Formation of hydrophobic coating on glass surface using atmospheric pressure non-thermal plasma in ambient air

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1 INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 37 (2004) PII: S (04) Formation of hydrophobic coating on glass surface using atmospheric pressure non-thermal plasma in ambient air Z Fang 1, Y Qiu 1 and E Kuffel 2 1 State Key Laboratory of Electrical Insulation for Power Equipment, Xi an Jiaotong University, Xi an , People s Republic of China 2 Department of Electrical and Computer Engineering, University of Manitoba, Winnipeg R3T 2N2, Canada myfz@263.net Received 3 March 2004 Published 28 July 2004 Online at stacks.iop.org/jphysd/37/2261 doi: / /37/16/007 Abstract Non-thermal plasmas under atmospheric pressure are of great interest in material surface processing because of their convenience, effectiveness and low cost. In this paper, the treatment of a glass surface for improving hydrophobicity using a non-thermal plasma generated by a dielectric barrier corona discharge (DBCD) with a needle array-to-plane electrode arrangement in atmospheric air is conducted, and the surface properties of the glass before and after the DBCD treatment are studied using contact angle measurement, surface resistance measurement and the wet flashover voltage test. The effects of the plasma dose (the product of average discharge power and treatment time) of DBCD on the surface modification are studied, and the mechanism of interaction between the plasma and glass surface is discussed. It is found that a layer of hydrophobic coating is formed on the glass surface through DBCD treatment, and the improvement of hydrophobicity depends on the plasma dose of the DBCD. It seems that there is an optimum plasma dose for the surface treatment. The test results of thermal ageing and chemical ageing show that the hydrophobic layer has quite stable characteristics. 1. Introduction In recent years, non-thermal plasmas under atmospheric pressure have attracted considerable interest for surface modification of materials [1, 2]. This environment friendly dry treatment can modify the surface properties of materials without changing the chemical and physical properties of the bulk. Furthermore, the operation of such a process at atmospheric pressure requires no vacuum equipment, and so the processing cost is reduced and the feasibility of continuous processing is improved, and therefore large-scale industrial applications are possible [3, 4]. Among various atmospheric pressure non-thermal plasmas, the dielectric barrier discharges (DBDs) are studied mostly for the easy formation of stable plasmas and their scalability, and numerous studies concerning surface modification of material using DBDs under atmospheric pressure have been undertaken [5 14], mainly for treatment of polymers and metals. These studies show that the highly reactive species in the discharge regime of DBDs can interact with the surfaces of materials and induce some physical and chemical changes (oxidation, polymerization, cross-linking, etching, etc) on them, and thus the surface properties, such as wettability, printability, adhesion and conductivity are improved. In some industrial applications, however, surface hydrophobicity is needed. For example, in high-voltage power transmission systems, the surface of glass or ceramic insulators needs good hydrophobic properties to prevent the occurrence of contamination flashover. In order to improve the hydrophobicity of the insulator surface, silicone oil and silicone grease are widely used; their excellent /04/ $ IOP Publishing Ltd Printed in the UK 2261

