- Email: [email protected]
Fabrication of a thin-layer PTFE coating exhibiting superhydrophobicity by supercritical CO2
Progress in Organic Coatings 111 (2017) 322–326
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Fabrication of a thin-layer PTFE coating exhibiting superhydrophobicity by supercritical CO2 Zhen-Xiu Zhang, Tao Zhang, Xin Zhang, Zhenxiang Xin, K. Prakashan
MARK
⁎
Key Laboratory of Rubber-Plastics, Ministry of Education /Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Thin-film deposition PTFE Supercritical carbon dioxide Superhydrophobicity
A new process for deposition of a thin-layer coating of Poly(tetrafluoroethylene) (PTFE) on a glass substrate exhibiting superhydrophobicity using supercritical carbon dioxide (sc-CO2) was reported. The process involved placing a PTFE source material and the glass substrate to be coated inside an autoclave in supercritical carbon dioxide (sc-CO2), maintained at 300 °C temperature and 24 MPa, for a time period. The variations in the surface morphology and wetting characteristics of both the coated glass substrate and the PTFE source specimen with increasing time period of the process have been investigated. The morphology of the surfaces was investigated by scanning electron microscopy while the wetting characteristics by contact angle measurement. The surface of the glass substrates was found to be increasingly deposited with minute globulets of PTFE with increasing time of the process. The static water contact angle measured on the coated substrates increased with increasing coating time and the coated substrates showed superhydrophobicity when the coating time exceeds a certain time period. The PTFE source specimen surface became increasingly rough with micron-sized surface swellings and protrusions appearing on the surface with increasing time of the process and showed superhydrophobicity when exposed in the supercritical environment for certain time period. The method may find use in fabricating a thinlayer coating of PTFE for potential microelectronics, microfluidics and sensor applications.
1. Introduction The poly(tetrafluroethylene) (PTFE) is a specialty polymer exhibiting a unique combination of excellent mechanical properties, thermal, chemical and electrical resistance properties, low coefficient of friction and low surface energy over a wide temperature range. A thinlayer of the PTFE with a thickness in the nano- to micro-meter range as applied on certain substrates has applications in the fields such as microelectonics [1], microfluidics [2], and sensors [3–5]. Because of the high chemical resistance of the PTFE polymer, its solution coating is difficult. In the well known PTFE coating applications, such as the nonstick cookware coatings and chemical and temperature resistant industrial coatings, the PTFE is applied as a thick layer (> 1 mm thickness). Such coating processes normally involve spraying of PTFE powders in the dry form or as dispersed in a solvent medium onto the substrate and the coated material then undergoing a high temperature sintering or baking step [6]. However, this method is not suitable for fabricating a thin-layer coating of PTFE for various micron-scale devices. Thin films of Teflon-like coatings have been deposited from PTFE target material sources using methods such as pulsed laser deposition
⁎
Corresponding author. E-mail address: [email protected] (K. Prakashan).
http://dx.doi.org/10.1016/j.porgcoat.2017.06.019 Received 14 June 2016; Received in revised form 3 March 2017; Accepted 12 June 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
[5,7], RF plasma sputtering [8], ion beam sputtering [9], RF-magnetron sputtering [10] and synchrotron radiation photodecomposition [11]. In these methods, in general, a PTFE target is bombarded with either high energy photons or plasma ions to cause disintegration and ablation of the target and the molecular fragments thereby produced then form a coating on the substrate to be coated. The supercritical carbon dioxide (sc-CO2), a fluid state of CO2 when it is held above its critical temperature and pressure (31.0 °C and 7.38 MPa, respectively [12]), is increasingly finding use as a ‘green solvent’ in many industrial processes because of its non-toxicity and other advantages [13–15]. A study by Vopilov et al. [16] reported successful fractionation of low molecular weight PTFE molecules from a recycled PTFE material described as ultradispersed polytetrafluoroethylene (UPTFE) powder material using sc-CO2 but the researches were not able to separate any low molecular weight fractions from virgin PTFE using the same method. In this paper we report a new method for fabrication of a thin-layer coating of PTFE exhibiting superhydrophobicity on a glass substrate, from a PTFE specimen material using sc-CO2 medium. The glass substrate to be coated and the PTFE source specimen were placed, without directly touching each other, in
Progress in Organic Coatings 111 (2017) 322–326
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duration and the syringe pump was activated to maintain the pressure at 24 MPa whenever a pressure drop noted. After the conclusion of each coating experiment conducted for the specific time duration, the glass substrate and the PTFE source specimen were taken out and investigated for their surface morphology and wetting characteristics.
sc-CO2 environment in an autoclave at the temperature 300 °C and pressure 24 MPa for varying time periods. The changes in the surface morphology and wetting characteristics of both the coated substrate and the PTFE source specimen were investigated after the varying time durations. The surface morphology has been investigated by scanning electron microscopy (SEM) while the wetting characteristics by measuring static water contact angle on the surfaces. The chemical nature of the coating deposited on the glass substrate was investigated by Energy Dispersive X-ray spectroscopy attached with the SEM machine (SEM-EDX). The changes parallelly occurring in the surface morphology and the wetting characteristics of the glass substrate and the PTFE source specimen with increasing time of the process have been correlated and discussed.
