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A novel method for the fabrication of superhydrophobic nylon net
Accepted Manuscript A Novel Method for the fabrication of superhydrophobic Nylon Net Xin Di, Wenbo Zhang, Deli Zang, Feng Liu, Yazhou Wang, Chengyu Wang PII: DOI: Reference:
S1385-8947(16)30944-5 http://dx.doi.org/10.1016/j.cej.2016.06.137 CEJ 15447
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
17 March 2016 9 June 2016 29 June 2016
Please cite this article as: X. Di, W. Zhang, D. Zang, F. Liu, Y. Wang, C. Wang, A Novel Method for the fabrication of superhydrophobic Nylon Net, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej. 2016.06.137
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A Novel Method for the fabrication of superhydrophobic Nylon Net Xin Di, Wenbo Zhang, Deli Zang, Feng Liu, Yazhou Wang, Chengyu Wang*, Key Laboratory of Bio-based Material Science and Technology, The Research Center of Wood Bionic Intelligence, Ministry of Education, Northeast Forestry University, Harbin 150040, China *Corresponding Author. E-mail: [email protected], Tel (Fax): +86-451-82190116 Abstract: In this study, we demonstrate a facile, cost-effective, and scalable method to fabricate superhydrophobic nylon net surface based on textured adhesive coating in combination with hydrophobization. The nylon net was immersed into an N-Methyl-2-pyrrolodone solution containing one-component polyurethane adhesive and then directly transferred into a glycerol aqueous solution to form a textured adhesive coating spontaneously. The silanization was further achieved through polar groups on the adhesive coating with alkylsilane compounds, forming self-assembled monolayers on the surface of nylon net. This resulted in our ability to tune the surface properties of the nylon net from being hydrophilic (18°) to superhydrophobic. The silanized adhesive coating endowed the nylon net with a water contact angle of 152° and a sliding angle less than 5°. When applied to oil-water separation, the superhydrophobic nylon mesh maintained its water repellency even after 60 cycles of oil-water separation and the separation efficiency (n-hexane) was over 95%. The method developed was also successfully applied to construct superhydrophobic filter
paper, fiberglass cloth and glass slide. Moreover, this method can be simplified when applied to construct superhydrophobic filter paper. Therefore, this cost-effective, easily-operated and environment friendly method offers great technological promise in the field of constructing superhydrophobic coatings on a large scale. Keywords: Nylon net; One-component PU; Superhydrophobic; Silanization; Oil-water separation; 1.
introduction It is well-known that the superhydrophobic property with water contact angle
(WCA) greater than 150° and sliding angle (SA) less than 10°[1-3] has motivated enormous interest because of its potential value in practical application. There are many useful properties about superhydrophobic surfaces, such as oil-water separation [4-8], self-cleaning [9, 10], anticorrosion [11], water repellency [12, 13], mechanical robustness [14, 15] and so on. The key point to fabricate superhydrophobic surface is the combination of micro-nano hierarchical structure with materials having low surface free energy [16, 17]. Up to now, kinds of methods have been developed to construct superhydrophobic surface, such as layer-by-layer assembly [18, 19], laser process [20, 21], solution-immersion method [22, 23], sol-gel technique [24, 25], chemical etching [26, 27], vapor deposition [28] and so forth. Nevertheless, some of the fabrication approaches are potentially costly, environmentally harmful and time-consuming in practical areas. With the development of offshore oil exploitation, water pollution has been a major threat to ecosystem. Therefore, researchers keep a watch eye on oil-water
separation materials to solve the problems of oil pollution. The separation materials for spilled oil disposal are being developed, such as activated carbon [29], separators [30], film membranes [31, 32], porous materials [33], and waste barley straw [34]. However, because of some common weaknesses: high cost, non-renewable, time-consuming, low separation performance, new materials with high separating efficiency, inexpensive and reusable capacities are urgently needed. In the new research, superhydrophobic surfaces have been designed with various properties such as low-cost, mechanical robustness, excellent separation performance and easy operation in terms of oil-water separation [35, 36]. Wang et al. [37] manufactured superhydrophobic fabrics and sponges with nanocrystals and thiols, which presented excellent features in oil-water separation. Wu et al. [38] adopted a chemical vapor deposition method to bind Fe3O4 nanoparticles onto polyurethane sponge surface and then the sponge was modified by tetraethoxysilane. Finally, the superhydrophobic sponge was successfully used for oil absorption and oil-water separation. In reality, most of studies utilize adhesives to bond substrates and inorganic particles together for constructing robust textured surfaces [39, 40]. However, there were no reports on utilizing adhesive solely to construct roughness on substrates, which will be easier to texture surfaces and reduce particle pollutants remarkably. In this study, a kind of commercial adhesive (one-component PU) with waterproof and anti-corrosion properties was applied to create a textured coating on nylon net surface and then the as-prepared sample was modified by Octadecyltrichlorosilane (OTS) to
