment corresponds to heating thewood in an inert atmosphere, until a maximum of250 �C, the colour is increased, but its photo-stability isimproved. It must be noted that this stability dependsstrongly on the wood species. The colour seems to bestable with softwoods tested and unstable for hard-woods [5].

Concerning the photochemical treatment [5] we canobtain a stable colour, with an accelerated (or not)photochemical reaction, under UV light. For exampleafter an impregnation of the wood surface with anequimolar mixture of benzophenone/amine (MDEA)and 1 h irradiation (15 mW/cm2 at 360 nm) the colour isequivalent to a photochemical treatment with 500 h(5 mW/cm2 at 360 nm), and the value of DE* is ratherhigh, reaching 20 for spruce. Interesting results havebeen obtained for Vancouver fir, oak and maple.

5.5. Surface protection by inorganic UV absorbers

It is well known that micronized transparent irondioxide, titanium dioxide and zinc dioxide [22,23] as wellas silica powder [24], are able to stabilise transparentcoated wood. Micronized rare metal oxides are alsoactually under test for the pre-treatment of wood.

Wood treated by preservatives, such as Cu(II) salts,presents a lowering of the ESR signal after photo-degradation [25]. This means that the preservatives playa role in wood photostabilisation. The beneficialproperties of Cr(VI) have been recognised in photo-stabilising wood surfaces [26].

6. Conclusions

Most of the results and progress presented in thisshort state of art paper on wood photodegradation havebeen presented during the different COST E 18 meet-ings, from 2001 until 2003.

We have a clear understanding of the mechanismswhich are at the origin of wood photodegradation andresearchers from industry and universities have beenable to propose different ways of protecting coatedwood systems. On a short term basis, we know thatgood protection is obtained by using appropriate typesand amounts of light stabilisers in primers and top coatscombined with the stabilisation of the wood surface/coating interface. Future activity is expected to concen-trate on improving the multicomponent protectionstrategy for the whole wood coating system.

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Photodegradation and photostabilisation of wood – the state of the art