Accepted Manuscript Facile preparation of super durable superhydrophobic materials Lei Wu, Junping Zhang, Bucheng Li, Ling Fan, Lingxiao Li, Aiqin Wang PII: DOI: Reference:

S0021-9797(14)00465-2 http://dx.doi.org/10.1016/j.jcis.2014.06.046 YJCIS 19669

To appear in:

Journal of Colloid and Interface Science

Received Date: Accepted Date:

10 April 2014 20 June 2014

Please cite this article as: L. Wu, J. Zhang, B. Li, L. Fan, L. Li, A. Wang, Facile preparation of super durable superhydrophobic materials, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis. 2014.06.046

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Facile preparation of super durable superhydrophobic materials Lei Wu,a,

b

Junping Zhang,*a Bucheng Li,a Ling Fan,a Lingxiao Li,a, c and Aiqin

Wang*a

a

Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical

Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. b

Graduate University of the Chinese Academy of Sciences, 100049 Beijing, P. R.

China. c

College of Petrochemical Technology, Lanzhou University of Technology, Lanzhou,

730050, P. R. China.

* Corresponding author: Prof. Dr. Junping Zhang, Tel: +86 931 4968251, E-mail: [email protected] Prof. Dr. Aiqin Wang, Tel: +86 931 4968118, E-mail: [email protected]

Abstract

The low stability, complicated and expensive fabrication procedures seriously hinder practical applications of superhydrophobic materials. Here we report an extremely simple method for preparing super durable superhydrophobic materials, e.g., textiles and sponges, by dip coating in fluoropolymers (FPs). The morphology, surface 1

chemical composition, mechanical, chemical and environmental stabilities of the superhydrophobic textiles were investigated. The results show how simple the preparation of super durable superhydrophobic textiles can be! The superhydrophobic textiles outperform their natural counterparts and most of the state-of-the-art synthetic superhydrophobic materials in stability. The intensive mechanical abrasion, long time immersion in various liquids and repeated washing have no obvious influence on the superhydrophobicity. Water drops are spherical in shape on the samples and could easily roll off after these harsh stability tests. In addition, this simple dip coating approach is applicable to various synthetic and natural textiles and can be easily scaled up. Furthermore, the results prove that a two-tier roughness is helpful but not essential with regard to the creation of super durable superhydrophobic textiles. The combination of microscale roughness of textiles and materials with very low surface tension is enough to form super durable superhydrophobic textiles. According to the same procedure, superhydrophobic polyurethane sponges can be prepared, which show high oil absorbency, oil/water separation efficiency and stability. Keywords: Superhydrophobic; Textiles; Sponges; Oil/water separation; Durable

1. Introduction Thousands of reports about superhydrophobic materials have been published since 2

2000 as inspired by self-cleaning and water-repellent properties of the lotus leaf in the nature world (Figure S1).1 It is well known now that the combination of proper surface roughness and materials with low surface energy is a successful way to prepare superhydrophobic surfaces.2-9 Superhydrophobic surfaces, characterized by high water contact angle (CA > 150°) and low CA hysteresis, have numerous promising applications in self-cleaning windows, non-wetting fabrics, anti-fogging, anti-icing and oil/water separation, etc.10-16 However, the low mechanical and chemical stability of superhydrophobic surfaces as well as the complicated and expensive fabrication procedures hamper their practical applications. On one hand, the microscale and/or nanoscale roughness are important in fabricating superhydrophobic surfaces, but inherently weak towards mechanical abrasion, especially the nanoscale roughness. The loss of roughness increases the contact area between water and the surface, which results in increase in CA hysteresis or even loss of the self-cleaning property.17-19 For most of the reported superhydrophobic surfaces, slight friction could cause complete loss of the superhydrophobicity. Many superhydrophobic coatings even cannot withstand finger touch. On the other hand, the materials with low surface energy for fabricating superhydrophobic surfaces are often chemically instable towards acid, base, organic solvents and UV irradiation, etc.20-22 3

Despite the importance of durability of superhydrophobic surfaces in applications, relatively little attention has been paid until very recently. Ras and his colleagues reviewed the advances in developing mechanically resilient superhydrophobic surfaces in 2011 and highlighted the importance of stability, which promotes the research in developing stable superhydrophobic surfaces.23 Generally, there are three strategies to fabricate durable superhydrophobic coatings including creating roughness from extra-substrate nanostructures,24-26 coating with nanocomposite27-30 and roughening bulk materials at the surface.31-34 However, most of the reported stable superhydrophobic coatings can only keep the superhydrophobicity under mild mechanical friction, e.g., sand microparticles impact, water jet and tape peeling.35,36 In addition, the stability of superhydrophobic materials is often evaluated by changes of water CA. Whereas CA hysteresis or sliding angle is seldom reported, which indicates that water drops are sticky on the surface after mechanical damage. The adhesion of water drops on a surface means loss of the self-cleaning property.23 So, it is more important but challenging to have the water drops roll off than to keep the high CA after mechanical damage. Only limited success has been achieved in preparing superhydrophobic materials with excellent mechanical durability, on which water drops could roll off after mechanical stability tests.27-30,37-40 Li et al. reported fabrication of laundering durable 4

