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Facile fabrication of superhydrophobic polytetrafluoroethylene surface by cold pressing and sintering
Applied Surface Science 257 (2011) 4821–4825
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile fabrication of superhydrophobic polytetrafluoroethylene surface by cold pressing and sintering Cheng Jiang a,b , Weixin Hou a,b , Qihua Wang a,∗ , Tingmei Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 14 December 2010 Accepted 15 December 2010 Available online 28 December 2010 Keywords: Polytetrafluoroethylene Superhydrophobic surface Cold pressing Sintering Mask
a b s t r a c t A series of superhydrophobic polytetrafluoroethylene (PTFE) surfaces were prepared by a facile cold pressing and sintering method, and their microstructures and wetting behaviors could be artificially tailored by altering sintering temperature and using different masks. Specifically, the microstructures mainly depended on the sintering temperature, whereas the wetting behaviors, water contact angle (WCA) and sliding angle (SA), greatly hinged on both the sintering temperature and mask. Then a preferable superhydrophobic surface with WCA of 162 ± 2◦ and SA of 7◦ could be obtained when the sintering temperature was 360 ◦ C and the 1000 grit abrasive paper was used as a mask. In addition, it was worth noting that the as-prepared surfaces exhibited excellent stability under UV illumination, which was the most key factor for them toward practical applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In nature, there are many superhydrophobic surfaces, such as lotus leaves [1], the legs of water striders [2], wings of cicadas [3], and so on. These natural superhydrophobic surfaces nearly all own micro/nano hierarchical structures and low surface energy materials on their surfaces. Inspired by these natural superhydrophobic surfaces, many artificial superhydrophobic surfaces emerged and were widely used in many fields. Many methods have been used for fabricating superhydrophobic surfaces, such as sol–gel [4,5], phase separation [6–8], layer-by-layer [9,10], chemical vapor deposition [11], template method [12–14], etc. However, most of these methods are difficult to process, time consuming, and expensive. The superhydrophobic surfaces cannot be prepared in large scale, limiting the applications of superhydrophobic surfaces. Therefore, facile methods for preparing superhydrophobic surfaces should be developed intensively. PTFE has an excellent resistance to chemical regent. It is the ideal material using in engineering fields. Recently, PTFE was widely used to fabricate superhydrophobic surfaces due to its low surface energy. Feng et al. [15] sprayed the PTFE emulsion on the mesh film to prepare a superhydrophobic PTFE coating mesh film. Luo et al. [16] fabricated a PTFE/poly(phenylene sulfide) superhydrophobic
∗ Corresponding author. Tel.: +86 931 4968180; fax: +86 931 4968180. E-mail address: [email protected] (Q. Wang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.088
coating with a porous and micrometer–nanometer-scale binary structure by adding (NH4 )2 CO3 and surfactant solutions into polymer emulsion. In order to improve the surface hydrophobic, Jucius et al. [17] used textured Ni mold to imprint texture on PTFE surface by hot pressing above the glass temperature of the PTFE. Schmid et al. [18] used nano-porous alumina to texture the PTFE surface, resulting in an increase of hydrophobicity of the PTFE surface. However, most of these methods were either complicated or expensive. In our previous work [13], the filter paper was used as a template to texture PTFE surface by cold pressing, and then sintered the PTFE in an oven at certain temperature to remove the template. By this simple method, PTFE superhydrophobic surfaces have been fabricated. In this article, the superhydrophobic PTFE surfaces with a special micro-papilla structure were prepared by the facile cold pressing and sintering method. Interestingly, different from previously reported superhydrophobic surfaces prepared by template method [12–14], it was found that this special microstructure mainly depended on the sintering temperature, instead of the masks used in the present experiment. 2. Experimental details 2.1. Materials PTFE (M-18F, Daikin Fluorochemicals Co., Ltd.); silicon wafers were respectively cleaned by ultrasonic washing in ethanol and water before use.
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Fig. 1. Variation of WCA of PTFE surfaces with sintering temperature: (a) 1000 grit abrasive paper as mask and (b) without mask.