2.2. Characterization The surface morphology of both the coated substrate and the PTFE source specimen was investigated by a field emission scanning electron microscope (FESEM, Philips XL-30 FEG). The chemical nature of the coating deposited on the glass substrate was investigated by using the energy dispersive X-ray spectroscopy (EDX) facility of the SEM instrument. The static water contact angles of the glass substrate and the PTFE source specimen were measured using a contact angle measurement system (JC2000D, Shanghai Chenzhong Digital Technology Limited, China). The measurements were done on distilled water droplets with a volume of 5 μl placed at varying locations on the surfaces at room temperature and pressure. Contact angles of five droplets were measured for each surface and the mean value was reported.
2. Experimental methods 2.1. Materials and process The PTFE material used as the source for the PTFE deposition was the DF-22 grade sheet material obtained from Dongyue Group Ltd, China. Thin slices less than 2 mm in thickness were cut from a PTFE sheet of 6 mm thickness and about 15 g of the sliced material was used as the source material for each coating experiment. Cleaned glass plates with dimensions 5 cm × 2 cm × 2 mm were used as the substrates for the coating process. A schematic of the experimental set up used for the process is shown in Fig. 1. It comprises an autoclave connected to a CO2 gas cylinder through a syringe pump. The autoclave has an inner volume of 80 cm3 and capacity to withstand a maximum pressure of 50 MPa and can be electrically heated up to a maximum temperature of 400 °C. The syringe pump of the set up is an electrically driven pressure pump with capacity to build up the CO2 pressure inside the autoclave up to 50 MPa. When the autoclave was heated to 300 °C temperature, the PTFE source specimen and the cleaned glass substrate were placed, without touching each other, inside the autoclave and its lid tightly closed. The CO2 gas was then pumped into the autoclave using the syringe pump and after exhausting the air inside for a while the outlet valve of the autoclave closed to build up the pressure inside to 24 MPa. When the pressure was reached to 24 MPa, the CO2 pumping stoped and the inlet valve closed. The set up was then kept at the temperature and pressure conditions for varying time periods of 12, 24, 36, 48, 60 and 72 hours (h). The set up was monitored during the entire process
3. Results and discussions The SEM micrographs of the coated glass substrates obtained after the coating experiments conducted for the different time periods are shown in Fig. 2. The SEM micrographs show that the glass substrates have increasingly covered with minute spherical shaped globulets when the time duration of the process increased. The amount of deposition as well as the surface coverage of the coating increased with increasing time of the process. The diameters of the deposited globulets vary roughly in the range of a few tens of nanometers to a few hundreds of nanometers. The values of the static water contact angles measured on these coated substrates are also shown in the respective SEM micrographs in Fig. 2. The static water contact angle measured on the coated substrates increased when the coating time increased. The contact angle shows a rapid increase from 123° to 148°, when coating time increased from 12 h to 36 h, but further increase in contact angle is small when coating time further increased. All the substrates that coated for more than 48 h have shown a contact angle greater than 150°. Surfaces showing a contact angle greater than 150° are generally considered as superhydrophobic. The superhydrophobicity of the coated substrates is Fig. 1. The schematic of the experimental set up used for the PTFE deposition process.
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Fig. 2. The SEM micrographs of the glass substrates coated for the different time periods: (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h (e) 60 h, and (f) 72 h.