2 Z Fang et al hydrophobic properties help make a protective coating on the insulator surface [15]. However, this coating on the insulator surface does not react chemically with the insulator surface molecules, and thus it cannot form a stable hydrophobic layer. Furthermore, such coatings would diminish more or less with time due to heavily contaminated environments [16]. Non-thermal plasmas under atmospheric pressure offer a new approach to testing insulator surfaces. In this paper, we used dielectric barrier corona discharges (DBCDs) to produce a non-thermal plasma for treatment of glass surfaces to form a stable and effective hydrophobic layer on them. Previously, we have used plane-to-plane DBDs and atmospheric pressure glow discharges (APGDs) to modify the surface properties of polytetrafluoroethylene (PTFE), and it was found that these treatments could introduce oxygen containing polar groups on the PTFE surface [17]. However, these two kinds of plasmas generated in the plane-to-plane electrode arrangement with a very short gas gap seem to be not applicable to insulators, which have a rather complicated shape. So in this paper, a needle array-to-plane electrode arrangement DBCD driven by a power frequency voltage supply is employed. As the insulator surface is generally coated with a very fine film of glass known as glaze [15], we used glass plates as test samples to demonstrate the effectiveness of the plasma treatment. The surface hydrophobicity and electrical properties of the glass before and after the DBCD treatment are studied using contact angle measurement, surface resistance measurement and the wet flashover voltage test. The effects of the plasma dose of the DBCD on the surface modification are studied, and the mechanism of interaction between the plasma and glass surface is discussed. 2. Experimental set-up and procedure The treatment was performed in ambient air. The experimental set-up for the treatment shown in figure 1 is similar to that used in our earlier research work [17], and the only difference lies in the electrode arrangement. The needle array used in this work had stainless steel needles with a tip radius of about 60 µm fixed on an electrode with an area of 50 mm 50 mm. A plane electrode made of brass with an area of 50 mm 50 mm was connected to the ground. The glass to be treated was placed on the lower plane electrode and also acted as a dielectric Figure 1. Schematic diagram of the experimental set-up: (1) needle array; (2) dielectric barrier (glass); (3) plane electrode. barrier. The plasma was created between the needle tip and the plane electrode. The inter-electrode air distance (from the needle tip to the glass) could be adjusted in the range 1 40 mm for the treatment. A 50 kv power frequency highvoltage testing transformer was used as a voltage source. The voltage applied to the electrodes was measured using a voltage divider, and is expressed using its root-mean-square value throughout this paper. The discharge current and transported charge are measured by placing a 50 coaxial resistor (r) and a 2 nf capacitor (C 0 ) between the bottom electrode and the ground, respectively. A Tek TDS3052 (500 MHz, 5 GS s 1 ) digital oscilloscope was used to record the voltage and current waveforms and the Lissajous figures. All samples were cleaned with detergent, rinsed with alcohol and distilled water in turn to remove surface contamination and dust and dried at room temperature for 2 h before plasma treatment. The following procedure was used for the treatment. First, the sample was put into the discharge region for pre-treatment for about several minutes. The aim of the pre-treatment was to clean and activate the glass surface, and this treatment will remove alkali ions and other contaminants. After this pre-treatment procedure, a thin layer of silicone oil (polydimethyl siloxane fluid) was sprayed onto the glass surface. Then, the coated sample was treated for several minutes. The plasma activates the molecules on the glass surface as well as molecules of the polydimethyl siloxane, thus promoting their interactions and forming a hydrophobic layer on the glass surface. Finally, the remaining silicone oil on the after-treatment glass surface was removed by rinsing in sodium hydroxide and ethanol solution. To determine the change in hydrophobicity of the glass surface, static contact angle measurements were made immediately after the treatment by dropping 2 µl distilled water on the sample surface and using the three-point measurement method. The values of the static contact angle shown in this paper are the averages of nine measured values. For measurement of surface resistance, a ZC hyperresistance apparatus was used, and a water sprayer was used to spray water at a pressure of 0.6 MPa directly onto the glass surface under test [18]. The surface wet flashover voltage was measured by placing two pieces of copper foil of 5 mm in width onto the glass surface as electrodes and under the same spraying condition as that of the surface resistance measurement. The topography and molecular structure of the glass surface before and after the treatment were investigated using a scanning electronic microscope (HITACHI S-2700) and infrared absorption measurement equipment (Shimadzu FTIR-8300), respectively. Figure 2 shows the typical waveforms of voltage, current and Lissajous figure of the needle array-to-plane DBCD. Unlike the plane-to-plane DBD, in which the current waveform has the same shape for the positive and negative half-cycles, the needle array-to-plane DBCD shows different properties at positive and negative half-cycles. This is because the needle electrodes possess a strong electric field around their tips and trigger the formation of a streamer at lower voltages across the inter-electrode gap than in a plane-to-plane DBD. So this kind of discharge exhibits properties that are inherent in both steadystate dc negative and positive point-plane coronas in addition to characteristics shown in plane-to-plane DBDs [19 21]. It may 2262

Formation of hydrophobic coating on glass surface Figure 3. Photograph of water drops on the glass surface before treatment (on the right-hand side) and after treatment (on the left-hand side).

