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Effects of operating parameters on plasma-induced PET surface treatment
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3081–3085 www.elsevier.com/locate/nimb
Effects of operating parameters on plasma-induced PET surface treatment R. Morent *, N. De Geyter, C. Leys Department of Applied Physics, Research Unit Plasma Technology (RUPT), Faculty of Engineering, Ghent University, Jozef Plateaustraat 22, B-9000 Ghent, Belgium Available online 24 March 2008
Abstract Plasma treatment is widely used to modify the surface properties of polymers, since it is a versatile, fast and environmentally benign technology. Different types of plasma sources can be utilized for surface modification, however, dielectric barrier discharges (DBDs) have received much attention due to their great flexibility. Therefore, in this paper, a DBD operating in air at medium pressure is used to modify the surface of a polyethylene terephthalate (PET) film. The influence of the main operating parameters (discharge power, gas flow, pressure and frequency) on the surface properties is studied in detail using contact angle measurements. Results show that the efficiency of the surface treatment increases with increasing discharge power, increasing pressure and decreasing gas flow. Results also show that the frequency of the discharge has no influence on the treatment efficiency. Ó 2008 Elsevier B.V. All rights reserved. PACS: 52.40.Hf; 81.05.Lg; 81.65.Mq Keywords: Dielectric barrier discharge; Polyethylene terephthalate; Contact angle measurements; Operating parameters
1. Introduction Plasma treatment is one of the most versatile techniques in surface modification. It has been widely used to improve the adhesion of coatings to metals and polymers [1], to increase the wettability and the printability of polymers [2], to enhance the biocompatibility of implants [3], . . . Plasmas, often considered as the fourth state of matter, are composed of an ionized gas containing a mixture of ions, electrons, radicals and photons. When these energetic active species collide with a surface, some of the chemical bonds will break making the surface more reactive. Depending on the type of gas in which the plasma is generated, certain new functional groups can be grafted on the surface [4]. Therefore, plasma surface modification can induce a specific surface chemistry without altering the bulk properties of the material [5]. *
Corresponding author. Tel.: +32 9 2644257; fax: +32 9 2644198. E-mail address: [email protected] (R. Morent).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.166
Dielectric barrier discharges (DBDs) or ‘‘silent” discharges are widely studied for the treatment of polymer films. A DBD is obtained between two electrodes, at least one of which is covered with a dielectric, when an AC high voltage is applied between the electrodes. The most interesting property of DBDs is that in most gases the breakdown starts at many points, followed by the development of independent current filaments (named microdischarges). These microdischarges are of nanosecond duration and are uniformly distributed over the dielectric surface [6]. In this paper, a DBD operating in air at medium pressure is used to alter the surface properties of a polyethylene terephthalate (PET) film. Despite the fact that DBDs are widely studied, there is currently little published work on the effects of varying operating parameters on the resulting surface treatment. Therefore, in this paper, the influence of the main operating parameters, i.e. treatment time, power, pressure, frequency and gas flow, on the surface properties is studied in detail using contact angle measurements.