In order to gauge the thickness of the coating deposited on the glass substrate the cross-sections of the coated glass substrates were observed using SEM. The cross-sectional SEM micrograph of a coated surface (coated for 48 h) is shown in Fig. 4. The heights of the deposited globulets vary but in the sub-micron scale. The thickness of the coating estimated is roughly about 0.25 μm for the coating deposited for 48 h. The cross-sections of the PTFE source specimens also were observed using SEM. The protrusions or swellings appeared on the PTFE source have heights of about a few micrometers. The surface roughening happening on the PTFE source specimen and the globulets deposition happening on the glass substrate during the process are intimately related. When the PTFE specimen was kept in the sc-CO2 under the relatively high temperature (300 °C) and pressure (24 MPa) conditions for long time (more than 12 h), the sc-CO2 was able to penetrate and swell the PTFE specimen at least at its surface. Some amount of PTFE, possibly low molecular weight molecules, got dissolved in the sc-CO2 and subsequently got deposited onto the glass substrate. The dissolution of the PTFE molecules in sc-CO2 and their subsequent deposition on the glass substrate continue with increasing time during the process, favored by the positive entropy change for the dispersing out of the PTFE molecules from a compact state in the PTFE
because the coatings are based on PTFE, an inherently hydrophobic material and there is a special micron-scale surface roughness on the coated surface. The SEM micrographs of the PTFE specimens used as the source material for the coating process carried out for varying time periods are shown in Fig. 3. The surface of the PTFE source specimen became increasingly rough with minute swellings and protrusions appearing on the surface as the time duration of the process increased. The surface swellings and protrusions seen on the PTFE source specimens have sizes in the range of a few micrometers as seen in the SEM micrographs of Fig. 3. The static water contact angles measured on these PTFE source specimens are shown in their respective SEM micrographs in Fig. 3. The contact angle on the PTFE source specimen increased with increasing time duration of the process. The contact angle rapidly increased from 121° to 151° when the coating time increased from 12 h to 36 h, but further raise in contact angle with increasing coating time is small. All the PTFE source specimens exposed to the coating environment for more than 36 h have shown a contact angle greater than 150°, showing superhydrophobicity. This is because when a micron-scale roughness is formed on the PTFE surface as a result of the sc-CO2 exposure, its hydrophobicity is greatly enhanced to become superhydrophobic.
Fig. 3. The SEM micrographs of the PTFE source specimens after the varying sc-CO2 exposure times: (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h (e) 60, and (f) 72 h.
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Fig. 4. The cross-sectional SEM micrographs of the coated glass substrate (a) and PTFE source specimen (b) after 48 h coating process.
Fig. 5. The EDX spectra of the PTFE coated glass (a) and PTFE source specimen (b) after 48 h coating process, and PTFE specimen not subjected to the process (c).
this case, PTFE molecules, possibly the low molecular weight ones, getting dissolved in the sc-CO2 and subsequently getting deposited on the glass substrate without degradation. The chemical nature of the coated glass substrate, PTFE source specimen and the virgin PTFE not subjected to the coating process was analyzed using SEM-EDX. The EDX spectrum and the elemental
specimen. Unlike the other reported methods used for obtaining PTFE like coatings such as the pulsed laser deposition [5,7], RF plasma sputtering [8], ion beam sputtering [9], RF-magnetron sputtering [10] and synchrotron radiation photodecomposition [11], here the PTFE molecules as such are not subjected to a destructive treatment, but they are removed out from the source specimen by a dissolution process. In 325
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subsequently got deposited on to the glass substrate. The globulets kinds of deposition as well as detection of high percentage of elemental oxygen in the coating deposited indicated the presence of CO2 within the coating. The method may find use in fabricating thin-layer coating of PTFE materials for potential microelectronics, microfluidics and sensor applications.
composition of the coated substrate are shown in Fig. 5(a). The main elements detected on the coated surface are carbon (59.18%), fluorine (22.67%) and oxygen (16.98%). The carbon and fluorine are of the PTFE molecules. The high amount of oxygen detected is unexpected for pure PTFE and is possibly comes from the CO2 traped with in the coated globulets, which may have been slightly foamed as a result of it. The ability of sc-CO2 to induce surface swellings and protrusions on the PTFE source specimen is already seen in SEM micrographs in Fig. 3. The EDX spectrum of a surface protrusion on the PTFE source specimen that subjected to a 48 h coating process is shown in Fig. 5(b). In addition to carbon (37.23%) and fluorine (61.52%), small amount of oxygen (1.25%) is also detected in the surface protrusion on the PTFE source specimen, indicating that the swelling is caused by the sc-CO2 penetration. The EDX spectrum of the pure PTFE specimen (Fig. 5(c)), that is not subjected to the coating process, shows only the elements carbon (39.47%) and fluorine (60.53%) as expected. In the sc-CO2 process conditions employed here, the deposited globulets is expected to be swelled and foamed by the CO2, especially as the deposited PTFE possibly consists of low molecular weight molecules. The foaming of polymers by saturating the polymers with sc-CO2 at high pressure and then rapidly releasing the pressure is well known and is a rapidly developing application for the sc-CO2 [13–15]. However, under the temperature and pressure conditions applied here the sc-CO2 was not successful in foaming the PTFE source specimen, except causing the micron-sized swellings and protrusions on the specimen surface.
