colour stabilisation of wood composites using polyethylene

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Colour stabilisation of wood composites usingpolyethylene glycol and melamine resin

Uwe Müller, Melanie Steiner

To cite this version:Uwe Müller, Melanie Steiner. Colour stabilisation of wood composites using polyethylene glycol andmelamine resin. European Journal of Wood and Wood Products, Springer Verlag, 2009, 68 (4),pp.435-443. �10.1007/s00107-009-0386-1�. �hal-00568260�

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For Peer Review

Draft Manuscript for Review

Colour-stabilisation of wood/melamine resin composites

without topcoat

Journal: Holz als Roh- und Werkstoff

Manuscript ID: HRW-08-0192.R2

Manuscript Type: ORIGINALARBEITEN / ORIGINALS

Date Submitted by the Author:

15-Jul-2009

Complete List of Authors: Müller, Uwe; Kompetenzzentrum Holz GmbH, Holz-Polymer-Verbunde Steiner, Melanie; Kompetenzzentrum Holz GmbH, Holz-Polymer-Verbunde

Keywords: colour-stabilisation , wood, melamine resin, poly ethylene glycol , photoyellowing, colourimetry, FTIR-ATR, composite

Editorial Office, TU MÃ¼nchen, Holzforschung MÃ¼nchen, Winzererstr. 45, 80797 MÃ¼nchen, Germany

Holz als Roh- und Werkstoff

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Colour stabilisation of wood composites using polyethylene glycol and melamine resin Uwe Müller, Melanie Steiner

Kompetenzzentrum Holz GmbH (Wood K plus), St.-Peter-Straße 25, 4021 Linz, Austria

Email: [email protected]

Abstract

Photo-yellowing of native and polyethylene glycol (PEG) modified wood and wood/melamine resin composites was studied by means of FTIR-ATR technique and colourimetry (CIE L*a*b* method). The discolouration ∆E shows a systematic asymptotic trend towards higher values with increasing irradiation time. Yellowing proceeds faster in natural wood compared to wood/melamine resin composites. Nevertheless, long-term irradiation experiments show that the total colour shift is similar for both.

Discolouration is significantly reduced by PEG treatment. In comparison to untreated wood, both glycol and melamine resin mainly reduce the irradiation-induced yellow shift. Moreover, PEG also shows an effect on the redness shift. Both effects result in decreased yellowing of the composite surface. An influence of the molecular weight of PEG was detected.

Zusammenfassung

Die Photovergilbung von unbehandeltem und Polyethylenglykol (PEG) behandeltem Holz bzw. Holz/Melaminharz-Kompositen wurde mit Hilfe der FTIR-ATR-Technik und Colorimetrie (CIE L*a*b* Methode) untersucht. Die Verfärbung ∆E zeigt dabei einen systematischen, asymptotischen Trend zu größeren Werten mit steigender Bestrahlungszeit. Die Vergilbung verläuft dabei im natürlichen Holz schneller als in den Holz/Melaminharz-Kompositen. Die maximale Farbveränderung ist in beiden Systemen jedoch letztendlich gleich.

Die Verfärbung wird durch PEG-Modifizierung signifikant reduziert. Im Vergleich zum unbehandelten Holz wird durch die Glykol-Modifizierung wie in den Holz/Melaminharz-Kompositen hauptsächlich die Gelbverschiebung reduziert. Zusätzlich zeigt PEG noch einen Effekt auf die Rotverschiebung. Beide Effekte münden in einer verringerten Vergilbung der Kompositoberfläche. Weiterhin wurde ein Einfluss der Molmasse des PEG auf diesen Effekt festgestellt.