3082
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
2. Experimental set-up 2.1. DBD set-up The used dielectric barrier discharge system is schematically presented in Fig. 1. The DBD is created between two circular copper electrodes, 40 mm in diameter. Both electrodes are covered by a ceramic (Al2O3) plate with a thickness of 0.7 mm and an area of 50 mm 50 mm. The air gap between the two ceramic plates is 2 mm. The upper electrode is connected to an AC high voltage source with a variable frequency (1–20 kHz), while the lower electrode is connected to earth through a resistor R or a capacitor C. Before starting the experiments, a polyethylene terephthalate (PET) film is placed on the lower ceramic plate. After introduction of the sample into the reactor, the discharge chamber is pumped down to several kPa, while an air flow (Air Liquide – Alphagaz 1) flows between the electrodes. The gas flow rate is controlled by a mass-flow controller (MSK Instruments). The gas outlet is connected to a rotary vane pump and a needle valve is used in such a way that the pressure is kept constant during plasma treatment. The voltage applied to the electrodes is measured using a high voltage probe (Tektronix P6015A), whereas the discharge current is obtained by measuring the voltage over a 100 X resistor connected in series to ground. The current and the voltage waveforms are recorded using an oscilloscope (Tektronix TDS210 – 60 MHz). The current waveform shows numerous short peaks superimposed on the capacitive current: these peaks are an indication of the microdischarge activity and each peak corresponds to a series of microdischarges [6]. The resistor can be replaced by a capacitor of 10 nF; the voltage measured across this capacitor is then proportional to the charge stored on the electrodes. This latter measurement, together with the applied voltage, is widely used to obtain the voltage-versus-charge plot or so-called Lissajous
Fig. 1. Experimental set-up of the DBD discharge (1. gas cylinder, 2. mass-flow controller, 3. plasma chamber, 4. pressure gauge, 5. needle valve, 6. pump).
figure [7]. The electrical energy consumed per voltage cycle Uel can be estimated by measuring the area of the Lissajous figure. The electric discharge power Pel can be obtained by multiplying Uel with the frequency of the feeding voltage [7]. 2.2. Contact angle measurements After plasma treatment, contact angles are obtained using a commercial Kru¨ss Easy Drop optical system (Kru¨ss GmbH, Germany). This system is equipped with a software operated high-precision liquid dispenser to precisely control the drop size of the used liquid. The drop image is then stored, via a monochrome interline CCD video camera, using PC-based acquisition and data processing. Using the computer software provided with the instrument, measurement of the static contact angles is fully automated. In this work, 2 ll distilled water drops are placed on the PET films after plasma treatment. The values of the static contact angles, shown in this paper, are obtained using Laplace–Young curve fitting based on the imaged sessile water drop profile and are the average of eight measured values. The standard deviation on the average contact angles is smaller than 2%. 3. Results 3.1. Influence of treatment time and power Fig. 2 depicts the evolution of the contact angle measured on the plasma treated PET films as a function of treatment time for different electrical discharge powers. The frequency of the DBD is kept constant at 10 kHz and the operating pressure is 5.0 kPa. The gas flow between the electrodes is set to 200 sccm (standard cubic centimetres per minute). The water contact angle of the untreated PET film is 74.8° and decreases to 33° for all discharge powers with increasing treatment time. However, when the PET film is treated during more than 1 s, the contact angle does not change anymore as a function of treatment time. This shows that there is a saturation of the plasma effect on the PET film. The large diminution of the contact angle after plasma treatment demonstrates the strongly increased wettability induced by the air DBD treatment even after very short treatment times. This behaviour can be attributed to the incorporation of oxygen-containing functionalities, resulting in a strong surface oxidation and therefore, an increased hydrophilicity of the polymer surface [8]. The molecular oxygen in the discharge is activated, ionized and dissociated resulting in the formation of extremely reactive oxygen species, which react immediately with the polymer surface [9]. As shown in a previous article, air plasma treatment of a PET film introduces C@O and O– C@O polar groups on the polymer surface [8]. Increasing the discharge power leads to a faster decrease in contact angle on the plasma treated PET films, as can be seen in Fig. 2. This can be explained by the fact that
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
3083
80 0.8 W 1.1 W 70
1.5 W
contact angle [°]
2.1 W 2.5 W
60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 2. Evolution of the contact angle of a PET film as a function of treatment time for different discharge powers.
