pigment and resin technology vol31 n1 2002

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Introduction

Contributed papers Surface modification of wood by plasma polymerisation

Wood is a remarkable material extensively used for construction and decoration. Like other biological materials, wood is susceptible to environmental degradation (Roux et al., 1988). When exposed outdoors a complex combination of chemical, mechanical, biological, and ultraviolet and visible-light induced changes contribute to its weathering. Therefore to ensure the long term value of wood substrates they are usually coated with various decorative and protective finishes such as paints, stains or varnishes. Transparent systems are often used because they allow the natural beauty of wood to remain visible. However, the durability of such transparent systems is limited. The most important degradations of an exterior wood coating come from the dimensional variations of wood which strain the coating and lead to its cracking. Therefore the durability of a wood-coating system should be increased through the stabilisation of these variations. Thermal or chemical treatments are effective but most of them modify the surface characteristics and consequently the coating adhesion is no more guaranteed (Podgorski and Roux, 1999). Plasma treatments have been found to be very effective in modifying the surface properties of various polymers, such as their wetting behaviour (Xiao, 1997). Plasma can be defined as a partially ionised gas, containing charged and neutral particles, including: electrons, positive or negative ions, radicals, excited atoms, and molecules (Kaplan and Rose, 1991). Plasmas can be broadly divided in two categories: cold and hot plasmas. Glow discharge techniques employed for the surface modification of organic materials are usually cold plasmas due to the thermal sensitive of the substrate. The plasma species interact with solid-phase substances generating chemical and morphological changes in the very top layers of the plasma-exposed substrates. The gas used has a major importance on the effects produced on the treated surfaces. The effect of plasma treatments may include: (a) surface cleaning; (b) ablation and degradation; (c) cross-linking; (d) surface oxidation; (e) polymerisation; (f) ion implantation. These effects may occur concurrently and depending on the processing

L. Podgorski C. Bousta F. Schambourg J. Maguin and B. Chevet The authors L. Podgorski, C. Bousta and F. Schambourg are based at the Centre Technique du Bois et de l’Ameublement (CTBA), Poˆle Construction, Alle´e de Boutaut, BP227, 33028 Bordeaux Cedex, France J. Maguin and B. Chevet are based at the Institut Franc¸ais Textile Habillement (IFTH), Direction Re´gionale de Lyon, Avenue Guy de Collongue, 69134 Ecully Cedex, France Keywords Wood, Wettability, Plasma, Coatings Abstract Plasma technology is often used in textile industries. The aim of this study is to transpose this technology to wood in order to protect it when used outdoors. First experiments have shown that this kind of treatment can be applied either to bare or finished wood. In this study, different plasma coatings of wood surface to create water repellent characteristics are presented. Treatment parameters were optimised and the surface characteristics of the plasma treated substrates were evaluated and compared. If this treatment is sufficiently resistant to weather it could be applied to wood without any other coatings. If it is not sufficiently resistant, it could be applied to a coated wood to extend the durability of the coating. This kind of deposit may also protect wood against fungi. Electronic access The current issue and full text archive of this journal is available at http://www/emeraldinsight.com/0369-9420.htm

Pigment and Resin Technology Volume 31 · Number 1 · 2001 · pp. 33–40 q MCB UP Limited · ISSN 0369-9420 DOI 10.1099/03699420210412575

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Surface modification of wood by plasma polymerisation

Pigment and Resin Technology

L. Podgorski et al.

Volume 31 · Number 1 · 2001 · 33–40

flexibility of the coating that is to say by decreasing it glass transition temperature (Tg). But in this case the coating is generally more sensitive to water. The waterproofing treatment could solve this sensitivity.