decrease surface free energy. The alkylsilane covalently bonded to the nylon net surface for the reason that the as-prepared nylon net surface treated by one-component PU possessed many polar groups. This novel technology of preparing superhydrophobic surfaces more simple compared to those traditional methods [41] and is also economical practical owing to the accessible and low-cost materials. When applied to oil-water separation, the superhydrophobic nylon net showed excellent separation performance. Moreover, this method is successfully applied to filter paper, fiberglass cloth and glass slide. 2. Materials and method 2.1 Materials One-component polyurethane (PU) was purchased from Shanghai Changcheng Fine chemistry Co., Ltd. N-Methyl-2-pyrrolodone (C5H9NO, NMP, Analytical reagent) was purchased from Tianjin Bodi Fine chemistry Co., Ltd. N-hexane was purchased from Tianjin Fuyu Co., Ltd. Octadecyltrichlorosilane (OTS) was provided by New Jersey. Glycerol was purchased from Tianjin Hengxing chemical manufacturing Co., Ltd. Sudan III was provided by Sinopharm Chemical Reagent Co., Ltd. Deionized water was self-made. Nylon net, glass side and fiberglass cloth were purchased locally. Filter paper was purchased from Hangzhou Special Paper Co., Ltd. All of the chemicals were used as received without further purification. 2.2 Synthesis of superhydrophobic nylon net In order to obtain textured surface, nylon net samples were immersed into an N-Methyl-2-pyrrolodone solution with the solute one-component PU (5g/L) for 2 h,
and then it was taken out and suspended in air about 30 s before being immersed into a mixture of glycerol and deionized water (glycerol aqueous solution, 7/3, v/v). After 6 h, the nylon net was taken out and rinsed with abundant deionized water to remove surface impurity and then dried at 60 °C for 2h. Finally the treated net was dipped into OTS/n-hexane (1%, v/v) for 2 h and subsequently the washed material was dried thoroughly to endow it with superhydrophobicity. 2.3 Characterization The surface morphology was observed by a scanning electron microscopy (SEM, FEI QUANTA200) operating at 15kV. A PicoPlus II AFM system from Molecular Imaging Inc. was used. AFM images were obtained at 512×512 resolution. The surface chemistry composition was investigated by Fourier transform infrared spectroscopy (FT-IR, Thermo Fisher Scientific, Nicolet 6700). For contact angles (CAs) measurements, 5µL deionized water dropped on the surface of the sample at room temperature, which was detected on a contact angle meter (Hitachi, CA-A) in at least five different places of the surface. 2.4 Oil-water separation and stability experiment Nylon net samples are 4 mm × 5 mm in size. An as-prepared nylon net covered the top of vial bottle. The separation method was to pass an immiscible mixture of n-hexane and water (1/1, v/v) through 80 mesh nylon net covering the bottle. The volumes of n-hexane before and after the Oil-water separation were measured as V0 and V1 by a graduated measuring glass cylinder with an accuracy of 0.1 ml respectively. The oil recovery efficiency is defined as Q (%) and calculated with equation (1):
Q =
(1)
, where V0 is the initial volume of the n-hexane and V1 is the final volume of the n-hexane. Chemical durability and structural stability of nylon net was investigated to ensure its practical use. Chemical durability was assessed by measuring contact angle of the synthetically sample which had been immersed into aqueous solution with pH values range from 1 to 13 for 12h. The structural stability of superhydrophobic product through cycling experiments of oil-water separation. 3. Results and discussion 3.1 Mechanism and surface morphology The mechanism for the formation of a superhydrophobic coating on nylon cloth fiber is showed in Fig. 1. As shown in Fig. 1a: Firstly, when nylon net was immersed into a liquid mixture of one-component PU and NMP, it was coated with the mixture because of the capillarity of the net and the stickiness of the adhesive. Secondly, When the PU-treated nylon net was immersed into glycerin aqueous solution, the NMP solvent on nylon net surface was removed with water while the polymer gel particles grew up radially [42]. The chemical reaction was shown in Fig. 1b. Thirdly, two reactions simultaneously occurred in the third step: I) direct hydrogen bonding between the -Si-OH groups of the hydrolyzed silane and the -N-H groups on the coated fiber; II) condensation of the –Si-OH groups to form a siloxane polymer ,which interact with N-H via hydrogen bonding. Finally, during the drying step, the -Si-O-Si- groups became bound to the fiber surface via a polycondensation.
Since isocyanate compounds in PU solution are sensible to water, it is easy to synthesize polyurethane in an organic system. Therefore, in this paper, one-component PU was firstly prepared in a NMP solution which was water-soluble and non-aqueous. And then the soaked nylon net was immersed into glycerin aqueous solution in order to obtain polymer gel particles. As shown in Fig. 1b, during the curing process, the reaction of isocyanate with water (main reaction) leads to the formation of urea with release of CO2[43]. As Fig. 1b shows, the rate of the chemical reaction between one-component PU and deionized water is fast. Therefore, to obtain suitable surface morphology, the ratio of water in glycerin solution has to be well controlled. Glycerol, a kind of inert solvent for one-component PU, could reduce the rate of the chemical reaction between one-component PU and deionized water and affect the formation of the micro-nano particles.