increasing the discharge power results in the presence of a higher amount of reactive oxygen species. This leads to a faster incorporation of oxygen-containing polar groups and therefore, a faster decrease in contact angle. 3.2. Influence of gas flow Fig. 3 shows the contact angle as a function of treatment time for gas flows ranging from 50 to 900 sccm. The discharge power and frequency are set to 1.1 W and 10 kHz, respectively. The operating pressure is kept constant at 5.0 kPa. Results show that the decrease in contact angle is slower when the gas flow is increased between the two
electrodes. At higher gas flows, the concentration of reactive oxygen species in the plasma region decreases, since a fraction of these species are blown away. Therefore, one can conclude that the used gas flow has a critical effect on the surface treatment of polymers and that it should be as low as possible in order to obtain the most efficient decrease in contact angle. 3.3. Influence of operating pressure Another parameter, that could influence the efficiency of the DBD treatment, is the operating pressure. Fig. 4 gives the contact angle as a function of treatment time for differ-
80 50 sccm 200 sccm 70
500 sccm
contact angle [°]
900 sccm 60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 3. Water contact angle of a PET film as a function of treatment time for different gas flows between the electrodes.
3084
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
80 0.5 kPa 1.0 kPa 70
2.0 kPa
contact angle [°]
3.0 kPa 5.0 kPa
60
7.0 kPa 9.0 kPa 50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 4. Decrease of water contact angle of plasma treated PET film as a function of treatment time for different operating pressures.
ent pressures. The discharge power and frequency are equal to 1.1 W and 10 kHz, respectively. The gas flow between the electrodes is maintained at 200 sccm. As shown in Fig. 4, the decrease in contact angle proceeds faster when the plasma treatment is performed at higher operating pressures. This can be easily explained: at higher pressure, the concentration of reactive oxygen species increases, leading to a more efficient decrease in contact angle.
The discharge power is kept constant at 1.1 W, while a constant gas flow of 200 sccm flows between the electrodes. The operating pressure is maintained at 5.0 kPa. Results clearly show that the decrease in contact angle is independent of the used frequency during plasma treatment. Therefore, one can conclude that the operating frequency does not influence the efficiency of the DBD treatment. 4. Conclusion
3.4. Influence of operating frequency The influence of the DBD frequency on the efficiency of the plasma treatment is also studied in detail. Fig. 5 depicts the evolution of the contact angle as a function of treatment time for different operating frequencies of the DBD.
In this paper, a PET film is treated with a DBD operating in air at medium pressure. After plasma treatment, the contact angle of the PET film considerably decreases due to the incorporation of oxygen-containing functionalities on the PET surface. The influence of different operating
80 2 kHz 5 kHz 70
10 kHz
contact angle [°]
15 kHz 60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 5. Influence of operating frequency on the evolution of the water contact angle of the PET film as a function of treatment time.
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
parameters, such as treatment time, power, gas flow, pressure and frequency on the plasma modification is studied in detail. Results show that the operating frequency has no influence on the efficiency of the DBD treatment. However, the decrease in contact angle is more efficient when increasing the operating pressure and the discharge power. In contrast, the gas flow between the electrodes should be decreased to increase the efficiency of the plasma treatment. References [1] E. Liston, L.M. Martinu, M.R. Wertheimer, J. Adhes. Sci. Technol. 7 (1993) 1091.
