Fabrication of Robust Water-Repellent Technology on Cotton Fabric via Reaction of Thiol-ene Click Chemistry

A robust superhydrophobic fabric coating was fabricated on cotton fabric under UV light, which was achieved by convenient surface modification with mercaptopropyltriethoxysilane, tetramethyltetravinylcyclotetrasiloxane, and octadecyl mercaptan. The modification of cotton fabric with 3-mercaptopropyltriethoxysilane introduces reactive mercapto groups, after which 2,4,6,8-tetramethyltetravinylcyclotetrasiloxane reacts with mercapto groups, and octadecyl mercaptan provides microscale roughness. The nonpolar carbon chains of thiol cause the cotton to have a low surface energy. As reported, the combination of microscale roughness with low surface energy has a superhydrophobic effect on cotton, which leads to a high contact angle of 161.8◦ and sliding angle of 8◦. Infrared spectroscopy, XPS, and SEM tests were used to characterize the chemical structure and morphological changes of the surface of cotton fabric before and after click reaction. The fabric after click reaction exhibited an oil–water mixture separation ability owing to its superhydrophobicity. Thus, the finished fabric could be used in the oil–water separation field. Importantly, the superhydrophobic textile displays resistance to laundering, mechanical abrasion, strong acidic and alkaline environments, and UV irradiation. We hope that this study can broaden the real-life applications of cotton fabric.


Introduction
Cellulose is the most abundant organic raw material [1], and cotton fabric is widely used in various industries. However, cotton fabric contains a large number of hydrophilic hydroxyl groups, and it is easily contaminated and wetted with water, which limits its scope of use to a certain extent [2,3]. Thus, the preparation of hydrophobic cotton is worth studying. Superhydrophobic surfaces are also used in the fields of self-cleaning, oil-water separation, anticorrosion, and antifog surfaces, so the preparation of superhydrophobic cotton fabric will broaden the application scope of this textile [4,5].
Superhydrophobic surfaces are those with a static contact angle of more than 150 • and a roll angle of less than 10 • [6,7]. Superhydrophobic properties can be achieved on many substrate surfaces, including wood [8,9], sponge [10], fabric [11,12], paper [13], metal [14], and other substrates. Fabrics have the advantages of light weight, flexibility, low cost, and wide application range [15,16]. There are two conditions for constructing a superhydrophobic surface: one is to reduce the surface energy of the low surface, and the other is to increase the surface roughness [6,7,17]. There are many methods for manufacturing superhydrophobic surfaces, including the sol-gel method [18], chemical vapor deposition [19], layer-by-layer self-assembly [20], etching [21], and plasma technology [22]. Although these production methods are effective and exhibit a certain durability, most of methods are complicated in terms of actual manufacturing. Some methods use fluorine-containing reagents that damage the ecological environment and are expensive. Thus, a simple, environmentally friendly, and inexpensive method for making superhydrophobic surfaces is urgently needed.
In 2001, Sharpless introduced click chemistry to the field of organic synthesis. This type of chemistry has the advantages of high product yield, easy separation and purification, insensitivity to oxygen and water, and mild and harmless solvent reaction conditions such as water. Thus, click chemistry is favored by an increasing number of scientists and has made great contributions in many fields such as drug development and biomedical materials [23][24][25]. Thiol-ene click chemistry is a new type of click chemistry developed in recent years. It has good selectivity and has become another important method for organic synthesis [26,27]. Due to the many advantages and high selectivity of thiol-ene click chemistry, it is used in the fields of material synthesis [26][27][28], polymers [29], and the preparation of functional carbohydrates [30] or modification of alkoxysilanes and silsesquioxanes [31]. Muzafarov et al. [32] combined hydrosilylation and thiol-ene chemistry to produce mono-and difunctional siloxane derivatives that can be used as initial monomers in the synthesis of polymers. Thiol-ene click chemistry has also been applied in the preparation of superhydrophobic surfaces [33]. For example, the self-healing superhydrophobic cotton fabric prepared by Gao Yang Liu et al. [34] has good friction resistance. Shanshan et al. reported a cotton fabric with excellent flame retardancy and superhydrophobicity prepared by the continuous deposition of a three-layer film of branched polyethyleneimine (bPEI), ammonium polyphosphate (APP), and fluorodecyl polyhedral oligosilsesquioxane (F-POSS) [35]. Chao-Hua Xue et al. [36] first etched polyester fibers with alkali to form a rough surface and then modified them with dodecylfluoroheptyl mercaptosilane to make the fabric superhydrophobic and superlipophilic by a click reaction on the fiber. These methods make the fabric superhydrophobic, but the fluorinated reagents used are not environmentally friendly, which has led to new requirements for researchers to use nonfluorinated reagents to make superhydrophobic surfaces.
In this work, we have studied a mild and effective method for generating superhydrophobic surfaces. Hereby, 3-mercaptopropyltriethoxysilane is used to introduce mercapto-reactive functional groups on cotton fabric. Further, 2,4,6,8-tetramethyltetravinylcyclotetrasiloxane reacts with a thiol under ultraviolet light to form a hydrophobic substance on the fiber surface, as shown in Figure 1. The superhydrophobic cotton fabric has good environmental resistance, presenting resistance to acid and alkali, water washing, abrasion, and ultraviolet light. The superhydrophobic fabric exhibits the function of oil-water separation, representing a potential application of cotton fabric.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 13 for 15 min, ethanol for 15 min, and deionized water for 15 min to remove waxes and fats. Then, the cotton fabric was dried at 25 °C for 12 h followed by drying at 80 °C for 2 h for the next preparation. The mercaptosilanes were attached on cotton fabric via chemical vapor deposition. In a container, cotton fabric (10 cm × 10 cm) and 0.3 mL of MPTES were added. Then, the container was sealed and mounted in an infrared-ray heating machine, followed by heating according to a heating program to 90 °C, which was held for 90 min [37]. Finally, the fabric was removed and washed with 100 mL of absolute ethanol in a conical flask for 2 h to remove residual MPTES, washed with deionized water, and dried at 80 °C under vacuum for 2 h. The fabric modified by MPTES was named fabric-SH.

