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Preparation of porous super-hydrophobic and super-oleophilic polyvinyl chloride surface with corrosion resistance property
Applied Surface Science 258 (2011) 1008–1013
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of porous super-hydrophobic and super-oleophilic polyvinyl chloride surface with corrosion resistance property Yingke Kang a , Jinyan Wang a , Guangbin Yang a , Xiujuan Xiong a , Xinhua Chen b,∗∗ , Laigui Yu a , Pingyu Zhang a,∗ a b
Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, PR China College of Chemistry and Chemical Engineering, Xuchang University, Xuchang 461000, PR China
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 16 June 2011 Accepted 24 July 2011 Available online 17 September 2011 Keywords: Polyvinyl chloride Super-hydrophobic surface Super-oleophilic Corrosion resistance
a b s t r a c t Porous super-hydrophobic polyvinyl chloride (PVC) surfaces were obtained via a facile solvent/nonsolvent coating process without introducing compounds with low surface energy. The microstructure, wetting behavior, and corrosion resistance of resultant super-hydrophobic PVC coatings were investigated in relation to the effects of dosage of glacial acetic acid and the temperature of drying the mixed PVC solution spread over glass slide substrate. As-prepared PVC coatings had porous microstructure, and the one obtained at a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and under a drying temperature of 17 ◦ C had a water contact angle of 150 ± 1.5◦ , showing super-hydrophobicity. In the meantime, it possessed very small contact angles for liquid paraffin and diiodomethane and good corrosion resistance against acid and alkali corrosive mediums, showing promising applications in self-cleaning, waterproof for outer wall of building, seawater resistant coating, and efficient separation of oil and water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Usually, a surface with a water contact angle of above 150◦ and small contact angle hysteresis is called super-hydrophobic surface [1,2]; and there are many natural super-hydrophobic species in nature, such as sacred lotus leaves [3], taro leaves [4], paddy leaves, butterfly’s wings and water strider’s legs [5]. Super-hydrophobic surfaces have promising applications in both biological processes and technological applications, such as oil/water separation [6,7], photoresponsive materials [8], biochemical separation [9], and targeted drug delivery [10]. This is why they have been attracting much attention in both industry and fundamental research. It has been found that the hydrophobicity of a surface can be enhanced by increasing surface roughness within a proper range, because air trapped between water droplet and solid surface is able to minimize the contact area [11–18]. In this respect, various super-hydrophobic surfaces and films have been successfully prepared by manipulating the surface chemistry and surface roughness [19–38]. Erbil et al. [39] found that the introduction of a proper non-solvent led to increase water contact angle of polypropylene coating, since it induced and enhanced the phase separation and hence increased
the surface roughness of the coating. Many other researchers had also focused on the fabrication of various super-hydrophobic surfaces via convenient and inexpensive routes and on investigation of their microstructure and performance as well [39–43]. We are particularly interested in surfaces with both superhydrophobic and super-oleophilic properties, since few publications are currently available about such surfaces that may be used to efficiently separate oil from water. Therefore, in the present research we pay special attention to polyvinyl chloride (PVC), one of the most commonly used and cheap thermoplastics with a wide range of uses in pipes and fittings, profiles, cables, flooring, films and sheets (in recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas; and at least 50% of the market is driven by the construction/housing industry [44]). A facile and inexpensive solvent/non-solvent coating route was thus established to prepare PVC coating possessing super-hydrophobic properties with reference to similar solvent/non-solvent method for fabricating super-hydrophobic PVC coating [45,46]. This article reports the preparation of the super-hydrophobic and super-oleophilic PVC coating and investigation of its properties and microstructure. 2. Experimental
∗ Corresponding author. Tel.: +86 378 3883594; fax: +86 378 3881358. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected], [email protected] (P. Zhang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.106
2.1. Materials Commercial grade poly(vinyl chloride) resin (2S-85, obtained from Botian Chemical Company of Tianjin, China; average
Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013
