Polyvinylidene fluoride (PVDF)/hydrophobic nano-silica (H-SiO₂) coated superhydrophobic porous materials for water/oil separation

Abstract

The separation of water and oil is a promising work due to the increasing worldwide oil pollution and leakage of chemical solvents. Superhydrophobic porous materials were prepared for their separation. In this work, various porous substrates of copper meshes, filter papers and polyurethane (PU) sponges were chosen to obtain superhydrophobic surfaces. Superhydrophobic surfaces were fabricated by polyvinylidene fluoride (PVDF) and hydrophobic nano-silica particle coatings through a spraying approach. The superhydrophobicity, stability of coatings and oil/water separation effect of these as-coated materials were studied. The results demonstrated that all the surfaces showed high water contact angles (>150°), good reusability and excellent oil selectively. Moreover, the coated filter paper was scaled up for practical use of removing trace water from mineral insulating oil in our group. It is promising that this superhydrophobic coating could be used in more applications.

---

Introduction

Crude oil is usually reported on due to leakage during exploitation, transport and storage. These leakages always damage the environment and ecosystems, as well leading to a severe waste.¹ Recently, oil pipelines ruptured in Russia, leading to a massive oil leakage into the Ob River, and now, the contaminated area has reached 10 hectares. The number of these similar accidents is increasing because of the development of society, and materials that can clean oil and chemical solvent pollution are in high demand.

Superhydrophobic materials have attracted great attention in practical applications due to their excellent properties of drag reduction,^2,3 anti-icing,^4,5 self-cleaning,^6,7 corrosion-resistance.⁸ Besides, an important application is oil/water separation, which could be used in cleaning oil on contaminated water.^9–11 As known, the superhydrophobic surface is influenced by two factors: one is the low-free-energy of the hydrophobic coating; the other is the roughness morphology of the surface.¹² So, coatings which have hydrophobic groups (such as –CH₃, –CH₂–, –F) can increase the superhydrophobicity. The roughness on multiple scales is also important to the superhydrophobicity of surfaces, just like surface protrusions to the hydrophobicity of lotus leaves.

Up to now, various hydrophobic coating materials have been reported to prepare superhydrophobic surface, including inorganic nanoparticles,^13–22 organic polymers,^23–25 organic–inorganic hybrid^26–29 and bio-polymers.^30–33 These coated materials always show excellent hydrophobicity. Polyvinylidene fluoride (PVDF) is a commercially hydrophobic fluoropolymer with low-free-energy and outstanding properties of wear-resistance, acid/bases-proof, thermal stability and mechanical properties.^34–39 Interestingly, a new crystalline phase (β-phase) of PVDF and a new group of C=C is formed in the heat treatment, which would contribute to the improvement of mechanical properties.³⁸ Considering the excellent cross-linking effect and various properties of PVDF, it may be used as a suitable material to fabricate superhydrophobic surface coating with some hydrophobic nanoparticles.

In this work, superhydrophobic composite surface coatings were obtained through spraying technique^40–42 and heat treatment. We prepared the superhydrophobic surfaces on copper meshes, polyurethane sponges and filter papers by PVDF and hydrophobic nano-silica particles. These composite coatings were designed to reduce surface free-energy and to improve the surface roughness, which met the two important points of forming superhydrophobic surfaces. For these fabrication processes, there were no need to use intricate equipment or expensive reagents like fluoroalkyl silanes in the coating materials. Spraying technique was used because of the minimum wastage of precursor solution and advantages of desired thickness.^43,44 Hence, our method was a simple and effective approach to prepare superhydrophobic surfaces. In the water/oil separation processes, oil permeated through the coated copper meshes and filter papers, but the water was resisted by the superhydrophobic surfaces. The coated sponges could absorb oil selectively and then oil was removed by squeezing.

Experimental section

1. Materials and sample preparation

Filter papers, polyurethane (PU) sponges and copper meshes were purchased from a local store. Polyvinylidene fluoride (PVDF) and hydrophobic nano-silica particles were purchased from Aladdin Reagent Co. Ltd. (Shanghai, China). N,N-Dimethylformamide (DMF), acetone, n-hexane, ethanol, nitric acid and petroleum ether were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All chemicals were analytic-grade and used without any further treatment. The preparation procedures were shown in Scheme 1.