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Introduction

Like most natural and synthetic polymers wood absorbs solar UV light. In this process, photolytic, photooxidative, and thermooxidative reactions (Andrady et al. 1998) occur which result in the degradation of wood (Rabek 1995, Scott 1990). The degradation ranges from surface discolouration in indoor applications to extensive loss of mechanical properties (Andrady et al. 1998, Derbyshire et al. 1995, Kiguchi and Evans 1998) for wood components in outdoor applications. Especially the combination of light, moisture and temperature changes leads to breakdown of the lignocellulosic network. Wood chemistry adapts the polymer stabilisation concept. Antioxidants, radical scavengers and UV absorbers were tested and reported as very effective in limiting surface discolouration (Feist and Hon 1984, Beyer et al. 2001, Hon 2001, Hayoz et al. 2003). Nevertheless, Derbyshire and Miller (1981), Kataoka et al. (2005, 2007) and Schaller and Rogez (2007) clearly showed that also VIS light causes lignin degradation and discolouration. These results indicate that wood cannot be fully protected by applying UV absorbers. As shown by the author in an earlier work (Müller et al. 2002) the use of UV absorbers and radical scavengers alone provides no effective protection for wood surfaces (without topcoat). As already stated above, lignin is the most photoactive wood component. However, extractives can also play an important role in photochemical reactions. Organic extractives, e.g., phenolic compounds, are well-known antioxidants (Mahoney 1969). Moreover, derivatives of lignin are used as commercial UV absorbers (Anonymus 2004). Extractives can act as UV absorbers, quench the excited state, trap free radicals, or act as hydrogen or electron donors. The influence of extractives on photodegradation of wood was investigated by Hon and Minemura (2001) and Pandey (2005). Moreover, density also seems to play an influential role. Kataoka et al. (2005) studied Japanese cedar and Japanese cypress to establish the effect of density. Western red cedar (Thuja plicata) is an example of a species with very high extractives content, which makes it biologically very durable, but which discolourates quickly (Hon and Minemura 2001) due to its low density . The extractives are concentrated in the cell walls and cell lumina (Hillis 1971). For trapping and quenching in a diffusion hindered matrix (like solid, crystalline structures), proximity of the reaction partners is advantageous to achieve high efficiency (Becker et al. 1991). From the photochemical point of view, high extractives content combined with high density is the ideal combination for high photostability. This explains the differences in surface discolouration between wood species (Oltean et al. 2008) with different extractives contents. Thus it is no surprise that wood species with high extractives contents and high density, e.g., larch and oak, show lower discolouration compared to other wood species (Oltean et al. 2008). The effect of H-donors on wood colour stability was investigated by Hon and Minemura (2001). The authors showed that a coating of polyethylene glycols (PEG) on bleached wood exerts a good con-trolling effect on discoloration. Moreover, PEG coating results in whitening of wood (Hon and Minemura 2001). Peroxy radicals, which were formed in a consecutive reaction from α-ether radicals generated by H-abstraction and oxygen, destroy the colouring structures. Several papers describe (Hansmann et al. 2006, Rapp and Peek 1999, Pittman et al. 1994, Inoue et al. 1993) increased resistance of melamine-treated wood versus natural and artificial weathering. Melamine-impregnated wood shows less discolouration and distinctive protection against photochemical lignin degradation and infestation by wood-staining fungi (Rapp and Peek 1999, Gsöls et al. 2003). Particularly greying of wood was reduced by melamine impregnation. Gindl et al. (2002) showed that the melamine resin is concentrated in the cell walls of the wood, like lignin. The 3-D-net of the melamine resin (Lukowsky 1999) decelerates the formation of lignin degeneration products (Rapp and Peek 1999). The rigid network minimizes diffusion-controlled reaction. Therefore, H-abstraction, energy transfer (singlet oxygen formation (Beyer et al. 1995)) and oxygen diffusion are hindered. Moreover, if the α-cleavage of lignin is reduced as well, the net yield of the process is determined by the diffusion of radicals out of the cage (Johnston and Wong 1984). In this study, the influence of melamine resins and PEG as H-donors on the degradation of wood and wood composites was investigated.

Experimental

European spruce (Picea excelsa L.) veneer with a moisture content of about 8% was used as native wood. All samples were polished with sandpaper (400 P) before use. The composites were produced from BK 40/90 (J. Rettenmaier & Söhne), oven-dried or polyethylene glycol (PEG)- impregnated (Pluriol E 600, Pluriol E 9000 powder, BASF), melamine ether resin (Hipe®esin MER or MPER, experimental products - where P means PEG modified, AMI Agrolinz Melamine International

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GmbH) and ethylene vinyl acetate copolymer (EVA, Escorene Ultra UL 40028CC, ExxonMobil Chemical). Table 1 shows the composition of the samples used. 33g batches of these materials were compounded using the internal mixer system Polylab from ThermoHaake (Rheocord 300p / Rheomix 540p) under following processing conditions: 100°C, 100 rpm, 3 min, roller rotors. The compounds were pressed at 23bar and 150°C for 15min in a laminate press (Bürkle LAMV 100) with a moulding frame. The cured specimens (12 x 12 x 0.45 cm³ and ~65g) were polished with sandpaper (400 P) prior to further testing. For impregnation with polyethylene glycol veneer or chips were stirred in a 10% aqueous glycol solution for 24 hours. Then the samples were filtered and dried at 30°C and 100 mbar for 24 hours and for 3h at 105°C.