2.2. Cold pressing of the PTFE materials The mold used in this experiment is a column with a diameter of 2.6 cm. The masks (400, 1000, 2000 grit abrasive papers, and silicon wafer) were placed at the bottom of the mold, and then the PTFE powder was added into the mold and cold pressed with 35 MPa for 20 min. The prepared PTFE block material was taken out of the mold and the mask was peeled off. 2.3. Sintering The PTFE materials were sintered at 300, 320, 330, 340, 360, 380, and 400 ◦ C for 5 h, respectively, and then naturally cooled to room temperature in an oven. 2.4. Characterization The microstructures of the prepared PTFE surfaces were observed by field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Japan). The surface roughness measures were carried out on surface roughness tester (type 2206, Harbin measuring & cutting tool works, China). The water contact angles of the PTFE surfaces were measured by DSA 100 optical contact-angle meter (DSA 100, Kruss company, Ltd., Germany) at ambient temperature. The average WCA values were obtained by measuring the same sample at five different positions with 5 L water droplets. Thermal analysis of PTFE was made on a Differential Scanning Calorimetry (DSC) instrument (STA 449 C, Netzsch, Germany) in air atmosphere at a heating rate of 10 ◦ C/min. The photographs of the water drops on the PTFE surface were captured with a digital camera (Olymbus). 3. Results and discussion PTFE, a commercially available hydrophobic polymer, WCA of the smooth PTFE surface is about 110–120◦ . In the present study, abrasive papers were firstly used to imprint the PTFE surfaces by cold pressing. Fig. 1 showed the variation of the WCAs of prepared PTFE surfaces with sintering temperature. When 1000 grit abrasive paper was used as a mask (Fig. 1a) and the sintering temperature was lower than 330 ◦ C, the as-prepared PTFE surface exhibited hydrophobicity with a water contact angle about 130–135◦ , and cannot meet the requirement of superhydrophobicity. As the sin-
tering temperature increase, the hydrophobicity of PTFE surfaces improved obviously. When the PTFE materials sintered at 340 ◦ C, the WCA of the PTFE surface was 154 ± 2◦ and SA was 14◦ . When the sintering temperature increased to 360 ◦ C, the as-prepared surface showed a superhydrophobic property with WCA 162 ± 2◦ and a lower SA about 7◦ . The hydrophobicity of the prepared surface decreased slightly while further increasing the temperature to 380 and 400 ◦ C. When no mask was used (Fig. 1b), the variation of WCAs of the as-prepared surfaces with sintering temperature was in correspondence with the surfaces prepared by 1000 grit abrasive paper. When the sintering temperature increased from 300 to 360 ◦ C, the WCAs of such PTFE surfaces increased from 124 ± 2◦ to 163 ± 2◦ . At 380 ◦ C and 400 ◦ C, the WCAs decreased slightly to 160 ± 2◦ and 155 ± 2◦ , respectively. The SAs of the above surfaces preparing without mask were all higher than 15◦ . This result further showed that the best sintering temperature was 360 ◦ C. The FE-SEM images gave an explanation of the wetting behaviors of the as-prepared PTFE surfaces, as shown in Fig. 2. When the sintering temperature was lower than 330 ◦ C (see Fig. 1 in supporting information), the surface, with only micro-scale grooves imprinted by abrasive paper but no any micro-papilla, was relatively smooth, as shown in Fig. 2a. With the increase of sintering temperature to 340 ◦ C, the PTFE surface gradually became rough as shown in Fig. 2b. Several micro-papillae and many nanoscale grooves were distributed on the surface, resulting in higher hydrophobicity. When the sintering temperatures were 360, 380 and 400 ◦ C, many micro-papillae were distributed on the surface with diameter about 1–4 m, and there were many nano-scale grooves on each micro-papilla (Fig. 2f) which composed of the micrometer–nanometer-scale binary structure roughness. Even on the smooth area of the surface (Fig. 2g), irregular nanostructures were also found. The microstructures of the surfaces sintering at various temperatures are different in densities and sizes of the micro-papillae. At 360 ◦ C (Fig. 2c), the density of micro-papillae on the surface was thick and the sizes were big. This rougher structure resulted in a larger WCA of 162 ± 2◦ and a lower SA about 7◦ . When the sintering temperature further