Scheme 1 Preparation procedures of copper mesh, filter paper and sponge.

1.1 Procedure for cleaning samples. Sponges and filter papers were ultrasonically cleaned in ethanol for 20 min and then cleaned in deionized water for 3 times. Then they were dried at 80 °C for 30 min. The copper meshes were first cleaned in (5 wt%) nitric acid and then ultrasonically cleaned in ethanol, deionized water successively for 20 min. The copper meshes were dried in nitrogen.

1.2 Preparation of PVDF/SiO₂ suspension. PVDF powder (1 wt%) was dissolved in N,N-dimethylformamide (DMF) by stirring for 20 min at room temperature till complete dissolution. Hydrophobic nano-silica particles (1 wt%) were added in the PVDF solution by stirring for 30 min and ultrasonication for 10 min till a homogeneous suspension was obtained.

1.3 Coating preparation. The PVDF/SiO₂ suspension was sprayed on the clean dried sponges, filter papers and copper meshes by a spray gun and then dried at 80 °C for 30 min.

2. Characterization techniques

Scanning electron microscope (SEM) and Energy Dispersive Spectrometer images were obtained by FEI Quanta 200 (FEI, Holland). Water contact-angles, shedding angles and the images that water droplets fell on surfaces were measured by the DSA 100 (KRŰSS, Germany) using water droplets (10 μL). The mechanical properties of the sponge was measured by a CMT6035 electromechanical universal testing machine (MTS systems company, China). The water and oil were separated through funnels.

Result and discussion

1. Superhydrophobic surfaces

The reason why oil permeate inside the superhydrophobic surfaces while water droplets stand on the surfaces is related to the surface Gibbs energy ΔG. For the spreading processes, according to Young's equationγ^s = γ^ls + γ^l cos θ, and Gibbs equation ΔG = γ^ls + γ^l − γ^s, we derive equation ΔG = −γ^l(cos θ − 1), where θ is contact angle, γ is the interfacial tension, the superscripts s and l are the solid, liquid phases, respectively. For superhydrophobic surfaces, water contact angles are larger than 150°, the ΔG > 0, meaning that water droplets can not spread spontaneously. When θ increases, ΔG increases, which means the harder water droplets spread. On the other hand, the oil contact angles can not be measured (close to 0°). Thus, the ΔG is nearly 0 and the surfaces can be wetted by oil easily.¹³

2. Copper meshes for the separation of oil/water mixtures

Metal meshes, such as copper meshes, are pore-structured that made of connected metal strands with good mechanical strength. After been changed to superhydrophobic surface, these superhydrophobic meshes with hundreds of tiny micron pores have frequently been used for separation of oil/water mixtures. In this paper, a PVDF/SiO₂ superhydrophobic coating was prepared by a simple spraying method. Fig. 1a showed that water droplets (pH = 1, 14) stood on the surface of coated copper mesh with spherical shapes. The water contact angle of this coated copper mesh was about 165.4 ± 2° (Fig. 1b), whereas that of oil could not be measured. The shedding angles⁴⁵ was about 4° (Fig. 1b). When water was dropped on the coated mesh, it rolled on the surfaces and fell down spontaneously (S. 1). It was believed that the nano-structured morphology and PVDF with low-surface-energy on the metal structures contributed to these unique properties. As shown in the scanning electron microscopy (SEM) image of the copper mesh (Fig. 2a), it exhibited a original smooth and neat porous mesh before treated, and coatings covered rough mesh after PVDF/SiO₂ superhydrophobic-functioned (Fig. 2b). Energy Dispersive Spectrometer (EDS) analysis also showed the existence of silicon and fluorine, meaning that the PVDF and nano-silica had been anchored on the surface (Fig. 2a and b). Water droplets that fell on the surface of as-coated mesh were out of shape at the time when they were in contact with the mesh surface, and then, bounced back and stood with spherical shape lastly (Fig. 1d₁–d₄). However, droplets of oil spread out and penetrated through the coated mesh quickly without bounced back. Due to this special surface wettability, the coatings did allow oil and organic solvents but not allow water to spread and penetrate through the mesh. As shown in Fig. 1c₁ and c₂, water (dyed red for easy observation) and oil, such as n-hexane, could be separated with such superhydrophobic copper mesh effectively without external force. When mixture of water and oil solution was poured onto the mesh, the oil quickly permeated through the mesh and fell into the beaker below while water was still on the as-coated mesh, with nearly no visible water (red) existing in the permeated oil. The stability of coatings were tested by water jetting test.⁴⁶ SEM figures (S4 a1 and a2†) showed that the coated mesh retained coatings after water jetting test. The ultrasonic-cleaning method and cycle life were also used to evaluate the stability. It was found that the coated copper mesh could keep superhydrophobicity after 30 minute ultrasonication in ethanol and could be reused for 20 times at lease, showing the good stability of coatings. In summary, it indicated that this superhydrophobic mesh was a good candidate in oil-polluted water treatments.

