Formation of superhydrophobic wood surfaces using an atmospheric pressure
dielectric barrier discharge in He-HMDSO mixtures
Olivier Levasseura, Luc Stafforda, Nicolas Gherardib,c, Nicolas Naudeb,c and Vincent Blanchardd
a
Département de Physique, Université de Montréal, Montréal (Québec) H3C 3J7, Canada
b
Université de Toulouse, UPS, INPT, LAPLACE, F-31062 Toulouse Cedex 9, France
c
CNRS, LAPLACE,F-31062 Toulouse cedex 9, France
d
FPInnovations, Québec (Québec) G1P 4R4, Canada
Abstract: This work examines the functionalization of sugar maple (Acer
saccharum) and black spruce (Picea mariana) wood surfaces using an
atmospheric-pressure dielectric barrier discharge in He and He/HMDSO
(hexamethyldisiloxane) gas mixtures. Wood samples were placed on one of the
electrodes and the plasma was sustained by applying a 3.5 kVpk-pk voltage at 12
kHz. Current-voltage characteristics in pure He showed that the introduction of
both wood species produced a filamentary discharge contrasting the glow
discharge encountered with glass samples. Optical emission spectroscopy
performed near the wood surface revealed strong N2 and N2+ emissions,
indicating that wood outgasing plays an important role in this regime change.
Analysis of the surface wettability through measurements of water contact angles
(WCA) indicated that samples treated in pure He became hydrophilic. On the
other hand, addition of HMDSO precursor produced superhydrophobic wood
surfaces with angles in the 120 - 140° range depending on treatment time and
wood specie, a very promising result for outdoor applications.
Keywords: Atmospheric pressure plasmas, plasma functionalization, wood.
1. Introduction
In several countries around the world, wood
is considered a fine, strong material, but one that
requires regular and meticulous maintenance, a
characteristic that makes it less desirable when
marketed for exterior applications compared to
composite materials. Being naturally hydrophilic,
the fast degradation of wood mainly arises from its
interaction with water. Degradation of wood can also
occur due to biological attacks and solar irradiation.
Over the years, important research efforts have been
devoted to the development of protective treatments
to prevent near-surface degradation in outdoor
applications without modifying the wood physical
appearance or strength. To control and modify
surface wettability of this polymer, the most
effective methods used wet chemicals to modify
bulk wood properties to increase its water
permeability [1]. The major problem of these
methods is that they all implied highly toxic or
corrosive chemicals such as chromate copper
arsenate, chromium trioxide, and creosote. An
alternative solution was heat treatments that also led
to more water-resistant wood [2]. However, albeit
not using chemicals, this method produced important
color change and significant decrease of the wood
bending strength [3].
Recently, we have started investigations on
the modification of wood surfaces using
atmospheric-pressure dielectric barrier discharges.
The advantages of such plasmas are numerous and
address major concerns of the wood industry:
atmospheric-pressure operation, high throughputs,
eco-friendly, and large flexibility. Our first set of
studies aimed at improving the adhesion of wood
with waterborne
urethane/acrylate
coatings.
The treatments were performed in an
atmospheric-pressure
plasma
controlled
by
dielectric-barrier, the details of which can be found
elsewhere [7]. This system includes a gas inlet line
located near the end of one of the electrodes that can
accommodate several carrier and precursors gases.
The plasma is created in a sealed, Al chamber
evacuated by a mechanical pump which allows
operation under controlled, atmospheric-pressure
conditions. In this work, the discharge is sustained
between a thin alumina sheet and a 3.2 mm-thick
wood sample. The discharge gap is fixed to 4 mm.
For all experimental conditions investigated, the
frequency and peak-to-peak voltage were maintained
at 12 kHz and 3.5 kV. Experiments were performed
in either He or He/HMDSO gas mixtures. The flow
rate of each gas was controlled using mass flow
meters. Two types of samples were used: sugar
maple (acer saccharum) and black spruce (picea
mariana). Unless otherwise specified, the plasma
chamber was pumped down to ~ 50 mTorr before
each experiment which allowed considerable
outgasing of wood. The plasma in the presence of
wood was analyzed through measurements of the
current-voltage characteristics and plasma emission
in the 300 to 900 nm range. Following plasma
exposure, the wettability with water of each sample
was characterized by measuring the contact angle ()
with a goniometer.