superhydrophobic cotton fabrics by γ-ray induced graft polymerization of 1H, 1H, 2H, 2H-nonafluorohexyl-1-acrylate.37 By mixing the fluorinated alkylsilanes with fluorinated-decyl polyhedral oligomeric silsesquioxane, tetraethyl orthosilicate or polydimethylsiloxane, Lin et al. prepared durable superhydrophobic materials which could withstand abrasion, plasma treatment and machine wash.27-30 We demonstrated facile

fabrication

of

durable

superhydrophobic

textiles

with

excellent

superhydrophobicity, mechanical, chemical and environmental stability by dip coating in a nanocomposite solution of fluoro-free organosilanes.41,42 However, some of the methods like γ-ray induced graft polymerization need tedious procedures, complex equipment and protection of inert gas, which hamper their massive industrial production. Also, the fluorinated alkylsilanes are too expensive for practical applications and some of the solvents like toluene,41,42 tetrahydrofuran27 and dimethylformamide30 are environmentally unfriendly. All these issues push us to explore a facile, economical and green strategy for scalable production of super durable superhydrophobic materials. Here, we demonstrate an extremely simple but long overlooked strategy for fabricating super durable and robust superhydrophobic materials including various textiles and polyurethane sponges. The superhydrophobic materials are prepared simply by dip coating in cost-effective fluoropolymers (FPs) with proper 5

concentration. No activation of textiles and polyurethane sponges is needed. Various textiles made from natural and manmade fibers, e.g., cotton, wool, polyester and polyacrylonitrile, can be successfully converted to super durable superhydrophobic textiles using the optimal coating parameters. The resulting superhydrophobic textiles were characterized by field emission scanning electron microscopy (FE-SEM) and Xray photoelectron spectroscopy (XPS). In addition, the mechanical (e.g., abrasion and machine wash), chemical (e.g., organic solvents, acid and base) and environmental (e.g., UV irradiation and extreme temperature) stability of the superhydrophobic coatings were evaluated in detail by evaluating the wetting properties. The textile related parameters such as the tensile strength and elongation at break were also evaluated besides the stability tests. In addition, a potential of the approach for largescale production of super durable superhydrophobic textiles was demonstrated. Moreover, the superhydrophobic polyurethane spongs with excellent chemical stabilities are promising materials in the selective cleanup of oil from water.

2. Experimental section

2.1. Materials

FPs were provided by Jin Tai Auxiliary Chemical Co. Ltd, Jiang Su, China. FPs 1# is a mixture of silane/siloxane and fluoropolymer (C < 8). FPs 2# is an aqueous

6

emulsion of a reactive polydimethylsiloxane with 23 wt% solid content. FPs 3# is a perfluorocarbon resin emulsion with 30 wt% solid content. FPs 4# is a mixture of fluorinated polymer, water and propylene glycol with a fluorinated polymer content of 25 wt%. Anhydrous ethanol, toluene, n-hexane, tetrachloroethylene, acetone, sodium hydrate, concentrated sulfuric acid, methylene blue, Sudan Red Ι and Oil Red O were purchased from China National Medicines Corporation Ltd. Commercial polyester fabrics were supplied by HM Group (China). Other textiles including cotton, polyester/cotton, wool, silk, viscose spun, acetate and polyester red II were kindly supplied by EMPA Testmaterials AG, Switzerland. For a full description of the samples please refer to Table 2. Commercial polyurethane sponges were supplied by Shaoxing Chen Feng Foam Corporation Ltd., Zhejiang, China. Washing powder was bought from local supermarket. All chemicals were used as received without further purification. Deionized water was used for all the experiments and tests.

2.2. Fabrication of superhydrophobic polyester textiles and sponges

Firstly, a piece of fabric (3 × 3 cm) or polyurethane sponge (1 × 1 × 1 cm) was washed in turn with deionized water and ethanol for several times, and then dried in an oven at 60 °C. FPs 2#, FPs 3# and FPs 4# were dispersed in water, and FPs 1# was diluted with toluene to certain concentration. Then, a piece of the cleaned fabric or polyurethane sponge was immersed in the as-prepared coating solution and 7

ultrasonicated for 30 min at 30 °C. Finally, the coated sample was annealed at 60 °C in an oven for 50 min except for the FPs 4# coated sample which was cured at 110 °C for 15 min.