Table 1: Sample composition – experimental scheme

Tabelle 1: Probenzusammensetzung – experimentelles Schema

wood type wood in %

MER in %

EVA in %

veneer native spruce veneer + PEG

chips 90 10 composite 1 chips + PEG 90 10

chips 80 10a) 10 composite 2 chips + PEG 80 10 10

a)for composite 2b 10% MPER ATR infrared spectra of the wood samples were recorded on a PERKIN ELMER FT-IR spectrophotometer (SPECTRUM ONE). The spectra were measured in ATR mode (golden gate single reflection ATR system, P/N 10500 series, SPECAC) at 4 cm-1 resolution with 10 scans per single measurement and averaging seven single measurements. All values are arithmetic means of 5 spots on each of 3 plates (totaling 15 measurements). The intensity data was calculated from absorption band areas relative to the intensity of the band at 895 cm-1 (wagging motion of the hydrogen on the C-1 position of the glucose ring in cellulose (Hon and Ifju 1978, Feist and Hon 1984), using the provided software. This reference is admissible because the C-1 position is stable in the oxidation process (Charter 1996, Durovič and Zellinger 1993, Hon 1981). Nevertheless, in composites or in polyethylene glycol-impregnated wood a superposition was observed at 895 cm-1. Therefore, in melamine resin composites the absorption band areas were related to the intensity of the band at 812 cm-1 (triazine ring sextant out-of-plane bend, Larkin et al. 1998), while for PEG-modified samples the absorption band areas were related to the intensity of the band at 667 cm-1 (C-OH out-of-plane bending mode; Liang and Marchessault 1959). Post-spectroscopic manipulation was kept at a minimum. The wood spectra were shifted only parallel to the wavenumber-axis so that the minimum between 2000 cm-1 and 1800 cm-1 was set to zero and normalised to the maximum at around 1018 cm-1. Photo-yellowing was assessed with the CIE L*a*b* method. The changes of lightness (L*), redness (a*) and yellowness (b*) were measured with a spectrophotometer (CM 2600d/2500d; MINOLTA). All values are arithmetic means of 5 spots and 3 plates. The light source for short-time irradiation was a xenon high-pressure arc lamp (XBO 100, NARVA) used without a filter (λ > 280 nm; measured incident light intensity Io = 17.5 mW�cm-2, ambient temperature 40 – 45°C, spectral distribution see Becker et al. 1991). The light source for long-time irradiation was an artificial weathering device, SUNTEST XLS+ (Atlas Material Testing Technology BV) was used (300 nm < λ < 800 nm; measured incident light intensity Io = 50 mW�cm-2, black panel temperature 65°C). The humidity was not controlled.

Results and discussion

Photodegradation and photo-yellowing

Previous results (Müller et al. 2003) show that the degradation kinetics of spruce is nearly independent from the xenon light sources, which mostly differ in intensity. The small differences in UV-B (xenon lamps have a low emission in this region; Becker et al. 1991)) do not result in different reactions. Irradiation with λ > 280 nm is useful for quick monitoring of the UV degradation of wood. The melamine resin used absorbs below 280 nm (Figure 1). Therefore, in wood/melamine resin composites this light is absorbed by the wood components (like lignin, cellulose etc.) alone.