3.1 Plasma characterization in presence of wood
samples
Figure 1 shows the current-voltage curves
obtained for a pure He plasma with either a glass or
a sugar maple sample on the electrode. Typically,
when the applied voltage to the gas, Vg (Vg is not
equal and not necessarily in phase with the applied
voltage as considerable potential drop and phase
shift occurs across both electrodes [8]), is greater
than that required for gas breakdown, one or more
current peaks of short duration appear. For a glass
substrate, we observe only one current peak per
cycle, with a constant duration of about 8µs. As
described in [9], this is characteristic of a
homogeneous discharge. On the other hand, with the
sugar maple electrode, several peaks could be
observed with their position and full width at half
maximum varying from a cycle to the other; a result
generally ascribed to a filamentary discharge.
Moreover, the intensity of the main current peak
decreases from ~ 40 mA for a glass sample to ~
10 mA for a wood substrate. Similar results were
observed with black spruce samples, suggesting that
the regime change is independent from the wood
specie placed on the electrode.
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
0.00
Glass 40
20
0
-20
-40
Wood 40
20
Current (mA)
2. Experimental details
3. Results and discussion
Voltage(kV)
Adhesion improvement up to 35 % after exposure to
a N2/O2 (1:2) plasma for 1 s was observed [4]; a
result ascribed to a degradation of the cellulose and
hemicellulose by UV photons and N2 metastables
followed by a reoxydation by oxygen [5].
Preliminary investigations also showed a significant
increase of the resistance to biological attacks of
white pine (pinus strobus) following exposure to the
same plasma [6]. In this work, we investigate the the
potential of such plasmas for the plasma-enhanced
chemical
vapor
deposition
of
functional,
nanostructured coatings on wood surfaces. The
plasma was sustained in helium using
hexamethyldisiloxane (HMDSO) as the precursor.
0
-20
-40
0.05
0.10
0.15
0.20
0.25
Time(ms)
Figure 1: Current-voltage characteristics of a He discharge
with either a glass substrate or a non-degassed sugar maple
sample. Operating frequency and peak-to-peak voltage are the
same for both materials.
The influence of a wood electrode on the
time-integrated plasma emission is shown in Fig. 2
for a nominally pure He plasma. In addition to the
expected spectral lines of He in the visible range,
Fig. 2 shows important emission from the second
positive system (SPS) of N2 and the first negative
system (FNS) of N2+ in the UV region. The presence
of N2 and N2+ in the plasma emission spectrum
results from the outgasing of the sample surface
following plasma exposure. This effect is even more
present in our case since wood is an extremely
porous material that can retain a large amount of gas.
5
10
Spectrum after 15s
Spectrum after 20 mins
4
Intensity (a.u.)
10
3
10
300
5
10
320
340 360
380
400
420
440
460 480
500
520
4
10
3
10
520 540 560 580 600 620 640 660 680 700 720 740 760 780 800
Wavelength (nm)
Figure 2: Temporal evolution of the emission from a He plasma
with a non-degassed sugar maple substrate. The operating
conditions are the same than in Fig. 1.
As the wood treatment time increases, i.e. as
the sample releases its gas, Fig. 2 shows that the
intensity of the N2 bands strongly decreases while
that of He increases. For example, the intensity of
the He line at 706.5 nm increases by a factor of 1.4
while the intensity of the N2 band head at 337nm
decreased by a factor of 3.2 between 15 s and 20
minutes. For black spruce, the same behavior could
be achieved after only 12 minutes (not shown); a
result that can probably be attributed to the more
porous structure of black spruce vs. sugar maple.
Nonetheless, nitrogen bands are always more or less
observable even after long treatment times since
wood takes a very long time to outgas.
3.2 Wettability with water after treatment in
He/HMDSO plasmas
After treatment in pure He for 4 minutes, the
surface became highly hydrophilic, with absorption
time less than 1s. This can probably be attributed to
the removal of the weak boundary layer and
extractives from the wood surface which decreases
the surface energy. In such conditions, the plasma is
playing a role comparable to sanding as reported in
[10]. The addition of HMDSO precursor to the He
plasma increases significantly the hydrophobicity of
the wood surface for all operating conditions
investigated, in good agreement with the results of
[11] and [12] obtained under low-pressure plasma
conditions. For example, the WCA of sugar maple
increases from ~ 30° before treatment to 137° after
treatment for 4 minutes in a He plasma with
100 ppm of HMDSO. Similar WCA were obtained
in plasmas with 20ppm and 100ppm of HMDSO,
suggesting that even at 20 ppm the surface is
completely covered by the hydrophobic coating. At
lower treatment time, the WCA decreases slightly,
with  going from 137° after 4 minutes to 122° after
15 s. Given the high roughness of wood samples,
this decrease can probably be attributed to a nonconformal mapping of the coating.
