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April 22, 2014
ABSTRACT
In this study, the possibilities of influencing the surface properties of sessile oak (Quercus
petraea), European ash (Fraxinus excelsior), Norway spruce (Picea abies), and European
Larch (Larix decidua) by a low-temperature atmospheric plasma treatment, namely, diffuse
coplanar surface barrier discharge, under various conditions were investigated. The effects of
varying the mutual distance between the electrode surface and plasma-treated wood surface
(0–1.2 mm), plasma treatment duration (3, 5, and 10 s) utilizable on industry lines, and the
atmosphere applied during the plasma treatment (air, N2, and CO2) were studied. The effects
of plasma on the wood surface were evaluated by measuring the contact angle, which
represents changes in surface polarity. In comparison with a reference untreated sample, the
plasma-activated samples exhibited the hydrophilization effect at distances lower than 0.5
mm from the electrode surface while exhibiting hydrophobization at distances greater than
0.5 mm. Further, conditions for the effective plasma modification of all tested wood samples
were found.
KEYWORDS
DCSBD plasma; modification; surface polarity; contact angle; wetting; hydrophilization;
hydrophobization
INTRODUCTION
Exterior wood is vulnerable to weather exposure; therefore, window facades and outdoor
wooden furniture need to be protected using varnishes or other protective coatings, with new
layers applied every few years. European legislation (ES 2004/42/EC, about coatings)
mandates the use of water-soluble coating materials. This law poses a serious challenge for
the wood handling and processing industry in Europe. In this study, we investigated an
approach for overcoming this challenge; our approach involves the application of atmospheric
plasma treatment on wood surfaces to activate them prior to coating. This activation provides
better conditions for good adhesion between the wood surface and coating. In addition, we
studied the possibilities offered by the application of diffuse coplanar surface barrier
discharge (DCSBD) plasma technology during the wood coating process. Changes in the
surface properties of wood modified by plasma discharge, such as surface free energy, contact
angle, or wettability, were studied under different plasma treatment conditions. Measurements
of wettability and surface free energy were performed under standard laboratory conditions.
Wettability was evaluated by measuring the water droplet contact angle by using the surface
energy evaluation (SEE) system.
In recent years, plasma technologies have been at the forefront of industrial application as
well as research and development of new methods for the protection of these products.
Although DCSBD plasma has great potential to become an efficient means for modifying the
surfaces of lignocellulosic materials, its application on these materials has still not been
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explored well. Plasma application is a universal and efficient technique that is being
employed to activate the surfaces of various polymeric materials. By producing highfrequency electrical discharge, plasma generates an ionized gas that can change the surface
properties of a material in contact with the plasma. Plasma modification is very effective for
altering the surface properties, especially wetting and adhesion, of various materials. One of
the advantages of surface activation is that it provides the possibility of modifying the
wetting and adhesion of wood and lignocellulosic materials toward those materials that
improve their resistance to degradation effects.
The laboratory-scale impact of DCSBD plasma has already been investigated on glass
fibers/glass (Kováčik et al. 2006), textile materials (Černák 2004; Ráheľ et al. 2000; Ráheľ et
al. 2003; Šimor et al. 2002; Šimor et al. 2003b; Štefečka 2003), foils (Šimor et al. 2003),
paper (Černák et al. 2003; Tóth et al. 2007), and wood-fiber materials (Beňová 2007; Hnát
2005; Nováková 2007; Odrášková et al. 2008; Szalay 2009).
MATERIALS AND METHODS
The following wood samples were tested in this study: sessile oak (Quercus petraea),
European ash (Fraxinus excelsior), Norway spruce (Picea abies), and European larch (Larix
decidua).
Preparation of Samples
Before subjecting them to plasma treatment, samples were stored in an air-conditioned room
(relative humidity: 50 ± 1%, temperature: 23 ± 2 °C) for a period of 24 h. Subsequently, the
samples were sanded with sandpaper (grit: 150). For the measurement of contact angle and
surface free energy, samples with sizes of 7 cm × 1.5 cm × 4 cm were used.