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Fig. 1: UV spectra of a melamine resin (type MER, acetonitrile/water)

Abb. 1: UV-Spektrum des Melaminharzes (Typ MER, Acetonitril/Wasser)

Fig. 2: Decay of lignin functionality at 1510 cm-1 and formation of carbonyl groups at 1730 cm-1 as a function of irradiation time (XBO 100 lamp, Io = 17.5 mW/cm2, λ > 280 nm, irradiation time 360 min, absorption was normalized at 1018 cm-1, left: composite 1, right: spruce (top), composite 2 (bottom);— before; --- after irradiation)

Abb. 2: Abbau der Ligninbande bei 1510 cm-1 und Bildung von Carbonylgruppen bei 1730 cm-1 als Funktion der Bestrahlungszeit (Lampe: XBO 100, Io = 17,5 mW/cm2, λ > 280 nm, Bestrahlungszeit 360 min, die Absorption wurde auf 1018 cm-1 normalisiert, links: Komposit 1, rechts: Fichte (oben), Komposit 2 (unten) — vor der Bestrahlung --- nach der Bestrahlung)

200 220 240 260 280 300

0,0

0,2

0,4

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1,0

MER

in CH3CN / H

2O 4:1

10 mg/l

1 mg/la

bso

rptio

n

λ in nm

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Figure 2 shows the changes in the IR spectra of the wood/melamine resin composite surfaces as a result of irradiation with a xenon arc lamp (λ > 280 nm). In the spectra, changes in the range below 1520 cm-1 are clearly visible. On the other hand, the triazine ring vibration at 812 cm-1 (Larkin et al. 1998) is nearly stable. These findings indicate that the changes at wave numbers below 1520 cm-1 result from lignin degradation (loss of the skeletal vibration at 1510 cm-1) rather than from degradation of the triazine ring, which also shows several absorption bands in this range (1563, 1551, 1501 cm-1: ν(-C=N-) (Larkin et al. 1998). Furthermore, the decay of lignin goes along with the formation of new carbonyl absorption in the region below 1700 cm-1 (conjugated and aromatic carbonyls as well as quinones) and in the 1700 – 1750 cm-1 region (nonconjugated aliphatic carbonyls). Therefore, in a spruce/melamine resin composite the IR absorption changes resulting from irradiation are similar to those in native spruce, where the absorption of light induces degradation of lignin and photooxidation of –CH2– or –CH(OH)– groups. The colour change of wood during irradiation or weathering is often described by the CIE L*a*b* method (e.g., Hon and Minemura 2001, Tolvaj and Mitsui 2005, Kataoka et al. 2005,2007), the most comprehensive colour space specified by the Commission Internationale d'Eclairage (CIE – International Commission on Illumination). The total colour difference ∆E is calculated by Eq. (1) (see DIN 6174).

2

12

2

12

2

12)**()**()**( bbaaLLE −+−+−=∆ (1)

where subscript 1 denotes the values before exposure and subscript 2 denotes the values after exposure, L* represents the grey value which varies between 0 (black) and 100 (white), positive values of (a*2-a*1) describe a red shift, negative values of (a*2-a*1) describe a green shift, positive values of (b*2-b*1) describe a yellow shift and negative values of (b*2-b*1) describe a blue shift. Figure 3 summarizes the changes of L*, a* and b* in native spruce and in the spruce/resin composite 1 as a result of the irradiation time. It shows that lightness and yellowness show nearly systematic trends with increasing irradiation time. The trend of the L*-value towards black, the increase of b*, and the slight change of a* show that in spruce as well as in composite 1 yellowing predominantly determines the ∆E changes.

Fig. 3: Lightness L*, redness a* and yellowness b* as functions of the irradiation time (�: spruce, �: composite 1; XBO lamp, experimental details see Figure 2)

Abb. 3: Lightness L*, Redness a* und Yellowness b* als Funktion der Bestrahlungszeit (�: Fichte, �: Komposit 1; Lampe: XBO, experimentelle Details vgl. Abbildung 2) The total colour difference ∆E shows a systematic trend to higher values with increasing irradiation time, see Figure 4. The formation of a local maximum (bleaching of chromophores – formed by UV irradiation), as shown in the work by Oltean et al. (2008) and Pandey (2005), was not observed in the time window applied. Yellowing proceeds faster in natural wood compared to composites. Gsöls et al. (2002) and Gindl et al. (2002) investigated melamine-impregnated wood and showed that

2,83,2

3,64,0

4,4

74

76

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82

84

86

2022

2426

2830

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360 min

90 min30 min

0 min

90 min

660 min

0 min

210 min

360 min

30 min

lightn

ess L

*

yellowness b*redness a*

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resin is deposited in the cell wall structure and hence in the vicinity of lignin. Therefore, it is possib-le that the melamine resin acts as a diffusion-hindering matrix and delays the formation of quinone as a reaction product of the lignin decay (e.g., Heitner 1993, Hon 2001, Schaller and Rogez 2007).