2.3. Measurement of water shedding angle (WSA)

Owing to the fact that the surfaces of some substrates such as textiles and sponges are macroscopically rough, it is very difficult to detect the full drop profile for CA measurement (Figure 1b). Consequently, the classical CA measurement, highly dependent on the method of drop shape analysis, is unsuited to reliably evaluate the wetting properties of the surfaces. Thus, WSA is used instead of CA and sliding angle according to a previous reported method (Figure S2).17-19,43 Typically, the samples were fixed onto glass slide and placed on the tilting table of the Contact Angle System OCA 20 (Dataphysics, Germany). A syringe was mounted above the tilting table with a fixed needle to a substrate distance of 10 mm. The syringe was positioned in a way that a drop falling from the needle would contact the substrate 20 mm from the bottom end of the sample. The needle with an inner diameter of 110 μm was used to produce liquid droplets with a volume of 7 ± 0.3 μL. To determine the WSA, measurements were started at an inclination angle of 50°. Droplets of liquid were released onto the sample at a minimum of three different positions. If all drops completely bounced or rolled down the sample, the inclination angle was reduced by 8

2° and the procedure repeated until one or more of the droplets would not completely roll down the surface. The lowest inclination angle at which all the drops completely rolled down or bounced off the surface was noted as the WSA.

2.4. Mechanical abrasion tests

The abrasion tests were performed according to a previously reported method (Figure S3).44 The sample was fixed onto the stainless steel column and moved repeatedly (40 cm for one cycle) on the abrasion partner at 5 KPa. In order to simulate the authentic utilization, the abrasion tests were performed by using A4 paper and sandpaper (2000 meshes) as the abrasion partners. The WSA after 10, 50, 100 and 200 cycles of A4 paper abrasion were recorded. Due to the severe abrasion condition when the sandpaper was used as the abrasion partner, we recorded the WSA after 10, 25, 50, 75 and 100 cycles. Before WSA measurement, the sandpaper abraded samples were washed with ethanol, and then dried at 60 °C because a lot of sands penetrated into the samples during the test.

2.5. Stability in solvents and oils

A piece of the sample was immersed in solvents and oils for 1h, 24h and 168h. Afterwards, all samples were dried in an oven at 60 °C before WSA measurement.

2.6. Laundering tests 9

The samples were washed in a washing machine with 10 pieces of cotton textiles (20 × 20 cm) and 0.15% (w/w) washing powder at room temperature for 50 cycles (30 min each). After each washing cycle, the fabric was washed in turn with deionized water and anhydrous ethanol for three times, and then dried in an oven at 60 °C before WSA measurement.

2.7. Oil absorbency and oil/water separation of polyurethane sponges

A piece of sample was immersed in oil at room temperature. The sample was taken out of the oil after 1 min, drained for several seconds and wiped with filter paper to remove excess oil. The oil absorbency k of the sample was determined by weighing the sample before and after oil absorption and calculated according to Eq. (1): k = (mt - mi)/mi × 100%

(1)

where mt is the weight of the wet sample with oil (g) and mi is the weight of dry sample (g). For the reusability tests, the sample was immersed in 10 mL of oils for 1 min to reach equilibrium, and then taken out, washed with n-hexane for three times and dried in an oven at 60 °C. This absorption-desorption procedure was repeated for 10 times. After each cycle, the WSA and oil absorbency were measured. For oil/water separation, a mixture of 20 mL of petroleum colored with Oil Red O and 50 mL of water colored with methylene blue was poured slowly into a beaker 10

through the coated sponge.

2.8. Characterization

The micrographs of the samples were taken using a field emission scanning electron microscope (SEM, JSM-6701F, JEOL). Before SEM observation, all samples were fixed on aluminum stubs and coated with gold ( ~ 7 nm). X-ray photoelectron spectra (XPS) were obtained using a VG ESCALAB 250 Xi spectrometer equipped with a Monochromated AlKα X-ray radiation source and a hemispherical electron analyzer. The spectra were recorded in the constant pass energy mode with a value of 100 eV, and all binding energies were calibrated using the C1s peak at 284.6 eV as the reference.

