BIBECHANA 18 (1) (2021) 19-25 19 BIBECHANA ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Department of Physics, Mahendra Morang A.M. Campus, TU, Biratnagar, Nepal Surface modification of polyamide by 50 Hz dielectric barrier discharge (DBD) produced in air at atmospheric pressure Rajesh Prakash Guragain1*, Hom Bahadur Baniya1,2, Santosh Dhungana1, Bishnu Prasad Pandey3 Ujjwal Man Joshi1, Deepak Prasad Subedi1 1Department of Physics, School of Science, Kathmandu University, Dhulikhel, Nepal 2Department of Physics, Tri-Chandra College, Tribhuvan University, Kathmandu, Nepal 3Department of Chemical Science and Engineering, Kathmandu University, Dhulikhel, Nepal *Email: rayessprakash@gmail.com Article Information: Received: February 14, 2020 Accepted: June 15, 2020 Keywords: Polyamide (PA) Contact Angle Surface Free Energy Hydrophilicity Optical Characterization ABSTRACT Industrial applications of the dielectric barrier discharge (DBD) have a long tradition. However, lack of understanding in some of its fundamental issues, such as the stochastic behaviors, is still a challenge for DBD researchers. The work was carried out at line frequency, 15 kV and at atmospheric pressure. This work focuses on the study of the electrical and optical characteristics of DBD at atmospheric pressure to determine a suitable condition for utilization of the device for surface modification of polyamides (PA) (Nylon 6/6). In this work, films were treated by dielectric barrier discharge and the effects on the morphology and chemistry of the material was studied. Surface characteristics were examined via contact angle measurements and SEM. The wettability tests revealed the improvement of the hydrophilic character of the surface of polyamide films as the water contact angle measured after the plasma treatments significantly decreased. The corresponding changes of the total surface energy revealed a significant increase in its polar component. The improvement of the wettability of PA strongly depends on the treatment time. The outcomes of the experiments proved that the modification of surface properties via plasma treatment reach to its saturation point after certain treatment time thus reducing the necessity of further treatment. DOI: https://doi.org/10.3126/bibechana.v18i1.27650 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons.org/licenses/by-nc/4.0/ 1. Introduction Discharge characterization of dielectric barrier discharge (DBD) are often done using both numerical and experimental methods [1,2]. The scientific method includes techniques like optical and electrical means. The high electron temperature in nonthermal plasma causes changes on the surface properties of treated sample while its bulk properties remaining unaffected because of the low http://nepjol.info/index.php/BIBECHANA mailto:rayessprakash@gmail.com https://doi.org/10.3126/bibechana.v18i1.27650 https://creativecommons.org/licenses/by-nc/4.0/ Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 20 ion temperature [3,4]. Most of the polymers treated industrially are heat sensitive, so treating them with hot plasma doesn't yield good results [5]. Here we discuss about the characterization of DBD and its application on the surface modification of polyamide at gas pressure condition. The surface modification of polymer includes change in wettability and adhesion as well as the enhancement or reduction of friction on the polymer surface [6]. The surface energy of the polymer can be either increased or decreased according to the need of the experiment and the functional groups can be either added or wiped out from the surface. Surface energy is a very important property for applications of polymers. The low surface energy of polyamide causes that its adhesive properties are insufficient for bonding and printing. The poor adhesion of