ZnO-Based UV Nanocomposites for Wood Coatings in Outdoor Applications

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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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no UVprotection

HALS/UVAImpregnation

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4 % ZnO 1 +Impregnation2 1

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Delta L* Delta b*

no ZnO4 % ZnO 1

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

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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,

DOI: 10.1002/mame.200900135

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



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

www.mme-journal.de 131

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.



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

DOI: 10.1002/mame.200900135


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–



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



Figure 1. SEM images of ZnO samples: (a) ZnO 1; (b) ZnO 2; (c) ZnO 3; (d) ZnO 4.

60

80

1001

45

3

]

6

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.



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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HALS/UVAImpregnation

2

4 % ZnO 1 +Impregnation2 1

colo

ur c

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Delta L* Delta b*

no ZnO4 % ZnO 1

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

ur c

hang

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-30-20-10

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�.



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



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.



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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[2] R. Flyunt, K. Czihal, F. Bauer, R. Mehnert, M. R. Buchmeiser, H.Bauch, R. Emmler, Proceedings of the 5th International WoodCoatings Congress, Prague, October 17–18, 2006.

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[19] DE 20 2005 021 576 U1 (2005); invs.: R. Schubert, L. Prager, R.Mehnert, M. Hinkefuß, R. Blau.

[20] DE 10 2006 042 063.2 (2006); invs.: R. Schubert, R. Mehnert.[21] DE 10 2008 024 149.0 (2008); invs.: R. Schubert, M. Hinkefuß,

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DOI: 10.1002/mame.200900135

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