3. Results and discussion

3.1. Preparation of super durable superhydrophobic polyester textiles

The durable superhydrophobic textiles were prepared by dip coating in solutions of four fluoropolymers (FPs 1# ~ FPs 4#). Firstly, the FPs were diluted with proper solvents (FPs 1# in toluene, FPs 2# ~ FPs 4# in water) to certain concentration. Subsequently, the FPs were bound onto the surface of the textiles via a simple dip coating method under ultrasonication. After cured in an oven for a period of time, we obtained super durable superhydrophobic textiles. 11

A hydrophobic substrate is helpful in preparing durable superhydrophobic surfaces by preventing the formation of hydrophilic defects as a result of wear.6 Thus, the hydrophobic (CA = 135.1 ± 10.2º) polyester textiles were used as the main model substrates in this study. The original polyester textiles are composed of many fibers (15 ~ 20 μm in diameter) with smooth surface (Figure 1a). In spite of high CA, water drops are sticky on the original polyester textiles and the polyester textiles could be completely wetted in a few seconds once immersed in water. Figure 1 The

hydrophobic

polyester

textiles

were

successfully

converted

to

superhydrophobic ones after coated with FPs 1# ~ FPs 4# according to the above procedure (Figures 1b and S2). The water drops show very high CA on the coated polyester textiles. However, it is impossible to get the accurate outline of the water droplets (dash line in Figure 1b) and then measure the CA and CA hysteresis exactly because the textile surface is macroscopically rough, pliant and non-reflective.4,14 Thus, WSA was used to evaluate water repellent properties of the coatings instead of CA and CA hysteresis in this study. All the coated polyester textiles show very good water repellent properties. Water drops could easily roll off the slightly tilted samples (WSA = 2 ~ 3°). A jet of water applied on the coated textiles could bounce off the surface without leaving a trace. The coated textiles are reflective in water because of 12

the existence of an air cushion between water and the textiles. Most of the area beneath the water droplet is the liquid/vapour interface and the ratio of liquid/solid interface is pretty small, which indicates that the interaction between water and the coating is very weak. The air cushions are stable over many weeks and the textiles remain completely dry after taken out. In addition, a 7 μL water droplet released from a height of 10 mm could bounce many times on the horizontal coated textiles (Movie S1). This means the kinetic energy of the water droplet is well conserved by the surface deformation and the dissipation of the kinetic energy by work of adhesion is very low during the impact against the surface.15 Moreover, the coated polyester textiles are also repellent towards other aqueous liquids beside water. Drops of coffee, milk, orange juice and vinegar are all spherical in shape on the coated polyester fabrics and could easily roll off (Figure S4). Figure 2 The surface morphology and chemical composition of the original and FPs coated polyester textiles were characterized by SEM and XPS (Figure 2). The F1s peak in the XPS spectrum of the original polyester textiles (Figure 2c) means they are treated with fluoro compound, thus are hydrophobic. In spite of evident change in the wetting property, no obvious change in surface morphology of the polyester textiles was observed after coated with FPs 1# ~ FPs 3#. This result is in accordance with the 13

previous reports.28,38 Only very sparse nanoparticles can be seen on the FPs 2# coated textile at very high magnification. Thus, the superhydrophobicity of the FPs 1# ~ FPs 3# coated polyester textiles is attributed to the low surface energy of the FPs 1# ~ FPs 3# and the microscale roughness of the textiles. As is well known, the XPS spectra show surface chemical composition with a detecting depth of a few nanometers. So, the C1s, O1s, F1s, Si2s and Si2p peaks in the XPS spectra of the coated samples (Figure 2f and 2i) are attributed to FPs 1# and FPs 2#. The FPs 3# is mainly composed of perfluorocarbon resin. Thus, very strong F1s, and weak C1s and O1s peaks were detected on the corresponding coated textiles. The FPs 4# is different from FPs 1# ~ FPs 3# and endows the textiles with a nanoscale roughness. The C1s, O1s, F1s and Cl2p peaks in the XPS spectrum of the FPs 4# coated textiles were detected. Figure 3 Although the FPs have different influences on surface morphology and chemical composition, all the coated samples show very low WSA in the range of investigated concentration of FPs (Figure 3). It can be concluded based on the analyses of surface morphology, chemical composition and wetting property that the nanoscale roughness is not essential in fabricating superhydrophobic textiles. The combination of the inherent microscale roughness of the textiles and the material with very low surface energy is enough to achieve excellent water repellent property. 14

3.2. Mechanical stability

Mechanical stability of the samples was evaluated by the abrasion tests. The FPs coated polyester fabrics show excellent mechanical stability. As is well known, the influence of abrasion on the samples is determined by many factors such as pressure, abrasion cycles, moving distance for one cycle and the abrasion partner. Lin et al. reported superhydrophobic textiles coated with fluoroalkyl silane modified silicone rubber/nanoparticle composite.27 The coated sample is intact and maintains the superhydrophobicity even after 28000 abrasion cycles at 12 kPa using untreated fabric as the abrasion partner. No difference in appearance can be seen in comparison with the sample before abrasion tests. We choose both A4 paper and sandpaper (2000 meshes) as the partners for the abrasion tests. Figure 4 Figure 5 The A4 paper with low roughness only has mild abrasion on the coatings rather than on the substrates. No obvious change in CA can be detected after 200 cycles of abrasion at 5 kPa using A4 paper as the abrasion partner. The changes in WSA with abrasion cycles are shown in Figures 4a and S5. WSA increases gradually with increasing the abrasion cycles. This means WSA is more sensitive than CA in evaluating wetting behavior of the samples after abrasion tests. This is because slight 15