3085
[2] N.Y. Cui, N.M.D. Brown, Appl. Surf. Sci. 189 (2002) 31. [3] P.K. Chu, J.Y. Chen, L.P. Wang, N. Huang, Mater. Sci. Eng.: Rep. 36 (2002) 143. [4] Y.J. Hwang, S. Matthews, M. McCord, M. Bourham, J. Electrochem. Soc. 151 (2004) C495. [5] H.U. Poll, U. Schladitz, S. Schreiter, Surf. Coat. Technol. 142–144 (2001) 489. [6] U. Kogelschatz, Plasma Chem. Plasma Process. 23 (2003) 1. [7] H.-E. Wagner, R. Brandenburg, K.V. Kozlov, A. Sonnenfeld, P. Michel, J.F. Behnke, Vacuum 71 (2003) 417. [8] N. De Geyter, R. Morent, C. Leys, L. Gengembre, E. Payen, Surf. Coat. Technol. 201 (2007) 7066. [9] G. Borcia, C.A. Anderson, N.M.D. Brown, Plasma Sources Sci. Technol. 12 (2003) 335.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 3081–3085 www.elsevier.com/locate/nimb
Effects of operating parameters on plasma-induced PET surface treatment R. Morent *, N. De Geyter, C. Leys Department of Applied Physics, Research Unit Plasma Technology (RUPT), Faculty of Engineering, Ghent University, Jozef Plateaustraat 22, B-9000 Ghent, Belgium Available online 24 March 2008
Abstract Plasma treatment is widely used to modify the surface properties of polymers, since it is a versatile, fast and environmentally benign technology. Different types of plasma sources can be utilized for surface modification, however, dielectric barrier discharges (DBDs) have received much attention due to their great flexibility. Therefore, in this paper, a DBD operating in air at medium pressure is used to modify the surface of a polyethylene terephthalate (PET) film. The influence of the main operating parameters (discharge power, gas flow, pressure and frequency) on the surface properties is studied in detail using contact angle measurements. Results show that the efficiency of the surface treatment increases with increasing discharge power, increasing pressure and decreasing gas flow. Results also show that the frequency of the discharge has no influence on the treatment efficiency. Ó 2008 Elsevier B.V. All rights reserved. PACS: 52.40.Hf; 81.05.Lg; 81.65.Mq Keywords: Dielectric barrier discharge; Polyethylene terephthalate; Contact angle measurements; Operating parameters
1. Introduction Plasma treatment is one of the most versatile techniques in surface modification. It has been widely used to improve the adhesion of coatings to metals and polymers [1], to increase the wettability and the printability of polymers [2], to enhance the biocompatibility of implants [3], . . . Plasmas, often considered as the fourth state of matter, are composed of an ionized gas containing a mixture of ions, electrons, radicals and photons. When these energetic active species collide with a surface, some of the chemical bonds will break making the surface more reactive. Depending on the type of gas in which the plasma is generated, certain new functional groups can be grafted on the surface [4]. Therefore, plasma surface modification can induce a specific surface chemistry without altering the bulk properties of the material [5]. *
Corresponding author. Tel.: +32 9 2644257; fax: +32 9 2644198. E-mail address: [email protected] (R. Morent).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.166
Dielectric barrier discharges (DBDs) or ‘‘silent” discharges are widely studied for the treatment of polymer films. A DBD is obtained between two electrodes, at least one of which is covered with a dielectric, when an AC high voltage is applied between the electrodes. The most interesting property of DBDs is that in most gases the breakdown starts at many points, followed by the development of independent current filaments (named microdischarges). These microdischarges are of nanosecond duration and are uniformly distributed over the dielectric surface [6]. In this paper, a DBD operating in air at medium pressure is used to alter the surface properties of a polyethylene terephthalate (PET) film. Despite the fact that DBDs are widely studied, there is currently little published work on the effects of varying operating parameters on the resulting surface treatment. Therefore, in this paper, the influence of the main operating parameters, i.e. treatment time, power, pressure, frequency and gas flow, on the surface properties is studied in detail using contact angle measurements.
3082
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
2. Experimental set-up 2.1. DBD set-up The used dielectric barrier discharge system is schematically presented in Fig. 1. The DBD is created between two circular copper electrodes, 40 mm in diameter. Both electrodes are covered by a ceramic (Al2O3) plate with a thickness of 0.7 mm and an area of 50 mm 50 mm. The air gap between the two ceramic plates is 2 mm. The upper electrode is connected to an AC high voltage source with a variable frequency (1–20 kHz), while the lower electrode is connected to earth through a resistor R or a capacitor C. Before starting the experiments, a polyethylene terephthalate (PET) film is placed on the lower ceramic plate. After introduction of the sample into the reactor, the discharge chamber is pumped down to several kPa, while an air flow (Air Liquide – Alphagaz 1) flows between the electrodes. The gas flow rate is controlled by a mass-flow controller (MSK Instruments). The gas outlet is connected to a rotary vane pump and a needle valve is used in such a way that the pressure is kept constant during plasma treatment. The voltage applied to the electrodes is measured using a high voltage probe (Tektronix P6015A), whereas the discharge current is obtained by measuring the voltage over a 100 X resistor connected in series to ground. The current and the voltage waveforms are recorded using an oscilloscope (Tektronix TDS210 – 60 MHz). The current waveform shows numerous short peaks superimposed on the capacitive current: these peaks are an indication of the microdischarge activity and each peak corresponds to a series of microdischarges [6]. The resistor can be replaced by a capacitor of 10 nF; the voltage measured across this capacitor is then proportional to the charge stored on the electrodes. This latter measurement, together with the applied voltage, is widely used to obtain the voltage-versus-charge plot or so-called Lissajous
Fig. 1. Experimental set-up of the DBD discharge (1. gas cylinder, 2. mass-flow controller, 3. plasma chamber, 4. pressure gauge, 5. needle valve, 6. pump).