increased to 380 ◦ C (Fig. 2d), the density of the papillae was too thick and the papillae embedded in the surface in half, leading to the decrease of roughness which resulted in a lower WCA about 158 ± 2◦ and a higher SA of 20◦ . The number and diameter of papilla on the surface decreased obviously after 400 ◦ C sintering, resulting in a decrease of hydrophobicity. The above results indicated that the sintering temperature influenced the morphologies of the PTFE surfaces, leading to the variation of the surface wetting behavior. Different types of abrasive papers (400 and 2000 grit) were also used to prepare PTFE samples, and the sintering temperature was 360 ◦ C according to above results. When the 400 and 2000 grit papers were used as the masks, the WCAs of the as-prepared surfaces were 156 ± 2◦ and 160 ± 2◦ , SAs 9◦ and 11◦ , respectively. The effect of the masks on the wetting behavior of the PTFE surfaces was slighter than that of sintering temperature. To clarify the effects of mask and sintering temperature, two comparative experiments were conducted. The PTFE materials were prepared without any masks. Unexpectedly, the as-prepared surfaces also showed superhydrophobicity after sintering at 360 ◦ C (Fig. 1b). In order to illustrate the effect of mask, a smooth silicon wafer was also used as a mask. The PTFE sample was sintered at 360 ◦ C for 5 h, and the surface also exhibited superhydrophobicity with a WCA about 152 ± 2◦ . The morphologies of the surfaces prepared in these two experiments were shown in Fig. 3. In comparison with the sample prepared by 1000 grit abrasive paper, the microstructures of PTFE surfaces using different masks (without mask and using silicon wafer as mask) were very similar, as shown in Fig. 3a–c. Micro-papillae can be observed on the prepared sur-
C. Jiang et al. / Applied Surface Science 257 (2011) 4821–4825
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Fig. 2. FE-SEM images of the PTFE surfaces using 1000 grit abrasive papers as masks and sintering at different temperatures: (a) 320 ◦ C; (b) 340 ◦ C; (c) 360 ◦ C; (d) 380 ◦ C; (e) 400 ◦ C; (f) the high-magnification image of the micro-papillae of the (c); (g) the high-magnification image of the smooth area of the (c).
faces, and both the papillae and the smooth area of the surfaces appeared nano-scale grooves. All of these three surfaces exhibit similar microstructures, indicating that the micro-papillae formed during the sintering process instead of imprinting by masks. The microstructure of the PTFE surface prepared by cold pressing, with neither mask nor sintering, was shown in Fig. 3d. It can be seen from the FE-SEM image that the surface was relatively smooth, without any micro-papillae and nano-scale grooves before sintering. Moreover, the nano-scale grooves on the PTFE surface prepared by using silicon wafer as a mask (Fig. 3c) were remarkably fewer than that on the surface using 1000 grit abrasive paper as a mask (Fig. 3a). The surface roughness (Ra) of the PTFE surfaces was also evaluated. The Ra of PTFE surface prepared by using 1000 grit abrasive paper, silicon wafer as a mask, and no mask was 2.55, 0.50, and
0.52 m, respectively. The above results indicated that the microstructure papillae of the PTFE surface were produced by sintering and abrasive papers which used in the cold pressing could increase the roughness of the surface. The photographs of water drops on PTFE surfaces were shown in Fig. 4. From the DSC curve of the PTFE (Fig. 5), it can get that PTFE begin to melt when the temperature is above 329 ◦ C and the melting point of the PTFE is around 344 ◦ C. When the sintering temperature was lower than 329 ◦ C, the PTFE bulk was in solid state. While at 330 ◦ C and 340 ◦ C, the PTFE cannot melt completely. So the sharp change of the temperature resulted in a small deformation that the PTFE surfaces were relatively smooth. While at 360 ◦ C and above, the PTFE melted completely and kept in fusion state. The quick decrease of the temperature during the cool process made the PTFE material
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Fig. 3. FE-SEM images of the PTFE surface using different masks for cold pressing and sintering at 360 ◦ C: (a) 1000 grit abrasive paper, (b) without any mask, and (c) silicon wafer. (d) PTFE surface prepared by cold pressing without mask and sintering.