Acknowledgments This research was supported by China Postdoctoral Science Foundation (Grant No. 2013M531561) and Promotive research fund for excellent young and middle-aged scientists of Shandong Province (BS2013CL018). References [1] J. Wang, H.K. Kim, F.G. Shi, B. Zhao, T.G. Nieh, Thickness dependence of morphology and mechanical properties of on-wafer low-k PTFE dielectric films, Thin Solid Films 377–378 (2000) 413–417. [2] H. Andersson, W. van der Wijngaart, P. Griss, F. Niklaus, G. Stemme, Hydrophobic valves of plasma deposited octafluorocyclobutane in DRIE channels, Sens. Actuators B 75 (2001) 136–141. [3] M. Wienecke, M.C. Bunescu, M. Pietrzak, K. Deistung, P. Fedtke, PTFE membrane electrodes with increased sensitivity for gas sensor applications, Synth. Met. 138 (2003) 165–171. [4] M. Nebel, S. Neugebauer, H. Kiesele, W. Schuhmann, Local reactivity of diamondlike carbon modified PTFE membranes used in SO2 Sensors, Electrochim. Acta 55 (2010) 7923–7928. [5] G. Kecskeméti, B. Hopp, T. Smausz, Z. Tóth, G. Szabó, Production of porous PTFE–Ag composite thin films by pulsed laser deposition, Appl. Surf. Sci. 258 (2012) 7982–7988. [6] Fact Sheet, Applying Teflon® Coatings, http://www.rjchase.com/application_ guidelines.pdf. [7] S.T. Li, E. Arenholz, J. Heitz, D. Bäuerle, Pulsed-laser deposition of crystalline Teflon (PTFE) films, Appl. Surf. Sci. 125 (1998) 17–22. [8] D.S. Bodas, A.B. Mandale, S.A. Gangal, Deposition of PTFE thin films by RF plasma sputtering on < 1 0 0 > silicon substrates, Appl. Surf. Sci. 245 (2005) 202–207. [9] J.L. He, W.Z. Li, L.D. Wang, J. Wang, H.D. Li, Deposition of PTFE thin films by ion beam sputtering and a study of the ion bombardment effect, Nucl. Instrum. Methods Phys. Res. B 135 (1998) 512–516. [10] H.M. Kim, S. Sohn, J.S. Ahn, Transparent and super-hydrophobic properties of PTFE films coated on glass substrate using RF-magnetron sputtering and Cat-CVD methods, Surf. Coat. Technol. 228 (2013) S389–S392. [11] T. Katoh, Y. Zhang, Deposition of Teflon-polymer thin films by synchrotron radiation photodecomposition, Appl. Surf. Sci. 138–139 (1999) 165–168. [12] R. Span, W. Wagner, A new equation of state for carbon dioxide covering the fluid region from the triple-Point temperatureto 1100 K at pressures up to 800 MPa, J. Phys. Chem. Ref. Data 25 (1996) 1509. [13] C. Boyère, C. Jérôme, A. Debuigne, Input of supercritical carbon dioxide to polymer synthesis: an overview, Eur. Polym. J. 1 (61) (2014) 45–63. [14] M. Sauceau, J. Fages, A. Common, C. Nikitine, E. Rodier, New challenges in polymer foaming: a review of extrusion processes assisted by supercritical carbon dioxide, Prog. Polym. Sci. 36 (2011) 749–766. [15] E. Kiran, Supercritical fluids and polymers – the year in review – 2014, J. Supercrit. Fluids 110 (2016) 126–153. [16] Yu E. Vopilov, L.N. Nikitin, G. Yu Yurkov, E.P. Kharitonova, A.R. Khokhlov, V.M. Bouznik, Effect of supercritical carbon dioxide on ultradispersed polytetrafluoroethylene, J. Supercrit. Fluids 62 (2012) 204–210.