damage to the coating may result in evident increase in adhesion force between the coating and water drops, and then the obvious change in WSA can be detected. Water droplets keep nearly sphere in shape (Figure 4b-e) and still could easily roll off the tilted samples after 200 cycles of abrasion. Although the concentration of FPs has no evident influences on WSA of the freshly coated samples as mentioned above, it does have great influences on the abrasion resistance (Figure S5). For example, the WSA of the polyester textiles coated with a FPs 1# concentration of 0.25 %, 0.5 % and 1.0 % is 1º, 2º and 3º, respectively. However, very big difference in WSA of the samples can be seen after 200 cycles of abrasion against A4 paper. Similar phenomena were also observed for the textiles coated with the other FPs. Figure 4a shows the WSA changes with abrasion cycles for the samples coated with the optimal concentration of FPs. Obvious difference in resistance to abrasion of A4 paper can be seen among the samples. The WSA is in the order of FPs 1# > FPs 2# > FPs 3# > FPs 4# after abrasion although WSA of the freshly coated samples is very close to each other as indicated with the circle in Figure 4a. For example, WSA of the FPs 1# coated polyester textiles increases from 2º to 31º after 200 abrasion cycles, whereas the WSA of the FPs 4# coated sample remains below 11º. Moreover, the slightly damaged superhydrophobicity (increased WSA) could be easily repaired by dip coating again. The repaired samples retained the same WSA 16

and abrasion resistance as the newly coated ones even after 10 abrasion-repair cycles (Figure 5). Figure 6 The variation of WSA with abrasion cycles was also investigated using sandpaper as the abrasion partner to further demonstrate the mechanical stability of the superhydrophobic materials. The sandpaper with high roughness has more intensive abrasion on both the coating and the substrate in comparison with A4 paper (Figure 6). The polyester textiles were seriously damaged by the sandpaper with increasing the abrasion cycles to 100 at 5.0 kPa (Figures 6b and S6). A half of the fabric even disappeared at the end of the tests as indicated by the dash circle. The SEM images also show that the newly introduced nanostructure of the FPs 4# coated samples was completely destroyed during abrasion (Figure 6c and d). A lot of fragments can be seen on the surface of the damaged filaments of the textiles. Meanwhile, the intensive abrasion with sandpaper also has an obvious effect on WSA (Figure 6a). The WSA increases gradually with increasing the abrasion cycles. Whereas the water drops still could surprisingly bounce off the residual samples, indicating super mechanical stability of the superhydrophobic materials (Movie S2). This is because the fabrics are coated uniformly in the dip coating process and the embedded fibers of the textiles are exposed to water drops once the fibers on the surface of fabric are removed during the 17

abrasion tests. The WSA is in the order of FPs 2# > FPs 1# > FPs 3# > FPs 4# after 100 abrasion cycles against sandpaper. The WSA of the FPs 4# coated polyester textiles is still below 11° after 100 abrasion cycles and is obviously lower than the others. This result indicates that the micro- and nanoscale hierarchical structures are not essential but helpful in fabricating durable superhydrophobic materials.

3.3. Chemical and environmental stability

Chemical and environmental stability are important properties for all the coatings. Any damage of the coating by chemical reagents or radiation may result in completely loss of the functionality. The superhydrophobic polyester textiles show nice chemical and environmental stability (Table 1). Immersion in 1 M H2SO4 aqueous solution for 24 h, strong UV irradiation for 1 h, freeze at -30 °C for 24 h and heat treatment at 200 °C for 2 h only result in slight increase in WSA. The 1 M NaOH aqueous solution has a more evident effect on WSA, especially for the silicone based FPs (FPs 1# and FPs 2#). The FPs 4# coated sample is the most stable according to these tests. Table 1 Figure 7 The effects of various frequently used solvents, e.g., water, ethanol and tetrachloroethylene, on superhydrophobicity of the coatings were also studied. No obvious changes in CA can be detected during the tests. The variation of WSA with 18