Fig. 4. Photographs of water drops with a volume of 5 L on PTFE surfaces: (a) and (b) using 1000 grit abrasive paper as mask and sintering at 320 ◦ C; (c) and (d) using 1000 grit abrasive paper as mask and sintering at 360 ◦ C; (e) and (f) without using mask but sintering at 360 ◦ C.
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and the WCAs of these surfaces were also higher than 150◦ when the sintering temperature was proper. The application of abrasive papers was beneficial to increase the surface roughness and decrease the SAs of as-prepared surfaces. The superhydrophobic surface showed UV stability even for 12 h irradiation. This facile and inexpensive method described above is suitable for large-scale production and can also be used to prepare other superhydrophobic PTFE composites materials. It is expected that this technique can be used in practical application to prepare the superhydrophobic surfaces. Acknowledgements The authors would like to acknowledge the financial support of the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51025517), the Innovative Group Foundation of NSFC (Grant No. 50721062) and the financial support of the National 973 project of China (2007CB607606). Fig. 5. The DSC curve of the PTFE material.
deformation more easily than in solid state. In the fusion state, the gas remove and porous elimination from the PTFE material also had a tremendous effect on the morphology of the surface. Higher temperature would also increase the inter particle collision chance of separate PTFE particles with surrounding particles, forming aggregates [19]. The micro-papilla structure may be produced by the above factors. Since the oxidation and degradation of polymer surface, polymer-based superhydrophobic surfaces are often unstable under UV illumination. However, the samples in this experiment showed excellent stability. The PTFE superhydrophobic surface which used 1000 grit abrasive paper as mask for cold pressing was exposed to 254 nm UV light at ambient air for 1, 2, and 12 h respectively. We found that the WCA of the PTFE surface decreased to 153◦ after 1 h exposure. After 2 h exposure, the WCA of the PTFE surface decreased to 151◦ . When the surface exposed to UV light for 12 h, the WCA remained 151◦ . The variation of WCA was slightly even after 12 h exposure, indicating that the as-prepared PTFE superhydrophobic surface was stable under UV light exposure. 4. Conclusion The PTFE superhydrophobic surfaces were prepared by a facile cold pressing and sintering method. Even without any masks, the micro-scale papillae can also form on the obtained PTFE surfaces
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2010.12.088. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
C. Neinhuis, W. Barthlott, Ann. Bot. 79 (1997) 667. X.F. Gao, L. Jiang, Nature 432 (2004) 36. W. Lee, M.-K. Jin, W.-C. Yoo, J.-K. Lee, Langmuir 20 (2004) 7665. N.J. Shirtcliffe, G. McHale, M.I. Newton, C.C. Perry, Langmuir 19 (2003) 5626. M. Manca, A. Cannavale, L. De Marco, A.S. Arico, R. Cingolani, G. Gigli, Langmuir 25 (2009) 6357. J.X. Qiongdan Xie, Adv. Mater. 16 (2004) 302. N. Zhao, Q. Xie, L. Weng, S. Wang, X. Zhang, J. Xu, Macromolecules 38 (2005) 8996. Y. Zhang, H. Wang, B. Yan, Y. Zhang, P. Yin, G. Shen, R. Yu, J. Mater. Chem. 18 (2008) 4442. S. Amigoni, E. Taffin de Givenchy, M. Dufay, F. Guittard, Langmuir 25 (2009) 11073. Y. Li, F. Liu, J.Q. Sun, Chem. Commun. (2009) 2730. Z.R. Zheng, Z.Y. Gu, R.T. Huo, Y.H. Ye, Appl. Surf. Sci. 255 (2009) 7263. W.K. Cho, I.S. Choi, Adv. Funct. Mater. 18 (2008) 1089. W. Hou, Q. Wang, J. Colloid Interface Sci. 333 (2009) 400. Z.Q. Yuan, H. Chen, J. Zhang, Appl. Surf. Sci. 254 (2008) 1593. L. Feng, Z.Y. Zhang, Z.H. Mai, Y.M. Ma, B.Q. Liu, L. Jiang, D.B. Zhu, Angew. Chem., Int. Ed. 43 (2004) 2012. Z. Luo, Z. Zhang, L. Hu, W. Liu, Z. Guo, H. Zhang, W. Wang, Adv. Mater. 20 (2008) 970. D. Jucius, A. Guobiene, V. Grigaliunas, Appl. Surf. Sci. 256 (2010) 2164. G. Schmid, M. Levering, T. Sawitowski, Z. Anorg, Allg. Chem. 633 (2007) 2147. Z. Luo, Z. Zhang, W. Wang, W. Liu, Q. Xue, Mater. Chem. Phys. 119 (2010) 40.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Facile fabrication of superhydrophobic polytetrafluoroethylene surface by cold pressing and sintering Cheng Jiang a,b , Weixin Hou a,b , Qihua Wang a,∗ , Tingmei Wang a a b