conditions and the reaction chamber design, one or more of these effects may predominate. Table I gives a comparison between low pressure plasmas and traditional chemical treatments. The deposition of coatings by plasma polymerisation produces materials with very interesting and sometimes unique application properties. Fluorine-containing monomers and silicone-containing monomers are two classes of molecules which are good candidates for the plasma deposition of hydrophobic coating on textiles. The objective of this study is to adapt the plasma technology used in textile industries to wood in order to make it waterproof. This new kind of coating is particularly interesting since it is invisible. Reduced water penetration into wood surfaces by deposition of tetrafluoromethane (CF4) and perfluoromethane (C3F6) (Podgorski et al., 2000), hexamethyldisiloxane (HMDSO) or polymethyldisiloxane (Denes et al., 1999; Denes and Young, 1999; Mahlberg et al., 1998), or butene (Magalhaes and Ferraira De Souza, 2000) has already been reported. First experiments have shown that this kind of treatment can be applied either to bare (Podgorski et al., 2000) or finished wood (Podgorski et al., 2000). According to the resistance against weathering of this new protection, two strategies can be considered: (1) If this deposit is sufficiently resistant to weathering, it could be applied on bare wood to keep its natural appearance and to use it without any traditional coating, (2) If this deposit is not sufficiently resistant to weathering it could be even though applied to a coated wood to extend the durability of the wood-coating system. As a matter of fact, the service life of such a system can be improved by increasing the

This kind of deposit may also protect wood against fungi since it creates a barrier against humidity. In this study, different plasma coatings of wood surface to create water repellent characteristics are presented: fluorine in gaseous phase (C3F6), fluorine in liquid phase (fluorinated acrylate with a C8F17 chain noted AC8F17) and silicone in gaseous phase (HMDSO). Treatment parameters were optimised. Surface characteristics of the plasma treated substrates were evaluated and compared in order to select treatment conditions leading to the lowest wettability possible (highest contact angles).

Experimental Materials Scot pine samples (150 mm£ 74 mm £ 18 mm) were used with an initial moisture content of 12%. They were quarter sawn in heartwood.

Plasma installation and treatments Plasma deposition in gaseous phase and in liquid phase was carried out at the French Institute for Textile and Garment Industries (IFTH) in Lyon. The equipment is made of five “gas” lines and one “liquid/gas” line. The “gas” lines use gas for different purposes: (a) cleaning (Ar, He, H2,. . .); (b) oxidation (air, O2, CO2,. . .); (c) nitriding (N2, NH3,. . .); (d) fluorination (CF4, fluorinecontaining monomers,. . .), thin film deposition (acetylene, propene,. . .).

Table I Comparison between plasma treatment and chemical treatment

Solvent Energy Type of reaction Potential development Energy consumption Pollution

Plasma

Chemical treatment

No solvent (gaseous phase) Electricity Complex high low low

Water Heat Simple Very low Important Important

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Surface modification of wood by plasma polymerisation

Pigment and Resin Technology

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Volume 31 · Number 1 · 2001 · 33–40

The “liquid/gas” line operates with liquid products whose vapour pressure is greater than the pressure used during the treatment. The treatment chamber is a vertical cylinder having a diameter of 45 cm and a height of 45 cm and therefore a volume close to 90 l. The upper part of the chamber is connected to a quartz tube in which the plasma is created thanks to a micro-wave generator operating at 2450 MHz. A pumping is performed to ensure a water extraction from the material to be treated and therefore a good plasma quality. Figure 1 shows the equipment. In the case of a plasma treatment in liquid phase, the monomers are sprayed (0.2 – 0.8 ml/sample) on wood samples which are then introduced into the plasma chamber. The following treatment parameters were studied in order to select those leading to the lowest wettability: . type of gas and liquid (Table II) . gas and liquid input (introduction in the upper or lower part of the chamber)

. .

. .

.

gas and liquid flow (0 –100 cm3/min) electric frequency of the vacuum pump(20– 50 Hz) treatment time (1– 15 min) distance between the samples and the plasma source (37 – 57 cm) power (700 –1100 W)

Experimental design was used to define treatment conditions which were selected for each experiment. These conditions are presented in Table III (gaseous phase) and Table IV (liquid phase).

Wood surface analysis Plasma deposition was characterised at the Technical Centre for Wood and Furniture (CTBA) by contact angle measurements using distilled water drops of 5 ml. Five drops were used on each samples and a mean contact angle u was calculated. The aim is to obtain surfaces with the poorest wettability possible (highest contact angles) as represented in Figure 2.

