Full Paper
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ZnO-Based UV Nanocomposites for WoodCoatings in Outdoor Applications
Franziska Weichelt, Rico Emmler, Roman Flyunt, Evelin Beyer,Michael R. Buchmeiser, Mario Beyer*
Nanocomposite UV coatings with adjustable properties for use on wood substrates in outdoorconditions were developed. Nanoscale ZnO was shown to be an efficient light absorber.Coatings were characterized in terms of elongation at brake, residual PI and double bondconversion, universal hardness, transparency, hydrophobicity, and yellowing. Coated sampleswere artificially weathered and studied withregard to their optical and mechanical proper-ties, as well as to changes in brightness, trans-parency, hydrophobicity, and waterpermeability. The prepared wood coatingsshowed an increased weather fastness andimproved optical properties. The suitabilityfor use in outdoor conditions was assured byoptimizing the elasticity of the coating anddecreasing its water permeability.
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Introduction
It iswell-established that lignin as one of themajor organic
component of wood decomposes photochemically under
irradiationwith lightwith l< 500nm.[1] Thedegradation is
observed up to a depth of 700mm. Phenolic chromophores
absorb the light and generate long-lived triplet states,
which in consecutive reactions lead to the degradation of
lignin. Oxygen and moisture accelerate the degradation of
lignin, which gives raise to low molecular degradation
products resulting in a loss of coating adhesion. The ideal
M. Beyer, R. EmmlerInstitute of Wood Technology, Zellescher Weg 24, D-01217Dresden, GermanyFax: þ49 351 466 2347; E-mail: [email protected]. Weichelt, R. Flyunt, E. Beyer, M. R. BuchmeiserLeibniz Institute of Surface Modification, Permoserstr. 15, D-04318Leipzig, GermanyM. R. BuchmeiserCurrent address: Universitat Leipzig, Institut fur TechnischeChemie, Linnestr. 3, D-04103 Leipzig, Germany
Macromol. Mater. Eng. 2010, 295, 130–136
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protective coating for wood substrates must therefore
possess sufficient filter efficiency up to 440nm and a high
barrier for oxygen and water vapor permeation. The latter
are typical features (together with high scratch, abrasion,
and impact resistance) of modern nanocomposite lac-
quers.[2–10]
Zincoxide showsnumerousoutstandingproperties, such
as chemical inertness or good optical characteristics with
respect to UV absorption.[11–13] It is thus a promising
candidate for the role of an efficient light absorber for UV
coatings applied on wood surfaces for outdoor use.
Experimental Part
Materials
Acrylates, methacrylates, and reactive thinners were purchased
from CYTEC Surface Specialties (Cray Valley, USA) or BASF
(Ludwigshafen, Germany). The coupling agents methacryloxypro-
pyltrimethoxysilane (MEMO) and vinyltrimethoxysilane (VTMO)
as well as surface additives were obtained from Evonik (Hanau,
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ZnO-Based UV Nanocomposites for Wood Coatings . . .
Table 1. Nanoscale ZnO-based UVAs.
Sample Obtained as Dimensions of
primary particles
nma)
ZnO 1 40wt.-% dispersion
in organic solvent
20
ZnO 2 powder 20
ZnO 3 powder 20
ZnO 4 powder 20–50
a)According to the product information of the providers.
Table 2. Contact angles of the cured lacquer B.
Lacquer system Contact angle
-
without surface additives 82.7� 0.3
þ1.5wt.-% polysiloxane resin 94.9� 0.5
þ1.5wt.-% silicone polyacrylate 99.6� 0.5
þ1.5wt.-% mixture of silicone acrylates 99.2� 1.0
Germany). Nanoscale zinc oxide (ZnO) was provided by three
suppliers (Table 1). Tinuvin 123 (CIBA Specialty Chemicals, Basel)
was used as a radical scavenger [hindered amine light stabilizer
(HALS), decanedioic acid, bis(2,2,6,6-tetramethyl-1-(octyloxy)-4-
piperidinyl) ester]. Tinuvin 400 is a mixture of 2-[4-[(2-hydroxy-
3-dodecyloxypropyl)oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethyl-
phenyl)-1,3,5-triazine and 2-[4-[(2-hydroxy-3-tridecyloxypropyl)
oxy]-2-hydroxyphenyl]-4,6-bis(2,4-dimethylphenyl)-1,3,5-triazine
in 1-methoxy-2-propanol. They both act as ultraviolet light
absorbers (UVAs) and were obtained from CIBA Specialty
Chemicals.