immersion time in solvents is related to the kinds of solvent and FPs (Figures 7 and S7). The WSA of FPs 4# coated polyester textiles remains almost the same as the freshly coated samples after immersion in acetone and water for 7 d. Slight increases in WSA to 4.5°, 6.0°, 6.5° and 8.0° were observed after immersion for 7 d in nhexane, tetrachloroethylene, toluene and ethanol, respectively. The WSA of the FPs 4# coated polyester textiles remains below 9° after immersion in all the investigated solvents for 7d, which is obviously below those coated with FPs 1# ~ FPs 3#. Water leads to the evident increase in WSA to 20° for the FPs 1# coated polyester textiles, whereas the WSA of the FPs 2# and FPs 3# coated textiles are more sensitive to nhexane and tetrachloroethylene. As is well known, tetrachloroethylene is an excellent solvent for organic materials and is widely used in dry cleaning. The excellent solubility of tetrachloroethylene for organic materials is fatal to various functional coatings on textiles. The FPs, especially FPs 4#, coated polyester textiles surprisingly

keep

their

excellent

superhydrophobicity

after

immersion

in

tetrachloroethylene for 7 d (Movie S3). Although the WSA increases after immersion for 7 d in some of the solvents, water drops could still easily roll off all the coated polyester textiles. This is attributed to the tight binding of the FPs to the polyester textiles and the inherent stability of the FPs towards various solvents.

3.4. Laundering stability 19

Laundering stability is a very important property of functional textiles. A laundering procedure is a combination of mechanical frictions (e.g., shearing forces with water and the wall of the container) and chemical interactions between the fabric and the detergent.37 These two factors facilitate the cleaning of a fabric during machine washing at constant temperature. Unfortunately these factors can also damage a textile coating.17-19 Both mechanical friction and the cleaning detergents included in the washing formulation could lead to a loss of superhydrophobicity. Thus, practical laundering stability tests were carried out in a washing machine to evaluate the effect of a combined mechanical and chemical stress during a washing cycle on the superhydrophobicity of the coated polyester fabrics (Figure 8). After washed for one cycle, the WSA of the polyester textiles coated with FPs 1# ~ FPs 4# increases to 13°, 12°, 9° and 7°, respectively. The WSA increases gradually with increasing the laundering cycles and remains below 23° after 50 laundering cycles. The WSA of FPs 3# and FPs 4# coated polyester textiles are even below 10°, which indicates excellent laundering durability of the coatings. A 10 mL water drop could easily bounce off the tilted FPs 4# coated sample even after 50 laundering cycles (Movie S4). The SEM observations reveal that the nanoscale roughness of the FPs 4# coated samples was damaged after 50 laundering cycles (Figure 8b and c). This result proves again that the nanoscale roughness is not essential for fabricating durable superhydrophobic 20

materials and is in accordance with the mechanical stability tests. Figure 8

3.5. Application on various textiles

This simple approach for the preparation of super durable superhydrophobic coatings is applicable to various synthetic and natural textiles such as wool, polyacrylonitrile and cotton (Table 2). Most of the investigated textiles are originally hydrophilic except for the wool fabric. The original textiles can be easily wetted and the water drops could completely penetrate into the textiles in a few seconds. All the textiles become superhydrophobic regardless of the composition and type of weave after dip coated in FPs 1#, FPs 3# and FPs 4# according to the same procedure. The FPs 2# is only effective for the wool textiles. All the superhydrophobic textiles are reflective in water indicating existence of an air cushion between the coating and water. Moreover, these superhydrophobic textiles also show comparable mechanical stability with the coated polyester textiles. The silk, cotton and hydrophilic polyester fabrics become superhydrophobic after coated with FPs 4#-5%. No obvious change in CA was detected during abrasion of the coated textiles using sandpaper as the abrasion partner. Although the textiles were seriously damaged during the abrasion tests, the water drops are still spherical in shape on them and could easily roll off the titled samples, indicating super stability of the superhydrophobic textiles (Figure 9). The simple and 21

inexpensive large-scale production of superhydrophobic materials is one of the major challenges to exploit the commercial potential of strongly water-repellent materials.45 The facile dip coating approach reported herein can be easily scaled up for producing durable superhydrophobic textiles. The textiles with a size of 1 × 1 m or even bigger can be uniformly coated according to the same procedure. Table 2 Figure 9 Of primary importance to a textile coating is that it does not affect their mechanical properties, e.g., tensile strength and flexibility. The mild dip coating approach in this study perfectly keeps the mechanical properties of textiles (Table 3). No visible change in the tensile strength and elongation at break can be detected. Further, the coated fabrics feel the same as the original textiles and no difference in appearance can be seen.