Figure 1 Schematic of the plasma equipment

Table II Nature of gas and liquid used for each plasma treatment Plasma treatment Fluorine in gaseous phase Silicone in gaseous phase Fluorine in liquid phase

Gas monomers C3F6 HMDSO

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Co-Gas promoting reaction of polymerisation

Liquid monomers

CF4 O2 CF4

AC8F17

Surface modification of wood by plasma polymerisation

Pigment and Resin Technology

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Volume 31 · Number 1 · 2001 · 33–40

Table III Plasma parameters studied for treatment in gaseous phase (Fluorine or silicone monomers) Experiment No.

Co-gas flow (cm3/min)

Gas flow (cm3/min)

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

Frequency (Hz)

Gas input (CF4 or O2)

Gas input (C3F6 or HMDSO)

80 80 20 80 80 80 20 20 20 80 20 20 50

20 80 80 20 80 80 80 20 20 20 80 20 50

1100 700 1100 1100 700 1100 1100 1100 700 700 700 700 900

37 57 37 57 57 37 57 57 57 37 37 37 47

1 1 15 1 15 15 1 15 15 15 1 1 8

20 20 20 50 20 50 50 20 50 50 50 20 35

low up up up low up low low up low low up up

low low up up up low up low low up low up up

1 2 3 4 5 6 7 8 9 10 11 12 15

Table IV Plasma parameters studied for treatment in liquid phase (Fluorine monomers) Experiment No. 1 2 3 4 5 6 7 8 9

Gas flow (cm3/min)

Monomer quantity (AC8F17) (ml)

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

Frequency (Hz)

20 80 20 80 20 80 20 80 50

0.2 0.2 0.8 0.8 0.2 0.2 0.8 0.8 0.6

700 700 700 700 1100 1100 1100 1100 900

57 37 37 57 57 37 37 57 47

9 1 9 1 1 9 1 9 5

50 50 20 20 20 20 50 50 35

Results and discussions

unmodified samples (mean contact angle = 108), indicating the creation of a hydrophobic surface. Except for experiment no. 5, plasma treatment with fluorine monomers is more effective than silicone. Wood samples treated with fluorine monomers have higher contact angle values for all plasma treatment conditions used. For each monomer, the best treatment conditions are presented in Table V.

Plasma treatment in gaseous phase Contact angle values obtained on wood surface treated with fluorine and silicone monomers in gaseous phase are presented in Figure 3. High water contact angle values indicate the hydrophobic character of plasma modified wood surface. Plasma modified samples exhibit very high water contact angle (768, contact angle , 1208) in comparison to the

Plasma treatment in liquid phase Figure 4 shows the influence of fluorine treatment in liquid phase on pine samples. A very important increase in the contact angle is observed due to the plasma treatment. Best results were obtained with experiment no. 3: the mean contact angle is very high (1448). Plasma conditions for this optimum treatment of wood in liquid phase are presented in Table VI.

Figure 2 Change in the surface wettability thanks to plasma treatment (waterproofing)

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Surface modification of wood by plasma polymerisation

Pigment and Resin Technology

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Volume 31 · Number 1 · 2001 · 33–40

Figure 3 Effect of fluorine and silicone plasma treatment (gaseous phase) on pine wettability

Figure 5 makes a comparison between plasma treatments in liquid and gaseous phase. For each plasma treatment, contact angle exhibit high values. On the one hand, treatments with fluorine monomers are more effective than treatment with silicone. On the other hand, a treatment in liquid phase gives better results than in gaseous phase. In liquid phase, some fluorine monomers probably penetrate into wood before polymerisation on

the wood surface. This penetration in wood may explain better wood surface properties.