molecular weight Mw = 100,000 g mol−1 ; coded as PVC) was used without any further treatment. Tetrahydrofuran (THF, obtained from Fuyu Fine Chemical Company of Tianjin, China) was chosen as the solvent. Glacial acetic acid (obtained from Fuyu Fine Chemical Company of Tianjin, China) was chosen as the non-solvent. Commercially obtained glass slide substrate was sequentially ultrasonically cleaned with distilled water, absolute ethanol and acetone, respectively, each for 10 min, followed by drying with nitrogen gas. 2.2. Preparation of porous super-hydrophobic PVC surfaces 0.1 g of PVC powder was dissolved slowly in 10.0 mL of tetrahydrofuran at ambient temperature to form a solution. Into resultant PVC solution was added 0.5–3.0 mL of glacial acetic acid (concentration 100%) at a rate of 1 mL s−1 . After being stirred for 5 min, a few droplets of resultant mixed solution were dripped onto the surface of cleaned glass slide with a burette and allowed to spread
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thereon to form a coating without disturbance. The coated glass slide was dried in atmosphere at pre-set temperatures (0 ◦ C, 10 ◦ C, 17 ◦ C, 24 ◦ C) for 24 h, yielding porous super-hydrophobic PVC surfaces. For a comparative study, PVC solution alone was directly dripped onto the surface of cleaned glass slide and dried in the same manner to form a smooth PVC surface. 2.3. Characterization of porous super-hydrophobic PVC coatings The morphologies of the porous surfaces were examined with a scanning electron microscope (SEM, Hitachi S-3200N; accelerating voltage 5 kV and 10 kV). Revolution TM v1.60b24 (4pi Analysis Inc.) was used for analysis of SEM images. The hydrophobicity of the PVC coatings was evaluated by measuring their contact angles with deionized water, liquid paraffin and diiodomethane, where the static contact angles (CA) of as-prepared coatings at room temperature in atmosphere were measured using a goniometer (Dropmaster 300 solid/liquid interface analyzer,
Fig. 1. SEM images of PVC surfaces obtained from the mixed PVC solutions containing different volume ratio of glacial acetic acid and tetrahydrofuran: (a) 0; (b) 0.5:10.0; (c) 1.0:10.0; (d) 15:10.0; (e) 2.0:10.0; and (f) 2.5:10.0. Insets refer to water contact angles on the as-prepared PVC surfaces.
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Fig. 2. Approaching, contacting, squeezing and departure processes of a 4 L water droplet suspending on a syringe with the super-hydrophobic surface obtained at a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and drying temperature of 17 ◦ C.
Kyowa Interface Science, Japan). At least five repeat measurements were conducted, each for newly formed sessile drops of about 4 L of deionized water, liquid paraffin and diiodomethane. The averages of the repeat measurements are cited in this article. Besides, the time for the liquid droplets to be fully absorbed by the porous coatings, time to-be-absorbed, was also recorded for each liquid droplet over a 20 min of interval. The surface roughness of the PVC coatings was determined using a YS2206B surface profiler (Harbin Measuring and Cutting Tool Group Company Ltd., China). 3. Results and discussion As shown in Fig. 1a, pure PVC coatings on the glass slide dried at various temperatures (0 ◦ C, 10 ◦ C, 17 ◦ C, 24 ◦ C) are transparent and smooth, and they have a water contact angle of only 87 ± 3◦ . This indicates that the volatilization of tetrahydrofuran has no influence on the surface roughness of pure PVC coatings, and preparing
conditions should be changed so as to obtain super-hydrophobic PVC surfaces by effectively increasing the surface roughness. Thus glacial acetic acid was added into the PVC solution at different volume ratios so as to induce the phase separation and increase the surface roughness. As we expected, the PVC coatings prepared from the mixed solution of PVC and glacial acetic acid were more rough and porous than those obtained from PVC solution alone (see Fig. 1b–f). In the meantime, the pore size (10–50 nm) of the PVC coatings increases with increasing concentration of glacial acetic acid in the mixed solution, largely due to entrapment of more air between the liquid droplet and solid surface therewith, leading to reduce contact area and increase contact angle. Table 1 shows the relationship between the contact angle of the PVC coatings and the content of glacial acetic acid in the mixed solution. It can be seen that the water contact angle of the PVC coatings increases with increasing content of glacial acetic acid when the volume ratio of glacial acetic acid and tetrahydrofuran in the mixed solution is lower than 2.5:10.0. When the volume ratio of glacial
Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013 Table 1 The relationship between the contact angle of PVC coatings (drying temperatures: 0 ◦ C, 10 ◦ C, 17 ◦ C, and 24 ◦ C) and content of glacial acetic acid in the mixed solution.