4. Conclusion A new process for depositing a superhydrophobic thin-layer coating of PTFE on a glass substrate was described. The process involved keeping a PTFE source specimen and the glass substrate to be coated under sc-CO2 at the temperature of 300 °C and pressure of 24 MPa for certain time period. The coating deposited on the glass substrate consisted of minute spherical shaped globulets of PTFE with sizes varying from a few tens of nanometers to a few hundreds of nanometers. The coating deposition as well as the coating coverage on the substrate increased with increasing time duration of the process and this was reflected on the static water contact angle measured on the coated substrates. The static water contact angle increased with increasing coating time and all substrates coated for more than 48 h showed a water contact angle greater than 150°. The superhydrophobicity of the coated substrates was attributed to the micron-scale surface roughness of the coated surfaces and to the inherent hydrophobicity of the PTFE. The PTFE source specimen became increasingly rough with time during the process as micron sized swellings and protrusions appeared on its surface. All PTFE source specimens subjected to the sc-CO2 process for more than 36 h have a contact angle greater than 150°. Under the relatively high temperature and pressure conditions employed in the process, the sc-CO2 was able to penetrate and swell the PTFE specimens, at least at their surfaces, and dissolve some amount of PTFE which
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Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Fabrication of a thin-layer PTFE coating exhibiting superhydrophobicity by supercritical CO2 Zhen-Xiu Zhang, Tao Zhang, Xin Zhang, Zhenxiang Xin, K. Prakashan
MARK
⁎
Key Laboratory of Rubber-Plastics, Ministry of Education /Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, 266042, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Thin-film deposition PTFE Supercritical carbon dioxide Superhydrophobicity
A new process for deposition of a thin-layer coating of Poly(tetrafluoroethylene) (PTFE) on a glass substrate exhibiting superhydrophobicity using supercritical carbon dioxide (sc-CO2) was reported. The process involved placing a PTFE source material and the glass substrate to be coated inside an autoclave in supercritical carbon dioxide (sc-CO2), maintained at 300 °C temperature and 24 MPa, for a time period. The variations in the surface morphology and wetting characteristics of both the coated glass substrate and the PTFE source specimen with increasing time period of the process have been investigated. The morphology of the surfaces was investigated by scanning electron microscopy while the wetting characteristics by contact angle measurement. The surface of the glass substrates was found to be increasingly deposited with minute globulets of PTFE with increasing time of the process. The static water contact angle measured on the coated substrates increased with increasing coating time and the coated substrates showed superhydrophobicity when the coating time exceeds a certain time period. The PTFE source specimen surface became increasingly rough with micron-sized surface swellings and protrusions appearing on the surface with increasing time of the process and showed superhydrophobicity when exposed in the supercritical environment for certain time period. The method may find use in fabricating a thinlayer coating of PTFE for potential microelectronics, microfluidics and sensor applications.
1. Introduction The poly(tetrafluroethylene) (PTFE) is a specialty polymer exhibiting a unique combination of excellent mechanical properties, thermal, chemical and electrical resistance properties, low coefficient of friction and low surface energy over a wide temperature range. A thinlayer of the PTFE with a thickness in the nano- to micro-meter range as applied on certain substrates has applications in the fields such as microelectonics [1], microfluidics [2], and sensors [3–5]. Because of the high chemical resistance of the PTFE polymer, its solution coating is difficult. In the well known PTFE coating applications, such as the nonstick cookware coatings and chemical and temperature resistant industrial coatings, the PTFE is applied as a thick layer (> 1 mm thickness). Such coating processes normally involve spraying of PTFE powders in the dry form or as dispersed in a solvent medium onto the substrate and the coated material then undergoing a high temperature sintering or baking step [6]. However, this method is not suitable for fabricating a thin-layer coating of PTFE for various micron-scale devices. Thin films of Teflon-like coatings have been deposited from PTFE target material sources using methods such as pulsed laser deposition
⁎
Corresponding author. E-mail address: [email protected] (K. Prakashan).
http://dx.doi.org/10.1016/j.porgcoat.2017.06.019 Received 14 June 2016; Received in revised form 3 March 2017; Accepted 12 June 2017 0300-9440/ © 2017 Elsevier B.V. All rights reserved.
[5,7], RF plasma sputtering [8], ion beam sputtering [9], RF-magnetron sputtering [10] and synchrotron radiation photodecomposition [11]. In these methods, in general, a PTFE target is bombarded with either high energy photons or plasma ions to cause disintegration and ablation of the target and the molecular fragments thereby produced then form a coating on the substrate to be coated. The supercritical carbon dioxide (sc-CO2), a fluid state of CO2 when it is held above its critical temperature and pressure (31.0 °C and 7.38 MPa, respectively [12]), is increasingly finding use as a ‘green solvent’ in many industrial processes because of its non-toxicity and other advantages [13–15]. A study by Vopilov et al. [16] reported successful fractionation of low molecular weight PTFE molecules from a recycled PTFE material described as ultradispersed polytetrafluoroethylene (UPTFE) powder material using sc-CO2 but the researches were not able to separate any low molecular weight fractions from virgin PTFE using the same method. In this paper we report a new method for fabrication of a thin-layer coating of PTFE exhibiting superhydrophobicity on a glass substrate, from a PTFE specimen material using sc-CO2 medium. The glass substrate to be coated and the PTFE source specimen were placed, without directly touching each other, in
Progress in Organic Coatings 111 (2017) 322–326
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duration and the syringe pump was activated to maintain the pressure at 24 MPa whenever a pressure drop noted. After the conclusion of each coating experiment conducted for the specific time duration, the glass substrate and the PTFE source specimen were taken out and investigated for their surface morphology and wetting characteristics.
sc-CO2 environment in an autoclave at the temperature 300 °C and pressure 24 MPa for varying time periods. The changes in the surface morphology and wetting characteristics of both the coated substrate and the PTFE source specimen were investigated after the varying time durations. The surface morphology has been investigated by scanning electron microscopy (SEM) while the wetting characteristics by measuring static water contact angle on the surfaces. The chemical nature of the coating deposited on the glass substrate was investigated by Energy Dispersive X-ray spectroscopy attached with the SEM machine (SEM-EDX). The changes parallelly occurring in the surface morphology and the wetting characteristics of the glass substrate and the PTFE source specimen with increasing time of the process have been correlated and discussed.