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Fig. 4: Colour change ∆E as a function of the irradiation time (XBO lamp, experimental details see Figure 2)

Abb. 4: Farbveränderung ∆E als Funktion der Bestrahlungszeit (Lampe: XBO, experimentelle Details vgl. Abbildung 2)

Fig. 5: Correlation between colour change and changes in IR absorption (the absorption was normalised to the band area at 895 cm-1 (spruce) and 667 cm-1 (spruce/PEG); � spruce, XBO lamp, � spruce/PEG, XBO lamp; � spruce, sun tester)

Abb. 5: Zusammenhang zwischen Farbveränderung und Änderungen der IR-Absorption (die Absorptionen wurden auf 895 cm-1 (Fichte) und 667 cm-1 (Fichte/PEG) normalisiert; � Fichte, Lampe: XBO, � Fichte/PEG, Lampe: XBO; � Fichte, Suntester)

0 50 100 150 200 250 300 350 400

0

2

4

6

8

spruce veneer

veneer + PEG 600

veneer + PEG 9000

composite 1

∆E

irradiation time in min

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For native spruce, it was found that the lignin decay (intensity changes of the IR band at 1510 cm-

1, expressed as the rel. ratio ∆A = A1510/A895) and ∆E are linearly correlated (Figure 5), see also Müller et al. 2003). This observation suggests that lignin decay is related to photo-yellowing. Unfortunately, the strong superposition of triazine and lignin bands impaired the quantitative analysis in this range. Nevertheless, the formation of carbonyl bands at 1730 cm-1 shows the same nonlinear behaviour for both (see Figure 6). Therefore, these observations allow the conclusion that the photochemistry of the materials is quite similar. The melamine resin acts as a photochemically inert matrix and hinders diffusion-controlled reactions through increased viscosity in the cell walls.

Fig. 6: Correlation between colour change and changes in IR absorption (aliphatic carbonyl functionality; absorption was normalised to the band area at 895 cm-1 (spruce) and 812 cm-1 (composite 1); XBO lamp, experimental details see Figure 2)

Abb. 6: Zusammenhang zwischen Farbveränderung und Änderungen in der IR-Absorption (aliphatische Carbonylbanden; die Absorptionen wurden auf 895 cm-1 (Fichte) und 812 cm-1 (Komposit 1) normalisiert, Lampe: XBO, experimentelle Details vgl. Abbildung 2)

Glycol modification

For allowing a chemical substance to penetrate into the cell wall its molecular weight should be low, especially for melamine solutions (Rosca et al. 2005). PEG molecular weight should not exceed 1000 g/mol (Wallström and Lindberg 1999, Norimoto 1996, 2001), and a weight percent gain of 35 marks the upper limit of modification of wood, due to wall saturation (Gsöls et al. 2002). The irradiation of PEG-impregnated wood veneer shows differences in the yellowing of the surface (see Figure 4). Wood impregnated with PEG 9000 showed strong yellowing especially in the initial phase. After 100 min of irradiation time the curve of colour change (∆E) flattens for PEG 9000-impregnated wood. After 360 min, natural and PEG 9000-impregnated wood show nearly the same colour change. However, impregnation of wood with PEG 600 shows a significant colour stabilising effect. Compared with untreated wood, impregnation with PEG 600 reduces yellowing by 50 percent. The different effects of PEG 600 and PEG 9000 can be explained by the different impregnation behaviour of the two types. In IR absorption spectra, lignin decay can be observed at 1510 cm-1. From the reduced yellowing one might conclude that lignin decay is also reduced. Nevertheless, in contrast to the decrease in yellowing significantly accelerated lignin decay was observed (see Figure 5). Taking the reaction mechanism into account one could argue that the strong lignin decay results from increased abundance of H-donors (PEG 600) in the photoinduced steps. This would indicate that PEG 600 is located in the cell walls and PEG 9000 in the lumina. Hence, it can be expected that only PEG 600, but not PEG 9000, accelerates the lignin decay. Moreover, it can be assumed that the reduced discolouration results from a photochemical reduction of the formed quinines, with PEG 600 acting