Polyamide to more polar polymers and other substrates represents a controversy, which might be solved with modification of the polymer [7]. For this reason, the securing of fine adhesion of printing inks or adhesives to PA surface necessitates to boost its surface energy by a convenient method of modification. So, industrial plasma treatment involves continuous and rapid treatment of huge quantity of polymeric sample every day. Reduction of treatment time of such polymeric samples by a few milliseconds can finally result out to the reduction of the treatment of thousands of samples by few minutes. So, determining optimal treatment time of the polymers sample is therefore, very important for industries which is the primary objective of the research. 2. Materials and Method The experimental setup of the research is as shown as in Fig. 1. A cylindrical polycarbonate tube is used as the plasma chamber. The PC chamber is preferred due to its transparent nature allowing the experimentalist to see what actually is happening inside the discharge chamber. Two circular brass electrodes, each diameter 5 cm and thickness 1 cm are 3.5 mm apart and a polycarbonate sheet of thickness 2mm is used as dielectric. The working voltage for the entire experiment is set to be 15000 V. Current and voltage waveform are obtained using the digital oscilloscope (Tektronix TDS2000). A high voltage probe (PINTEK HVP- 28HF) is used for the determination of voltage across the electrodes. Similarly, Optical characterization of DBD at both the conditions is carried using ocean optics (USB2000+). The samples of polyamides (Nylon 6/6) are cut into (3 cm Γ— 1.5 cm) rectangular pieces. They are then washed by isopropanol and put into an ultrasonic bath for further cleaning. Samples are placed on the polycarbonate sheet between the electrodes. Thus, treated sample are taken for surface characterization. Contact angle was measured using a contact angle goniometer (Rame-Hart Model 200). In this work, LEO (500) /Zeiss Field- emission scanning electron microscope (SEM) was used to examine surface morphology. Fig. 1: Schematic diagram of the experimental Setup. 3. Results and Discussion Electrical characterization The value of electron density (𝑛𝑒) has been calculated by using Power Balance Method [8,9]. Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 21 This method is based upon the principle that the energy lost by the plasma parameters is equal to the power delivered by the source. The balance between the input power from high voltage power supply (P) and power lost in the plasma leads to. ............(1) 2 ab e b lost P n Av E = Fig. 2: Electrical characterization of DBD at atmospheric condition. Fig.2 shows the current and voltage waveform as a function of time of the dielectric barrier discharge generated using line frequency. The calculation was made by using the values given below: Applied voltage = 15000 𝑉, Discharge current = 0.0386 π‘šπ΄, Inter- Electrode separation = 3.5 π‘šπ‘š, Applied frequency = 50 𝐻𝑧, Area of each electrode = 20.43 π‘π‘š2, Bohm velocity (𝑣𝑏) = 2 Γ— 105 π‘π‘š/𝑠, πΈπ‘™π‘œπ‘ π‘‘ = 50 𝑒𝑉 (in our condition) and π‘ƒπ‘Žπ‘ is the total power absorbed by the capacitively coupled plasma. Using these values, the electron density (𝑛𝑒) was found to be 2.52 Γ— 1013π‘π‘šβˆ’3 at atmospheric condition. Optical characterization For optical characterization of DBD, a small hole is made on the cylindrical polycarbonate tube and, an optical fiber is inserted there. The discharge of the DBD is made to pass through the optical fibre and the spectra is recorded by the Ocean Optics (USB2000+) at atmospheric conditions. Fig. 3 shows the spectra of the discharge and their corresponding intensities, and wavelength at an atmospheric pressure condition. Fig. 3: Spectra of the discharge at atmospheric pressure. In this method four suitable lines; two for NII (204.02 nm, 655.85 nm) and two for N III (245.21nm, 378.57 nm) were chosen from spectral lines of nitrogen obtained from the discharge. The optical characterization is carried out using line intensity ratio method [10,11]. 