150
Contact angle (◦ ) 0 ◦C
0 0.5:10.0 1.0:10.0 1.5:10.0 2.0:10.0 2.5:10.0 3.0:10.0
160
140 10 ◦ C
17 ◦ C
24 ◦ C
88.2 90.6 83.3 113.6 105.2 97.6 119.7 110.8 109.5 122.5 123.2 143.4 136.6 134.5 146.1 149.0 150.5 151.5 No continuous coating was obtained
90.8 88.8 89.7 92.1 105.3 109.1 109.7
CA / Deg.
V(HAc) /V(THF)
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pH=1 pH=4 pH=7 pH=10 pH=13
130 120 110 100 90 80
acetic acid and tetrahydrofuran is higher than 3:10.0, no continuous coating can be obtained via drying at 0 ◦ C, 10 ◦ C, and 17 ◦ C. Super-hydrophobic PVC surfaces can always be obtained by drying the mixed solution spread over the glass slide, with a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0, at 0 ◦ C, 10 ◦ C, and 17 ◦ C. The reason might lie in that glacial acetic acid was volatized so fast above 20 ◦ C and hence it did not cause any damage to the porous surfaces. In one word, glacial acetic acid has an important effect on the surface topography of the PVC coatings obtained at a proper temperature, and the super-hydrophobicity of the PVC surfaces can be effectively manipulated by adjusting the dosage of glacial acetic acid as well as the volatilization temperature of the solution. Super-hydrophobic surface should have a water contact angle greater than 150◦ and small contact angle hysteresis. Fig. 2 schematically shows the approaching, contacting, squeezing and departure processes of a 4 L water droplet suspending on a syringe with the super-hydrophobic surface obtained by spreading the mixed PVC solution of a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 over the glass slide and drying at 17 ◦ C. The arrows show the direction towards which the platform of the goniometer moves. It can be seen that the as-prepared superhydrophobic surface has a very little adhesion to water. Since air can be easily trapped by the pores of the porous PVC surfaces, water droplets on such surfaces can only contact with the tips of the polymer particles, and it cannot penetrate into the pores, due to the presence of air pockets trapped in the pores [47,48]. Cassie and Baxter [13] proposed that the contact angle r on such
0
0.5:10.0 1.0:10.0 1.5:10.0 2.0:10.0 2.5:10.0
V(HAc) / V(THF) Fig. 3. The contact angles (◦ ) of solutions with different pH values on as-prepared PVC coating (drying temperature was 17 ◦ C).