We also investigated a self-made nanoscale ZnO sample, which
was synthesized as follows. Zinc acetatedihydrate (252 g, 1.15mol)
was dissolved in 3.8 L of ethanol and the solution was heated to
reflux in a Normag 6 L batch reactor equipped with a condenser, a
thermometer and a stirrer. A solution of sodium hydroxide (92 g,
2.3mol) in 115mL of ethanol and 345mL of water was added via a
peristaltic pump (starting velocity: 300 rpm for 10min, then
increase to 600 rpm for 60min). During that time a white
precipitate formed. The dispersion was then allowed to cool to
room temperature. The precipitate was filtrated over a glass frit
(pore size 4), thoroughly washed with a mixture of water and
ethanol anddried at 90 8C. Finally, thenanopowderwas calcined in
a muffle furnace at 350 8C for 1 h.
The lacquers used in this work were lacquer A, consisting of
70wt.-% of aliphatic urethane diacrylate A and 30wt.-% of a
reactive thinner, and lacquer B, consisting of 70wt.-% of aliphatic
urethane diacrylate B and 30wt.-% of a reactive thinner.
This study was limited to wood designed for vertical outdoor
applications, where high scratch resistance is not required. For this
reason lacquers A and B were not reinforced with nano- and
microparticles, such as SiO2 and corundum.
Thephotoinitiator (PI) employed in thisworkwasDarocure4265
(CIBA Specialty Chemicals, used on a 2wt.-% base), which is a 1:1
mixture of 2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO)
and 2-hydroxy-2-methyl-1-phenyl-propan-1-one (Darocure 1173).
This PI having a second absorption maximum around 380nm is
especially suited for the UV curing of pigmented wood finishes as
well aswhite UV inks. Spruce and pinewood sampleswere chosen
for the present work. Prior to lacquer application, part of the wood
samples were pre-treated with an aqueous wood impregnation
Macromol. Mater. Eng. 2010, 295, 130–136
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H5100 (3H-Lacke, Hiddenhausen, Germany) containing two
different lignin protecting formulations. They were applied as
follows:
Impregnation 1: containing 2wt.-% of a solid lignin protecting
agent 1.
Impregnation 2: containing 5% of water-based lignin protecting
formulation 2.
Spruce wood was chosen as substrate for the coating trials and
artificialweathering experiments, since thiswood species is one of
the most frequently used materials for wooden facade sidings in
Central Europe.
Coating and Curing Procedures
Glass and polycarbonate plates were coated using a 50–100mm
doctor blade and used for all measurements presented in Table 2
and 3. To obtain a superior adhesion of the coatings, all wood
sampleswere treatedwithfinesandpaper (400grit).Woodsamples
were coatedusingaBuerkle roller coater. Thecoatingweightwas in
the range of 60–70g �m�2 realized by applying two layers. The first
layer was typically 5–10% heavier than the second. Curing of the
first layer under a N2 atmosphere led to adhesion problems of the
second layer. Therefore, the first layer was always cured under air,
while the second was cured under a N2 atmosphere. There, the
residualO2 concentrationwaskept below100ppm. In caseofwood
samples, an overall dose of 4 500mJ � cm�2 for each layer was
reached by a threefold curing at a conveyor speed of 4m �min�1
applying full power (120W � cm�1) to the Hg lamp. Glass or
polycarbonate sampleswere cured in one step at a conveyor speed
of 1.3m �min�1.