3 Formation of hydrophobic coating on glass surface Figure 3. Photograph of water drops on the glass surface before treatment (on the right-hand side) and after treatment (on the left-hand side). (This figure is in colour only in the electronic version) Figure 2. Measured voltage and current waveforms (a) and Lissajous figure (b) of the needle array-to-plane discharge. be noted that, unlike the Lissajous figure in the plane-toplane DBD, which is very close to a parallelogram [6, 17], the Lissajous figure of this type of DBCD shows a nearly continuous discharge in each half-cycle of the applied voltage. In summary, the atmospheric non-thermal plasma with a larger cross-sectional area than that in DBD could be uniformly produced using a needle array-to-plane DBCD with a lower discharge initial voltage than that of the plane-to-plane DBD [19, 21, 22]. 3. Experimental results Surface treatments are performed by setting the air gap to 1.5 mm and varying the average discharge power and treatment time. The electric energy dissipated in the reactor in each voltage cycle is calculated by measuring the area of the Lissajous figure, and the average discharge power can be obtained by multiplying the energy per voltage cycle and the frequency of the ac power (50 Hz). We describe the changes in the surface in the plasma processes as a function of the mean electrical energy input into the plasma, i.e. the product of the discharge power and treatment time (in millijoules), which is also known as plasma dose in the literature. Figure 4. Variation of contact angle with the plasma dose: (1) the average discharge power is kept at 175 mw; (2) the treatment time is kept at 11 min. Figure 3 shows a photograph of water drops on the glass surface before (on the right side) and after the plasma treatment (on the left side). It is seen that the hydrophobicity of the glass surface after the treatment is greatly increased. Figures 4 6 show the variations of water contact angle, surface resistance and wet flashover voltage of the glass surface with the plasma dose in two sets of experiments, in which the average discharge power is kept at 175 mw and the treatment time is kept at 11 min, respectively. As can be seen from these figures, the contact angle, surface resistance and wet flashover voltage all increase with increasing plasma dose, and reach a maximum of 121, and 19.1 kv at a dose of 1925 mj, respectively. With a further increase in plasma dose, no apparent changes in these three parameters are observed even with a small decrease. It may be noted that the trend of the surface resistance and wet flashover voltage variations is different from that of the contact angle. In the case of the contact angle (figure 4), the two curves 2263

4 Z Fang et al Figure 5. Variation of surface resistance with the plasma dose: (1) the average discharge power is kept at 175 mw; (2) the treatment time is kept at 11 min. Figure 6. Variation of wet flashover voltage with the plasma dose: (1) the average discharge power is kept at 175 mw; (2) the treatment time is kept at 11 min. obtained are very close, suggesting that the average discharge power and treatment time seem to have the same effect on the contact angle of the glass surface. In the cases of surface resistance and wet flashover voltage (figures 5 and 6), the two curves are somewhat different before the saturation point (1925 mj) is reached, which suggests incomplete utilization of the plasma dose when the treatment time is too short. 4. Discussion It is suggested that the plasma treatment makes the surface become hydrophobic due to the interaction of the plasma with the glass surface and silicone oil. The decrease in hydrophobicity when the plasma dose is in excess of a certain critical value is probably due to plasma etching, which destroys the surface chemical bonds, leading to an increase of polar groups and deterioration of the surface hydrophobic layer. As solids with high surface energy are hydrophilic and tend to promote wetting, and solids with low surface energy are hydrophobic, the contact angle method is widely used for characterizing the surface changes in plasma treatment for its simplicity [23]. In this paper the hydrophobicity of the glass surface is characterized by the water contact angle, surface resistance and wet flashover voltage for convenience in practical industrial applications. A more detailed and quantitative study of the hydrophobicity of the plasma treated glass surface can be achieved by estimating the surface energy and its polar and dispersive parts using the method presented in [24]. Surface treatment using plasmas is a process of interaction between the plasma and the surface to be treated. In the case of DBCDs, there are many active species, and so the glass surface and the polydimethyl siloxane molecules are exposed to a highly reactive regime, with some chemical bonds being cut off. The bombardment of active species (mainly ions) on the glass surface can remove the alkali ions and hydroxide, and generate radicals on the surface, as shown in figure 7(b). The breaking of polydimethyl siloxane molecules ((CH 3 ) 3 SiO[Si(CH 3 ) 2 O] n Si(CH 3 ) 3 ) can generate CH 3, and (CH 3 ) 3 SiO[Si(CH 3 ) 2 O] n Si(CH) + 2 with some other groups, and they can react with the radicals on the glass surface and form a layer of hydrophobic coating on it (figure 7(c)). Figure 8 shows the FTIR spectra of the glass surface before and after the plasma treatment. It can be seen that three absorption peaks appear after the plasma treatment, which are not seen in the untreated sample. The peak at a wavenumber of approximately 1460 cm 1 corresponds to the CH 3 asymmetric bending in Si(CH 3 ); the peak at 1355 cm 1 is due to CH 2 vibration in Si CH 2 Si; and the peak at 2955 cm 1 is attributed to CH 3 symmetric vibration. Also, the Si O Si absorption peak at 1030 cm 1 for the untreated glass compared with that for plasma treated at 1070 cm 1 indicates a shift, which means that new groups are chemically bonded with Si. These results show that a new material is chemically bonded on the surface after the plasma treatment, which prevents the invasion of water molecules to the polar bonds in the bulk of the glass and contributes to the improvement in hydrophobicity of the glass surface. In order to investigate the stability of the hydrophobic surface induced by the plasma treatment, thermal ageing and chemical ageing tests were performed. Thermal ageing tests were conducted by putting the treated sample (with the average discharge power of 175 mw and treatment time of 11 min) into an oven, and the temperature inside was kept at 120 C. Table 1 shows the variation of the surface resistance with the thermal ageing time. It is seen that the high temperature does not make the surface resistance decrease. Chemical ageing tests were conducted by immersing the treated samples (with the average discharge power of 175 mw and treatment time of 11 min) first into 10% concentration sulfuric acid solution and then into 5% concentration sodium hydrate, both for 30 min. Prior to the measurement, the samples were taken out from the solution and thoroughly cleaned with water. It is found that there is no apparent change in surface resistance after immersion in the acid and alkaline solutions. 5. Conclusions The non-thermal plasma generated by a DBCD with a needle array-to-plane electrode arrangement was used to improve 2264