Plasma Treatment Conditions
For evaluating the plasma treatment effect, a DCSBD plasma system with an adjustable
power output of up to 420 W was used. This system was a planar source of low-temperature
plasma (Simor et al. 2002). DCSBD electrodes, consisting of 15 pairs of silver strip electrodes
(length: 200 mm, width: 1 mm) embedded 0.5 mm below the surface of 96% Al2O3 ceramics,
were energized by 14 kHz sinusoidal voltage, supplied by a HV generator (LIFETECH
VF700). The mutual distance between the silver strip electrodes was 1 mm.
The plasma treatment duration was set to 3, 5, and 10 s. As a working atmosphere,
compressed air, N2, and CO2 were used. The effect of mutual distance between the sample
surface and DCSBD electrode surface (termed “gap” in the figures) was also investigated.
The mutual distance was varied using the stock of 0.13-mm-thick microscope cover slips
from 0.133 mm to 1.064 mm. The thickness of the individual plates was 0.133 mm.
Contact Angle
For the evaluation of the effect of DCSBD plasma treatment on the change in the surface free
energy of the samples, a SEE system was used. This system evaluates the surface free energy
of a material by measuring the contact angle of liquids with different polarities. To evaluate
the impact of wood surface modification caused by plasma treatment, changes in the water
droplet contact angle for various distances from the electrode surface were monitored. In this
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method, the shape of a liquid droplet on the solid surface of the studied substance is optically
read. Contact angle is defined as the angle formed between the plane of the substance and the
tangential plane to the droplet surface on the solid interface between the droplet and
surrounding atmosphere.
RESULTS AND DISCUSSION
Fig. 2–5 show the dependences of the contact angle (θ, °) on the distance between the sample
surface and electrode surface (gap, mm) for various DCSBD plasma treatment durations (3, 5,
and 10 s) in the air atmosphere for spruce (Fig. 2), oak (Fig. 3), ash (Fig. 4), and larch (Fig.
5). It was found that compared to a reference untreated spruce wood surface, the contact angle
of the plasma-treated samples decreased up to a distance of 0.4 mm at all the tested treatment
durations. The contact angle of water droplets on the reference spruce sample was 51.35°. A
decrease in the contact angle implies that the sample surface became more hydrophilic than
the reference sample surface. In practical terms, this means that plasma treatment at shorter
distances result in better wetting of the spruce wood surface. At distances between 0.5 mm
and 0.9 mm, the contact angle increased at all the tested treatment durations, and the sample
surface became hydrophobic.
The thickness of the generated DCSBD plasma layer was about 0.35 mm (Kováčik et al.
2006). Direct contact between the sample surface and plasma layer in the air atmosphere
caused the generation of polar chemical groups on the sample surface. However, an increase
in the distance between the sample and electrode surfaces caused an opposite behavior and the
sample surface became hydrophobic without the addition of any additional chemical into the
reaction system.
Table 1 summarizes the results for the maximum hydrophilizing and hydrophobizing effects
of the DCSBD plasma on all samples tested in air, CO2, and N2 atmospheres, for a treatment
time of 5 s. Hydrophilizing and hydrophobizing effects were compared against a reference
untreated sample. The results reveal that by changing the atmosphere, maximum
hydrophobizing and hydrophilizing effects were obtained for various distances in the case of
all samples. From these results, the following general conclusion can be drawn: for all
samples, , the hydrophilizing effect occurs at distances shorter than 0.5 mm, while the
hydrophobizing effect occurs at distances greater than 0.5 mm. These changes in the patterns
of the sample surfaces were due to reactions during the plasma treatment, reflected by
changes in the surface wettability of the samples due to the formation of various types of
functional groups on the lignin, hydrocarbons, or extractives. At shorter distances, the
generation of new polar groups caused hydrophilization of the plasma-treated sample surface.
By means of electron spectroscopy for chemical analysis (ESCA), Belgacem et al. (1995) and
others (Avramidis et al. 2009, Busnel et al. 2010, Lecoq et al. 2008) found that plasma
treatment results in the formation of various functional groups such as aldehydes/ketones and
carboxylic groups on cellulose, lignin, and wood samples. These results were also confirmed
by Calvimontes et al. (2011). Using XPS analysis, they confirmed that plasma causes
reactions, resulting in the decomposition of the polymer chain, oxidative reactions, and
formation of aldehydes and carboxylic acids.