a porous surface comprised of solid and air might be expressed by the following equation: cos r = f1 cos s − f2 = (1 − f2 ) cos s − f2 where f1 and f2 are the fractions of solid surface and air in composite surface, respectively, (in other words, f1 + f2 = 1), s is the equilibrium contact angle on a flat solid surface. According to the equation, if the fraction of air (f2 ) is high enough (see Table 2), corresponding to an increased surface roughness (see Table 3), it will be feasible to prepare a super-hydrophobic surface. Such a super-hydrophobic surface structure can well trap air, resulting in lowered sliding angle [49]. Paying attention to the corrosion resistance of PVC against various acidic liquids and some basic liquids of lower concentrations, we evaluated the wettability of different liquids with different pH values on the porous super-hydrophobic PVC surface. Fig. 3 shows the contact angles (◦ ) of solutions with different pH values on asprepared PVC coatings (drying temperature was 17 ◦ C). It can be seen that all the tested liquids have similar contact angles on the as-prepared super-hydrophobic PVC surfaces. This means that the
Table 2 The fractions of air in composite surfaces as-prepared at 17 ◦ C. Items
Values
VHAc /VTHF CA (◦ ) f2 (%)
0 83.3 0
0.5:10.0 97.6 22.3
1.0:10.0 109.5 40.4
1.5:10.0 143.4 82.3
2.0:10.0 146.1 84.8
2.5:10.0 151.5 89.2
Table 3 The roughness of the composite surfaces as-prepared at 17 ◦ C. Items
Values
VHAc /VTHF Ra (m)
0 0.105
0.5:10.0 0.991
1.0:10.0 1.073
1.5:10.0 2.238
2.0:10.0 2.803
2.5:10.0 4.108
Table 4 The contact angles (◦ ) of the porous super-hydrophobic PVC surfaces (volume ratio of glacial acetic acid to tetrahydrofuran 2.5:10.0; drying temperature 17 ◦ C) when immersed in different solutions for 14 days. Contact angle (◦ )
Solutions
H2 O HCl (pH 1) NaOH (pH 13) C2 H5 OH
HCl (pH 1)
H2 O
NaOH (pH 13)
Diiodomethane
Liquid paraffin
153.1 155.1 152.6 153.2
152.5 151.9 152.5 153.3
152.3 152.9 153.1 152.3
0.3 0.9 0.5 0.5
5.3 5.8 6.2 5.1
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Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013
Fig. 4. A drop of diiodomethane was almost completely spread out on the porous super-hydrophobic PVC surface (volume ratio of glacial acetic acid to tetrahydrofuran 2.5:10.0; drying temperature 17 ◦ C).
found that glacial acetic acid had an important effect on the surface topography of the PVC coatings obtained at a proper temperature; and the super-hydrophobicity of PVC surfaces can be effectively manipulated by properly adjusting the dosage of glacial acetic acid in the mixed PVC solution as well as the volatilization temperature of the solution. The PVC coatings obtained from the mixed PVC solution containing 2.5:10.0 (volume ratio) of glacial acetic acid to THF and under a drying temperature of 17 ◦ C possess contact angles of above 150◦ for water and very small contact angles for liquid paraffin and diiodomethane, showing good super-hydrophobicity and super-oleophilicity. Besides, they have good corrosion resistance against acidic and alkali corrosive mediums, showing promising applications in self-cleaning, waterproof for outer wall of building, seawater resistant coating, and separation of oil and water. Fig. 5. The SEM of the porous film which was immersed in the 0.1 mol L−1 hydrochloride acid.
Acknowledgements porous PVC coating should have good corrosion resistance even in the acidic and basic corrosive mediums. At the same time, as-prepared super-hydrophobic PVC surface prepared under a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and drying temperature of 17 ◦ C was super-oleophilic. Namely, it allowed 4 L of liquid paraffin droplet to fast spread in less than 3 s and showed a small contact angle of about 8◦ for liquid paraffin. Also it allowed almost complete fast spreading of diiodomethane (see Fig. 4). We also immersed the prepared surfaces in different solutions for 14 days; Fig. 5 shows the SEM of the immersed surface and confirms that the surface topography was not damaged by the solutions. Then we tested the contact angles, found all the tested liquids have similar contact angles on the as-prepared super-hydrophobic PVC surface (shown in Table 4). It was shown that the films inherited excellent corrosion resistance and can be used continuously for practical purposes. 4. Conclusions Porous super-hydrophobic and super-oleophilic PVC coatings were prepared via a facile solvent/non-solvent coating process below 20 ◦ C without addition of low-surface-energy compounds. The microstructure and wetting behavior plus corrosion resistance of the PVC coatings were investigated with respect to dependence on the dosage of glacial acetic acid and drying temperature of the mixed PVC liquid spread over the glass slide substrate. It has been