0 2 4 6 8 10

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spruce

composite 1

rel. r

atio (

A17

30

cm

-1 /A

refe

ren

ce)

∆E

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as H-donor. In fact, in the presence of PEG 600 the formation of quinone structures is dramatically reduced. In the IR spectra, no formation of carbonyl bands at 1650 cm-1 as in natural wood (see Figure 2 and 7) is observed. This is in line with the above thesis. Nevertheless, accelerated lignin decay was observed for both glycol derivatives; see Figure 7.

Fig. 7: IR spectra of spruce and PEG-modified and irradiated spruce (absorption was normalised at 1018 cm-1, XBO lamp, experimental details see Figure 2)

Abb. 7: IR-Spektren von Fichte und PEG modifizierter und bestrahlter Fichte (die Absorptionen wurden auf 1018 cm-1 normalisiert, Lampe: XBO, experimentelle Details vgl. Abbildung 2) IR and colour measurements detect signals in different depths from the wood surface. For the ATR technique it is known that the depth of penetration of the infrared radiation into the sample depends on the angle of incidence, the wavelength of the infrared radiation, and the refractive indices of the ATR crystal and the sample (Harrick 1980, Scherzer 2002). Therefore, the detectable depth profile is less than 4 µm. The depth of penetration of visible light is much higher, however. Kataoka et al. (2005, 2007) studied this problem for light of different wavelengths and depending on wood density. Kataoka et al. (2007) measured the penetration of light with a photodetector. For Cryptomeria japonica D. the depth of 10% transmittance varies from 33 µm (246 nm) over 63 µm (341 nm) to 279 µm (496 nm) and for a 1% transmittance from 66 µm (246 nm) over 131 µm (341 nm) to 585 µm (496 nm). Moreover, up to 400 nm the photodegradation depth correlates very well with the 1% transmittance range (~ 300 µm). These papers show that light penetration is deeper than 70 µm for UV light and around 200 µm for visible light, which were the values most often derived from the work by Hon and Ifju (1978). Apparently, PEG 600 penetrates and protects wood at least down to the depth that is relevant for colourimetry. Figure 8 summarizes the total colour changes ∆E of wood, composites and PEG 600-modified samples resulting from irradiation. It shows that discolouration can be reduced by PEG 600 modification. In all samples impregnated with PEG 600 a reduction of the shift can be observed. Compared to the unmodified wood both glycol and melamine resin mainly reduce the yellow shift (positive ∆b*). The effect on redness a* is low in the composites. In wood a slight green shift (negative ∆a*) was observed as a result of the modification. Especially composite 2 in combination with PEG modification shows good colour stability. Experiments for over 400 h in the sun tester confirm the results obtained in short-term trials. Figure 9 shows that discolouration can be reduced significantly by PEG modification. Compared to untreated wood both glycol and melamine resin mainly reduce the yellow shift (positive ∆b*). Moreover, PEG also shows an effect on the redness shift ∆a*. Both effects result in a decreased ∆E-value of the irradiated sample. Besides, colour stability had to be judged cautiously when there was no presence of humidity (which is notoriously low in normal running modes of light irradiation machines), and the effect of water proved detrimental for discoloration and weathering rates; see Turkulin and Sell (2002) and Turkulin et al. (2004).

1400 1500 1600 1700 1800 1900

0,00

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spruce

spruce + PEG 600 (360 min)

spruce + PEG 9000 (360 min)

ab

so

rban

ce in

a.

u.

wavenumber in cm-1

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Fig. 8: Changes of total colour difference ∆E as a result of irradiation of wood veneer, composite 1 and composite 2 (XBO lamp, irradiation time 360 min, experimental details see Figure 2, where a indicates PEG-modified spruce and b indicates PEG-modified resin)

Abb. 8: Änderungen der Farbe ∆E als Resultat der Bestrahlung von Fichtenfurnier, Komposit 1 und Komposit 2 (Lampe XBO, Bestrahlungszeit 360 min, experimentelle