Samples cured by Hg lamp displayed high gloss in the range of
80–90U at 608. In case the second layer of a coating made of an
identical formulation was microfolded by the action of a 172nm
excimer lamp followed by curing with a Hg lamp, the gloss of the
resulting sample was low (typically 5–10U at 608). The set-up for
physical matting is described elsewhere.[5,8,14–21]
Analytical Techniques
The conversion of the �C¼C� double bonds in the coatings
was determined by IR spectroscopy measuring the ratios of
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F. Weichelt, R. Emmler, R. Flyunt, E. Beyer, M. R. Buchmeiser, M. Beyer
Table 3. Dependence of universal hardness, amount of residual double bonds and PI for lacquer B containing standard additives on curingconditions and on the presence of ZnO.
Curing conditions ZnO Dose Universal
hardness
Residual double
bonds
Residual
photoinitiator
mJ � cm�2 N �mm�2 % wt.-%
15m �min�1 no 390 110 7.1 1.32
10m �min�1 no 570 114 6.8 1.18
5m �min�1 no 1 140 118 5.1 0.98
1.3m �min�1 no 4 500 128 2.6 0.42
3� UV at 4 m �min�1 no 4 500 152 2.5 0.39
3� UV at 4 m �min�1 4wt.-% ZnO 1 4500 131 2.7 0.43
3� UV at 4 m �min�1 2wt.-% ZnO 2 4500 135 2.7 –a)
3� UV at 4 m �min�1 2wt.-% ZnO 3 4500 139 2.8 –a)
3� UV at 4 m �min�1 2wt.-% ZnO 1 4500 136 2.9 –a)
a)Not determined.
132
peak areas for absorbance at 810 cm�1 for cured and uncured
probe after the normalization of C¼O absorbance peak, typically
at 1 720 cm�1. The residual PIs were determined by means of
high-performance liquid chromatography (HPLC) analysis after
their extraction from the cured coatings with acetonitrile. For
this purpose, a 0.200 g sample of the cured coating was placed in
2mL of acetonitrile and sonicated for 10min at 25 8C in an
ultrasonic bath. The sample was filtered and subject to HPLC.
Quantification of the PIs was achieved on a Varian Pursuit�RsUltra C-18 reversed phase column using acetonitrile/water
(80:20 vol.-%) as a mobile phase at a flow rate of 0.4mL �min�1
using UV detection at 255nm.
Contact angles were measured on a Contact Angle System OCA
20 (DataPhysics InstrumentsGmbH, Filderstadt, Germany). Values
for the elongation at break of the differentmonomers and lacquers
were measured with the aid of a dynamic mechanical analyzer
DMA7e (PerkinElmer). The elasticity of the coatingswas evaluated
according to ENV 13696. The impact resistance of the coatingswas
tested according to EN 438-2 using the small ball device (Erichsen
GmbH, Hemer, Germany). Martens hardness was measured
according to EN ISO 14577-1 with a Fischerscope HP100V XY
(Helmut Fisher GmbH & Co., Sindelfingen, Germany). The water
vaporpermeability testwas conductedaccording toZDINEN927-4.
Water permeabilitymeasurementswere done according toDIN EN
927-5. Outdoor weathering resistance tests were performed
according to DIN EN 927-3. Artificial weathering was performed
inaccordancewithDINEN ISO11341applyingaCI 3000XENOTEST
device (AtlasMTT,Chicago,USA). Colormeasurementswere carried
out according to DIN 5033-7 by the use of a CM 3610d spectro-
photometer (Minolta, Japan). Gloss was determined according
to DIN 67530 using a reflectometer REFO 3-D (Dr. Bruno Lange
GmbH, Berlin, Germany). Haze measurements were done with a
Haze-Gard plus device (Byk-Gardner GmbH, Geretsried, Germany).
UV Spectra were recorded using a Shimadzu 2101 UV–Vis
Spectrophotometer.
Macromol. Mater. Eng. 2010, 295, 130–136
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Results and Discussion
Choice of a Lacquer
The major criteria for lacquers designed for outdoor
applications are:
(i) high reactivity and possibly high elasticity;
(ii) low yellowing of the lacquer’s components and high
chemical resistance under outdoor conditions.
