Applied Science and Convergence Technology 2019; 28(4): 101-106
Published online July 31, 2019
Copyright © The Korean Vacuum Society.
Seung-Hyun Honga , Tae-Hee Kimb , * , and Sooseok Choia , b , *
aDepartment of Nuclear and Engineering, Jeju National University, Jeju 63243, Republic of Korea
bInstitute for Nuclear Science and Technology, Jeju National University, Jeju 63243, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-CommercialLicense (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution,and reproduction in any medium, provided the original work is properly cited.
Gilding arc plasma is a non-equilibrium phase and can be generated at atmospheric pressure. Surface treatments are a popular application of this form of plasma because of its relatively low temperature. Polytetrafluoroethylene (PTFE) film is naturally hydrophobic with low surface energy caused by its very stable covalent carbon bonds. In this study, the surface of PTFE film was made hydrophilic by treating it with gliding arc plasma. Tests were conducted with argon, oxygen, air, and nitrogen as discharge gases to test the effects of different species of active radicals formed by the arc. The treated film surfaces were analyzed after surface treatment with different gases and for different times, and the contact angle of the PTFE was 80° before treatment. OH radicals from the ambient air were generated by the gliding arc and these attached to the surface of PTFE. However, the contact angle increased again over time, and when oxygen or nitrogen were used during the plasma treatment, the PTFE surface returned to its original hydrophobic state after about 8 days. The OH radicals attached to the surface likely reacted with ambient air and left the surface over the course of this process.
Keywords: Gliding arc plasma, Polytetrafluoroethylene, Surface modification, Surface treatment, Hydrophilicity, Hydrophilic surface
Surface treatments using atmospheric plasma are simpler, economical, and more flexible than vacuum-plasma surface treatments [1–5]. Many studies have used gliding arc plasma, dielectric barrier discharge (DBD) [6,7], RF discharge [8,9], corona discharge , and microwave plasma  for atmospheric-plasma surface treatments. A gliding arc is an oscillating periodic phenomenon that develops between at least two opposite-sign electrodes. When voltage is first applied, an arc spans the narrowest points of the electrodes, and this arc is dragged downstream by the flow of some gas. The discharge grows in length as it travels down the widening gap between the electrodes until it reaches a critical value at which the arc disappears; another arc immediately forms across the shortest path between the two electrodes; thus, the phenomenon is cyclic [12,13]. The arc length and number of repetitions can be controlled easily by adjusting the gas flow rate or the voltage. The cyclic nature of this process means that the gliding arc plasma simultaneously has a high-temperature plasma region and a low-temperature plasma region. Because only the low-temperature plasma comes in contact with the material being treated, materials with low heat resistance such as polymer film can be treated. Gliding-arc plasma is well-suited for surface treatments, and it is more environmentally friendly than mechanical or wet-chemical processes. Gliding-arc plasma treatments are also not limited by the thickness of the material because the plasma only touches the material like a torch. Therefore, gliding arc plasma is useful in many industrial applications that require coating, painting, printing, or bonding . Figure 1 shows the optical emission spectra of a gliding arc plasma using air as the discharge gas. These observations were taken with a spectrometer (HR2000+, Ocean Optics). Spectral fingerprints of NO (A-X), N2 (C-B) and OH (A-X) appear in this figure. The intensities of these features increased as the samples were treated with higher flow rates. We know that OH and NO radicals are the most common highly reactive species generated in atmospheric plasma [15–17]. NO radicals are known to be involved in acid effects , and OH radicals are the most important mechanism by which hydrophilicity is increased by treatment with gliding arc plasma. The mechanisms by which atmospheric plasma generates OH radicals can be divided into three types. First, electrons collide with water vapor molecules in the air (H2O + e → H + OH + e), second, metastable atoms dissociate (H2O + Arm → OH + H + Ar), and third, excited oxygen atoms react with water-vapor molecules (H2O+ O → 2OH) . When gliding-arc plasma is used to irradiate a polymer surface, hydroxyl groups like OH radicals adhere to the surface. These hydroxyl groups are hydrophilic, so they change the surface of the polymer from hydrophobic to hydrophilic .
PTFE (polytetrafluoroethylene) is a polymer that has strong atomic bonds between carbon and fluorine. This atomic bonding grants PTFE many useful properties like chemical and thermal resistance, low coefficient of friction, and electrical stability. PTFE was discovered in 1938 and has since been applied in a wide variety of products. PTFE is used, for example, as non-stick coating for kitchen utensils, electrical cables, and waterproof fabrics . Owing to the high electro-negativity of fluorine-bonded carbon, the surface of PTFE is hydrophobic, which makes it is difficult to adhere to other materials, as is needed in forming cabling. Hydrophilicity is also a problem when printing on or dyeing the material, and some biomedical applications need a more hydrophilic surface. We have tested a gliding-arc plasma treatment that attaches OH radicals to the PTFE film surface in order to make it useful in applications that require a hydrophilic surface.