Fig. 1 (a) Water droplets (pH = 1, 14) stand on the surface of coated copper mesh; (b) photographs of a water droplet on the coated copper mesh with contact angle of 165.4° and shedding angle (about 4°); (c₁ and c₂) separation of water (dyed red) and oil by the coated copper mesh; (d₁–d₄) photographs showing the interaction of a water droplet with the coated copper mesh.

Fig. 2 (a) EDS and SEM images of the original copper mesh; (b) EDS and SEM images of the PVDF/SiO₂ coated copper mesh.

3. Filter paper for the separation of oil/water mixtures

Filter papers are common flexible, inexpensive fiber material that have been frequently used for water/oil separation. Moreover, they usually show excellent mechanical strength owing to their flexibility. In this work, a superhydrophobic filter paper was prepared for separation of trace water from mineral insulating oil by a facile spraying technique with PVDF/hydrophobic SiO₂ nano-particles. The PVDF could provide low-free-energy of the surface and hydrophobic nano-SiO₂ particles formed rough surface, which met the two main requirements of superhydrophobicity. The chemical compositions of our filter paper surfaces were characterized by EDS and morphology by SEM. The EDS analysis detected the exist of silicon and fluorine elements, meaning the PVDF and hydrophobic SiO₂ were covered on the surface (Fig. 3a and b). As shown in SEM images (Fig. 3a and b), the skeletal structures of coated filter paper exhibited uniformly coatings with a dense layer, but the original ones showed smooth. Such superhydrophobic coatings consisted of PVDF and hydrophobic nano-silica particles composition, leading to a significant change in wettability of surfaces. The coated filter paper showed superhydrophobicity with water droplets (pH = 1, 14) exhibiting spherical shapes on the surface (Fig. 4a). The water contact angle of original paper changed from 76.7 ± 1° to 158.9 ± 2° after superhydrophobic treatment (Fig. 4b₁ and b₂). When water was dropped on the as-coated filter paper, it rolled on the surface and fell down spontaneously without adhering (S. 2). Ultra-high-speed video was used to capture the moment that water dropped on such as-coated paper surface (Fig. 4d₁–d₄). As shown, the water droplet deformed elastically when it bumped into the surface and then bounced up to a spherical shape till it stood on the surface. Moreover, the water jetting test,⁴⁶ ultrasonic-cleaning method and cycle life were used to evaluate the stability of coated paper. SEM figures (S4 b1 and b2†) showed the retained coatings after water jetting test, which was attributed to the tight binding of the PVDF and fiber of paper. After separation of oil and water, the contaminated filter paper could be ultrasonic-cleaned in ethanol and dried for reuse at least 36 times. Fig. 4c₁ and c₂ showed the high selectivity of this as-coated filter paper in separation of water (dyed red) and oil, such as n-hexane. When oil/water mixture was poured into the funnel, the oil passed through the filter paper and flowed along the wall of the funnel and dropped into the beaker underneath, while the water stayed in the funnel. Particularly, when this superhydrophobic paper was used to separate trace water from mineral insulating oil,⁴⁷ it could decrease the water content from 90.4 ppm to about 52 ppm (Fig. 5), which reached some special standard. Furthermore, due to its advantages of simple fabrication, excellent durability and high separation efficiency, our group have scaled up the production of large samples for the practical use in trace water remove from mineral insulating oil in voltage transformer.