Monomers that fulfill the first criterion are based on
epoxy-, polyester-, polyether-, or aromatic urethane acry-
lates. However, they are discarded for outdoor applications,
since they do not fulfill the second requirement because of
their hydrolysis and/or photochemical degradation. Alter-
natively, the aliphatic urethane acrylates or their mixtures
with aliphatic acrylates are recommended for the coatings
with a high weathering resistance.
High reactivity translates into a reduced amount of
energy consumed and higher conversion of the starting
compounds. High elasticity is needed, becausewood under
outdoor conditions suffers from strong dimensional
changes and, therefore, the corresponding coatings have
to be sufficiently elastic. As a first approach, the values of
elongation at break can be taken as a measure for coating
elasticity. Theyweremeasured for the startingmaterials as
well as for different lacquer systems. Those reactive
thinners (mono- anddiacrylates) recommended for outdoor
applications are characterized by relatively low elasticity
with elongations in the range of 10–17%.Here,we tested an
aliphatic urethane triacrylate recommended for outdoor
applications. It displayedaveryhighvalue for elongationat
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ZnO-Based UV Nanocomposites for Wood Coatings . . .
break (75%), however, this monomer possessed generally
bad initial adhesion (directly after UV curing) to the
impregnated wood samples. Therefore, two alternative
lacquers A and B (for the description see Experimental Part)
were developed. They showed a superior primary adhesion
to the impregnated wood. Despite the higher values for
elongation at break of lacquer A as compared to lacquer B
(26 vs. 12%), the latter showed better weatherability and
impact resistance compared to lacquer A. Therefore, all
further investigations were exclusively carried out using
lacquer B.
Evaluation of Hydrophobicity
Theexperimentallydeterminedvalues for thecontactangle
were taken as a preliminary measure for the hydrophobi-
cityof the coatings (Table2). LacquerBwas characterizedby
a high contact angle of about 838, which could be further
increased up to 99.28 by adding 1.5wt.-% of a mixture of
silicone acrylates.
In contrast to silicone polyacrylate additives, silicone
acrylates copolymerize with the acrylic matrix and hence
do not migrate within the cured coating. In addition, they
provide coatings with a perfect optical appearance. It
should also be noted that silicone acrylate-based coatings
display a slower decrease in the contact angle during
weathering thancoatingswithother surfaceadditives, that
is, their hydrophobicity is more long-lasting. In view of
these advantages,we choose amixture of silicone acrylates
at a 1.5wt.-% level as an additive (referred to as ‘‘standard
additive’’) for all further investigations.
Optimization of Curing Conditions
Many parameters are important for the durability of
coatings. For example, the universal hardness is considered
as a measure of surface hardness of the coating, that is,
crosslinking density at the surface. In UV curing, cross-
linking is directly related to the total dose absorbed by the
coating and its reactivity. In order to determine the
optimum curing conditions in terms of residual double
bonds and total dose to be applied, we measured both
double bond conversion and universal hardness applying
different total doses. The results for lacquer B are
summarized in Table 3. As can be seen, the values of the
universal hardness increased by only 14% from 110 to
128N �mm�2 upon a 10-fold increase in the absorbed dose
(from 390 to 4 500mJ � cm�2).
When a same dose of 4 500mJ � cm�2 was applied to a
sample that was passed three times under a Hg lamp at a
conveyor speed of 4m �min�1, the universal hardness
increased for another 15% reaching 152N �mm�2 (2.5mol-%
residualdoublebonds).We thenaddedvariousamounts (2–
Macromol. Mater. Eng. 2010, 295, 130–136
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4wt.-%) of ZnO to the lacquer. No influence of the type of
ZnO on the residual double bond content (they were in the
range between 2.7 and 2.9%) and contact angles was
observed. Therefore, we conclude that ZnO does not
change the polymerization depth. The universal hardness
obtained for the resulting ZnO-containing coatings was
slightly lower (9–14%) which can be explained by a minor
reduction of crosslinking degree in the presence of ZnO
nanoparticles.