The authors are grateful to the Ministry of Science and Technology of China (project of “973” Plan, grant No. 2007CB607606) and National Natural Science Foundation of China (Grant No. 50975077) for the financial support to this research.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation of porous super-hydrophobic and super-oleophilic polyvinyl chloride surface with corrosion resistance property Yingke Kang a , Jinyan Wang a , Guangbin Yang a , Xiujuan Xiong a , Xinhua Chen b,∗∗ , Laigui Yu a , Pingyu Zhang a,∗ a b
Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, PR China College of Chemistry and Chemical Engineering, Xuchang University, Xuchang 461000, PR China
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 16 June 2011 Accepted 24 July 2011 Available online 17 September 2011 Keywords: Polyvinyl chloride Super-hydrophobic surface Super-oleophilic Corrosion resistance
a b s t r a c t Porous super-hydrophobic polyvinyl chloride (PVC) surfaces were obtained via a facile solvent/nonsolvent coating process without introducing compounds with low surface energy. The microstructure, wetting behavior, and corrosion resistance of resultant super-hydrophobic PVC coatings were investigated in relation to the effects of dosage of glacial acetic acid and the temperature of drying the mixed PVC solution spread over glass slide substrate. As-prepared PVC coatings had porous microstructure, and the one obtained at a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and under a drying temperature of 17 ◦ C had a water contact angle of 150 ± 1.5◦ , showing super-hydrophobicity. In the meantime, it possessed very small contact angles for liquid paraffin and diiodomethane and good corrosion resistance against acid and alkali corrosive mediums, showing promising applications in self-cleaning, waterproof for outer wall of building, seawater resistant coating, and efficient separation of oil and water. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Usually, a surface with a water contact angle of above 150◦ and small contact angle hysteresis is called super-hydrophobic surface [1,2]; and there are many natural super-hydrophobic species in nature, such as sacred lotus leaves [3], taro leaves [4], paddy leaves, butterfly’s wings and water strider’s legs [5]. Super-hydrophobic surfaces have promising applications in both biological processes and technological applications, such as oil/water separation [6,7], photoresponsive materials [8], biochemical separation [9], and targeted drug delivery [10]. This is why they have been attracting much attention in both industry and fundamental research. It has been found that the hydrophobicity of a surface can be enhanced by increasing surface roughness within a proper range, because air trapped between water droplet and solid surface is able to minimize the contact area [11–18]. In this respect, various super-hydrophobic surfaces and films have been successfully prepared by manipulating the surface chemistry and surface roughness [19–38]. Erbil et al. [39] found that the introduction of a proper non-solvent led to increase water contact angle of polypropylene coating, since it induced and enhanced the phase separation and hence increased
the surface roughness of the coating. Many other researchers had also focused on the fabrication of various super-hydrophobic surfaces via convenient and inexpensive routes and on investigation of their microstructure and performance as well [39–43]. We are particularly interested in surfaces with both superhydrophobic and super-oleophilic properties, since few publications are currently available about such surfaces that may be used to efficiently separate oil from water. Therefore, in the present research we pay special attention to polyvinyl chloride (PVC), one of the most commonly used and cheap thermoplastics with a wide range of uses in pipes and fittings, profiles, cables, flooring, films and sheets (in recent years, PVC has been replacing traditional building materials such as wood, concrete and clay in many areas; and at least 50% of the market is driven by the construction/housing industry [44]). A facile and inexpensive solvent/non-solvent coating route was thus established to prepare PVC coating possessing super-hydrophobic properties with reference to similar solvent/non-solvent method for fabricating super-hydrophobic PVC coating [45,46]. This article reports the preparation of the super-hydrophobic and super-oleophilic PVC coating and investigation of its properties and microstructure. 2. Experimental
∗ Corresponding author. Tel.: +86 378 3883594; fax: +86 378 3881358. ∗∗ Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected], [email protected] (P. Zhang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.106
2.1. Materials Commercial grade poly(vinyl chloride) resin (2S-85, obtained from Botian Chemical Company of Tianjin, China; average
Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013