Fig. 1 (a) Schematic illustration for superhydrophobic nylon net; (b) Curing reaction process of one-component PU in deionized water. 3.2 Surface morphology The surface morphology of nylon net was shown in Fig. 2. SEM images of the
pristine sample (Fig. 1a and 1b) showed that the pristine nylon net possessed a smooth surface, which was regularly knitted. The particles in the size from a few hundred nanometers to several micrometers came into being on the nylon surface after the curing process.
Fig. 2 SEM images of (a-b) pristine nylon net and (c-d) superhydrophobic nylon net different magnifications. To obtain the roughness features, pristine surface and superhydrophobic nylon net surface was analyzed by PicoPlus II AFM from Molecular Imaging (MI) Corporation (AZ, USA). The AFM images shown in Fig. 6 was indicative of the surface roughening impact associated with the existing particle system.
Fig. 3 AFM images of single fibers of the nylon net: (a) pristine fiber and (b) superhydrophobicsuperhydrophobic fiber. 3.3 Surface wettability of nylon net surface The wettability of nylon net was studied by measuring contact angle on sample surface. As shown in Table 1, two main conclusions were obtained: i) When the volume ratio of glycerol and water remained unchanged, the WCAs of the as-prepared nylon net increased at first and decreased later with the increase of the concentration of NMP; ii) When the concentration of NMP was 10g/L, the WCAs increased with the increase of concentration of glycerol. When the concentration of NMP was 30g/L, the WCAs increased at first and flattened later with the increase of various ratio of glycerol in glycerin solution. When the concentration of NMP was 50g/L and 70g/L, the WCAs increased at first and decreased later with the increase of various ratio of glycerol in glycerin solution. There are two possible reasons for the above main conclusions. Firstly, when the concentration of PU solution is low, because of the low viscosity of PU solution, the formed adhesive particles on nylon net surface can peel off easily in a low concentration of glycerin solution during the curing process, which makes it
considerably difficult to form surface roughness. Secondly, when the concentration of PU solution is high, it is difficult to form adhesive particles in a high concentration of glycerin solution. Therefore, the ratio of glycerol in glycerin solution has to be well controlled. Table 1 The effect of different mass-volume concentration of one-component PU solution and various ratio of glycerol in glycerin solution on water contact angle of hydrophobic. The concentration of PU in NMP(g/L)
10g/L
30g/L
50g/L
70g/L
The volume ratio of glycerol and water
WCA(°)
WCA(°)
WCA(°)
WCA(°)
1:9
114
143
148
142
3:7
140
147
148
147
1:1
145
148
150
145
7:3
146
149
152
145
9:1
148
149
131
119
Fig. 4 Variations in the WCA of as-prepared nylon net with the volume ratio of glycerol and water at given concentration of PU in NMP. The WCA of the pristine net sample was 18° as seen in Fig. 5a, which could be explained by the imino groups on the sample surface. As presented in Fig. 5b, the WCA of OTS-treated nylon mesh surface was 125°, revealing that only low surface energy without enough roughness would not lead to superhydrophobicity. As shown in Fig. 5c, the textured nylon surface showed a WCA of 0 °. After treated with OTS, the textured nylon surface became superhydrophobic (WCA = 152 °), as shown in Fig. 5d.
Fig. 5 Images of water droplets on different samples: 5µL water droplet on (a) pristine nylon net, (b) OTS-treated nylon net, (c) textured nylon net (d) superhydrophobic nylon net. Generally, for a totally smooth surface, the wettability is described by means of Young equation, which is recognized as the theoretical principle of surface wettability. Young equation: cos =
(2)
where γsv, γsl, γlv are the interfacial free energy of solid/vapor, solid/liquid, and liquid/vapor, respectively. Obviously in a real case, surfaces are unlikely to be completely smooth, which means that Young’s equation does not tally with the actual situation. In order to overcome this apparent drawback, Wenzel modified Eq.(1) and answered the connect angle θr of liquid droplet on a rough homogeneous surface: = r
(3)
where r is the roughness factor; θr and θ represent the contact angle on a rough surface and Young's contact angle on a smooth surface, respectively. For material with contact angle less than 90°, the real value of θr will decrease along with the increase of
surface roughness. For a deep consideration of wettability, Cassie proposed a novel roughness equation which can be applied to heterogeneous roughness and low energy surface. = −
(4)
where f1 is fraction of liquid/solid and f2 is fraction of liquid/vapor, θc and θ are the water contact angle on rough and flat surfaces, respectively. In this model, the superhydrophobic property of the sample surface will be enhanced with the increase of air cushion part proportion. Here, the value of θc on the superhydrophobic surface is 152° and the θ on the smooth net surface (modified by OTS) is 125°. Thus, the fraction of air is calculated by Eq. (3) to be 0.75. 3.4 Surface composition The one-component PU and OTS coating on nylon net surface were detected by FT-IR instrument. Fig. 6 displayed that peak at 1726 cm−1 represented an obvious C=O stretching vibration on account of one-component PU [44]. The absorption peak at 1115 cm−1 was attributed to the C-O stretching vibration [45] ,which was also from one-component PU. Four absorption peaks at 3300 cm−1, 2927 cm−1 , 2858 cm−1 and 1025 cm−1 were assigned to N-H bonds, -CH3 bonds, -CH2 bonds and Si-O-Si bonds, respectively [46, 47]. In summary, it is indicated that the well-produced superhydrophobic nylon net was attributed to the combination of the one-component PU with OTS.