Details vgl. Abbildung 2, wobei a für PEG modifizierte Fichte und b für PEG modifiziertes Harz steht)

01

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∆L*

∆b*∆a*

0 100 200 300 400 500

0

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2

S

∆E

irradiation time in h

Fig. 9: Changes of lightness L*, redness a*, yellowness b* (left) and total colour difference ∆E (right) as a result of irradiation of spruce veneer, composite 2 and composite 2a (suntester, ∆a*, ∆b* and ∆L* after 432 and 216 h (spruce veneer), ∆E as a function of irradiation time, where S indicates spruce veneer, 2 indicates composite 2, and a indicates PEG-modified spruce)

Abb. 9: Änderungen in Lightness L*, Redness a*, Yellowness b* (links) und der Farbe ∆E (rechts) als Resultat der Bestrahlung von Fichtenfurnier, Komposit 2 und Komposit 2a (Suntester, ∆a*, ∆b* und ∆L* nach 432 und 216 h (Fichtenfurnier), ∆E als Funktion der Bestrahlungszeit, wobei S für Fichtenfurnier, 2 für Komposite 2 und a für PEG modifizierte Fichte steht)

0

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without PEG

PEG modified spruce

PEG modified resin

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Conclusion

Photo-yellowing of wood and wood/melamine resin composites was studied using the FTIR-ATR technique and colourimetry (CIE L*a*b* method). The total colour difference ∆E shows a systematic trend to higher values with increasing irradiation time. Yellowing proceeds faster in natural wood compared to wood/melamine resin composites. Nevertheless, long-term irradiation experiments show that the total colour shift is similar for both. The formation of carbonyl bands at 1730 cm-1 shows the same nonlinear behaviour for both samples. These observations allow the conclusion that the photochemistry of these materials is quite similar. The melamine resin that was used absorbs below 280 nm. Therefore, under irradiation with light λ > 280 nm the resin assumes no light absorbing function. In other words, the resin acts as a photochemically inert matrix. Presumably, the hindered yellowing is a result of the rigid network formation in the cell walls which minimizes diffusion-controlled quinone formation and oxidation of the cellulosic building blocks. Impregnation with PEG 600 reduces yellowing by 50 percent, while PEG 9000-impregnated wood shows yellowing similar to natural wood. This can be explained by the different impregnation behaviour (cell wall vs. lumen) of the two PEG types. For all PEG 600-modified samples (wood, wood/resin composites) a reduction of the shift to darker colours can be observed. Compared to unmodified wood both glycol and melamine resin mainly reduce the yellow shift (positive ∆b*). The effect of PEG on redness a* is low in the composites. In wood a slight green shift (negative ∆a*) was observed as a result of the modification. The short-term experiments show that PEG-modified composites exhibit good colour stability. Exposures of 400 h in a commercial xenon-lamp equipped with a weathering cabinet confirm the results obtained during short exposures to high-pressure arc lamp. . Discolouration is reduced significantly by PEG. Compared to untreated wood, both glycol and melamine resin mainly reduce the yellow shift (positive ∆b*) resulting from irradiation. Moreover, PEG also shows an effect on the redness shift ∆a*. Both effects result in a decreased ∆E-value of the irradiated samples.

Acknowledgements

The authors gratefully acknowledge the support of the Competence Centre for Wood Composites and Wood Chemistry (Wood K plus), funded by the Austrian Federal Government and the provincial governments of Upper Austria, Lower Austria and Carinthia. Special thanks are due to AMI Agrolinz Melamine International GmbH for providing and improving the new melamine resins.

References

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Figure Legend

Fig. 1: UV spectra of a melamine resin (type MER, acetonitrile/water)

Abb. 1: UV-Spektrum des Melaminharzes (Typ MER, Acetonitril/Wasser)

Fig. 2: Decay of the lignin functionality at 1510 cm-1 and formation of carbonyl groups at 1730 cm-1 as function of the irradiation time (XBO 100 lamp, Io = 17.5 mW/cm2, λ > 280 nm, irradiation time 360 min, absorption was normalized at 1018 cm-1, left: composite 1, right: spruce (top), composite 2 (bottom);— before; --- after irradiation)