𝑅1 𝑅2 = 𝐼1 𝐼2⁄ 𝐼3 𝐼4⁄ = ( π΄π‘π‘ž 𝐴π‘₯𝑦 ) ( 𝑔𝑝 𝑔π‘₯ ) ( πœ†π‘₯𝑦 πœ†π‘π‘ž ) ( 𝐴𝑒𝑣 π΄π‘Ÿπ‘  ) ( 𝑔𝑒 π‘”π‘Ÿ ) ( πœ†π‘Ÿπ‘  πœ†π‘’π‘£ ) 𝑒π‘₯𝑝 [βˆ’ πΈπ‘βˆ’πΈπ‘₯βˆ’πΈπ‘Ÿ+𝐸𝑒 𝐾𝐡𝑇𝑒 ]………... (2) Here, in equation (2), 𝑅 is the ratio of the intensity of two lines, 𝐼 is the intensity of the spectral line, 𝐴𝑖𝑗 is the transition probability of the transition 𝑖 ⟢ 𝑗, 𝑔𝑖 is the statistical weight of the upper level, πœ† is the wavelength of the line radiation, 𝐸𝑖 is the energy of the upper level, 𝐾𝐡 is Boltzmann constant and 𝑇𝑒 is the electron temperature. The values of πœ† and 𝐼 are obtained from the observation, and the values of 𝐴𝑖𝑗 , 𝑔𝑖 and 𝐸𝑖 are obtained Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 22 from the National Institute of Standards and Technology (NIST) Atomic Spectra Database [12]. Table 1: shows the values of transition probability (𝐴𝑗𝑖), statistical weight of upper level (𝑔𝑖) and energy of the upper level (𝐸𝑖) for NII and NIII lines obtained from NIST database. Nitrogen Lines (𝝀) Transition probability (π‘¨π’‹π’Š) (π‘Ίβˆ’πŸ) Statistical weight of upper level (π’ˆπ’Š) Energy of upper level (π‘¬π’Š) (eV) NII (204.02nm) 2.11Γ—107 5 21.62 NII (655.85nm) 1.43Γ—106 5 41.73 NIII (245.21nm) 1.21Γ—107 6 23.26 NIII (378.57nm) 2.40Γ—106 5 40.59 Using the above data in equation (3.6) mentioned above, we obtain, 𝑅1 𝑅2 = 0.506𝑒π‘₯𝑝 [ 2.78 𝐾𝐡𝑇𝑒 ] Taking 𝑇𝑒 = 0.5 𝑒𝑉 , corresponding intensity ratio 𝑅1 𝑅2 = 131.4. Similarly, the other values of 𝑇𝑒 and their corresponding intensity ratio 𝑅1 𝑅2 are listed in Table 2. Table 2: Value of 𝑅1 𝑅2 for different 𝑇𝑒. Electron Temperature (eV) Intensity Ratio π‘ΉπŸ π‘ΉπŸ 0.5 131.47 0.6 52.04 0.7 26.84 0.8 16.34 0.9 11.10 1 8.15 1.1 6.33 1.2 5.13 1.3 4.29 1.4 3.68 1.5 3.22 1.6 2.87 1.7 2.59 1.8 2.37 Fig. 4: Optical characterization of DBD at atmospheric condition. Fig.4 is the graph plotted between electron temperature 𝑇𝑒 and corresponding intensity ratio 𝑅1 𝑅2 . This graph is used to determine the electron temperature using the value of 𝑅1 𝑅2 obtained from the intensity ratio. From the OES spectra, the value of the ratio of the intensity of spectral lines is found to be 8.58 which corresponds to the electron temperature of 0.97 eV. Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 23 Contact angle and surface free energy measurements The static contact angle measurements are made before the treatment and immediately after the treatment by dropping 4 πœ‡π‘™ of distilled water (H2O) and glycerol (C3H8O3) on the surface of the PA samples. For smooth and homogeneous surface, the water contact angle and surface free energy is measured at the equilibrium according to the Young's equation, and Owens Wendt Kaeble methods respectively [13,14]. cos ..........(3)sv sl lv     βˆ’ = where, sv is the surface free energy of the solid substrate, sl is the interfacial tension between the solid and the liquid and lv is the surface tension of the liquid. For two liquids i and j , ( ) ( ) 1 1 2 2(1 cos ) 2 2 ......(4)d d p p li i li s li s     + = + ( ) ( ) 1 1 2 2(1 cos ) 2 2 ......