A very important aspect in coatings for outdoor
applications is the content of residual PI in the cured
coating. Residual PI will serve as a source of free radicals,
which in turn accelerate aging and degradation of the
coating. As can be seen from Table 3, such parameters as
universal hardness, content of residual double bonds and
contact angle are acceptable for outdoor weathering
already at a low dose of 390mJ � cm�2 (1� UV at conveyor
speed of 15m �min�1). However, about 66% of starting PI
remains in the coating. Acceptable values (approximately
20% with respect to the initial PI concentration) was only
achieved at a dose of 4 500mJ � cm�2 (1.3m �min�1, 1� UV
or 4m �min�1, 3� UV). Only those wood samples cured at
such conditions were taken for weathering tests.
Optimization of the Light Protective Function
The canning electron microscopy (SEM) images of the four
appliednanoscale zinc oxides, namedas ZnO1–4below, are
shown in Figure 1(a)–(d). The primary species of all ZnO
samplesare indeednanoparticleswithanaverage sizeof ca.
20–70nm.ZnO1,available asadispersion,wasadded to the
lacquers and stirred for 5min. The resultingnanocomposite
was stable (some sedimentation was observed only after
weeks of storage) and no tendency to form agglomerates
was observed. Therefore, nanocomposites based on ZnO 1
were used as such without filtration. Samples containing
ZnO 2–4 (17wt.-%) were dispersed in isobornyl acrylate at
high shearing force using a laboratory dissolver (ca.
5 000 rpm, 2 h). After addition of the PI to the dispersions,
glass plates were coated using a 100 and 12mm doctor
blade, respectively, and subject to UV curing. ZnO 2-based
coatings were very homogeneous and the optical appear-
ance of the cured coatings was equally good for the lacquer
with or without filtration through 30mm nylon filter. The
coating made of a lacquer based on ZnO 3 was inhomo-
geneous in case a 100mmdoctor blade was used indicative
for a strong agglomeration. A very fast sedimentationupon
centrifugation within a short time was observed, too.
However, the optical quality was significantly improved in
case the coating thickness was reduced to 12mm. An
additional in situ surface modification of ZnO 3 particles
with trialkoxysilanes resulted in no improvement. The
lacquer based on ZnO 4 behaved similarly to one based on
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F. Weichelt, R. Emmler, R. Flyunt, E. Beyer, M. R. Buchmeiser, M. Beyer
Figure 1. SEM images of ZnO samples: (a) ZnO 1; (b) ZnO 2; (c) ZnO 3; (d) ZnO 4.
60
80
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134
ZnO 3. Thus, lacquers based on ZnO 2–4 were immediately
applied after filtration in order to avoid sedimentation of
these additives.
Light transmittance and transparency measurements
were carried out with quartz samples coated with ZnO 1-
based formulations (2wt.-% inCN435, aliphatic triacrylate)
containing no UV stabilizer, or alternatively, a mixture of a
HALS and organicUVAs or thenanoscale zinc oxides ZnO1–
4 (Figure 2). The blank samplewithout any ZnO displayed a
haze of about 1%,whereas the coatingbased on2wt.-%ZnO
1hadahazeofapproximately4%.ConversionofCN435was
almost quantitative (amount of residual double bond
approximately 1%) showing that there was no significant
inhibition of the UV-triggered polymerization. The mea-
surement of the UV–Vis transmission spectra of 60mm
lacquer films with the above described formulations
revealed that there is a relatively steep increase in the
transmittance curve at 400nm if the conventional HALS/
Figure 2. UV transmission and transparency of cured coatings of60mm thickness containing different nanoscale ZnO species.
3000
20
40T [%
Figure 3. Transm(without PI) contno ZnO; 2, 4wt.-2wt.-% ZnO 4; 6,123.
Macromol. Mater. Eng. 2010, 295, 130–136
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
UVA mixture is used (Figure 3). Addition
of ZnO extends the light absorption into
the visible region thus decreasing the
irradiation intensity at thewood surface.