molecular weight Mw = 100,000 g mol−1 ; coded as PVC) was used without any further treatment. Tetrahydrofuran (THF, obtained from Fuyu Fine Chemical Company of Tianjin, China) was chosen as the solvent. Glacial acetic acid (obtained from Fuyu Fine Chemical Company of Tianjin, China) was chosen as the non-solvent. Commercially obtained glass slide substrate was sequentially ultrasonically cleaned with distilled water, absolute ethanol and acetone, respectively, each for 10 min, followed by drying with nitrogen gas. 2.2. Preparation of porous super-hydrophobic PVC surfaces 0.1 g of PVC powder was dissolved slowly in 10.0 mL of tetrahydrofuran at ambient temperature to form a solution. Into resultant PVC solution was added 0.5–3.0 mL of glacial acetic acid (concentration 100%) at a rate of 1 mL s−1 . After being stirred for 5 min, a few droplets of resultant mixed solution were dripped onto the surface of cleaned glass slide with a burette and allowed to spread
1009
thereon to form a coating without disturbance. The coated glass slide was dried in atmosphere at pre-set temperatures (0 ◦ C, 10 ◦ C, 17 ◦ C, 24 ◦ C) for 24 h, yielding porous super-hydrophobic PVC surfaces. For a comparative study, PVC solution alone was directly dripped onto the surface of cleaned glass slide and dried in the same manner to form a smooth PVC surface. 2.3. Characterization of porous super-hydrophobic PVC coatings The morphologies of the porous surfaces were examined with a scanning electron microscope (SEM, Hitachi S-3200N; accelerating voltage 5 kV and 10 kV). Revolution TM v1.60b24 (4pi Analysis Inc.) was used for analysis of SEM images. The hydrophobicity of the PVC coatings was evaluated by measuring their contact angles with deionized water, liquid paraffin and diiodomethane, where the static contact angles (CA) of as-prepared coatings at room temperature in atmosphere were measured using a goniometer (Dropmaster 300 solid/liquid interface analyzer,
Fig. 1. SEM images of PVC surfaces obtained from the mixed PVC solutions containing different volume ratio of glacial acetic acid and tetrahydrofuran: (a) 0; (b) 0.5:10.0; (c) 1.0:10.0; (d) 15:10.0; (e) 2.0:10.0; and (f) 2.5:10.0. Insets refer to water contact angles on the as-prepared PVC surfaces.
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Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013
Fig. 2. Approaching, contacting, squeezing and departure processes of a 4 L water droplet suspending on a syringe with the super-hydrophobic surface obtained at a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and drying temperature of 17 ◦ C.
Kyowa Interface Science, Japan). At least five repeat measurements were conducted, each for newly formed sessile drops of about 4 L of deionized water, liquid paraffin and diiodomethane. The averages of the repeat measurements are cited in this article. Besides, the time for the liquid droplets to be fully absorbed by the porous coatings, time to-be-absorbed, was also recorded for each liquid droplet over a 20 min of interval. The surface roughness of the PVC coatings was determined using a YS2206B surface profiler (Harbin Measuring and Cutting Tool Group Company Ltd., China). 3. Results and discussion As shown in Fig. 1a, pure PVC coatings on the glass slide dried at various temperatures (0 ◦ C, 10 ◦ C, 17 ◦ C, 24 ◦ C) are transparent and smooth, and they have a water contact angle of only 87 ± 3◦ . This indicates that the volatilization of tetrahydrofuran has no influence on the surface roughness of pure PVC coatings, and preparing
conditions should be changed so as to obtain super-hydrophobic PVC surfaces by effectively increasing the surface roughness. Thus glacial acetic acid was added into the PVC solution at different volume ratios so as to induce the phase separation and increase the surface roughness. As we expected, the PVC coatings prepared from the mixed solution of PVC and glacial acetic acid were more rough and porous than those obtained from PVC solution alone (see Fig. 1b–f). In the meantime, the pore size (10–50 nm) of the PVC coatings increases with increasing concentration of glacial acetic acid in the mixed solution, largely due to entrapment of more air between the liquid droplet and solid surface therewith, leading to reduce contact area and increase contact angle. Table 1 shows the relationship between the contact angle of the PVC coatings and the content of glacial acetic acid in the mixed solution. It can be seen that the water contact angle of the PVC coatings increases with increasing content of glacial acetic acid when the volume ratio of glacial acetic acid and tetrahydrofuran in the mixed solution is lower than 2.5:10.0. When the volume ratio of glacial
Y. Kang et al. / Applied Surface Science 258 (2011) 1008–1013 Table 1 The relationship between the contact angle of PVC coatings (drying temperatures: 0 ◦ C, 10 ◦ C, 17 ◦ C, and 24 ◦ C) and content of glacial acetic acid in the mixed solution.