(5)d d p p lj j lj s lj s     + = + Surface free energy (Ξ³) was calculated in terms of polar p s and dispersive d s components. The addition of these two components eventually gives the total surface free energy of the solid. Fig. [5, 6] show the variation of water contact angle and surface free energy of PA surface as the function of treatment time. The contact angle is found to decrease with the increase in treatment time. After few minutes it finally saturates. Treatment of PA by atmospheric pressure dielectric barrier discharge is able to change its contact angle from 57.510 (control) to a minimum value of 29.010 after 30 seconds. When the treatment time exceeds 30 seconds, the measured water contact angle nearly seems to reach a saturation state suggesting that the physical and chemical changes induced Fig. 5: Contact angle measurement at atmospheric pressure. Fig. 6: Surface energy measurement of Polyamide at atmospheric pressure. by the plasma is also in saturation state [15]. Also, the treatment of PA by atmospheric plasma is able to change its surface energy from 49.84 mJ/m2 (control) to about 64.91 mJ/ m2. Similar trend is also observed for the polar component and it is mainly due to the incorporation of the polar species such as carbonyl (C=O) and carboxyl (-COOH) groups on the treated PA surface. The dispersive Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 24 component does not have any contribution to increase the wettability of the PA surface [16]. It is found that the further treatment of PA also does not cause any significant changes in the surface free energy reducing the requirement of further treatment. SEM analysis Fig. [7-8] show the SEM micrographs of control and plasma treated PA films at atmospheric pressure. As can be seen in the micrograph, the untreated polymer surfaces is characterized by organized lamellae. This texture of the untreated sample of the polymer pertains to the process of the polymer film production (which is possibly induced by the film blow procedure during production) and not due to plasma treatment. It can be seen that the surfaces of the air plasma treated polymer is changed with slightly increased non-uniform roughness compared to the untreated polymers. The gradual increase in the particle grain size with the image scan area can be attributed to the etching of the polymer surface by plasma treatment [17]. Fig. 7: SEM micrograph of control sample of Polyamide. Fig. 8: SEM micrograph of plasma treated sample of Polyamide at 1 minute. 4. Conclusions The results of this research showed that a relatively homogeneous discharge can be achieved at an atmospheric pressure condition. Treatment of PA using dielectric barrier resulted in improvement on hydrophilicity of the polymeric material. The improvement of wettability of PA strongly depends on the treatment time. During the experiment, contact angle of polymers after plasma treatment was found to decrease effectively whereas, corresponding surface free energy was increased. SEM micrographs of the plasma treated PA samples revealed the change in the surface morphology of the film. It was also seen that the change in surface properties of the treated sample reached its saturation point, eliminating the requirement of further treatment. So, the polymer industries can therefore save the energy by calculating the optimal treatment time for the sample over which there is noticeable change on the surface properties. Rajesh Prakash Guragain et al. / BIBECHANA 18 (1) (2021) 19-25 25 References [1] M. Petit, N. Jidenko, A. Goldman, M. Goldman & J.P. Borra, Electrical characterization of gas discharges using a numerical treatment. Application to dielectric barrier discharges, Review of Scientific Instruments. 73 (2002) 2705-2712. https://doi.org/10.1063/1.1484236 [2] G. Nersisyan and W. G. Graham, Characterization of a dielectric barrier discharge operating in an open reactor with flowing helium, IOP Publishing Ltd, Plasma Sources Science and Technology. 13 (2004) 582-587. https://doi.org/10.1088/0963-0252/13/4/005 [3] M. Kuzuya, T. Izumi, Y. Sasai, & S. Kondo, Sodium carboxylate effect of non-cross-linked hydrogel on plasma-induced radical formation as studied by electron spin resonance, Thin Solid Films. 