Fig. 6 FT-IR spectra of (a) pristine net surface, (b) one-component PU treated net surface and (c) superhydrophobic net surface. 3.5 Oil-water separation and stability experiment Robustness plays an important role in practical applications, so chemical durability and cycling experiments of oil-water separation of the as-prepared nylon net were investigated. The WCAs of the samples were stable between 152° to 150° showing that the superhydrophobic nylon net owned excellent chemical durability against erosion of acid and alkali. Furthermore, we evaluated the robustness of superhydrophobic product through cycling experiments of oil-water separation. The separation was repeated 60 times, displaying an excellent structural stability with WCA no less than 149° (Fig. 7a) and recovery efficiency of n-hexane over 95% (Fig. 7b).
Fig. 7 (a) Variations of water contact angle of as-prepared nylon net surface with pH ranging from 1 to 13. (b) Variation of water contact angle and separation efficiency of superhydrophobic nylon net with recycle numbers of oil-water separation. Fig. 8 showed the process of the superhydrophobic nylon surface for removal of oil from water. When oil-water mixture was poured onto the surface, the oil would penetrate nylon mesh quickly and fall into the vessel. Meanwhile, water was blocked and tumbled out of nylon surface into outboard beaker along the exterior of vessel. As a result, the superhydrophobic nylon net exhibited an excellent separation performance as shown in Fig. 8e.
Fig. 8 Images of oil removal by superhydrophobic nylon net. (a) vial bottle sealed with superhydrophobic nylon net, (b-e) observable process of oil-water separation and n-hexane stained by Sudan III.
3.6 Application in constructing superhydrophobic filter paper, fiberglass cloth and slide glass Fig. 9 displayed optical images of liquid drops on superhydrophobic substrates (left) and original substrates (right). The results showed that when the mass-volume concentration of one-component PU was 50g/L and the ratio of glycerol and deionized water was 7/3, the obtained nylon net had the best hydrophobicity after being treated with OTS. In addition, according to above mentioned optimal condition, the optimization was successfully applied to the construction of superhydrophobic filter paper and fiberglass cloth directly with WCAs of 152° (Fig. 9a) and 151°(Fig. 9b) respectively. Moreover, because of the fine adsorptive effect, the filter paper can be textured adequately after being treated by deionized water only and further treatment with OTS make it super-hydrophobic with a WCA as high as 151° (Fig. 9c), displaying an easier and more inexpensive fabrication compared to other surfaces. Furthermore, a further study has been carried out to construct super-hydrophobic glass slide. The experiment exhibited that when the mass-volume concentration of one-component PU was 100g/L and the volume ratio of glycerol and deionized water was 1/1, the obtained glass slide had the best hydrophobic property after being treated with OTS. As is showed in Fig. 9d, the WCA of the superhydrophobic glass slide was 156°. Fig. 9 (a-d) also displayed that all of the original substrates were hydrophilic.
Fig. 9 Optical images of liquid drops on superhydrophobic substrates (left) and original substrates (right). (a) filter paper (b) fiberglass cloth (c) filter paper, (d) glass slide.4. Summary In summary, we have proposed a novel, flexible and inexpensive method to manufacture superhydrophobic nylon net surface. The result represented that superhydrophobic nylon net surface was obtained through combining adhesive particles with OTS. The as-prepared nylon net displayed excellent superhydrophobicity, outstanding oil-water separation capability and stability. This constructing approach was also applicable to the fabrication of superhydrophbic filter paper, fiberglass cloth and glass slide. Therefore, this cost-effective, easily-operated and environment friendly method
offers
great
technological promise
in the
field
of
constructing
superhydrophobic coatings on a large scale. Acknowledgements： ： This research was supported by the Fundamental Research Funds for the Central Universities (DL12EB05-01) and program for New Century Excellent Talents in University (NCET-10-0311). References: [1] Q.F. Xu, B. Mondal, A.M. Lyons, Fabricating superhydrophobic polymer surfaces with excellent
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Highlights This method utilizes adhesive solely to construct roughness on substrates. This approach is successfully applied to filter paper, fiberglass cloth and glass slide. The superhydrophobic nylon net maintained its water repellency even after 60 cycles of oil-water separation.