Influence of each treatment parameter on contact angle (treatment in liquid phase) For each treatment parameter, the mean contact angle uh obtained with the high level of the parameter and the mean contact angle

Table V Plasma conditions for the best treatment of wood in gaseous phase

Plasma parameters Fluorine monomers Silicone monomers

Gas flow (CF4 or O2) (cm3/min) 80 20

Gas flow (C3F6 or HMDSO) (cm3/min) 80 80

Power (W) 1100 1100

Distance samples/ plasma source (cm) 37 37

Treatment time (min) 15 15

Frequency (Hz) 50 20

Figure 4 Effect of fluorine treatment in liquid phase on pine wettability

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Gas input (CF4 or O2) Up Up

Gas input (C3F6 or HMDSO) low up

Mean contact angle (8) 119.6 111.2

Surface modification of wood by plasma polymerisation

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Volume 31 · Number 1 · 2001 · 33–40

Table VI Plasma conditions for the best treatment of wood in liquid phase Gas flow (CF4) (cm3/min)

Monomer quantity (AC8F17) (ml)

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

Frequency (Hz)

Contact angle values (8)

0.8

700

37

9

20

144.3

20

Figure 5 Effectiveness of the different plasma treatments

Table VII Influence of the gas flow level on contact angle Exp No.

Gas flow (cm /min) Level 3

Monomer quantity AC8F17 (ml)

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

1 3 5 7

20 20 20 20

low

0.2 0.8 0.2 0.8

700 700 1100 1100

57 37 57 37

9 9 1 1

2 4 6 8

80 80 80 80

high

0.2 0.8 0.2 0.8

700 700 1100 1100

37 57 37 57

1 1 9 9

Frequency (Hz)

Contact angle (8)

50 128.2 20 144.3 20 135.5 50 108.9 Mean contact angle ut = 129.2 50 134.0 20 142.9 20 120.8 50 136.1 Mean contact angle uh = 133.5 uh2 ut = 4.2

Table VIII Influence of the quantity monomer level on contact angle Exp. No.

Monomer quantity AC8F17 (ml) Level

Gas flow (cm3/min)

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

1 5 2 6

0.2 0.2 0.2 0.2

low

20 20 80 80

700 1100 700 1100

57 37 37 37

9 1 9 1

3 4 7 8

0.80 0.8 0.8 0.8

high

20 80 20 80

700 700 1100 1100

37 57 37 57

9 1 1 9

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Frequency (Hz)

Contact angle (8)

50 128.2 20 135.5 50 134.0 50 120.8 Mean contact angle ut = 129.6 20 144.3 20 142.9 50 108.9 50 136.1 Mean contact angle uh = 133.1 uh – ut = 3.4

Surface modification of wood by plasma polymerisation

Pigment and Resin Technology

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Volume 31 · Number 1 · 2001 · 33–40

Figure 6 Interaction between the power and the distance sample/plasma source

ul obtained for the low level have been compared. For the gas flow and the monomer quantity, the procedure and results are detailed in Tables VII and VIII respectively. The same procedure was used for each treatment parameter. Results are presented in Table IX. Table IX shows that high power and high frequency have negative effects on contact angles: the plasma treatment is less effective when performed at high power (u = 125.38 at 1100 W) than at low power (u = 137.48 at 700 W). Gas flow, monomer quantity, distance sample/plasma source and treatment time have positive effects on contact angle which are maximum when treatments are performed at the high level of these parameters (Table IX). Power and distance between samples and plasma source are linked as shown in Figure 6. If samples are closed to the plasma source, a low power is sufficient to record high contact angles whereas a high power is required if samples are far from the plasma source. Figure 6 shows that contact angles are more important if distance and power are at their maximum or minimum at the same time. In other cases, plasma treatments are less effective. Thanks to Table IX and Figure 6, it is possible to select plasma treatment parameters to obtain the highest contact angle and therefore the best water-repellent properties. Gas flow, monomer quantity and treatment time should be at their high level (positive effect in Table IX). Frequency should be at its low level (negative effect in Table IX). Power and distance should be at their low level (Figure 6).

These theoretical plasma conditions are presented in Table X. Except for gas flow, these conditions are similar to those presented in Table VI. Thanks to these treatment parameters, contact angles higher than 144.38 may be obtained.

Conclusions The aim of this study is to adapt plasma technology used in textile industries to wood in order to make it waterproof. If it turns out to be sufficiently resistant, it is conceivable that it will be no longer necessary to cover wood with finishing products. Because this film is invisible, the unfinished, natural look of the wood, currently so soughtafter by architects, would be retained.