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School, Chinese Academy of Sciences, Beijing 100039, PR China
a r t i c l e
i n f o
Article history: Received 24 June 2010 Received in revised form 14 December 2010 Accepted 15 December 2010 Available online 28 December 2010 Keywords: Polytetrafluoroethylene Superhydrophobic surface Cold pressing Sintering Mask
a b s t r a c t A series of superhydrophobic polytetrafluoroethylene (PTFE) surfaces were prepared by a facile cold pressing and sintering method, and their microstructures and wetting behaviors could be artificially tailored by altering sintering temperature and using different masks. Specifically, the microstructures mainly depended on the sintering temperature, whereas the wetting behaviors, water contact angle (WCA) and sliding angle (SA), greatly hinged on both the sintering temperature and mask. Then a preferable superhydrophobic surface with WCA of 162 ± 2◦ and SA of 7◦ could be obtained when the sintering temperature was 360 ◦ C and the 1000 grit abrasive paper was used as a mask. In addition, it was worth noting that the as-prepared surfaces exhibited excellent stability under UV illumination, which was the most key factor for them toward practical applications. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In nature, there are many superhydrophobic surfaces, such as lotus leaves [1], the legs of water striders [2], wings of cicadas [3], and so on. These natural superhydrophobic surfaces nearly all own micro/nano hierarchical structures and low surface energy materials on their surfaces. Inspired by these natural superhydrophobic surfaces, many artificial superhydrophobic surfaces emerged and were widely used in many fields. Many methods have been used for fabricating superhydrophobic surfaces, such as sol–gel [4,5], phase separation [6–8], layer-by-layer [9,10], chemical vapor deposition [11], template method [12–14], etc. However, most of these methods are difficult to process, time consuming, and expensive. The superhydrophobic surfaces cannot be prepared in large scale, limiting the applications of superhydrophobic surfaces. Therefore, facile methods for preparing superhydrophobic surfaces should be developed intensively. PTFE has an excellent resistance to chemical regent. It is the ideal material using in engineering fields. Recently, PTFE was widely used to fabricate superhydrophobic surfaces due to its low surface energy. Feng et al. [15] sprayed the PTFE emulsion on the mesh film to prepare a superhydrophobic PTFE coating mesh film. Luo et al. [16] fabricated a PTFE/poly(phenylene sulfide) superhydrophobic
∗ Corresponding author. Tel.: +86 931 4968180; fax: +86 931 4968180. E-mail address: [email protected] (Q. Wang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.088
coating with a porous and micrometer–nanometer-scale binary structure by adding (NH4 )2 CO3 and surfactant solutions into polymer emulsion. In order to improve the surface hydrophobic, Jucius et al. [17] used textured Ni mold to imprint texture on PTFE surface by hot pressing above the glass temperature of the PTFE. Schmid et al. [18] used nano-porous alumina to texture the PTFE surface, resulting in an increase of hydrophobicity of the PTFE surface. However, most of these methods were either complicated or expensive. In our previous work [13], the filter paper was used as a template to texture PTFE surface by cold pressing, and then sintered the PTFE in an oven at certain temperature to remove the template. By this simple method, PTFE superhydrophobic surfaces have been fabricated. In this article, the superhydrophobic PTFE surfaces with a special micro-papilla structure were prepared by the facile cold pressing and sintering method. Interestingly, different from previously reported superhydrophobic surfaces prepared by template method [12–14], it was found that this special microstructure mainly depended on the sintering temperature, instead of the masks used in the present experiment. 2. Experimental details 2.1. Materials PTFE (M-18F, Daikin Fluorochemicals Co., Ltd.); silicon wafers were respectively cleaned by ultrasonic washing in ethanol and water before use.