CONCLUSIONS
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In this study, the possibilities of surface modifications in wood samples (sessile oak,
European ash, Norway spruce, and European larch) by DCSBD plasma treatment in various
atmospheres (air, N2, and CO2,), for various industrially applicable treatment durations (3, 5,
and 10 s), and at various distances between the wood surface and plasma electrode surface
were investigated. It was found that the atmosphere employed for modifying the surface
wettability of the sample affected its surface properties after the plasma treatment. The air
atmosphere was found to be the best industrially applicable atmosphere because it provides
results similar to those provided by the other two tested atmospheres but is considerably
cheaper. The effect of distance between the sample and electrode surfaces was found to be
essential for changing the polarity of the sample surface. In general, for all samples, at
distances shorter than 0.5 mm, hydrophilization occurred, while at distances greater than 0.5
mm, hydrophobization occurred.
REFERENCES
Avramidis G, Hauswald E, Militz H, ViÖl W, Wolkenhauer A (2009) Plasma treatment of
wood and wood-based materials to generate hydrophilic or hydrophobic surface
characteristics. Wood Mater Sci Eng 4: 52-60.
Belgacem MN, Czeremuszkin G, Sapieha S, Gandini A (1995) Surface characterization of
cellulose fibres by XPS and inverse gas chromatography. Cellulose 2: 145-157.
Beňová L (2007) Methods of study of modification and degradation process of lignocellulosic
materials. MS thesis, Faculty of Chemical and Food Technology, Slovak University of
Technology, Bratislava. 78 pp. In Slovak.
Busnel F, Blanchard V, Pregnet J, Stafford L, Riedl B, Blanchet P, Sarkissian A (2010)
Modification of sugar maple (Acer saccharum) and black spruce (Picea mariana) wood
surfaces in a dielectric barrier discharge (DBD) at atmospheric pressure. Adhes Sci Technol
24(8): 1401-1413.
Calvimontes A, Mauersberger P, Nitschke M, Dutschk V, Simon F (2011) Effects of oxygen
plasma on cellulose surface. Cellulose 18: 803.
Černák M (2004) Method and apparatus for treatment of textile materials. International Patent
Application Number PCT/SK02/000008 (WO 02/095115).
Černák M, Ráhel J, Šimor M (2003) Electrode element for generation of diffuse coplanar
barrier discharge and the method of its manufacturing. Slovak Patent Application PP 01362003.
Chan CM (1994) Contact angle measurement. Pages 35-45 in Polymer surface modification
and characterization. Hanser/Gardner Publications, Inc., Cincinnati, OH.
Gindl M, Sinn G, Gindl W, Reiterer A, Tschegg S (2001) A comparison of different methods
to calculate the surface free energy of wood using contact angle measurements. Colloids Surf
A 181(1): 279-287.
Good RJ (1992) Contact angle, wetting and adhesion: A critical review. J Adhes Sci Tehnol 6:
1269.
4
Good RJ, van Oss CJ (1992) Chapter 1 in ME Schrader and G Loeb, eds. Modern approaches
to wettability: Theory and applications. Plenum Press, New York.
Hnát M (2005) Effect of low-energy plasma to the surface properties of wood. BC thesis,
Faculty of Chemical and Food Technology, Slovak University of Technology, Bratislava. In
Slovak.
Kalnins MA, Knaebe MT (1992) Wettability of weathered wood. J Adhes Sci Technol 6(12):
1325.
Kováčik D, Zahoranová A, Ráheľ J, Buček A, Černák M (2006) Kontinuálna povrchová
úprava PP netkaných textílií pomocou plazmového zariadenia využívajúceho DCSBD výboj.
Moderní trendy ve fyzice plazmatu a pevných látek II. Hustopeče, Česká republika, 69 pp. In
Slovak.
Kwok DY (1998) Contact angles and surface energetic. PhD thesis, University of Toronto,
Toronto.