This might be taken as an important
indicator for the efficiency of nanoscale
ZnO as light absorbers. In view of these
measurements, ZnO loadings of 4wt.-%
of ZnO 1 and 2wt.-% of ZnO 2–4 were
finally chosen for coatings of ca. 60mm
thickness.
Light Protection of Spruce Wood
The performance of the ZnO-based nano-
composites was evaluated by comparing
their resistance toartificialweathering to
the one of a system without ZnO as well
as to one based on the conventional UV
protective system (UV absorbers in a
combination with HALS). As mentioned
above, in some variants the wood was
additionally pre-treated with a water-
borne wood impregnation, each containing one of two
different lignin protecting formulations 1 and 2 before
applying the UV coating. The effect of these different UV-
stabilizing measures on the discoloration of coated spruce
wood is summarized in Figure 4. Here, the brightness
change DL� of the sample coated with the nanocomposite
lacquer without UV-protecting additives was �20. This
sample also underwent strong yellowing (the Db� was
equal to 15). Addition of 0.5wt.-% of benzotriazole UVAand
HALS (conventional UVA) to the lacquer formulation led to
the expected improvement of the light fastness. When the
700600500400
2
[nm]Wavelength
ission spectra for a 50mm layer of lacquer Baining 1.5wt.-% mixture of silicone acrylates. 1,% ZnO 1; 3, 2wt.-% ZnO 2; 4, 2wt.-% ZnO 3; 5,no ZnOþ 2wt.-% Tinuvin 400þ 1wt.-% Tinuvin
DOI: 10.1002/mame.200900135
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4 % ZnO 1 +Impregnation2 1
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Figure 4. Change of brightness (DL�) and b� value (Db�) of sprucewood samples coated with nanocomposite UV lacquer in depen-dence on the UV-protective system (weathering time 1 500h).
Figure 6. Brightness loss DL� and yellowing Db� after 1 500h ofweathering of spruce wood samples coated with the UV nano-composite B containing three different variants of nanoscale ZnO(no lignin protecting impregnation).
UVA/HALS system was replaced by 4wt.-% of ZnO 1,
comparable results were obtained with regard to DL�.
However, the yellowing factor Db� decreased. High color
stability could be reached in case the wood surface was
treated with the lignin protecting impregnations. Particu-
larly with formulation 2, only a minor loss of brightness
was observed after 1 500h weathering. Best results were,
however, obtainedusingacombinationof ligninprotecting
impregnation and ZnO in the top coat.
Figure 5 shows the time profile of color changes during
1 500hofartificialweatheringof twodifferent samples. It is
-30-20-10
010203040
16001400120010008006004002000weathering time in h
colo
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010203040
16001400120010008006004002000weathering time in h
col
our c
hang
e
B
A
Figure 5. Time profile of color changes of spruce wood coatedwith UV nanocomposite. (A)Without UV protection; (B) 2wt.-% ofZnO 2, lignin protecting formulation 2.^: DL�,�: DE�,~: Db�,&:Da�.
Macromol. Mater. Eng. 2010, 295, 130–136
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
obvious that the samplewithout UV protection underwent
the strongest discoloration within the first 200h. The
brightness loss DL� reached the value of �12 and the
discolorationDE�wasabout20. Incontrast, thewoodcoated
with ZnO-containing UV nanocomposite coating showed
only a minor discoloration, which remained stable
throughout the whole weathering cycle.
Minor differenceswere observed between the nanoscale
ZnO used in this series. As was mentioned in the
experimental part, the maximum concentrations were
not the same for all variants due to different dispersion
behavior. However, at comparable concentrations it can be
seen (Figure 6) that, apart from comparably small devia-
tions in theDL�, theDb� values that characterize the yellow-
blue color shift differed considerably. This behavior is
obviously caused by different particle shapes and disper-
sion characteristics.