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Fig. 1. Variation of WCA of PTFE surfaces with sintering temperature: (a) 1000 grit abrasive paper as mask and (b) without mask.
2.2. Cold pressing of the PTFE materials The mold used in this experiment is a column with a diameter of 2.6 cm. The masks (400, 1000, 2000 grit abrasive papers, and silicon wafer) were placed at the bottom of the mold, and then the PTFE powder was added into the mold and cold pressed with 35 MPa for 20 min. The prepared PTFE block material was taken out of the mold and the mask was peeled off. 2.3. Sintering The PTFE materials were sintered at 300, 320, 330, 340, 360, 380, and 400 ◦ C for 5 h, respectively, and then naturally cooled to room temperature in an oven. 2.4. Characterization The microstructures of the prepared PTFE surfaces were observed by field emission scanning electron microscopy (FE-SEM, JSM-6701F, JEOL, Japan). The surface roughness measures were carried out on surface roughness tester (type 2206, Harbin measuring & cutting tool works, China). The water contact angles of the PTFE surfaces were measured by DSA 100 optical contact-angle meter (DSA 100, Kruss company, Ltd., Germany) at ambient temperature. The average WCA values were obtained by measuring the same sample at five different positions with 5 L water droplets. Thermal analysis of PTFE was made on a Differential Scanning Calorimetry (DSC) instrument (STA 449 C, Netzsch, Germany) in air atmosphere at a heating rate of 10 ◦ C/min. The photographs of the water drops on the PTFE surface were captured with a digital camera (Olymbus). 3. Results and discussion PTFE, a commercially available hydrophobic polymer, WCA of the smooth PTFE surface is about 110–120◦ . In the present study, abrasive papers were firstly used to imprint the PTFE surfaces by cold pressing. Fig. 1 showed the variation of the WCAs of prepared PTFE surfaces with sintering temperature. When 1000 grit abrasive paper was used as a mask (Fig. 1a) and the sintering temperature was lower than 330 ◦ C, the as-prepared PTFE surface exhibited hydrophobicity with a water contact angle about 130–135◦ , and cannot meet the requirement of superhydrophobicity. As the sin-
tering temperature increase, the hydrophobicity of PTFE surfaces improved obviously. When the PTFE materials sintered at 340 ◦ C, the WCA of the PTFE surface was 154 ± 2◦ and SA was 14◦ . When the sintering temperature increased to 360 ◦ C, the as-prepared surface showed a superhydrophobic property with WCA 162 ± 2◦ and a lower SA about 7◦ . The hydrophobicity of the prepared surface decreased slightly while further increasing the temperature to 380 and 400 ◦ C. When no mask was used (Fig. 1b), the variation of WCAs of the as-prepared surfaces with sintering temperature was in correspondence with the surfaces prepared by 1000 grit abrasive paper. When the sintering temperature increased from 300 to 360 ◦ C, the WCAs of such PTFE surfaces increased from 124 ± 2◦ to 163 ± 2◦ . At 380 ◦ C and 400 ◦ C, the WCAs decreased slightly to 160 ± 2◦ and 155 ± 2◦ , respectively. The SAs of the above surfaces preparing without mask were all higher than 15◦ . This result further showed that the best sintering temperature was 360 ◦ C. The FE-SEM images gave an explanation of the wetting behaviors of the as-prepared PTFE surfaces, as shown in Fig. 2. When the