Fig. 3 (a) EDS and SEM images of the original filter paper; (b) EDS and SEM images of the PVDF/SiO₂ coated filter paper.

Fig. 4 (a) Water droplets (pH = 1, 14) stand on the surface of the coated filter paper; (b₁ and b₂) water contact angles of filter paper before and after PVDF/SiO₂ coated; (c₁ and c₂) separation of water (dyed red) and oil by the coated filter paper; (d₁–d₄) photographs showing the interaction of a water droplet with the coated filter paper.

Fig. 5 Water content of mineral insulating oil before and after filtration by the coated filter paper.

4. Superhydrophobic sponge for the separation of oil/water mixtures

PU sponge is considered as a promising high-capacity absorbent due to its high porosity, larger surface area and good elasticity. In view of its inherent hydrophobicity, it was used as substrate to prepare superhydrophobic absorbent for oil cleanup. SEM showed the sponge images before and after treatment (Fig. 6a and b). Clearly, the surface of original sponge was smooth and flat, while the coated sponge exhibited rough coatings distributing on the skeletal structure. They constituted the hierarchical roughness that required for superhydrophobicity. Energy Dispersive Spectrometer (EDS) analysis also proved the existence of silicon and fluorine on the surface (Fig. 6a and b). These meant that the PVDF and nano-silica had been anchored on the surfaces. As shown in Fig. 7a, water droplets (pH = 1, 14) could easily stand on the surface of coated sponge with spherical shapes rather than permeating into the pores. Water droplets rolled over the surfaces easily instead of adhering to it (S. 3). The contact angles of the original and as-coated sponges were measured to be approximately 64.3 ± 3° and 153.1 ± 2°(Fig. 7b₁ and b₂), however oil droplets spread into the pores quickly when they were measured. The water shedding angle of coated sponge was measured about 7° (inserted Fig. 7b₂). It was because of the hydrophobic group (–F,–CH₂–) and hydrophobic nano-silica that coated on the surface.

Fig. 6 (a) EDS and SEM images of the original sponge; (b) EDS and SEM images of the PVDF/SiO₂ coated sponge.

Fig. 7 (a) Water droplets (pH = 1, 14) stand on the surface of the coated sponge; (b₁ and b₂) water contact angles of the original and PVDF/SiO₂ coated sponge; inserted figure, shedding angle of PVDF/SiO₂ coated sponge; (c₁–c₃) separation of water and crude oil by the coated sponge and squeezing process.

As shown in Fig. 7c, when the coated sponge was dipped into water/oil mixture, it was spontaneously wetted by oil in seconds because of the hydrophobicity and capillary action.¹³ The absorbed oil was removed and collected through simple squeezing process due to the excellent elasticity. After oil removing, the sponge floated on water due to its low density and superhydrophobicity. A result exhibited that no obvious oil floated on water, which demonstrated the high separation efficiency.

As good elastic properties of the as-coated sponges, it could be reused until the oil and water were separated, and then the sponge could be cleaned by n-hexane and dried for next reuse, thus avoiding secondary environmental pollution. This as-coated sponge could be reused for more than 200 times and also showed excellent elasticity and good mechanical durability (Fig. 8a). The oil-absorption capacity, defined as k= (m_s − m_i)/m_i, were measured as 15–25 times of their original weight, relating to the densities of various oils, where m_s is the weight of saturated oil sponge, and m_i is the weight of initial sponge. Moreover, the sponges showed good ability of collecting oils selectively from salty water (the salinity was 0, 3.5%, 7.0%). The selectivity of sponge was investigated even it did not reached the saturation oil adsorption. As shown in Fig. 8b, the mass rates of increased water and initial sponge were almost the same with the increased weight of oil. It exhibited that the superhydrophobic sponge did not absorb water even there were remaining interstice, explaining high oil selectivity of this as-coated sponge. The stability of PVDF/SiO₂ coatings was tested by water jetting test and ultrasonic treatment in ethanol for half an hour. SEM figures of S4(c1 and c2†) showed that the coatings were still covered on the skeleton of sponge, exhibiting excellent binding capacity of coatings. These could imply the coated sponge as a good candidate for the cleaning up of spilled oil.

