Surface properties of sol gel treated thermally modified wood

ORIGINAL PAPER

Surface properties of sol–gel treated thermally modified wood

Boris Mahltig • Martin Arnold • Per Lothman

Received: 15 February 2010 / Accepted: 22 April 2010 / Published online: 29 April 2010

� Springer Science+Business Media, LLC 2010

Abstract The application of inorganic sol coating agents

is a versatile method for wood surface functionalisation.

However, the use of sols for the surface finishing of ther-

mally modified wood (TMT) so far has not been investi-

gated thoroughly. This paper reports on the surface

properties of TMT treated with modified silica sols. The

silica sols are modified with the inorganic colour pigment

iron oxide red. Pigment distribution and height profiles of

sol treated TMT are characterised by optical microscopy in

3D mode and by scanning electron microscopy. Selected

evaluations are also repeated after artificial weathering of

the coated wood specimens. Hydrophobic surface proper-

ties are determined using contact angle measurements. The

coloration can be adjusted by the degree of pigmentation

of the applied nanosol. Moreover, the water repellency

of TMT is significantly enhanced by the sol treatment.

Therefore, the application of pigment modified nanosols

could lead to TMT with improved weathering stability and

a wider coloration spectrum.

Keywords Thermally modified wood �Surface treatment � Weathering � SiO2 � Iron oxide �Pigments

1 Introduction

Thermally modified wood (TMT) is produced from natural

wood by thermal processing at around 200 �C in a controlled

atmosphere. Commercially it is frequently called Thermo-

Wood. TMT is known for an improved dimensional stability

and an improved resistance against fungal destruction

compared to natural wood [1–7]. Although the durability in

natural weathering is somewhat enhanced for heat-treated

wood compared to untreated wood [8], an additional surface

treatment is necessary for TMT in order to reach sufficient

UV resistance and color stability for outdoor applications

[9]. A possible method to improve wood properties is the

application of inorganic sols [10]. Nanosols contain inor-

ganic nanometer sized particles in an organic or aqueous

solvent [11]. The properties of nanosols can be easily

modified in a wide range by chemical or physical modifi-

cation [12, 13]. The modification of natural wood by inor-

ganic silicon compounds and sol–gel systems is a well

established method, e.g. to enhance dimensional stability

[14], improve antimicrobial properties and fire-resistance

[15–20]. Attention has been paid especially to water repel-

lency and improvement of weathering durability [21–23].

Aloui et al. reported on the comparison of organic UV

absorbers with inorganic pigments as additives for clearco-

ating systems on wood [24]. However, the application of

B. Mahltig (&)

Gesellschaft zur Forderung von Medizin-, Bio- und

Umwelttechnologien e.V., GMBU e.V., Bautzner Landstrasse,

01454 Dresden-Rossendorf, Germany

e-mail: [email protected]

M. Arnold

Wood Laboratory, Empa, Swiss Federal Laboratories

for Materials Testing and Research, Uberlandstrasse 129,

8600 Dubendorf, Switzerland

P. Lothman

BioNanotechnology and Multifunctional Interfaces Research

Group, Institut fur Botanik, Biomimetics, Technische Universitat

Dresden, 01062 Dresden, Germany

P. Lothman

Departement de Genie Physique and Regroupement Quebecois

sur les Materiaux de Pointe (RQMP), Ecole Polytechnique de

Montreal, Station Centre-ville, P.O. Box 6079, Montreal,

QC, Canada

123

J Sol-Gel Sci Technol (2010) 55:221–227

DOI 10.1007/s10971-010-2236-3

inorganic nanosols for the surface finishing of TMT has not

yet been investigated thoroughly. The purpose of this study

is to characterise intensively TMT surfaces after treatment

with inorganic nanosols modified with the inorganic colour

pigment iron oxide red. The combination of modified silica

sols with pigments of iron oxide red for TMT treatment is up

to now only little investigated [10]. Surface properties of

TMT with and without nanosol treatment are characterised

by optical and scanning electron microscopy and by contact

angle measurements.