3.6. Superhydrophobic polyurethane sponges for oil absorption and oil/water separation

This simple approach can also be used to prepare superhydrophobic polyurethane sponges besides various textiles (Figure 10). Different from most of the previous literatures about preparing superhydrophobic polyurethane sponges, no pre-activation of the sponge is needed in this study. The original sponge can be wetted by both water 22

and oil, whereas the coated sponge can only be selectively wetted by oil. Water drops show very high CA on the coated sponge and could easily roll off WSA = 2°. A jet of water could perfectly bounce off the surface without leaving a trace, which is mainly attributed to the strong binding between the 3D porous skeleton and the superhydrophobic coating. In spite of evident changes in wettability, no obvious change in surface morphology of the sponge was observed after coated with FPs 4#. Table 3 Figure 10 Figure 11 The superhydrophobic sponge can be used to selectively absorb oils from water. The absorbency for various oils is in the range of 9.2 to 41.7 g/g depending on their density (Figure 11a). Once dropped on the surface of water with floating oils, the sponge absorbed the oils completely within a few seconds regardless of the density of oils (Figure 11b). The superhydrophobic sponge can also be used to absorb heavy oils under water (Figure 11c). The superhydrophobic sponge is surrounded by an air cushion when it is immersed in water by an external force. The air layer disappeared at once when the coated sponge contacted the heavy oils under water. The heavy oils can be absorbed in a few seconds and taken out of water with the sponge. Interestingly, no dripping of absorbed oil could be observed in the handling process 23

indicating firm absorption of oil by the coated material. The absorbed oils could be collected easily by squeezing the sponge with hand. Figure 12 For expanding their practical applicability in the separation of large amounts of oil pollutants from water surface, the superhydrophobic sponge was fixed at the opening of a tube and combined with an external vacuum pump for continuous oil absorption (Figure 12a).46 Once immersed at the oil/water interface (Figure 12b), the superhydrophobic sponge was quickly wetted by petroleum on the surface of water, but was completely resistant to water because of its superoleophilicity and superhydrophobicity. Afterwards, the floating petroleum was continuously pumped into the vessel through the superhydrophobic sponge at a vacuum degree of 0.035 MPa (Figure 12c). Finally, only transparent, clean water was leaving in the beaker and no water was visible to the naked eye in the collected oil (Figure 12d), which indicates a high efficiency in oil/water separation. The superhydrophobic sponge can also be used for oil/water separation owing to its moderate oil absorbency (Figure 13). A mixture of oil and water (20 mL of petroleum colored with Oil Red O and 50 mL of water colored with methylene blue) was poured slowly into the beaker through the coated sponge. A part of the oil was quickly absorbed by the sponge, and the excess oil penetrated the sponge and dropped into the 24

bottle beneath it. Meanwhile, more and more water was collected on the surface of the sponge. Figure 13 The superhydrophobic sponge can be repeatedly used for oil absorption and oil/water separation. The long-time immersion in oil has very little effect on superhydrophobicity of the samples (Figure S8). The WSA is still below 9° after kept in chloroform, toluene and petroleum even for 7 d. Water drops are spherical in shape and could easily roll off the samples after the stability test in oils. In addition, the sponge still maintains its excellent superhydrophobicity and high oil absorbency after 10 cycles of absorption/desorption (Figure S9). The excellent reusability in terms of superhydrophobicity and oil absorbency makes the superhydrophobic sponges very promising in the cleanup of oil from water.

4. Conclusions

We explored a simple, cost-effective and scalable approach for preparing super durable superhydrophobic materials by dip coating in FPs. The kind of FPs and their concentration have great influences on WSA and stability of the superhydrophobic coatings. The textiles coated under the optimal conditions show excellent superhydrophobicity, mechanical (e.g., abrasion and laundering), environmental (e.g., UV irradiation, very low and high temperatures) and chemical (e.g., acid, base and 25

organic solvents) stabilities. Water drops keep spherical in shape and could easily roll off the coated textiles after harsh mechanical abrasion, long time immersion in various solvents and repeated laundering. In addition, this facile approach is scalable and can be easily applied to various synthetic and natural textiles, e.g., wool, polyacrylonitrile and cotton. Moreover, according to the same procedure, superhydrophobic polyurethane sponges can be prepared, which show high oil absorbency, oil/water separation efficiency and stability. We have made a solid step forward in preparing super durable superhydrophobic materials and broken the bottleneck for their practical applications. We believe that this facile approach could be used to prepare durable superhydrophobic coatings on a variety of substrates and find applications in many fields.

Acknowledgements

We are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences. We thank Mrs. Jiamei Liu for XPS analysis.