Fig. 8 (a) Stress–strain curves of the coated sponge in the process of repeated mechanical compression; (b) mass rate of increased water and sponge under different amount of oil in different salty water.

Conclusions

In summary, a water-repellent coating that made of PVDF and hydrophobic nano-silica particles was fabricated by a facile, single-step spraying approach in this work. This water-repellent coating was used on the preparation of various porous substrates of copper meshes, filter papers, and PU sponges. It was found that all the treated materials exhibited superhydrophobicity (contact angles >150°) and excellent mechanical stability (reusability). These superhydrophobic porous materials were used for separation of oil and water. The results showed that the copper mesh could separate water and oil mixture selectively and efficiently. The as-coated PU sponge could absorb oils rapidly and selectively, following the simple squeezing process to reuse. The treated filter paper was used for separation of trace water from mineral insulating oil and showed good results. We also have scaled up the filter papers for practical use in trace water remove from mineral insulating oil in voltage transformer. It was believed that this superhydrophobic coating and spraying technique could provide a facile approach for fabrication of superhydrophobic materials and their expanding application.

Acknowledgements

This research was supported by “the Fundamental Research Funds for the Central Universities” (2015203020208).

References

1. L. Peng, H. Li, Y. Zhang, P. Yu and Y. Luo, RSC Adv., 2014, 4, 46470–46475.
2. M. Cheng, M. Song, H. Dong and F. Shi, Small, 2015, 11, 1665–1671.
3. Y. Tang, T. Fu, Q. Liu and W. Luo, Mater. Sci. Technol., 2015, 31, 730–736.
4. K. Li, X. Zeng, H. Li and X. Lai, Appl. Surf. Sci., 2015, 346, 458–463.
5. Z. Zuo, R. Liao, C. Guo, Y. Yuan, X. Zhao, A. Zhuang and Y. Zhang, Appl. Surf. Sci., 2015, 331, 132–139.
6. X. Yang, L. Zhu, Y. Chen, B. Bao, J. Xu and W. Zhou, Appl. Surf. Sci., 2015, 349, 916–923.
7. W. Li and Z. Kang, Surf. Coat. Technol., 2014, 253, 205–213.
8. Y. H. Fan, Z. J. Chen, J. Liang, Y. Wang and H. Chen, Surf. Coat. Technol., 2014, 244, 1–8.
9. L. Li, B. Li, L. Wu, X. Zhao and J. Zhang, Chem. Commun., 2014, 50, 7831–7833.
10. B. Li, L. Wu, L. Li, S. Seeger, J. Zhang and A. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 11581–11588.
11. L. Wu, J. Zhang, B. Li and A. Wang, Polym. Chem., 2014, 5, 2382–2390.
12. X. Zhang, T. Geng, Y. Guo, Z. Zhang and P. Zhang, Chem. Eng. J., 2013, 231, 414–419.
13. L. Peng, S. Yuan, G. Yan, P. Yu and Y. Luo, J. Appl. Polym. Sci., 2014, 131, 40886.
14. A. Asthana, T. Maitra, R. Büchel, M. K. Tiwari and D. Poulikakos, ACS Appl. Mater. Interfaces, 2014, 6, 8859–8867.
15. S. Sethi and A. Dhinojwala, Langmuir, 2009, 25, 4311–4313.