2 Experimental part

2.1 Materials

Beech (Fagus sylvatica) TMT specimens were prepared

from thermally treated boards produced by Mitteramsko-

gler GmbH (Gaflenz, Austria). Thermal modification was

done in special drying chambers using the following pro-

cedure: heating to the drying temperature of approximately

100 �C, drying to a wood moisture content of 2–4%,

heating in a controlled atmosphere to 200 �C. The speci-

mens were surface machined by either planing only or with

an additional sanding step (drum sander, grit size 100).

Modified silica sols with solid content of 7.6 wt% were

prepared by acidic hydrolysis of tetraethoxysilane as

described in literature [25, 26]. For hydrolysis, to 20 ml

tetraethoxysilane in 84 ml ethanol an amount of 4 ml

0.01 N HCl was added and the resulting mixture is stirred

for at least 24 h. Afterwards 2 wt% of PEMA was dis-

solved in the silica sol. PEMA is a polyacrylate (polyeth-

ylmethacrylate) soluble in ethanol. This polymer is

commercially supplied by the Rohm GmbH (Darmstadt,

Germany) under the name Plexigum [27]. Subsequently 10

or 50% iron oxide red pigment paste was added and stirred

thoroughly. The pigment paste was supplied by HABICH

GesmbH (Leiben-Weitenegg, Austria) and contained 40

wt% of iron oxide red in methoxypropanol (Fe2O3, C.I.

Pigment Red 101; CAS-Nr.: 1332-37-2). The used pigment

is micronized and contains particles usually with diameters

of few micrometers. An amount of 0.3 L nanosol was

applied onto one square meter of specimen surface by

spraying (LPHV-gun, Binks M 1-G, USA). The resulting

coating solution contains two solvents ethanol and

methoxypropanol. After drying at room temperature for not

less than 5 h, the treated specimens were thermally cured at

60 �C for 1 h.

As reference for contact angle measurements TMT

samples are treated with only on single component (silica

sol, PEMA solution in ethanol or pure iron oxide red

pigment paste). The preparation and drying regime is

similar for these samples as described above.

2.2 Methods

The optical properties of the specimen surface were inves-

tigated by UV/Vis-spectroscopy in arrangement of diffuse

reflection. Colour measurements were performed with a

spectrometer MCS 400 (Zeiss, Germany). The topography

of the surfaces was characterised by light microscopy and

scanning electron microscopy (Leo 420 scanning electron

microscope Leo, Germany). 3D Light microscopy was

performed with a digital light microscope VHX-100

(Keyence, Japan). Apart from the lateral structures, also the

height of surface structures can be characterised by this type

of light microscope. Contact angle measurements were used

to determine the hydrophobic properties of the surfaces.

The water-contact angles were measured using a contact

angle system OCA from dataphysics (Germany). After

applying a drop of water on the sample surface the contact

angles were measured as a function of time and the average

contact angle was calculated from at least five different

measurements at five different locations on the specimen

surface. Artificial weathering was performed for 2,000 h

according to EN 927-6:2006 in a QUV accelerated weath-

ering tester based on UVA-340 fluorescent UV lamps

(Q-LAB, USA). The exposure cycle consists of an initial

24 h condensation phase to generate moisture stress in the

wood substrate, followed by intervals of 2.5 h UV-light and

0.5 h water spray to degrade and leach the sample surface.

Leaching is an integral part of the used exposure cycle.

3 Results and discussion

3.1 Surface coloration

With the application of pigmented nanosol the dark brown

color of TMT changes to the color of the used red pigment

(Fig. 1). By increasing the pigment content of the sol, the

original colour of TMT is covered gradually, while the

underlying wood structure remains partly visible. Thus,

the intensity of the coloration can be adjusted according to

the desired effects, as demonstrated by UV/Vis reflectance

spectroscopy (Fig. 2).

3.2 Surface morphology

The surface topography as characterised by 3D-optical

microscopy reveals highly structured surfaces on untreated

TMT after planing and sanding (Fig. 3). Regarding ana-

tomical features, axially oriented fibre bundles and cut

222 J Sol-Gel Sci Technol (2010) 55:221–227

123

open vessels typical for beech wood can clearly be dis-

tinguished. Planing appears to lead to a rougher and more

irregular surface, while on sanded surfaces some of the

anatomical features are clogged by dust, leading to an

apparently more homogenous structure. Therefore, most

investigations with pigmented nanosol were performed on

sanded surfaces.