The addition of ZnO to the lacquer and impregnation
with lignin protectors did not only decrease the discolora-
tion of thewood surface. SinceUV irradiation ofwood leads
to radical formation and autoxidation reactions of the
ligninwithin the upperwood cell layers, hydrophilic lignin
decay products emerge and can be mobilized by water
vapor. This results in aweakening of the adhesion between
thewoodsurfaceandthetopcoatand leads todelamination
of the latter. Such behavior was in fact observed with the
wood samples possessing no or only insufficient UV
protection. Thus, large parts of the coating were delami-
nated as indicated by white or gray areas. In contrast, in
addition to a decreased discoloration, the samples contain-
ing ZnO undergo pronounced postponed delamination of
the organic coating from the wood surface due to the
reduced lignin photooxidation.
The results for selected coating combinations are
compiled in Figure 7. Also included are experiments with
physical matting by irradiation of the wet coating surface
witha172nmexcimer lampbeforeUVcuring. Thismatting
resulted in a comparable or even slightly increased UV
www.mme-journal.de 135
F. Weichelt, R. Emmler, R. Flyunt, E. Beyer, M. R. Buchmeiser, M. Beyer
Figure 7. CIELab color distance DE� of spruce wood samples pre-treated with lignin-protecting impregnation 2 and coated withUV nanocomposite after 1 000 and 1 500h of artificial weath-ering. Samples: 1, 2wt.-% ZnO 2, high gloss; 2, 2wt.-% of ZnO 2,matted; 3, 2wt.-% of ZnO 3, high gloss; 4, no ZnO, high gloss; 5, noZnO, matted; 6, 2wt.-% of ZnO 4, high gloss.
136
stabilization effects [compare samples 1 and 2 (ZnO 2) or 4
and 5 (no ZnO) in Figure 7].
With lacquer formulation B, ZnO 3 showed the highest
UV protection effect as becomes evident by comparing the
color differences of the irradiated sampleswith those of the
non-irradiated one.
Generally, the weathered nanocomposite derived coat-
ing layers contained only a small number of cracks. The
water-uptake of wood coated with this lacquer was
determined after 72h water storage in accordance with
DIN EN 927-5 and was found to be 49 g �m�2. This lacquer
formulation provides therefore a highmechanical stability
in combinationwith lowwater permeability. Thus, defined
limits for dimensionally stable and limited dimensionally
stable coatings given by this norm are �175 and
�250 g �m�2, respectively.
Conclusion
Wood for outdoor use can be efficiently protected from
discoloration by semitransparent coatings obtained
through application of a UV-cured nanocomposite lacquer
based on nanoscale ZnO. The investigated ZnO products
showed much higher stabilizing efficiency compared
to a conventional UV-protecting formulation. The best
results were achieved in case the wood was additionally
pre-treated with a water-based wood impregnation con-
taining a special lignin protector combination. The ZnO
additionally prevents any delamination of the coating
from the wood surface triggered by UV light and moisture.
Wood coatings with low gloss were obtained by physical
matting using 172nm irradiation generated by excimer
lamp. Coating formulations that were characterized by
a high elasticity and low water permeability are
considered thepreferredmaterial for a long-timeprotection
of wood.
Macromol. Mater. Eng. 2010, 295, 130–136
� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Acknowledgements: The authors are indebted to C. Kuhnel,I. Reinhardt, S. Wenk, and B. Brendler for technical assistanceand to Dr.W. Knolle for valuable discussion. The work was carriedout within the frames of the AiF Project (AiF no. 15 301 BR),supported by the Federal Ministry for Economics and Technology(BMWi) via the IGF program of the Arbeitsgemeinschaft Indus-trieller Forschungsvereinigungen ‘‘Otto von Guericke’’ e.V. (AiF).The final report can be obtained from the European ResearchSociety of Thin Films (EFDS).
Received: May 18, 2009; Revised: September 4, 2009; Publishedonline: December 1, 2009; DOI: 10.1002/mame.200900135
Keywords: coatings; hydrophobicity; nanoparticles; UV protec-tion; wood
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DOI: 10.1002/mame.200900135