Table IX Influence of the different treatment parameter on contact angle (treatment in liquid phase)

Gas flow Low level Mean contact angle ut for the low level High level Mean contact angle uh for the high level Du = uh – ut Effect of the parameter on contact angle

3

Monomer quality

Plasma treatment parameters Distance samples/ Power plasma source

Treatment time

Frequency

20 cm /min

0.2 ml

700 W

37 cm

1 min

20 Hz

129.28 80 cm3/cm

129.68 0.8 ml

137.48 1100 W

127.08 57 cm

130.38 9 min

135.98 50 Hz

133.18 3.58

125.38 2 12.18

135.78 8.78

132.48 2.18

126.88 2 9.18

2

+

+

2

133.58 4.38 +

+

(+: the parameter has a positive effect (the mean contact angle increases when the parameter level increases); 2 : the parameter has a negative effect (the mean contact angle decreases when the parameter level increases))

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Volume 31 · Number 1 · 2001 · 33–40

Denes, A.R., Tshabalala, M.A., Rowell, R., Denes, F. and Young, R.A. (1999), “Hexamethyldisiloxane-plasma coating of wood surfaces for creating water repellent characteristics”, Holzforschung, Vol. 53, pp. 318-26. Kaplan, S.L. and Rose, P.W. (1991), “Plasma surface treatment of plastics to enhance adhesion”, International Journal of Adhesion and Adhesives, Vol. 11 No. 2, pp. 109-13. Magalhaes, W.L.E. and Ferraira De Souza, M. (2000). 1-butene-cold plasma coating of solid softwood. Second Woodcoatings Congress, Paper 32, 12 pages, 23– 25 October 2000, The Hague (NL). Mahlberg, R., Niemi, H.E.M., Denes, F. and Rowell, R.M. (1998), “Effect of oxygen and hexamethyldisiloxane plasma on morphology, wettability and adhesion properties of polypropylene and lignocellulosics”, International Journal of Adhesion and Adhesives, Vol. 18, pp. 283-97. Podgorski, L. and Roux, M.L. (1999), “Wood modification to improve the durability of coatings”, Surface Coating International, Vol. 82 No. 12, pp. 590-6. Podgorski, L., Chevet, B., Onic, L. and Merlin, A. (2000), “Modification of wood wettability by plasma and corona treatments”, International Journal of Adhesion and Adhesives, Vol. 20 No. 2, pp. 103-11. Podgorski, L., Schambourg, F.J. and Maguin, B. (2000). Chevet wood waterproofing: a new kind of coating. Second Woodcoatings Congress, Paper 29, 7 pages, 23– 25 October 2000, The Hague (NL). Roux, M.L., Wozniak, E., Miller, E.R., Boxall, J., Bo¨ttcher, P., Kropf, F. and Sell, J. (1988), “Natural weathering of various surfcae coatings on five species at four European sites”, Holz als Roh- und Werkstoff, Vol. 46, pp. 165-70. Xiao, G.Z. (1997), “Effects of solvents on the surface properties of oxygen plasma-treated polyethylene and propylene films”, J. Adhesion Sci. Technol, Vol. 11 No. 5, pp. 655-63.

Table X Plasma parameters for an optimum treatment of wood (liquid phase)

Gas flow (CF4) (cm3/min) 80

Monomer quantity (AC8F17) (ml) 0.8

Power (W)

Distance samples/ plasma source (cm)

Treatment time (min)

Frequency (Hz)

700

37

9

20

If the film is not sufficient in itself, the deposit on the coated wood might help to extend the service life of the finish and ensure that wood will regain market shares for outdoor uses. By protecting wood from moisture the microfilm envisaged may act as a barrier against fungi and all the more so because this type of deposit shows an antifungal effectiveness for treated textiles. Test samples of pine were waterproofed by gaseous fluorine or silicone plasma treatment or liquid fluorine plasma treatment. The water wettability tests attest to the effectiveness of these treatments. Best results were obtained in liquid phase. Treatments parameters best suited to wood were selected. The resistance to accelerated weathering and to fungi degradation of the plasma treated wood will be evaluated and compared to unmodified wood properties.

References Denes, A.R. and Young, R.A. (1999), “Reduction of weathering degradation of wood through plasma polymer coating”, Holzforschung, Vol. 53, pp. 632-40.

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