Lecoq E, Clément F, Panousis E, Loiseau JF, Held B, Castetbon A, Guimon C (2008) Pinus
pinaster surface treatment realized in spatial and temporal afterglow DBD conditions. Eur
Phys J: Appl Phys 42: 47-53.
Neumann AW (1974) Contact angles and their temperature dependence: Thermodynamic
status, measurement, interpretation and application. Adv Colloid Interface Sci 4: 105.
Neumann AW, Good RJ, Hope CJ, Sejpal M (1974) An equation-of-state approach to
determine surface tensions of low-energy solids from contact angles. J Colloid Interface Sci
49(2): 291-304.
Nguyen T, Johns WE (1979) The effects of aging and extractives on the surface free energy of
Douglas-fir and red wood. Wood Sci Technol 13: 29.
Nováková E (2007) Study the effects of plasma treatment on the surface of wood materials.
MS thesis, Faculty of Chemical and Food Technology, Slovak University of Technology,
Bratislava. 44 pp. In Slovak.
Ondrášková M, Ráheľ J, Zahoranová A, Tiňo R, Černák M (2008) Plasma activation of wood
surface by diffuse coplanar surface barrier discharge. Plasma Chem Plasma Process 28: 203211.
Owens DK, Wendt RC (1969) Estimation of the surface free energy of polymers. J Appl
Polym Sci 13:1741-1747.
Ráhel J, Černák M, Hudec I, Brablec A, Trunec D (2000) Atmospheric-pressure plasma
treatment of ultra-light-molecular-weight polypropylene fabric. Czech J Phys 50, Suppl S3:
445-448.
Ráheľ J, Šimor M, Černák M, Štefečka M, Imahori Y, Kando M (2003) Hydrophilization of
polypropylene nonwoven fabric using surface barrier discharge. Surf Coat Technol 169-170:
604-608.
5
Šimor M, Rahel J, Černák M, Imahori Y, Štefecka M, Kando M (2003) Atmospheric-pressure
plasma treatment of polyester nonwoven fabrics for electroless plating. Surf Coat Technol 172
(1): 1-15.
Simor M, Rahel J, Vojtek P, Černak M Brablec A (2002) Atmospheric-pressure diffuse
coplanar surface discharge for surface treatments. Appl Phys Lett 81(15): 2716-2718.
Šimor M, Rahel J, Vojtek P, Brablec A, Černák M (2002) Atmospheric-pressure diffuse
coplanar surface discharge for surface treatments. Appl Phys Lett 81: 2716-2718.
Šimor M, Sťahel P, Brablec A, Navrátil Z, Kováčik D, Zahoranová A, Buršíková V, Černák
M (2003) Deposition of polymer films in the diffuse coplanar surface discharge. In R
D'Agostino, P Favia, F Fracassi and F Palumbo, eds. (Taormina, IMIP, CNR Bari),
Proceedings of the 16th International Symposium on Plasma Chemistry, June 22-27 2003,
Taormina, Italy. Full paper on CD Rom.
Šmatko L, Jablonsky M, Tiňo R, Šurina I (2010) Physico-chemical surface treatment of
lignocellulosic materials: The effect of low temperature atmospheric plasma. Tatranské
Matliare: 1st International conference Renewable Energy Sources, 2010. In Slovak.
Tiňo R, Beňová L, Katuščák S (2008) Evaluation of intake capability of LCM surface by
adsorption dyeding method and its utilization for evaluation of changes in surface polarity
caused by DCSBD low-energy plasma. Chemicke Listy 102(15): 1194-1197 (special issue).
Tiňo R, Bugnicourt E (2011) Plasma paints a brighter picture wood surface modification
process enhances exterior durability. Eur Coat J 5: 30-33.
Tóth A, Černáková L, Černák M, Kunovská K (2007) Surface analysis of groundwood paper
treated by diffuse coplanar surface barrier discharge (DCSBD) type atmospheric plasma in air
and in nitrogen. Holzforschung 61: 528-531.
Contact:
Sandi McIntyre
Editage / Cactus Communications
sandi.mcintyre@editage.com
267-332-0051, ext. 106
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