On nanosol treated TMT surfaces the pigment layer was

not homogeneously distributed (Fig. 4). With 3D micro-

copy it is possible to relate the original microscopic picture

to a digitised surface topography of the investigated sur-

face. From this it is apparent that the applied pigments are

preferentially deposited in the cavities of the wood surface

(e.g. cut open vessels), while the raised structural features

(e.g. fibre bundles of cell walls) are less covered with

pigments.

Fig. 1 Macroscopic appearance of TMT surfaces without and with nanosol treatment (left: untreated TMT, center: nanosol under addition

of 10% pigment paste, right: nanosol under addition of 50% pigment paste)

300 400 500 600 700 800 9000

10

20

30

40

50

60

70

80

90

100

untreated TMT

TMT / nanosol with 10% pigment paste

TMT / nanosol with 50% pigment paste

refle

ctio

n [%

]

wavelength [nm]

Fig. 2 UV/Vis reflectance spectra of TMT surfaces

Fig. 3 3D light microscopy images of untreated TMT surfaces

(above: planed surface; below: sanded surface)

J Sol-Gel Sci Technol (2010) 55:221–227 223

123

However, this phenomenon depends on the pigment

concentration in the used nanosol (Fig. 5). With a low

pigment concentration, the wood surface is not fully cov-

ered (Fig. 4), while with a high pigment concentration a

complete coverage and a regular red coloration can be

achieved (Fig. 5).

These observations are confirmed and complemented by

scanning electron microscopy (Fig. 6). Compared to the

rough, fibrous surface of untreated TMT, the surfaces after

application of the nanosol are smoother and existing cav-

ities are filled by the applied sol. Pigment particles on the

wooden surface can be observed with diameters up to

2 lm. With a lower pigment concentration in the applied

sol the distribution of the pigment particles is still some-

what inhomogeneous. Nevertheless, also on surface areas

without deposited pigment particles the TMT surface

appears to be smoother compared to the untreated surface.

Probably, the silica component of the nanosol is covering

the whole wooden surface rather homogeneously, while the

pigment component is deposited preferentially in surface

depressions. With the application of a sol with higher

pigment concentration, the TMT surface is completely

covered with densely packed pigment particles. This

homogeneous layer leads to an opaque and intense

coloration.

After 2,000 h of artificial weathering, uncoated TMT

surfaces show structural changes typical for weathering

degradation (loose fibres, destroyed pits), while nanosol

treated surfaces still appear to be partly protected by the

pigment layer (Fig. 7). However the pigment layer on the

nanosol treated TMT is partly eroded (Figs. 7 and 8). With

the high pigment concentration, sizeable pieces of the

pigment layer break away during the weathering process

(Fig. 7). With a lower pigment concentration the pigment

layer is less brittle and the weathering erosion of the pig-

ment layer appears to take place more intensily in the

surface depressions, while the raised structural features are

still covered by the pigment containing nanosol (Fig. 8).

The localised erosion is probably the result of a stronger

abrasion on the surface in areas without nanosol protection.

Surface areas with an intact nanosol coating have a higher

weathering resistance and remain therefore as protective

coating spots on the wooden surface. This is indicating that

a pigment containing nanosol coating could be a possible

means to realise a weathering protection of TMT surfaces.

Fig. 4 3D light microscopy image of the TMT surface after treat-

ment with pigment containing nanosol (pigment concentration

10%)—given is a comparison between the true color image (above)

and the calculated height profile image (below)

Fig. 5 Light microscopy image of the TMT surface after treat-

ment with pigment containing nanosol (pigment concentration




123

Fig. 6 SEM images TMT surfaces without and with nanosol treatment

Fig. 7 SEM images of untreated and nanosol treated surfaces after 2,000 h of artificial weathering


123

3.3 Water repellent properties

The wetting behavior of TMT with and without nanosol

treatment is shown in Fig. 9 by contact angle values as a

function of time. Planed or sanded TMT specimens are

compared to planed natural beech wood. Without nanosol

treatment, TMT initially exhibits a slightly higher contact

angle compared to natural beech. However, on surfaces

without nanosol treatment the contact angles decrease with

measurement time and after 60 s no difference can be

observed between the specimens. After 240 s the drop of

water was almost completely absorbed. Eventhough espe-

cially the sanded TMT exhibits a higher water-contact

angle at the beginning, over a longer time measurement

there is no relevant difference in water repellency between

TMT and natural wood without nanosol treatment. The

decrease of the contact angle as function of time is prob-

ably caused by sinking of the water drop into the wood.

