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For Peer Review

Colour stabilisation of wood composites using
polyethylene glycol and melamine resin
Uwe Müller, Melanie Steiner
To cite this version:
Uwe Müller, Melanie Steiner. Colour stabilisation of wood composites using polyethylene glycol
and melamine resin. European Journal of Wood and Wood Products, Springer Verlag, 2009,
68 (4), pp.435-443. <10.1007/s00107-009-0386-1>. <hal-00568260>
HAL Id: hal-00568260
https://hal.archives-ouvertes.fr/hal-00568260
Submitted on 23 Feb 2011
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Holz als Roh- und Werkstoff
Draft Manuscript for Review
Colour-stabilisation of wood/melamine resin composites
without topcoat
r
Fo
Journal:
Manuscript ID:
Manuscript Type:
Complete List of Authors:
15-Jul-2009
Müller, Uwe; Kompetenzzentrum Holz GmbH, Holz-PolymerVerbunde
Steiner, Melanie; Kompetenzzentrum Holz GmbH, Holz-PolymerVerbunde
er
Keywords:
ORIGINALARBEITEN / ORIGINALS
Pe
Date Submitted by the
Author:
HRW-08-0192.R2
colour-stabilisation , wood, melamine resin, poly ethylene glycol ,
photoyellowing, colourimetry, FTIR-ATR, composite
ew
vi
Re
Editorial Office, TU MÃ¼nchen, Holzforschung MÃ¼nchen, Winzererstr. 45, 80797 MÃ¼nchen, Germany
Page 1 of 15
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.
Fo
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
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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.
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Die Verfärbung wird durch PEG-Modifizierung signifikant reduziert. Im Vergleich zum
unbehandelten Holz wird durch die Glykol-Modifizierung wie in den Holz/MelaminharzKompositen 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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1
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 controlling 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.
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Page 2 of 15
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
2
Page 3 of 15
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
native spruce
composite 1
composite 2
veneer
veneer + PEG
chips
chips + PEG
chips
chips + PEG
MER
in %
90
90
80
80
10
10
10a)
10
EVA
in %
10
10
rP
a)
wood
in %
Fo
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 PEGmodified 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 mWcm-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 mWcm-2, black
panel temperature 65°C). The humidity was not controlled.
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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.
3
1,0
0,8
absorption
10 mg/l
1 mg/l
0,6
MER
in CH3CN / H2O 4:1
0,4
0,2
0,0
Fo
200
220
rP
240
260
280
300
λ in nm
Fig. 1: UV spectra of a melamine resin (type MER, acetonitrile/water)
Abb. 1: UV-Spektrum des Melaminharzes (Typ MER, Acetonitril/Wasser)
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Page 4 of 15
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)
4
Page 5 of 15
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).
∆E = ( L *2 − L *1 )2 + (a *2 −a *1 )2 + (b *2 −b *1 )2
(1)
Fo
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.
rP
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.
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0 min
90 min
30 min
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210 min
360 min 660 min
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lightness L*
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0 min
76
210 min
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2,8
30 min
3,2
360 min
90 min
3,6
redn
4,0
ess
a*
24
4,4
22
20
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28
s
nes
w
lo
yel
30
b*
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
5
resin is deposited in the cell wall structure and hence in the vicinity of lignin. Therefore, it is possible 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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Page 6 of 15
6
Page 7 of 15
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∆E
4
2
spruce veneer
veneer + PEG 600
veneer + PEG 9000
composite 1
0
0
50
100
150
200
250
300
350
400
irradiation time in min
rP
Fo
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)
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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)
7
For native spruce, it was found that the lignin decay (intensity changes of the IR band at 1510 cm1
, 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.
-1
6
spruce
composite 1
5
4
3
2
0
ee
1
rP
rel. ratio (A1730 cm /Areference)
7
Fo
2
4
6
8
10
∆E
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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)
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Abb. 6: Zusammenhang zwischen Farbveränderung und Änderungen in der IRAbsorption (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
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Page 8 of 15
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 9000impregnated 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
8
Page 9 of 15
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.
0,40
absorbance in a. u.
0,35
spruce
spruce + PEG 600 (360 min)
spruce + PEG 9000 (360 min)
0,30
0,25
0,20
0,15
0,10
Fo
0,05
0,00
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wavenumber in cm
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)
ee
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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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).
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without PEG
PEG modified spruce
PEG modified resin
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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)
ee
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)
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0
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∆E
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Page 10 of 15
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∆b
*
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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)
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Page 11 of 15
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.
ee
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Acknowledgements
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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.
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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)
Fo
Fig. 3: Lightness L*, redness a* and yellowness b* as functions of irradiation time (:
spruce, : composite 1; XBO lamp, experimental details see Figure 2)
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Abb. 3: Lightness L*, Redness a* und Yellowness b* als Funktion der Bestrahlungszeit
(: Fichte, : Komposit 1; Lampe: XBO, experimentelle Details vgl. Abbildung 2)
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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)
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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)
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Abb. 5: Zusammenhang zwischen Farbveränderung und Änderungen in den IRAbsorptionen (die Absorptionen wurden auf 895 cm-1 (Fichte) und 667 cm-1 (Fichte/PEG)
normalisiert; Fichte, Lampe: XBO, Fichte/PEG, Lampe: XBO; Fichte, Suntester)
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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 IRAbsorption (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)
14
Page 15 of 15
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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