S1385-8947(16)30944-5 http://dx.doi.org/10.1016/j.cej.2016.06.137 CEJ 15447
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
17 March 2016 9 June 2016 29 June 2016
Please cite this article as: X. Di, W. Zhang, D. Zang, F. Liu, Y. Wang, C. Wang, A Novel Method for the fabrication of superhydrophobic Nylon Net, Chemical Engineering Journal (2016), doi: http://dx.doi.org/10.1016/j.cej. 2016.06.137
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A Novel Method for the fabrication of superhydrophobic Nylon Net Xin Di, Wenbo Zhang, Deli Zang, Feng Liu, Yazhou Wang, Chengyu Wang*, Key Laboratory of Bio-based Material Science and Technology, The Research Center of Wood Bionic Intelligence, Ministry of Education, Northeast Forestry University, Harbin 150040, China *Corresponding Author. E-mail: [email protected], Tel (Fax): +86-451-82190116 Abstract: In this study, we demonstrate a facile, cost-effective, and scalable method to fabricate superhydrophobic nylon net surface based on textured adhesive coating in combination with hydrophobization. The nylon net was immersed into an N-Methyl-2-pyrrolodone solution containing one-component polyurethane adhesive and then directly transferred into a glycerol aqueous solution to form a textured adhesive coating spontaneously. The silanization was further achieved through polar groups on the adhesive coating with alkylsilane compounds, forming self-assembled monolayers on the surface of nylon net. This resulted in our ability to tune the surface properties of the nylon net from being hydrophilic (18°) to superhydrophobic. The silanized adhesive coating endowed the nylon net with a water contact angle of 152° and a sliding angle less than 5°. When applied to oil-water separation, the superhydrophobic nylon mesh maintained its water repellency even after 60 cycles of oil-water separation and the separation efficiency (n-hexane) was over 95%. The method developed was also successfully applied to construct superhydrophobic filter
paper, fiberglass cloth and glass slide. Moreover, this method can be simplified when applied to construct superhydrophobic filter paper. Therefore, this cost-effective, easily-operated and environment friendly method offers great technological promise in the field of constructing superhydrophobic coatings on a large scale. Keywords: Nylon net; One-component PU; Superhydrophobic; Silanization; Oil-water separation; 1.
introduction It is well-known that the superhydrophobic property with water contact angle
(WCA) greater than 150° and sliding angle (SA) less than 10°[1-3] has motivated enormous interest because of its potential value in practical application. There are many useful properties about superhydrophobic surfaces, such as oil-water separation [4-8], self-cleaning [9, 10], anticorrosion [11], water repellency [12, 13], mechanical robustness [14, 15] and so on. The key point to fabricate superhydrophobic surface is the combination of micro-nano hierarchical structure with materials having low surface free energy [16, 17]. Up to now, kinds of methods have been developed to construct superhydrophobic surface, such as layer-by-layer assembly [18, 19], laser process [20, 21], solution-immersion method [22, 23], sol-gel technique [24, 25], chemical etching [26, 27], vapor deposition [28] and so forth. Nevertheless, some of the fabrication approaches are potentially costly, environmentally harmful and time-consuming in practical areas. With the development of offshore oil exploitation, water pollution has been a major threat to ecosystem. Therefore, researchers keep a watch eye on oil-water
separation materials to solve the problems of oil pollution. The separation materials for spilled oil disposal are being developed, such as activated carbon [29], separators [30], film membranes [31, 32], porous materials [33], and waste barley straw [34]. However, because of some common weaknesses: high cost, non-renewable, time-consuming, low separation performance, new materials with high separating efficiency, inexpensive and reusable capacities are urgently needed. In the new research, superhydrophobic surfaces have been designed with various properties such as low-cost, mechanical robustness, excellent separation performance and easy operation in terms of oil-water separation [35, 36]. Wang et al. [37] manufactured superhydrophobic fabrics and sponges with nanocrystals and thiols, which presented excellent features in oil-water separation. Wu et al. [38] adopted a chemical vapor deposition method to bind Fe3O4 nanoparticles onto polyurethane sponge surface and then the sponge was modified by tetraethoxysilane. Finally, the superhydrophobic sponge was successfully used for oil absorption and oil-water separation. In reality, most of studies utilize adhesives to bond substrates and inorganic particles together for constructing robust textured surfaces [39, 40]. However, there were no reports on utilizing adhesive solely to construct roughness on substrates, which will be easier to texture surfaces and reduce particle pollutants remarkably. In this study, a kind of commercial adhesive (one-component PU) with waterproof and anti-corrosion properties was applied to create a textured coating on nylon net surface and then the as-prepared sample was modified by Octadecyltrichlorosilane (OTS) to