Abb. 2: Abbau der Ligninbande bei 1510 cm-1 und Bildung von Carbonylgruppen bei 1730 cm-1 als Funktion der Bestrahlungszeit (Lampe: XBO 100, Io = 17,5 mW/cm2, λ > 280 nm, Bestrahlungszeit 360 min, die Absorption wurde auf 1018 cm-1 normalisiert, links: Komposit 1, rechts: Fichte (oben), Komposit 2 (unten) — vor der Bestrahlung --- nach der Bestrahlung)

Fig. 3: Lightness L*, redness a* and yellowness b* as functions of irradiation time (�: spruce, �: composite 1; XBO lamp, experimental details see Figure 2)

Abb. 3: Lightness L*, Redness a* und Yellowness b* als Funktion der Bestrahlungszeit (�: Fichte, �: Komposit 1; Lampe: XBO, experimentelle Details vgl. Abbildung 2)

Fig. 4: Colour change ∆E as a function of irradiation time (XBO lamp, experimental details see Figure 2)

Abb. 4: Farbveränderung ∆E als Funktion der Bestrahlungszeit (Lampe: XBO, experimentelle Details vgl. Abbildung 2)

Fig. 5: Correlation between colour change and changes in IR absorption (absorption was normalised to the band area at 895 cm-1 (spruce) and 667 cm-1 (spruce/PEG); � spruce, XBO lamp, � spruce/PEG, XBO lamp; � spruce, suntester)

Abb. 5: Zusammenhang zwischen Farbveränderung und Änderungen in den IR-Absorptionen (die Absorptionen wurden auf 895 cm-1 (Fichte) und 667 cm-1 (Fichte/PEG) normalisiert; � Fichte, Lampe: XBO, � Fichte/PEG, Lampe: XBO; � Fichte, Suntester)

Fig. 6: Correlation between colour change and changes in IR absorption (aliphatic carbonyl functionality; absorption was normalised to the band area at 895 cm-1 (spruce) and 812 cm-1 (composite 1); XBO lamp, experimental details see Figure 2)

Abb. 6: Zusammenhang zwischen Farbveränderung und Änderungen in der IR-Absorption (aliphatische Carbonylbanden; die Absorptionen wurden auf 895 cm-1 (Fichte) und 812 cm-1 (Komposit 1) normalisiert, Lampe: XBO, experimentelle Details vgl. Abbildung 2)

Fig. 7: IR spectra of spruce and PEG-modified and irradiated spruce (absorption was normalised at 1018 cm-1, XBO lamp, experimental details see Figure 2)

Abb. 7: IR-Spektren von Fichte und PEG modifizierter und bestrahlter Fichte (die Absorptionen wurden auf 1018 cm-1 normalisiert, Lampe: XBO, experimentelle Details vgl. Abbildung 2)

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Fig. 8: Changes of total colour difference ∆E as a result of irradiation of wood veneer, composite 1 and composite 2 (XBO lamp, irradiation time 360 min, experimental details see Figure 2, where a indicates PEG modified spruce and b indicates PEG-modified resin)

Abb. 8: Änderungen der Farbe ∆E als Resultat der Bestrahlung von Fichtenfurnier, Komposit 1 und Komposit 2 (Lampe XBO, Bestrahlungszeit 360 min, experimental Details

vgl. Abbildung 2, wobei a für PEG modifizierte Fichte und b für PEG modifiziertes Harz steht

Fig. 9: Changes of lightness L*, redness a*, yellowness b* (left) and total colour difference ∆E (right) as a result of irradiation of spruce veneer, composite 2 and composite 2a (suntester, ∆a*, ∆b* and ∆L* after 432 and 216 h (spruce veneer), ∆E as a function of irradiation time, where S indicates spruce veneer, 2 indicates composite 2, and a indicates PEG modified-spruce)

Abb. 9: Änderungen in Lightness L*, Redness a*, Yellowness b* (links) und der Farbe ∆E (rechts) als Resultat der Bestrahlung von Fichtenfurnier, Komposit 2 und Komposit 2a (Suntester, ∆a*, ∆b* und ∆L* nach 432 und 216 h (Fichtenfurnier), ∆E als Funktion der Bestrahlungszeit, wobei S für Fichtenfurnier, 2 für Komposite 2 und a für PEG modifizierte Fichte steht)

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colour stabilisation of wood composites using polyethylene

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