figure [7]. The electrical energy consumed per voltage cycle Uel can be estimated by measuring the area of the Lissajous figure. The electric discharge power Pel can be obtained by multiplying Uel with the frequency of the feeding voltage [7]. 2.2. Contact angle measurements After plasma treatment, contact angles are obtained using a commercial Kru¨ss Easy Drop optical system (Kru¨ss GmbH, Germany). This system is equipped with a software operated high-precision liquid dispenser to precisely control the drop size of the used liquid. The drop image is then stored, via a monochrome interline CCD video camera, using PC-based acquisition and data processing. Using the computer software provided with the instrument, measurement of the static contact angles is fully automated. In this work, 2 ll distilled water drops are placed on the PET films after plasma treatment. The values of the static contact angles, shown in this paper, are obtained using Laplace–Young curve fitting based on the imaged sessile water drop profile and are the average of eight measured values. The standard deviation on the average contact angles is smaller than 2%. 3. Results 3.1. Influence of treatment time and power Fig. 2 depicts the evolution of the contact angle measured on the plasma treated PET films as a function of treatment time for different electrical discharge powers. The frequency of the DBD is kept constant at 10 kHz and the operating pressure is 5.0 kPa. The gas flow between the electrodes is set to 200 sccm (standard cubic centimetres per minute). The water contact angle of the untreated PET film is 74.8° and decreases to 33° for all discharge powers with increasing treatment time. However, when the PET film is treated during more than 1 s, the contact angle does not change anymore as a function of treatment time. This shows that there is a saturation of the plasma effect on the PET film. The large diminution of the contact angle after plasma treatment demonstrates the strongly increased wettability induced by the air DBD treatment even after very short treatment times. This behaviour can be attributed to the incorporation of oxygen-containing functionalities, resulting in a strong surface oxidation and therefore, an increased hydrophilicity of the polymer surface [8]. The molecular oxygen in the discharge is activated, ionized and dissociated resulting in the formation of extremely reactive oxygen species, which react immediately with the polymer surface [9]. As shown in a previous article, air plasma treatment of a PET film introduces C@O and O– C@O polar groups on the polymer surface [8]. Increasing the discharge power leads to a faster decrease in contact angle on the plasma treated PET films, as can be seen in Fig. 2. This can be explained by the fact that
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
3083
80 0.8 W 1.1 W 70
1.5 W
contact angle [°]
2.1 W 2.5 W
60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 2. Evolution of the contact angle of a PET film as a function of treatment time for different discharge powers.
increasing the discharge power results in the presence of a higher amount of reactive oxygen species. This leads to a faster incorporation of oxygen-containing polar groups and therefore, a faster decrease in contact angle. 3.2. Influence of gas flow Fig. 3 shows the contact angle as a function of treatment time for gas flows ranging from 50 to 900 sccm. The discharge power and frequency are set to 1.1 W and 10 kHz, respectively. The operating pressure is kept constant at 5.0 kPa. Results show that the decrease in contact angle is slower when the gas flow is increased between the two
electrodes. At higher gas flows, the concentration of reactive oxygen species in the plasma region decreases, since a fraction of these species are blown away. Therefore, one can conclude that the used gas flow has a critical effect on the surface treatment of polymers and that it should be as low as possible in order to obtain the most efficient decrease in contact angle. 3.3. Influence of operating pressure Another parameter, that could influence the efficiency of the DBD treatment, is the operating pressure. Fig. 4 gives the contact angle as a function of treatment time for differ-
80 50 sccm 200 sccm 70
500 sccm
contact angle [°]
900 sccm 60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 3. Water contact angle of a PET film as a function of treatment time for different gas flows between the electrodes.