decrease surface free energy. The alkylsilane covalently bonded to the nylon net surface for the reason that the as-prepared nylon net surface treated by one-component PU possessed many polar groups. This novel technology of preparing superhydrophobic surfaces more simple compared to those traditional methods [41] and is also economical practical owing to the accessible and low-cost materials. When applied to oil-water separation, the superhydrophobic nylon net showed excellent separation performance. Moreover, this method is successfully applied to filter paper, fiberglass cloth and glass slide. 2. Materials and method 2.1 Materials One-component polyurethane (PU) was purchased from Shanghai Changcheng Fine chemistry Co., Ltd. N-Methyl-2-pyrrolodone (C5H9NO, NMP, Analytical reagent) was purchased from Tianjin Bodi Fine chemistry Co., Ltd. N-hexane was purchased from Tianjin Fuyu Co., Ltd. Octadecyltrichlorosilane (OTS) was provided by New Jersey. Glycerol was purchased from Tianjin Hengxing chemical manufacturing Co., Ltd. Sudan III was provided by Sinopharm Chemical Reagent Co., Ltd. Deionized water was self-made. Nylon net, glass side and fiberglass cloth were purchased locally. Filter paper was purchased from Hangzhou Special Paper Co., Ltd. All of the chemicals were used as received without further purification. 2.2 Synthesis of superhydrophobic nylon net In order to obtain textured surface, nylon net samples were immersed into an N-Methyl-2-pyrrolodone solution with the solute one-component PU (5g/L) for 2 h,
and then it was taken out and suspended in air about 30 s before being immersed into a mixture of glycerol and deionized water (glycerol aqueous solution, 7/3, v/v). After 6 h, the nylon net was taken out and rinsed with abundant deionized water to remove surface impurity and then dried at 60 °C for 2h. Finally the treated net was dipped into OTS/n-hexane (1%, v/v) for 2 h and subsequently the washed material was dried thoroughly to endow it with superhydrophobicity. 2.3 Characterization The surface morphology was observed by a scanning electron microscopy (SEM, FEI QUANTA200) operating at 15kV. A PicoPlus II AFM system from Molecular Imaging Inc. was used. AFM images were obtained at 512×512 resolution. The surface chemistry composition was investigated by Fourier transform infrared spectroscopy (FT-IR, Thermo Fisher Scientific, Nicolet 6700). For contact angles (CAs) measurements, 5µL deionized water dropped on the surface of the sample at room temperature, which was detected on a contact angle meter (Hitachi, CA-A) in at least five different places of the surface. 2.4 Oil-water separation and stability experiment Nylon net samples are 4 mm × 5 mm in size. An as-prepared nylon net covered the top of vial bottle. The separation method was to pass an immiscible mixture of n-hexane and water (1/1, v/v) through 80 mesh nylon net covering the bottle. The volumes of n-hexane before and after the Oil-water separation were measured as V0 and V1 by a graduated measuring glass cylinder with an accuracy of 0.1 ml respectively. The oil recovery efficiency is defined as Q (%) and calculated with equation (1):
Q =
(1)
, where V0 is the initial volume of the n-hexane and V1 is the final volume of the n-hexane. Chemical durability and structural stability of nylon net was investigated to ensure its practical use. Chemical durability was assessed by measuring contact angle of the synthetically sample which had been immersed into aqueous solution with pH values range from 1 to 13 for 12h. The structural stability of superhydrophobic product through cycling experiments of oil-water separation. 3. Results and discussion 3.1 Mechanism and surface morphology The mechanism for the formation of a superhydrophobic coating on nylon cloth fiber is showed in Fig. 1. As shown in Fig. 1a: Firstly, when nylon net was immersed into a liquid mixture of one-component PU and NMP, it was coated with the mixture because of the capillarity of the net and the stickiness of the adhesive. Secondly, When the PU-treated nylon net was immersed into glycerin aqueous solution, the NMP solvent on nylon net surface was removed with water while the polymer gel particles grew up radially [42]. The chemical reaction was shown in Fig. 1b. Thirdly, two reactions simultaneously occurred in the third step: I) direct hydrogen bonding between the -Si-OH groups of the hydrolyzed silane and the -N-H groups on the coated fiber; II) condensation of the –Si-OH groups to form a siloxane polymer ,which interact with N-H via hydrogen bonding. Finally, during the drying step, the -Si-O-Si- groups became bound to the fiber surface via a polycondensation.
Since isocyanate compounds in PU solution are sensible to water, it is easy to synthesize polyurethane in an organic system. Therefore, in this paper, one-component PU was firstly prepared in a NMP solution which was water-soluble and non-aqueous. And then the soaked nylon net was immersed into glycerin aqueous solution in order to obtain polymer gel particles. As shown in Fig. 1b, during the curing process, the reaction of isocyanate with water (main reaction) leads to the formation of urea with release of CO2[43]. As Fig. 1b shows, the rate of the chemical reaction between one-component PU and deionized water is fast. Therefore, to obtain suitable surface morphology, the ratio of water in glycerin solution has to be well controlled. Glycerol, a kind of inert solvent for one-component PU, could reduce the rate of the chemical reaction between one-component PU and deionized water and affect the formation of the micro-nano particles.