457 (2004)12–19. https://doi.org/10.1016/j.tsf.2003.12.032 [4] H. B. Baniya, R. Shrestha, R. P. Guragain, M. B. Kshetri, B. P. Pandey, & D. P. Subedi, Generation and Characterization of an Atmospheric-Pressure Plasma Jet (APPJ) and Its Application in the Surface Modification of Polyethylene Terephthalate, International Journal of Polymer Science. (2020) 1–7. doi:10.1155/2020/9247642 [5] P. Fabbri, & M. Messori, Surface Modification of Polymers, Modification of Polymer Properties. (2017) 109–130. https://doi.org/10.1016/C2014-0-02434-3 [6] D. Hegemann, H. Brunner, & C. Oehr, Plasma treatment of polymers for surface and adhesion improvement, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms. 208 (2003) 281–286. https://doi.org/10.1016/S0168-583X(03)00644-X [7] I. NovΓ‘k, A. Popelka, M. ValentΓ­n, I. ChodΓ‘k, M. Ε pΓ­rkovΓ‘, A. TΓ³th, M. MarΓ΄nek, Surface Behavior of Polyamide 6 Modified by Barrier Plasma in Oxygen and Nitrogen, International Journal of Polymer Analysis and Characterization. 19 (2014) 31–38. https://doi.org/10.1080/1023666X.2014.850907 [8] N. Balcon, A. Aanesland, & R. Boswell, Pulsed RF discharges, glow and filamentary mode at atmospheric pressure in argon, Plasma Sources Science and Technology. 16 (2007) 217–225. https://doi.org/10.1088/0963-0252/16/2/002 [9] R. P. Guragain, S. Gautam, R. Shrestha, & D. P. Subedi, Surface Modification of Polycarbonate by Treatment with 50Hz Dielectric Barrier Discharge at Near Atmospheric Pressure, International Journal of Science and Research (IJSR). 5 (2016) 1468-1470. [10] A. Sarani, A. Y. Nikiforov, N. D. Geyter, R. Morent, C. Leys, Characterization of an atmospheric pressure plasma jet and its application for treatment of non-woven textiles, Proceeding of the 20th International Symposium on Plasma Chemistry, PA, USA. (2011). http: //hdl.handle.net/1854/LU-1339873. [11] H. B. Baniya, R. P. Guragain, B. Baniya, G. Qin, and D. P. Subedi, Improvement of hydrophilicity of polyamide using atmospheric pressure plasma jet, BIBECHANA. 17 (2020) 133-138. https://doi.org/10.3126/bibechana.v17i0.26869 [12] A. Kramida, Yu. Ralchenko, J. Reader, & NIST ASD Team, NIST Atomic Spectra Database (ver. 5.7.1) (2019). [Online]. https://doi.org/10.18434/T4W30F [13] Y. Yuan & T. R. Lee, Contact Angle and Wetting Properties, Springer Series in Surface Sciences. (2013) 3–34. https://doi.org/10.1007/978-3-642-34243-1_1 [14] R. P. Guragain, S. Gautam. D. P. Subedi, & R. Shrestha, Effect of Plasma Treatment on the Surface of Polyethylene Terephthalate with 50Hz Dielectric Barrier Discharge at Near-Atmospheric Pressure, International Journal of Recent Research and Review (IJRRR). IX (2016) 34-37 [15] D. Pappas, A. Bujanda, J. D. Demaree, J. K. Hirvonen, W. Kosik, R. Jensen, & S. McKnight, Surface modification of polyamide fibers and films using atmospheric plasmas, Surface and Coatings Technology.201(2006)4384–4388. https://doi.org/10.1016/j.surfcoat.2006.08.068 [16] Z. Gao, Modification of surface properties of polyamide 6 films with atmospheric pressure plasma, Applied Surface Science. 257 (2011) 6068–6072. https://doi.org/10.1016/j.apsusc.2011.01.132 [17] Z. Gao, S. Peng, J. Sun, L. Yao, & Y. Qiu, Influence of processing parameters on atmospheric pressure plasma etching of polyamide 6 films, Applied Surface Science. 255 (2009)7683–7688. https://doi.org/10.1016/j.apsusc.2009.04.137 https://doi.org/10.1063/1.1484236 https://iopscience.iop.org/journal/0963-0252 https://doi.org/10.1088/0963-0252/13/4/005 https://doi.org/10.1016/j.tsf.2003.12.032 https://doi.org/10.1016/C2014-0-02434-3 https://doi.org/10.1016/S0168-583X(03)00644-X https://doi.org/10.1080/1023666X.2014.850907 https://doi.org/10.1088/0963-0252/16/2/002 https://doi.org/10.3126/bibechana.v17i0.26869 https://doi.org/10.18434/T4W30F https://doi.org/10.1007/978-3-642-34243-1_1 https://doi.org/10.1016/j.surfcoat.2006.08.068 https://doi.org/10.1016/j.apsusc.2011.01.132 https://doi.org/10.1016/j.apsusc.2009.04.137