This type of water absorption is obviously decelerated by

the used coatings.

As comparison the contact angles of TMT after appli-

cation of the single components are determined (Fig. 10).

By application of the polymer PEMA high contact angles

of around 100� can be achieved on TMT and by this certain

water repellent properties can be reached on TMT. This is a

significant hydrophobic effect, however in literature are

also contact angles of 140� reported for wood after sol–gel

application, for example by using fluorinated silane com-

pounds [28]. For this, it should be clear that the PEMA can

indicate hydrophobic properties on TMT but other addi-

tives could probably lead to stronger water repellent

effects. The treatment with a pure silica sol does not

change significantly the water contact angle compared to

untreated TMT (Fig. 10). In contrast, TMT treated only

with iron oxide red pigment contains a strong hydrophi-

licity and water drop placed on its surface is immediately

absorbed. With this background of applying together

hydrophobic and hydrophilic components of the nanosol

Fig. 8 3D light microscopy image of a nanosol treated TMT surface

after 2,000 h of accelerated weathering (pigment concentration



0 30 60 90 120 150 180 210 2400

10

20

30

40

50

60

70

80

90

100

110

natural beech (planed)

TMT (planed)

TMT (sanded)

nanosol treated TMT (10% pigment) (planed)

nanosol treated TMT (10% pigment) (sanded)

nanosol treated TMT (50% pigment) (planed)

nanosol treated TMT (50% pigment) (sanded)cont

act a

ngle

of w

ater

[°]

time until measurement [sec]

Fig. 9 Contact angle of water drops on different surfaces

0 30 60 90 120 150 180 210 240

0

10

20

30

40

50

60

70

80

90

100

110

cont

act a

ngle

of w

ater

[°]

time until measurement [sec]

TMT treated with silica sol TMT treated with PEMA solution TMT treated with iron oxide red paste

Fig. 10 Contact angle of water drops on sanded TMT surfaces

treated with liquid containing only one component


123

the question raised, if the nanosol treatment will change the

TMT properties to hydrophilic or hydrophobic.

After nanosol application the contact angle is significantly

increased to values of about 100� (Fig. 9). During mea-

surement the contact angle decreases only about 45� to val-

ues of more than 65�. It can be stated, that by nanosol

treatment water repellent properties can be induced on

TMT. Moreover, this effect appears to be stronger on sanded

TMT specimens and with higher concentrations of the iron

oxide pigment in the nanosol. This observation is particularly

interesting, since iron oxide is the hydrophilic compound. An

explanation could be that the hydrophilic iron oxide is also

coated and sealed by the hydrophobic polymer component

PEMA. By this, an increased contact angle with increasing

iron oxide pigment concentration may be explained by

different roughness as result of more deposited pigment

particles on the TMT surface.

4 Conclusions

The application of pigment modified silica nanosols on

TMT was investigated. The pigment particles are prefer-

entially deposited in the surface depressions of the rough

TMT surface, which results in an inhomogenous pigment

distribution. Nevertheless, the hydrophobic properties of

TMT can be significantly enhanced by nanosol treatment.

Furthermore a coloration of TMT can be achieved and

easily adjusted by the amount of pigment used for nanosol

modification. Therefore, the application of pigment modi-

fied nanosol on TMT could in future lead to an improved

weathering stability and adjustable coloration of this wood

based material. This may lead to novel outdoor applica-

tions of durable TMT.

Acknowledgments For financial support we owe many thanks to

the European Community. This work has been done under the EC

program ‘Competitive and Sustainable Growth’ (Nanowood project:

CRAFT-1999-71678).

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Surface properties of sol gel treated thermally modified wood

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