150
Contact angle (◦ ) 0 ◦C
0 0.5:10.0 1.0:10.0 1.5:10.0 2.0:10.0 2.5:10.0 3.0:10.0
160
140 10 ◦ C
17 ◦ C
24 ◦ C
88.2 90.6 83.3 113.6 105.2 97.6 119.7 110.8 109.5 122.5 123.2 143.4 136.6 134.5 146.1 149.0 150.5 151.5 No continuous coating was obtained
90.8 88.8 89.7 92.1 105.3 109.1 109.7
CA / Deg.
V(HAc) /V(THF)
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pH=1 pH=4 pH=7 pH=10 pH=13
130 120 110 100 90 80
acetic acid and tetrahydrofuran is higher than 3:10.0, no continuous coating can be obtained via drying at 0 ◦ C, 10 ◦ C, and 17 ◦ C. Super-hydrophobic PVC surfaces can always be obtained by drying the mixed solution spread over the glass slide, with a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0, at 0 ◦ C, 10 ◦ C, and 17 ◦ C. The reason might lie in that glacial acetic acid was volatized so fast above 20 ◦ C and hence it did not cause any damage to the porous surfaces. In one word, glacial acetic acid has an important effect on the surface topography of the PVC coatings obtained at a proper temperature, and the super-hydrophobicity of the PVC surfaces can be effectively manipulated by adjusting the dosage of glacial acetic acid as well as the volatilization temperature of the solution. Super-hydrophobic surface should have a water contact angle greater than 150◦ and small contact angle hysteresis. Fig. 2 schematically shows the approaching, contacting, squeezing and departure processes of a 4 L water droplet suspending on a syringe with the super-hydrophobic surface obtained by spreading the mixed PVC solution of a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 over the glass slide and drying at 17 ◦ C. The arrows show the direction towards which the platform of the goniometer moves. It can be seen that the as-prepared superhydrophobic surface has a very little adhesion to water. Since air can be easily trapped by the pores of the porous PVC surfaces, water droplets on such surfaces can only contact with the tips of the polymer particles, and it cannot penetrate into the pores, due to the presence of air pockets trapped in the pores [47,48]. Cassie and Baxter [13] proposed that the contact angle r on such
0
0.5:10.0 1.0:10.0 1.5:10.0 2.0:10.0 2.5:10.0
V(HAc) / V(THF) Fig. 3. The contact angles (◦ ) of solutions with different pH values on as-prepared PVC coating (drying temperature was 17 ◦ C).