2.2. Characterization The surface morphology of both the coated substrate and the PTFE source specimen was investigated by a field emission scanning electron microscope (FESEM, Philips XL-30 FEG). The chemical nature of the coating deposited on the glass substrate was investigated by using the energy dispersive X-ray spectroscopy (EDX) facility of the SEM instrument. The static water contact angles of the glass substrate and the PTFE source specimen were measured using a contact angle measurement system (JC2000D, Shanghai Chenzhong Digital Technology Limited, China). The measurements were done on distilled water droplets with a volume of 5 μl placed at varying locations on the surfaces at room temperature and pressure. Contact angles of five droplets were measured for each surface and the mean value was reported.
2. Experimental methods 2.1. Materials and process The PTFE material used as the source for the PTFE deposition was the DF-22 grade sheet material obtained from Dongyue Group Ltd, China. Thin slices less than 2 mm in thickness were cut from a PTFE sheet of 6 mm thickness and about 15 g of the sliced material was used as the source material for each coating experiment. Cleaned glass plates with dimensions 5 cm × 2 cm × 2 mm were used as the substrates for the coating process. A schematic of the experimental set up used for the process is shown in Fig. 1. It comprises an autoclave connected to a CO2 gas cylinder through a syringe pump. The autoclave has an inner volume of 80 cm3 and capacity to withstand a maximum pressure of 50 MPa and can be electrically heated up to a maximum temperature of 400 °C. The syringe pump of the set up is an electrically driven pressure pump with capacity to build up the CO2 pressure inside the autoclave up to 50 MPa. When the autoclave was heated to 300 °C temperature, the PTFE source specimen and the cleaned glass substrate were placed, without touching each other, inside the autoclave and its lid tightly closed. The CO2 gas was then pumped into the autoclave using the syringe pump and after exhausting the air inside for a while the outlet valve of the autoclave closed to build up the pressure inside to 24 MPa. When the pressure was reached to 24 MPa, the CO2 pumping stoped and the inlet valve closed. The set up was then kept at the temperature and pressure conditions for varying time periods of 12, 24, 36, 48, 60 and 72 hours (h). The set up was monitored during the entire process
3. Results and discussions The SEM micrographs of the coated glass substrates obtained after the coating experiments conducted for the different time periods are shown in Fig. 2. The SEM micrographs show that the glass substrates have increasingly covered with minute spherical shaped globulets when the time duration of the process increased. The amount of deposition as well as the surface coverage of the coating increased with increasing time of the process. The diameters of the deposited globulets vary roughly in the range of a few tens of nanometers to a few hundreds of nanometers. The values of the static water contact angles measured on these coated substrates are also shown in the respective SEM micrographs in Fig. 2. The static water contact angle measured on the coated substrates increased when the coating time increased. The contact angle shows a rapid increase from 123° to 148°, when coating time increased from 12 h to 36 h, but further increase in contact angle is small when coating time further increased. All the substrates that coated for more than 48 h have shown a contact angle greater than 150°. Surfaces showing a contact angle greater than 150° are generally considered as superhydrophobic. The superhydrophobicity of the coated substrates is Fig. 1. The schematic of the experimental set up used for the PTFE deposition process.
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Fig. 2. The SEM micrographs of the glass substrates coated for the different time periods: (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h (e) 60 h, and (f) 72 h.