16. W. Zhang, X. Lu, Z. Xin, C. Zhou and J. Liu, RSC Adv., 2015, 5, 55513–55519.
17. S. C. Guo, F. Wu, L. Fang, C. Y. Mao and Y. Y. Dou, Mater. Technol., 2015, 30, 43–49.
18. D. D. Nguyen, N. H. Tai, S. B. Lee and W. S. Kuo, Energy Environ. Sci., 2012, 5, 7908–7912.
19. Y. Liu, J. Zhou, E. Zhu, J. Tang, X. Liu and W. Tang, Carbon, 2015, 82, 264–272.
20. J. N. Wang, Y. L. Zhang, Y. Liu, W. Zheng, L. P. Lee and H. B. Sun, Nanoscale, 2015, 7, 7101–7114.
21. L. Wang, X. Chen, D. Yan, Y. Yang and Z. Chu, Ceram. Int., 2015, 41, 9801–9805.
22. A. Srinivasa Rao and S. Sakthivel, J. Alloys Compd., 2015, 644, 906–915.
23. S. Yun, H. Luo and Y. Gao, J. Mater. Chem. A, 2014, 2, 14542–14549.
24. M. Psarski, D. Pawlak, J. Grobelny and G. Celichowski, J. Adhes. Sci. Technol., 2015, 29, 2035–2048.
25. G. Jiang, R. Hu, X. Wang, X. Xi, R. Wang, Z. Wei, X. Li and B. Tang, J. Text. Inst., 2013, 104, 790–797.
26. X. Yang, Z. Yin, F. Chen, J. Hu and Y. Yang, Sci. Total Environ., 2015, 529, 182–190.
27. S. He, S. Zhang and C. Lu, Colloids Surf., A, 2011, 387, 86–91.
28. C. C. Chang, K. H. Wei and W. C. Chen, J. Electrochem. Soc., 2003, 150, F147–F150.
29. Q. P. Liu, L. X. Gao, Z. W. Gao and L. Yang, Mater. Lett., 2007, 61, 4456–4458.
30. M. C. Choi, G. Sung, S. Nagappan, M. H. Han and C. S. Ha, J. Nanosci. Nanotechnol., 2012, 12, 5788–5793.
31. H. Sharififard, M. Soleimani and F. Z. Ashtiani, J. Taiwan Inst. Chem. Eng., 2012, 43, 696–703.
32. W. W. Y. Chow, S. Herwik, P. Ruther, E. Göthelid and S. Oscarsson, Appl. Surf. Sci., 2012, 258, 7864–7871.
33. A. M. Stepan, F. Ansari, L. Berglund and P. Gatenholm, Compos. Sci. Technol., 2014, 98, 72–78.
34. Y. Yu, H. Chen, Y. Liu, V. S. J. Craig, L. H. Li, Y. Chen and A. Tricoli, Polymer, 2014, 55, 5616–5622.
35. B. N. Sahoo and K. Balasubramanian, RSC Adv., 2015, 5, 6743–6751.
36. N. Jia, Q. Xing, X. Liu, J. Sun, G. Xia, W. Huang and R. Song, J. Colloid Interface Sci., 2015, 453, 169–176.
37. Y. Xiang, F. Liu and L. Xue, J. Membr. Sci., 2015, 476, 321–329.
38. H. Wang, Z. Liu, E. Wang, R. Yuan, D. Gao, X. Zhang and Y. Zhu, Appl. Surf. Sci., 2015, 332, 518–524.
39. B. N. Sahoo and K. Balasubramanian, J. Colloid Interface Sci., 2014, 436, 111–121.
40. J. Li, L. Yan, H. Li, J. Li, F. Zha and Z. Lei, RSC Adv., 2015, 5, 53802–53808.
41. Y. Li, S. Chen, M. Wu and J. Sun, Adv. Mater., 2014, 26, 3344–3348.
42. W. Wu, X. Wang, X. Liu and F. Zhou, ACS Appl. Mater. Interfaces, 2009, 1, 1656–1661.
43. J. Li, R. Wu, Z. Jing, L. Yan, F. Zha and Z. Lei, Langmuir, 2015, 31, 10702–10707.
44. H. Kim and Y. Cho, Colloids Surf., A, 2015, 465, 77–86.
45. J. Zhang and S. Seeger, Adv. Funct. Mater., 2011, 21, 4699–4704.
46. B. Li and J. Zhang, Carbon, 2015, 93, 648–658.
47. L. Zhou, P. Yu, Y. He, H. Xia, X. Guo and Y. Luo, RSC Adv., 2015, 5, 92947–92953.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17728f

---