Formation of hydrophobic coating on glass surface (a) (b) Figure 8. Infrared absorption of the glass surface before (a) and after (b) the plasma treatment. Table 1.

SEM observation of the glass surface before and after the DBD treatment: (a) untreated, (b) after the pre-treatment, (c) after the treatment. the hydrophobicity of a glass surface.

sectional area under atmospheric pressure.

5 Formation of hydrophobic coating on glass surface (a) (b) Figure 8. Infrared absorption of the glass surface before (a) and after (b) the plasma treatment. Table 1. Variation of surface resistance with thermal ageing time. (c) Time (h) Surface resistance (10 10 ) References Figure 7. SEM observation of the glass surface before and after the DBD treatment: (a) untreated, (b) after the pre-treatment, (c) after the treatment. the hydrophobicity of a glass surface. The DBCD exhibits properties that are inherent in both steady-state dc negative and positive point-plane coronas and in plane-to-plane DBDs, and it can generate plasmas in a relatively large sectional area under atmospheric pressure. After the plasma treatment, a layer of hydrophobic coating is formed on the glass surface, and the contact angle, surface resistance and wet flashover voltage are all increased. It is found that the change in surface properties depends on the plasma dose. The higher the plasma dose, the better the improvement in hydrophobicity until the critical plasma dose is reached. In this paper, the optimal condition for the treatment is at a plasma dose of 1925 mj. The ageing test results show that the surface coating has quite good stability. [1] Kunhardt E E 2000 IEEE Trans. Plasmas Sci [2] Napartovich A P 2001 Plasma Polym [3] Shenton M J and Stevens G C 2001 J. Phys. D: Appl. Phys [4] Pochner K, Neff W and Lobert R 1995 Surf. Coat. Technol. 74/ [5] Elisasson B and Kogelschatz U 1991 IEEE Trans. Plasmas Sci [6] Kogelschatz U 2003 Plasma Chem. Plasma Process [7] Cui N Y and Brown M D 2002 Appl. Surf. Sci [8] Wagner H E, Brandenburga R and Kozlovb K V 2003 Vacuum [9] Borcia G, Anderson C A and Brown M D 2003 Plasma Sources Sci. Technol [10] Dumitrascu N, Borcia G, Apetroaei N and Popa G 2002 Plasma Sources Sci. Technol [11] Rehn P, Wolkenhauer A, Bente M, Forster S and Viol W 2003 Surf. Coat. Technol. 174/ [12] Massines F, Gouda G, Gherardi N, Duran M and Croquesel E 2001 Plasma Polym [13] Golobovskii Y B, Maiorov V A, Behnke J and Behnke J F 2002 J. Phys. D: Appl. Phys [14] Kodama S, Habaki H, Sekiguchi H and Kawasaki J 2002 Thin Soild Films [15] Gorur R S, Cherney E and de Tourreil C 1995 IEEE Trans. Power Delivery [16] Eldridge K, Xu J, Yin W and Jeffery A M 1999 IEEE Trans. Power Delivery [17] Fang Z, Qiu Y and Luo Y 2003 J. Phys. D: Appl. Phys [18] Gorur R S et al 1999 IEEE Trans. Power Delivery

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