sintering temperature was lower than 330 ◦ C (see Fig. 1 in supporting information), the surface, with only micro-scale grooves imprinted by abrasive paper but no any micro-papilla, was relatively smooth, as shown in Fig. 2a. With the increase of sintering temperature to 340 ◦ C, the PTFE surface gradually became rough as shown in Fig. 2b. Several micro-papillae and many nanoscale grooves were distributed on the surface, resulting in higher hydrophobicity. When the sintering temperatures were 360, 380 and 400 ◦ C, many micro-papillae were distributed on the surface with diameter about 1–4 m, and there were many nano-scale grooves on each micro-papilla (Fig. 2f) which composed of the micrometer–nanometer-scale binary structure roughness. Even on the smooth area of the surface (Fig. 2g), irregular nanostructures were also found. The microstructures of the surfaces sintering at various temperatures are different in densities and sizes of the micro-papillae. At 360 ◦ C (Fig. 2c), the density of micro-papillae on the surface was thick and the sizes were big. This rougher structure resulted in a larger WCA of 162 ± 2◦ and a lower SA about 7◦ . When the sintering temperature further increased to 380 ◦ C (Fig. 2d), the density of the papillae was too thick and the papillae embedded in the surface in half, leading to the decrease of roughness which resulted in a lower WCA about 158 ± 2◦ and a higher SA of 20◦ . The number and diameter of papilla on the surface decreased obviously after 400 ◦ C sintering, resulting in a decrease of hydrophobicity. The above results indicated that the sintering temperature influenced the morphologies of the PTFE surfaces, leading to the variation of the surface wetting behavior. Different types of abrasive papers (400 and 2000 grit) were also used to prepare PTFE samples, and the sintering temperature was 360 ◦ C according to above results. When the 400 and 2000 grit papers were used as the masks, the WCAs of the as-prepared surfaces were 156 ± 2◦ and 160 ± 2◦ , SAs 9◦ and 11◦ , respectively. The effect of the masks on the wetting behavior of the PTFE surfaces was slighter than that of sintering temperature. To clarify the effects of mask and sintering temperature, two comparative experiments were conducted. The PTFE materials were prepared without any masks. Unexpectedly, the as-prepared surfaces also showed superhydrophobicity after sintering at 360 ◦ C (Fig. 1b). In order to illustrate the effect of mask, a smooth silicon wafer was also used as a mask. The PTFE sample was sintered at 360 ◦ C for 5 h, and the surface also exhibited superhydrophobicity with a WCA about 152 ± 2◦ . The morphologies of the surfaces prepared in these two experiments were shown in Fig. 3. In comparison with the sample prepared by 1000 grit abrasive paper, the microstructures of PTFE surfaces using different masks (without mask and using silicon wafer as mask) were very similar, as shown in Fig. 3a–c. Micro-papillae can be observed on the prepared sur-
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Fig. 2. FE-SEM images of the PTFE surfaces using 1000 grit abrasive papers as masks and sintering at different temperatures: (a) 320 ◦ C; (b) 340 ◦ C; (c) 360 ◦ C; (d) 380 ◦ C; (e) 400 ◦ C; (f) the high-magnification image of the micro-papillae of the (c); (g) the high-magnification image of the smooth area of the (c).