a porous surface comprised of solid and air might be expressed by the following equation: cos r = f1 cos s − f2 = (1 − f2 ) cos s − f2 where f1 and f2 are the fractions of solid surface and air in composite surface, respectively, (in other words, f1 + f2 = 1), s is the equilibrium contact angle on a flat solid surface. According to the equation, if the fraction of air (f2 ) is high enough (see Table 2), corresponding to an increased surface roughness (see Table 3), it will be feasible to prepare a super-hydrophobic surface. Such a super-hydrophobic surface structure can well trap air, resulting in lowered sliding angle [49]. Paying attention to the corrosion resistance of PVC against various acidic liquids and some basic liquids of lower concentrations, we evaluated the wettability of different liquids with different pH values on the porous super-hydrophobic PVC surface. Fig. 3 shows the contact angles (◦ ) of solutions with different pH values on asprepared PVC coatings (drying temperature was 17 ◦ C). It can be seen that all the tested liquids have similar contact angles on the as-prepared super-hydrophobic PVC surfaces. This means that the
Table 2 The fractions of air in composite surfaces as-prepared at 17 ◦ C. Items
Values
VHAc /VTHF CA (◦ ) f2 (%)
0 83.3 0
0.5:10.0 97.6 22.3
1.0:10.0 109.5 40.4
1.5:10.0 143.4 82.3
2.0:10.0 146.1 84.8
2.5:10.0 151.5 89.2
Table 3 The roughness of the composite surfaces as-prepared at 17 ◦ C. Items
Values
VHAc /VTHF Ra (m)
0 0.105
0.5:10.0 0.991
1.0:10.0 1.073
1.5:10.0 2.238
2.0:10.0 2.803
2.5:10.0 4.108
Table 4 The contact angles (◦ ) of the porous super-hydrophobic PVC surfaces (volume ratio of glacial acetic acid to tetrahydrofuran 2.5:10.0; drying temperature 17 ◦ C) when immersed in different solutions for 14 days. Contact angle (◦ )
Solutions
H2 O HCl (pH 1) NaOH (pH 13) C2 H5 OH
HCl (pH 1)
H2 O
NaOH (pH 13)
Diiodomethane
Liquid paraffin
153.1 155.1 152.6 153.2
152.5 151.9 152.5 153.3
152.3 152.9 153.1 152.3
0.3 0.9 0.5 0.5
5.3 5.8 6.2 5.1
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Fig. 4. A drop of diiodomethane was almost completely spread out on the porous super-hydrophobic PVC surface (volume ratio of glacial acetic acid to tetrahydrofuran 2.5:10.0; drying temperature 17 ◦ C).
found that glacial acetic acid had an important effect on the surface topography of the PVC coatings obtained at a proper temperature; and the super-hydrophobicity of PVC surfaces can be effectively manipulated by properly adjusting the dosage of glacial acetic acid in the mixed PVC solution as well as the volatilization temperature of the solution. The PVC coatings obtained from the mixed PVC solution containing 2.5:10.0 (volume ratio) of glacial acetic acid to THF and under a drying temperature of 17 ◦ C possess contact angles of above 150◦ for water and very small contact angles for liquid paraffin and diiodomethane, showing good super-hydrophobicity and super-oleophilicity. Besides, they have good corrosion resistance against acidic and alkali corrosive mediums, showing promising applications in self-cleaning, waterproof for outer wall of building, seawater resistant coating, and separation of oil and water. Fig. 5. The SEM of the porous film which was immersed in the 0.1 mol L−1 hydrochloride acid.
Acknowledgements porous PVC coating should have good corrosion resistance even in the acidic and basic corrosive mediums. At the same time, as-prepared super-hydrophobic PVC surface prepared under a glacial acetic acid to tetrahydrofuran volume ratio of 2.5:10.0 and drying temperature of 17 ◦ C was super-oleophilic. Namely, it allowed 4 L of liquid paraffin droplet to fast spread in less than 3 s and showed a small contact angle of about 8◦ for liquid paraffin. Also it allowed almost complete fast spreading of diiodomethane (see Fig. 4). We also immersed the prepared surfaces in different solutions for 14 days; Fig. 5 shows the SEM of the immersed surface and confirms that the surface topography was not damaged by the solutions. Then we tested the contact angles, found all the tested liquids have similar contact angles on the as-prepared super-hydrophobic PVC surface (shown in Table 4). It was shown that the films inherited excellent corrosion resistance and can be used continuously for practical purposes. 4. Conclusions Porous super-hydrophobic and super-oleophilic PVC coatings were prepared via a facile solvent/non-solvent coating process below 20 ◦ C without addition of low-surface-energy compounds. The microstructure and wetting behavior plus corrosion resistance of the PVC coatings were investigated with respect to dependence on the dosage of glacial acetic acid and drying temperature of the mixed PVC liquid spread over the glass slide substrate. It has been
The authors are grateful to the Ministry of Science and Technology of China (project of “973” Plan, grant No. 2007CB607606) and National Natural Science Foundation of China (Grant No. 50975077) for the financial support to this research.
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