3084
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
80 0.5 kPa 1.0 kPa 70
2.0 kPa
contact angle [°]
3.0 kPa 5.0 kPa
60
7.0 kPa 9.0 kPa 50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 4. Decrease of water contact angle of plasma treated PET film as a function of treatment time for different operating pressures.
ent pressures. The discharge power and frequency are equal to 1.1 W and 10 kHz, respectively. The gas flow between the electrodes is maintained at 200 sccm. As shown in Fig. 4, the decrease in contact angle proceeds faster when the plasma treatment is performed at higher operating pressures. This can be easily explained: at higher pressure, the concentration of reactive oxygen species increases, leading to a more efficient decrease in contact angle.
The discharge power is kept constant at 1.1 W, while a constant gas flow of 200 sccm flows between the electrodes. The operating pressure is maintained at 5.0 kPa. Results clearly show that the decrease in contact angle is independent of the used frequency during plasma treatment. Therefore, one can conclude that the operating frequency does not influence the efficiency of the DBD treatment. 4. Conclusion
3.4. Influence of operating frequency The influence of the DBD frequency on the efficiency of the plasma treatment is also studied in detail. Fig. 5 depicts the evolution of the contact angle as a function of treatment time for different operating frequencies of the DBD.
In this paper, a PET film is treated with a DBD operating in air at medium pressure. After plasma treatment, the contact angle of the PET film considerably decreases due to the incorporation of oxygen-containing functionalities on the PET surface. The influence of different operating
80 2 kHz 5 kHz 70
10 kHz
contact angle [°]
15 kHz 60
50
40
30 0
0.5
1
1.5
2
treatment time [s] Fig. 5. Influence of operating frequency on the evolution of the water contact angle of the PET film as a function of treatment time.
R. Morent et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 3081–3085
parameters, such as treatment time, power, gas flow, pressure and frequency on the plasma modification is studied in detail. Results show that the operating frequency has no influence on the efficiency of the DBD treatment. However, the decrease in contact angle is more efficient when increasing the operating pressure and the discharge power. In contrast, the gas flow between the electrodes should be decreased to increase the efficiency of the plasma treatment. References [1] E. Liston, L.M. Martinu, M.R. Wertheimer, J. Adhes. Sci. Technol. 7 (1993) 1091.
3085
[2] N.Y. Cui, N.M.D. Brown, Appl. Surf. Sci. 189 (2002) 31. [3] P.K. Chu, J.Y. Chen, L.P. Wang, N. Huang, Mater. Sci. Eng.: Rep. 36 (2002) 143. [4] Y.J. Hwang, S. Matthews, M. McCord, M. Bourham, J. Electrochem. Soc. 151 (2004) C495. [5] H.U. Poll, U. Schladitz, S. Schreiter, Surf. Coat. Technol. 142–144 (2001) 489. [6] U. Kogelschatz, Plasma Chem. Plasma Process. 23 (2003) 1. [7] H.-E. Wagner, R. Brandenburg, K.V. Kozlov, A. Sonnenfeld, P. Michel, J.F. Behnke, Vacuum 71 (2003) 417. [8] N. De Geyter, R. Morent, C. Leys, L. Gengembre, E. Payen, Surf. Coat. Technol. 201 (2007) 7066. [9] G. Borcia, C.A. Anderson, N.M.D. Brown, Plasma Sources Sci. Technol. 12 (2003) 335.