faces, and both the papillae and the smooth area of the surfaces appeared nano-scale grooves. All of these three surfaces exhibit similar microstructures, indicating that the micro-papillae formed during the sintering process instead of imprinting by masks. The microstructure of the PTFE surface prepared by cold pressing, with neither mask nor sintering, was shown in Fig. 3d. It can be seen from the FE-SEM image that the surface was relatively smooth, without any micro-papillae and nano-scale grooves before sintering. Moreover, the nano-scale grooves on the PTFE surface prepared by using silicon wafer as a mask (Fig. 3c) were remarkably fewer than that on the surface using 1000 grit abrasive paper as a mask (Fig. 3a). The surface roughness (Ra) of the PTFE surfaces was also evaluated. The Ra of PTFE surface prepared by using 1000 grit abrasive paper, silicon wafer as a mask, and no mask was 2.55, 0.50, and
0.52 m, respectively. The above results indicated that the microstructure papillae of the PTFE surface were produced by sintering and abrasive papers which used in the cold pressing could increase the roughness of the surface. The photographs of water drops on PTFE surfaces were shown in Fig. 4. From the DSC curve of the PTFE (Fig. 5), it can get that PTFE begin to melt when the temperature is above 329 ◦ C and the melting point of the PTFE is around 344 ◦ C. When the sintering temperature was lower than 329 ◦ C, the PTFE bulk was in solid state. While at 330 ◦ C and 340 ◦ C, the PTFE cannot melt completely. So the sharp change of the temperature resulted in a small deformation that the PTFE surfaces were relatively smooth. While at 360 ◦ C and above, the PTFE melted completely and kept in fusion state. The quick decrease of the temperature during the cool process made the PTFE material
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Fig. 3. FE-SEM images of the PTFE surface using different masks for cold pressing and sintering at 360 ◦ C: (a) 1000 grit abrasive paper, (b) without any mask, and (c) silicon wafer. (d) PTFE surface prepared by cold pressing without mask and sintering.
Fig. 4. Photographs of water drops with a volume of 5 L on PTFE surfaces: (a) and (b) using 1000 grit abrasive paper as mask and sintering at 320 ◦ C; (c) and (d) using 1000 grit abrasive paper as mask and sintering at 360 ◦ C; (e) and (f) without using mask but sintering at 360 ◦ C.
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and the WCAs of these surfaces were also higher than 150◦ when the sintering temperature was proper. The application of abrasive papers was beneficial to increase the surface roughness and decrease the SAs of as-prepared surfaces. The superhydrophobic surface showed UV stability even for 12 h irradiation. This facile and inexpensive method described above is suitable for large-scale production and can also be used to prepare other superhydrophobic PTFE composites materials. It is expected that this technique can be used in practical application to prepare the superhydrophobic surfaces. Acknowledgements The authors would like to acknowledge the financial support of the National Science Foundation for Distinguished Young Scholars of China (Grant No. 51025517), the Innovative Group Foundation of NSFC (Grant No. 50721062) and the financial support of the National 973 project of China (2007CB607606). Fig. 5. The DSC curve of the PTFE material.
deformation more easily than in solid state. In the fusion state, the gas remove and porous elimination from the PTFE material also had a tremendous effect on the morphology of the surface. Higher temperature would also increase the inter particle collision chance of separate PTFE particles with surrounding particles, forming aggregates [19]. The micro-papilla structure may be produced by the above factors. Since the oxidation and degradation of polymer surface, polymer-based superhydrophobic surfaces are often unstable under UV illumination. However, the samples in this experiment showed excellent stability. The PTFE superhydrophobic surface which used 1000 grit abrasive paper as mask for cold pressing was exposed to 254 nm UV light at ambient air for 1, 2, and 12 h respectively. We found that the WCA of the PTFE surface decreased to 153◦ after 1 h exposure. After 2 h exposure, the WCA of the PTFE surface decreased to 151◦ . When the surface exposed to UV light for 12 h, the WCA remained 151◦ . The variation of WCA was slightly even after 12 h exposure, indicating that the as-prepared PTFE superhydrophobic surface was stable under UV light exposure. 4. Conclusion The PTFE superhydrophobic surfaces were prepared by a facile cold pressing and sintering method. Even without any masks, the micro-scale papillae can also form on the obtained PTFE surfaces
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.apsusc.2010.12.088. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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