References

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30

Table 1. WSA of the superhydrophobic polyester textiles after treated under various conditions. WSA (º) FPs 1#

FPs 2#

FPs 3#

FPs

After preparation

2

3

3

2

UV (300 ~ 400 nm, 300 W, 10 cm), 1h

10

11

8

4

-30 ºC, 24 h

7

9

8

4

200 ºC, 2 h

5

7

10

5

1 M H2SO4, 24 h

8

7

6

5

1 M NaOH, 24 h

11

15

10

5

31

Table 2. WSA of the superhydrophobic textiles coated with different FPs. No. Product names

Type of Original

WSA of coated samples (°)

weave

CA (°)

FPs 1# FPs 2# FPs 3# FPs 4#

211 Cotton, percale

plain

0

20±0.6 -

19±0.6 15±0.6

213 Polyester/cotton

plain

0

11±1

15±0.6 10±0.6

214 Cotton twill fabric

twill

0

15±0.6 -

402 Wool muslin

plain

127±10.1

2±0.6

403 Silk crêpe

crêpe

0

15±0.6 -

11±0.6 1±0.6

0

7±0.6

-

20±0.6 14±0.6

plain

0

8±0.6

-

18±0.6 10±0.6

for plain

0

10±0.6 -

14±0.6 8±0.6

plain

0

4±0.3

-

405 2,

5

acetate, taffeta

-

20±0.6 9±0.6

15±0.6 14±0.6 10±0.6

endless fibers 408 Polyacrylonitrile 413 Cotton crockmeter II

Polyester red

-

2±0.6

32

Table 3. Mechanical properties of the pristine and coated fabrics. Parameters

403 silk

Polyester white

Polyester red

Pristine Coated Pristine Coated Pristine Coated Thickness (mm)

0.18

0.21

0.19

0.23

0.36

0.36

Tensile strength (MPa)

218.5

173.2

121.8

116.0

108.8

121.1

52.8

44.3

55.5

35.3

54.3

Elongation at break (%) 31.2

33

Figure captions Figure 1. SEM and digital images of the (a) original and (b) FPs 4#-5% coated polyester textiles. The water drops are colored with methylene blue. Figure 2. SEM images and XPS spectra of (a-c) original, (d-f) FPs 1#-0.5%, (g-i) FPs 2#-0.2%, (j-l) FPs 3#-5% and (m-o) FPs 4#-5% coated polyester textiles. Figure 3. Variation of WSA of water drops on the coated polyester textiles with concentration of FPs 1# ~ FPs 4#. Figure 4. (a) WSA changes depending on abrasion cycles using A4 paper as the abrasion partner. Images of water drops on the FPs 4#-5% coated polyester fabrics after (b) 10, (c) 50, (d) 100 and (e) 200 abrasion cycles. The water drops in (b-e) are colored with methylene blue. Figure 5. WSA changes depending on abrasion-repair cycles of the FPs coated polyester textiles. The abrasion tests were carried out using A4 paper as the abrasion partner at 5.0 kPa. Every abrasion-repair cycle is composed of 200 times of abrasion and one time of repair. Figure 6. (a) WSA changes depending on abrasion cycles using sandpaper (2000 meshes) as the abrasion partner. (b) Digital images of water drops on the FPs 4#-5% coated fabrics after different abrasion cycles against sandpaper. The water drops in (d) are colored with methylene blue. (c, d) SEM images of FPs 4#-5% coated polyester fabrics after 100 abrasion cycles against sandpaper. 34

Figure 7. WSA changes of the FPs 4#-5% coated polyester fabrics depending on immersion time in various solvents. Figure 8. (a) WSA changes depending on laundering cycles (30 min each). Insert in (b) is water drops bouncing off the FPs 4#-5% coated polyester fabric after 50 cycles of machine wash. (b, c) SEM images of the FPs 4#-5% coated polyester fabric after 50 cycles of machine wash. Figure 9. (a) WSA changes depending on abrasion cycles of the FPs 4#-5% coated fabrics against sandpaper. Images of water drops on the coated (b) silk, (c) cotton and (d) polyester red fabrics after different abrasion cycles. The water drops are colored with methylene blue. Figure 10. Wettability and morphology of (a) original and (b) FPs 4#-5% coated polyurethane sponges. The water and petroleum drops (10 μL) are colored with methylene blue and Oil Red O, respectively. Figure 11. (a) Oil absorbency of the superhydrophobic sponge. Removal of (b) floating oil (colored with Oil Red O) on the surface of water and (c) heavy oil (colored with Sudan Red Ι) under water. Figure 12. Pump assisted continuous absorption of oil on water surface. Floating petroleum was colored with Oil Red O. Figure 13. Oil/water separation using the superhydrophobic sponge. Water and petroleum are colored with methylene blue and Oil Red O, respectively. 35

Figure 1

36

Figure 2

37

Figure 3

38

Figure 4

39

Figure 5

40

Figure 6

41

Figure 7

42

Figure 8

43

Figure 9

44

Figure 10

45

Figure 11

46

Figure 12

47

Figure 13

48

Graphical abstract

49

Highlights

 A facile approach for preparing super durable superhydrophobic materials.

 The materials exhibit excellent mechanical and chemical stabilities.

 The superhydrophobic materials can be used for effective oil/water separation.

50