In order to gauge the thickness of the coating deposited on the glass substrate the cross-sections of the coated glass substrates were observed using SEM. The cross-sectional SEM micrograph of a coated surface (coated for 48 h) is shown in Fig. 4. The heights of the deposited globulets vary but in the sub-micron scale. The thickness of the coating estimated is roughly about 0.25 μm for the coating deposited for 48 h. The cross-sections of the PTFE source specimens also were observed using SEM. The protrusions or swellings appeared on the PTFE source have heights of about a few micrometers. The surface roughening happening on the PTFE source specimen and the globulets deposition happening on the glass substrate during the process are intimately related. When the PTFE specimen was kept in the sc-CO2 under the relatively high temperature (300 °C) and pressure (24 MPa) conditions for long time (more than 12 h), the sc-CO2 was able to penetrate and swell the PTFE specimen at least at its surface. Some amount of PTFE, possibly low molecular weight molecules, got dissolved in the sc-CO2 and subsequently got deposited onto the glass substrate. The dissolution of the PTFE molecules in sc-CO2 and their subsequent deposition on the glass substrate continue with increasing time during the process, favored by the positive entropy change for the dispersing out of the PTFE molecules from a compact state in the PTFE
because the coatings are based on PTFE, an inherently hydrophobic material and there is a special micron-scale surface roughness on the coated surface. The SEM micrographs of the PTFE specimens used as the source material for the coating process carried out for varying time periods are shown in Fig. 3. The surface of the PTFE source specimen became increasingly rough with minute swellings and protrusions appearing on the surface as the time duration of the process increased. The surface swellings and protrusions seen on the PTFE source specimens have sizes in the range of a few micrometers as seen in the SEM micrographs of Fig. 3. The static water contact angles measured on these PTFE source specimens are shown in their respective SEM micrographs in Fig. 3. The contact angle on the PTFE source specimen increased with increasing time duration of the process. The contact angle rapidly increased from 121° to 151° when the coating time increased from 12 h to 36 h, but further raise in contact angle with increasing coating time is small. All the PTFE source specimens exposed to the coating environment for more than 36 h have shown a contact angle greater than 150°, showing superhydrophobicity. This is because when a micron-scale roughness is formed on the PTFE surface as a result of the sc-CO2 exposure, its hydrophobicity is greatly enhanced to become superhydrophobic.
Fig. 3. The SEM micrographs of the PTFE source specimens after the varying sc-CO2 exposure times: (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h (e) 60, and (f) 72 h.
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Fig. 4. The cross-sectional SEM micrographs of the coated glass substrate (a) and PTFE source specimen (b) after 48 h coating process.
Fig. 5. The EDX spectra of the PTFE coated glass (a) and PTFE source specimen (b) after 48 h coating process, and PTFE specimen not subjected to the process (c).
this case, PTFE molecules, possibly the low molecular weight ones, getting dissolved in the sc-CO2 and subsequently getting deposited on the glass substrate without degradation. The chemical nature of the coated glass substrate, PTFE source specimen and the virgin PTFE not subjected to the coating process was analyzed using SEM-EDX. The EDX spectrum and the elemental
specimen. Unlike the other reported methods used for obtaining PTFE like coatings such as the pulsed laser deposition [5,7], RF plasma sputtering [8], ion beam sputtering [9], RF-magnetron sputtering [10] and synchrotron radiation photodecomposition [11], here the PTFE molecules as such are not subjected to a destructive treatment, but they are removed out from the source specimen by a dissolution process. In 325
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subsequently got deposited on to the glass substrate. The globulets kinds of deposition as well as detection of high percentage of elemental oxygen in the coating deposited indicated the presence of CO2 within the coating. The method may find use in fabricating thin-layer coating of PTFE materials for potential microelectronics, microfluidics and sensor applications.
composition of the coated substrate are shown in Fig. 5(a). The main elements detected on the coated surface are carbon (59.18%), fluorine (22.67%) and oxygen (16.98%). The carbon and fluorine are of the PTFE molecules. The high amount of oxygen detected is unexpected for pure PTFE and is possibly comes from the CO2 traped with in the coated globulets, which may have been slightly foamed as a result of it. The ability of sc-CO2 to induce surface swellings and protrusions on the PTFE source specimen is already seen in SEM micrographs in Fig. 3. The EDX spectrum of a surface protrusion on the PTFE source specimen that subjected to a 48 h coating process is shown in Fig. 5(b). In addition to carbon (37.23%) and fluorine (61.52%), small amount of oxygen (1.25%) is also detected in the surface protrusion on the PTFE source specimen, indicating that the swelling is caused by the sc-CO2 penetration. The EDX spectrum of the pure PTFE specimen (Fig. 5(c)), that is not subjected to the coating process, shows only the elements carbon (39.47%) and fluorine (60.53%) as expected. In the sc-CO2 process conditions employed here, the deposited globulets is expected to be swelled and foamed by the CO2, especially as the deposited PTFE possibly consists of low molecular weight molecules. The foaming of polymers by saturating the polymers with sc-CO2 at high pressure and then rapidly releasing the pressure is well known and is a rapidly developing application for the sc-CO2 [13–15]. However, under the temperature and pressure conditions applied here the sc-CO2 was not successful in foaming the PTFE source specimen, except causing the micron-sized swellings and protrusions on the specimen surface.