When replacing the carrier gas by nitrogen,
we observed hyperhydrophilic surfaces for both
sugar maple and black spruce, with the water droplet
being completely absorbed within the first second of
contact with the wood surface. This behavior can
probably be attributed to the formation of
hydrophilic SiOx or SiONx coatings. Indeed, in
He/HMDSO plasmas, given the low current and
discharge duration, the HMDSO fragmentation
degrees are probably low, which is likely to lead to
the grafting of hydrophobic Si-CH3 groups. In
contrast, N2 plasmas are characterized by higher
currents and discharge times allowing for higher
HMDSO fragmentation degrees leading to the
deposition of hydrophilic SiOx functions [13].
In order to examine the aging characteristics
of the superhydrophobic coatings obtained after
treatment in He/HMDSO mixtures, the wood
samples were put aside and the WCA were measured
Water contact angle (degrees)
at different times after the plasma treatment for a
period of 4 months. The results are shown in Fig. 3
for sugar maple samples treated for treatment time, t,
ranging from 15s to 4 minutes. For t = 4 minutes in a
He plasma with 100 ppm of HMDSO, WCA values
remain constant for up to four months after the
treatment, showing an excellent dimensional
stability of the coating. However, shorter treatment
times result in a decaying behavior of the surface
hydrophobicity over time. For example, for t = 30s,
 decreases by almost 30° after four months of
natural aging. For t = 15 s, the decrease of  is more
drastic, with  decreasing from 130° to 75° in only
30 days. Afterward, wettability seems to stabilize
around 75°. Since the optimal dimensional stability
of the wood coating is achieved after a treatment
> 1 minute, this suggests that a minimal coating
thickness is required for the wettability to be
maintained over long aging periods.
demonstrated an excellent stability of the WCA, a
very promising result for outdoor applications.
Nevertheless, our study of the discharge stability
have shown that the plasma was filamentary for both
species and for all experimental conditions
investigated, which could be problematic for the
growth of high-quality coatings. Significant
emission from the SPS of N2 and FNS of N2+ was
observed. The incorporation of such impurities in the
plasma is likely to alter the discharge kinetics, more
particularly through the decrease of the number
density of He metastables responsible for the
dissociation of the precursor and thus for the growth
of the coating. Detailed investigations on the
influence of wood outgasing on the discharge
properties are in progress.
References
[1] A.S. Ross and W.C. Feist, American Paint &
Coatings Journal, 78, 41 (1993)
160
150
15 s
140
4 mins
30 s
[2] M. Pétrissans et al. Holzforschung, 57, 301
(2003)
[3] P. Bekhta and P. Niemz, Holzforschung, 57, 539
(2003)
130
120
[4] F. Busnel et al., J. Adhes. Sci. Technol. 24, 1401
(2010)
110
100
[5] J. Prégent et al., Proc. of the SVC, April 17-22
2010, Orlando, FL.
90
80
70
0
20
40
60
120
Time since treatment (days)
Figure 3:Temporal evolution of sugar maple samples
hydrophobicity for 15s, 30s and 4 mins treatment times in a
He/HMDSO (100ppm) plasma.
4. Conclusion
In summary, our measurements have shown
that sugar maple and black spruce wood samples
treated in He/HMDSO discharges controlled by
dielectric barrier became superhydrophobic with
contact angles in the 120 - 140° range depending on
the operating conditions. For samples with a
relatively
thick
coating,
long-term
aging
[6] V. Blanchard et al., private communication
FPInnovations.
[7] F. Massines et al., J. Appl. Phys. 83, 2950 (1998)
[8] F. Massines et al. Plasma Phys. Control. Fusion,
47, B577 (2005)
[9] F. Massines et al. The Eur. Phys. J. Appl. Phys.
47, 22805 (2009)
[10] A. Wolkenhauer et al. Int. J. Adh. Adhes, 29,
18, (2009)
[11] A. R. Denes et al., Holzforschung, 53, 318
(1999)
[12] S. Zanini et al., Wood Science Technology, 42,
149 (2008)
[13] I. Enache et al., Plasma Processes and Polymers
4, 806, (2007).