Figure 2 shows a schematic of the gliding-arc plasma system we used for testing surface treatments. The system consists of two curved electrodes, an AC power supply, and a gas-flow inlet. The two stainless-steel electrodes are 100 mm long and the gas inlet diameter is 4 mm. The shortest distance between the electrodes is 6 mm, the longest distance between the electrodes is 20 mm, and the electrodes are 3 mm from the treatment target. When a 25 kHz power source is connected to the two electrodes and the discharge gas is injected, a gliding arc plasma is generated. Oxygen, nitrogen, argon, and air were tested as discharge gases. Square PTFE samples were cut to 3 cm on a side, and they were cleaned by ethanol and dried thoroughly before treatment. In all tests, the flow rate of the discharge gas and the treatment time were 15 slpm and 30 min, respectively.
The contact angle is a good indicator of whether a polymer surface is hydrophilic or hydrophobic. Contact-angle measurements were used to confirm the hydrophilicity of the PTFE surface. A contact angle measurement device (Phoenix-150, SEO Company) was used to measure contact angles. When taking these measurements, about 9
The shape of the gliding arc with different discharge gases is shown in Fig. 3. As the flow rate of the discharge gas increases, the arc-cycle duration decreases and the length of the arc increases. At flow rates of about 15 slpm or more, however, the arc length was fixed. Thus, the arc length can only be increased so much by increasing the flow rate, and the arc length also depends on factors like the distance between the electrodes and the voltage. The arc shape was similar when using oxygen, argon, nitrogen, and air as the discharge gas. However, the arc length of the plasma was relatively short when using helium as the discharge gas. The density of helium is lower than that of the ambient air, so the helium flow generates a weak drag force for elongating the generated arc. This short arc did not make good contact with the surface of the PTFE sample. The sample could be crumpled up to make contact with the arc, but then the results were difficult to compare with those from the other samples. Therefore, helium was not used as a discharge gas in our tests.
The morphology of the PTFE samples was examined with FE-SEM analysis. Figures 4(a) and 4(b) show the untreated PTFE film surfaces at different locations, and Figure 4(c) and 4(d) show the PTFE film surfaces at different locations after gliding-arc plasma treatment for 30 min in oxygen. These images show that the morphologies of the untreated and treated surfaces are practically the same. The surface of the treated sample was damaged slightly, but this damage did not affect the physical properties of the PTFE film. This result confirms that the low-temperature region of the gliding arc in fact treats the surface. Further, the SEM images suggest that the increased hydrophilicity is not related to a physical change to the surface of the PTFE but instead is explained by a chemical change. The low risk of thermal damage means that the process can be used to treat heat-sensitive materials like polymers.
Figure 5 compares the AFM analysis results of the (a) pristine sample and (b) sample after treatment for 30 min in oxygen. The untreated sample was flat, and the surface was less uniform after treatment. In several studies, the surface of plasma-treated PTFE has been shown to be rougher because of plasma-etching reactions, which tend to increase the contact angle of the surface . In the present work, however, the contact angle decreased. This means that the chemical species attached to the surface have a greater influence than the physical changes made to the surface.
Figure 6 shows the results of using oxygen for the discharge gas. The untreated PTFE sample had a contact angle of 81.46° and the contact angle of the treated sample was 44.98°.
The contact angle after treatment is plotted against the treatment time in
Figure 7 plots the changes in the contact angle over time after treatment. The contact angle was the lowest when treated with oxygen plasma, but it increased rapidly with time. Treatments with argon and air plasma yielded the most-durable changes to the surface wettability, and nitrogen yielded the least-durable change. The radicals attached to the surface by the treatment tend to react with the ambient air and disappear over time, returning the PTFE surface to its original hydrophilic state.
C1s spectra from untreated and treated PTFE samples are shown in Fig. 8. The spectrum has peaks at the binding energies of 292 and 284 eV, which correspond to (CF2) and (C–C) bonds in PTFE, respectively. Figure 9 shows the F1s spectra, with a main peak at 689 eV, which represents the (F–C–F) structure in PTFE [22,23].
The surface compositions of the PTFE samples as measured by XPS are shown in
Surface treatments using gliding arc plasma to make the surface of PTFE film hydrophilic were tested with several discharge gases. The surface can be made hydrophilic by gliding arc plasma treatment while leaving the surface morphology intact. Oxygen proves to be the best discharge gas for efficiently changing the wettability of the surface. During the treatment, reactive species such as OH radicals are attached to the PTFE film surface. Over time, however, these radicals react with ambient air so that the contact angle increases over time until the surface returns to being hydrophobic. Analysis of changes to the PTFE surface show that the surface wettability can be changed easily by a gliding arc plasma under atmospheric pressure.
This project is supported by the R&D Center for the Reduction of non-CO2 Greenhouse Gases (201700 2430003) funded by Korea Ministry of Environment (MOE) as a Global Top Environment R&D Program and the National Research Foundation of the Republic of Korea (NRF-2015M2B2A9030393).
Contact angles of untreated and treated PTFE using different discharge gases.
Enthalpies of dissociation and ionization of the discharge-gas components.
|O2 (kJ/mol)||O (kJ/mol)||N2 (kJ/mol)||N (kJ/mol)||Ar (kJ/mol)|
|Dissociation (M2 → 2M)||498||-||945||-||-|
|Ionization (P → P+ + e)||1167||1310||1505||1402||1520|
Contact angles for different treatment times under oxygen gas.
|Treatment time||30 s||1 min||5 min||10 min||15 min||30 min|
Atomic composition of untreated and treated PTFE film by XPS analysis.
|Atomic percentage (%)||Elemental ratio|