Fig. 1 (a) Schematic illustration for superhydrophobic nylon net; (b) Curing reaction process of one-component PU in deionized water. 3.2 Surface morphology The surface morphology of nylon net was shown in Fig. 2. SEM images of the
pristine sample (Fig. 1a and 1b) showed that the pristine nylon net possessed a smooth surface, which was regularly knitted. The particles in the size from a few hundred nanometers to several micrometers came into being on the nylon surface after the curing process.
Fig. 2 SEM images of (a-b) pristine nylon net and (c-d) superhydrophobic nylon net different magnifications. To obtain the roughness features, pristine surface and superhydrophobic nylon net surface was analyzed by PicoPlus II AFM from Molecular Imaging (MI) Corporation (AZ, USA). The AFM images shown in Fig. 6 was indicative of the surface roughening impact associated with the existing particle system.
Fig. 3 AFM images of single fibers of the nylon net: (a) pristine fiber and (b) superhydrophobicsuperhydrophobic fiber. 3.3 Surface wettability of nylon net surface The wettability of nylon net was studied by measuring contact angle on sample surface. As shown in Table 1, two main conclusions were obtained: i) When the volume ratio of glycerol and water remained unchanged, the WCAs of the as-prepared nylon net increased at first and decreased later with the increase of the concentration of NMP; ii) When the concentration of NMP was 10g/L, the WCAs increased with the increase of concentration of glycerol. When the concentration of NMP was 30g/L, the WCAs increased at first and flattened later with the increase of various ratio of glycerol in glycerin solution. When the concentration of NMP was 50g/L and 70g/L, the WCAs increased at first and decreased later with the increase of various ratio of glycerol in glycerin solution. There are two possible reasons for the above main conclusions. Firstly, when the concentration of PU solution is low, because of the low viscosity of PU solution, the formed adhesive particles on nylon net surface can peel off easily in a low concentration of glycerin solution during the curing process, which makes it
considerably difficult to form surface roughness. Secondly, when the concentration of PU solution is high, it is difficult to form adhesive particles in a high concentration of glycerin solution. Therefore, the ratio of glycerol in glycerin solution has to be well controlled. Table 1 The effect of different mass-volume concentration of one-component PU solution and various ratio of glycerol in glycerin solution on water contact angle of hydrophobic. The concentration of PU in NMP(g/L)
10g/L
30g/L
50g/L
70g/L
The volume ratio of glycerol and water
WCA(°)
WCA(°)
WCA(°)
WCA(°)
1:9
114
143
148
142
3:7
140
147
148
147
1:1
145
148
150
145
7:3
146
149
152
145
9:1
148
149
131
119
Fig. 4 Variations in the WCA of as-prepared nylon net with the volume ratio of glycerol and water at given concentration of PU in NMP. The WCA of the pristine net sample was 18° as seen in Fig. 5a, which could be explained by the imino groups on the sample surface. As presented in Fig. 5b, the WCA of OTS-treated nylon mesh surface was 125°, revealing that only low surface energy without enough roughness would not lead to superhydrophobicity. As shown in Fig. 5c, the textured nylon surface showed a WCA of 0 °. After treated with OTS, the textured nylon surface became superhydrophobic (WCA = 152 °), as shown in Fig. 5d.
Fig. 5 Images of water droplets on different samples: 5µL water droplet on (a) pristine nylon net, (b) OTS-treated nylon net, (c) textured nylon net (d) superhydrophobic nylon net. Generally, for a totally smooth surface, the wettability is described by means of Young equation, which is recognized as the theoretical principle of surface wettability. Young equation: cos =
(2)
where γsv, γsl, γlv are the interfacial free energy of solid/vapor, solid/liquid, and liquid/vapor, respectively. Obviously in a real case, surfaces are unlikely to be completely smooth, which means that Young’s equation does not tally with the actual situation. In order to overcome this apparent drawback, Wenzel modified Eq.(1) and answered the connect angle θr of liquid droplet on a rough homogeneous surface: = r
(3)
where r is the roughness factor; θr and θ represent the contact angle on a rough surface and Young's contact angle on a smooth surface, respectively. For material with contact angle less than 90°, the real value of θr will decrease along with the increase of
surface roughness. For a deep consideration of wettability, Cassie proposed a novel roughness equation which can be applied to heterogeneous roughness and low energy surface. = −
(4)
where f1 is fraction of liquid/solid and f2 is fraction of liquid/vapor, θc and θ are the water contact angle on rough and flat surfaces, respectively. In this model, the superhydrophobic property of the sample surface will be enhanced with the increase of air cushion part proportion. Here, the value of θc on the superhydrophobic surface is 152° and the θ on the smooth net surface (modified by OTS) is 125°. Thus, the fraction of air is calculated by Eq. (3) to be 0.75. 3.4 Surface composition The one-component PU and OTS coating on nylon net surface were detected by FT-IR instrument. Fig. 6 displayed that peak at 1726 cm−1 represented an obvious C=O stretching vibration on account of one-component PU [44]. The absorption peak at 1115 cm−1 was attributed to the C-O stretching vibration [45] ,which was also from one-component PU. Four absorption peaks at 3300 cm−1, 2927 cm−1 , 2858 cm−1 and 1025 cm−1 were assigned to N-H bonds, -CH3 bonds, -CH2 bonds and Si-O-Si bonds, respectively [46, 47]. In summary, it is indicated that the well-produced superhydrophobic nylon net was attributed to the combination of the one-component PU with OTS.