Acknowledgments This research was supported by China Postdoctoral Science Foundation (Grant No. 2013M531561) and Promotive research fund for excellent young and middle-aged scientists of Shandong Province (BS2013CL018). References [1] J. Wang, H.K. Kim, F.G. Shi, B. Zhao, T.G. Nieh, Thickness dependence of morphology and mechanical properties of on-wafer low-k PTFE dielectric films, Thin Solid Films 377–378 (2000) 413–417. [2] H. Andersson, W. van der Wijngaart, P. Griss, F. Niklaus, G. Stemme, Hydrophobic valves of plasma deposited octafluorocyclobutane in DRIE channels, Sens. Actuators B 75 (2001) 136–141. [3] M. Wienecke, M.C. Bunescu, M. Pietrzak, K. Deistung, P. Fedtke, PTFE membrane electrodes with increased sensitivity for gas sensor applications, Synth. Met. 138 (2003) 165–171. [4] M. Nebel, S. Neugebauer, H. Kiesele, W. Schuhmann, Local reactivity of diamondlike carbon modified PTFE membranes used in SO2 Sensors, Electrochim. Acta 55 (2010) 7923–7928. [5] G. Kecskeméti, B. Hopp, T. Smausz, Z. Tóth, G. Szabó, Production of porous PTFE–Ag composite thin films by pulsed laser deposition, Appl. Surf. Sci. 258 (2012) 7982–7988. [6] Fact Sheet, Applying Teflon® Coatings, http://www.rjchase.com/application_ guidelines.pdf. [7] S.T. Li, E. Arenholz, J. Heitz, D. Bäuerle, Pulsed-laser deposition of crystalline Teflon (PTFE) films, Appl. Surf. Sci. 125 (1998) 17–22. [8] D.S. Bodas, A.B. Mandale, S.A. Gangal, Deposition of PTFE thin films by RF plasma sputtering on < 1 0 0 > silicon substrates, Appl. Surf. Sci. 245 (2005) 202–207. [9] J.L. He, W.Z. Li, L.D. Wang, J. Wang, H.D. Li, Deposition of PTFE thin films by ion beam sputtering and a study of the ion bombardment effect, Nucl. Instrum. Methods Phys. Res. B 135 (1998) 512–516. [10] H.M. Kim, S. Sohn, J.S. Ahn, Transparent and super-hydrophobic properties of PTFE films coated on glass substrate using RF-magnetron sputtering and Cat-CVD methods, Surf. Coat. Technol. 228 (2013) S389–S392. [11] T. Katoh, Y. Zhang, Deposition of Teflon-polymer thin films by synchrotron radiation photodecomposition, Appl. Surf. Sci. 138–139 (1999) 165–168. [12] R. Span, W. Wagner, A new equation of state for carbon dioxide covering the fluid region from the triple-Point temperatureto 1100 K at pressures up to 800 MPa, J. Phys. Chem. Ref. Data 25 (1996) 1509. [13] C. Boyère, C. Jérôme, A. Debuigne, Input of supercritical carbon dioxide to polymer synthesis: an overview, Eur. Polym. J. 1 (61) (2014) 45–63. [14] M. Sauceau, J. Fages, A. Common, C. Nikitine, E. Rodier, New challenges in polymer foaming: a review of extrusion processes assisted by supercritical carbon dioxide, Prog. Polym. Sci. 36 (2011) 749–766. [15] E. Kiran, Supercritical fluids and polymers – the year in review – 2014, J. Supercrit. Fluids 110 (2016) 126–153. [16] Yu E. Vopilov, L.N. Nikitin, G. Yu Yurkov, E.P. Kharitonova, A.R. Khokhlov, V.M. Bouznik, Effect of supercritical carbon dioxide on ultradispersed polytetrafluoroethylene, J. Supercrit. Fluids 62 (2012) 204–210.
4. Conclusion A new process for depositing a superhydrophobic thin-layer coating of PTFE on a glass substrate was described. The process involved keeping a PTFE source specimen and the glass substrate to be coated under sc-CO2 at the temperature of 300 °C and pressure of 24 MPa for certain time period. The coating deposited on the glass substrate consisted of minute spherical shaped globulets of PTFE with sizes varying from a few tens of nanometers to a few hundreds of nanometers. The coating deposition as well as the coating coverage on the substrate increased with increasing time duration of the process and this was reflected on the static water contact angle measured on the coated substrates. The static water contact angle increased with increasing coating time and all substrates coated for more than 48 h showed a water contact angle greater than 150°. The superhydrophobicity of the coated substrates was attributed to the micron-scale surface roughness of the coated surfaces and to the inherent hydrophobicity of the PTFE. The PTFE source specimen became increasingly rough with time during the process as micron sized swellings and protrusions appeared on its surface. All PTFE source specimens subjected to the sc-CO2 process for more than 36 h have a contact angle greater than 150°. Under the relatively high temperature and pressure conditions employed in the process, the sc-CO2 was able to penetrate and swell the PTFE specimens, at least at their surfaces, and dissolve some amount of PTFE which
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