Fig. 6 FT-IR spectra of (a) pristine net surface, (b) one-component PU treated net surface and (c) superhydrophobic net surface. 3.5 Oil-water separation and stability experiment Robustness plays an important role in practical applications, so chemical durability and cycling experiments of oil-water separation of the as-prepared nylon net were investigated. The WCAs of the samples were stable between 152° to 150° showing that the superhydrophobic nylon net owned excellent chemical durability against erosion of acid and alkali. Furthermore, we evaluated the robustness of superhydrophobic product through cycling experiments of oil-water separation. The separation was repeated 60 times, displaying an excellent structural stability with WCA no less than 149° (Fig. 7a) and recovery efficiency of n-hexane over 95% (Fig. 7b).
Fig. 7 (a) Variations of water contact angle of as-prepared nylon net surface with pH ranging from 1 to 13. (b) Variation of water contact angle and separation efficiency of superhydrophobic nylon net with recycle numbers of oil-water separation. Fig. 8 showed the process of the superhydrophobic nylon surface for removal of oil from water. When oil-water mixture was poured onto the surface, the oil would penetrate nylon mesh quickly and fall into the vessel. Meanwhile, water was blocked and tumbled out of nylon surface into outboard beaker along the exterior of vessel. As a result, the superhydrophobic nylon net exhibited an excellent separation performance as shown in Fig. 8e.
Fig. 8 Images of oil removal by superhydrophobic nylon net. (a) vial bottle sealed with superhydrophobic nylon net, (b-e) observable process of oil-water separation and n-hexane stained by Sudan III.
3.6 Application in constructing superhydrophobic filter paper, fiberglass cloth and slide glass Fig. 9 displayed optical images of liquid drops on superhydrophobic substrates (left) and original substrates (right). The results showed that when the mass-volume concentration of one-component PU was 50g/L and the ratio of glycerol and deionized water was 7/3, the obtained nylon net had the best hydrophobicity after being treated with OTS. In addition, according to above mentioned optimal condition, the optimization was successfully applied to the construction of superhydrophobic filter paper and fiberglass cloth directly with WCAs of 152° (Fig. 9a) and 151°(Fig. 9b) respectively. Moreover, because of the fine adsorptive effect, the filter paper can be textured adequately after being treated by deionized water only and further treatment with OTS make it super-hydrophobic with a WCA as high as 151° (Fig. 9c), displaying an easier and more inexpensive fabrication compared to other surfaces. Furthermore, a further study has been carried out to construct super-hydrophobic glass slide. The experiment exhibited that when the mass-volume concentration of one-component PU was 100g/L and the volume ratio of glycerol and deionized water was 1/1, the obtained glass slide had the best hydrophobic property after being treated with OTS. As is showed in Fig. 9d, the WCA of the superhydrophobic glass slide was 156°. Fig. 9 (a-d) also displayed that all of the original substrates were hydrophilic.
Fig. 9 Optical images of liquid drops on superhydrophobic substrates (left) and original substrates (right). (a) filter paper (b) fiberglass cloth (c) filter paper, (d) glass slide.4. Summary In summary, we have proposed a novel, flexible and inexpensive method to manufacture superhydrophobic nylon net surface. The result represented that superhydrophobic nylon net surface was obtained through combining adhesive particles with OTS. The as-prepared nylon net displayed excellent superhydrophobicity, outstanding oil-water separation capability and stability. This constructing approach was also applicable to the fabrication of superhydrophbic filter paper, fiberglass cloth and glass slide. Therefore, this cost-effective, easily-operated and environment friendly method
offers
great
technological promise
in the
field
of
constructing
superhydrophobic coatings on a large scale. Acknowledgements： ： This research was supported by the Fundamental Research Funds for the Central Universities (DL12EB05-01) and program for New Century Excellent Talents in University (NCET-10-0311). References: [1] Q.F. Xu, B. Mondal, A.M. Lyons, Fabricating superhydrophobic polymer surfaces with excellent
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Highlights This method utilizes adhesive solely to construct roughness on substrates. This approach is successfully applied to filter paper, fiberglass cloth and glass slide. The superhydrophobic nylon net maintained its water repellency even after 60 cycles of oil-water separation.