Superhydrophobic Fabric Finishing Process
A mercapto-modified cotton fabric sample (5 cm × 5 cm) was immersed in a mixture containing 50 mL of ethyl acetate and the proper proportion of octadecyl mercaptan (thiol), 2,4,6,8tetramethyltetravinylcyclotetrasiloxane (TMTVSi) and 2,2-dimethoxy-2-phenylacetophenone (DMPA). The reaction system was sealed and irradiated with a UV lamp (250 W, λ = 365 nm) several times at room temperature. After the reaction, the fabric sample was removed, washed with absolute ethanol for 4 h to remove unreacted reactants, and dried at 80 °C under vacuum. The fabric was named finished fabric. The effect of different photoreaction times and reactant molar ratios on the hydrophobicity of the fabric was explored. The contact angle and sliding angle were used as evaluation indicators to select the best process conditions.

Characterization
The surface morphology of original cotton fabric, mercapto-modified cotton fabric, and superhydrophobic fabric were sputter-coated with gold and analyzed by scanning electron microscopy (SEM, Hitachi-S4800, Tokyo, Japan,). The SEM images were obtained in vacuum with an accelerating voltage of 3 kV. Elemental analysis was performed by energy-dispersive spectroscopy

Preparation of Mercapto-Modified Cotton Fabric
The original cotton fabric was washed to remove physical impurities from the surface, using the following conditions: 10 cm × 10 cm cotton fabric samples, 100 • C, 2 g/L soap tablets, 5 g/L Na 2 CO 3 , and a bath ratio of 1:50. After pretreatment, the cotton fabric was ultrasonically cleaned with acetone for 15 min, ethanol for 15 min, and deionized water for 15 min to remove waxes and fats. Then, the cotton fabric was dried at 25 • C for 12 h followed by drying at 80 • C for 2 h for the next preparation.
The mercaptosilanes were attached on cotton fabric via chemical vapor deposition. In a container, cotton fabric (10 cm × 10 cm) and 0.3 mL of MPTES were added. Then, the container was sealed and mounted in an infrared-ray heating machine, followed by heating according to a heating program to 90 • C, which was held for 90 min [37]. Finally, the fabric was removed and washed with 100 mL of absolute ethanol in a conical flask for 2 h to remove residual MPTES, washed with deionized water, and dried at 80 • C under vacuum for 2 h. The fabric modified by MPTES was named fabric-SH.

Superhydrophobic Fabric Finishing Process
A mercapto-modified cotton fabric sample (5 cm × 5 cm) was immersed in a mixture containing 50 mL of ethyl acetate and the proper proportion of octadecyl mercaptan (thiol), 2,4,6,8-tetramethyltetravinylcyclotetrasiloxane (TMTVSi) and 2,2-dimethoxy-2-phenylacetophenone (DMPA). The reaction system was sealed and irradiated with a UV lamp (250 W, λ = 365 nm) several times at room temperature. After the reaction, the fabric sample was removed, washed with absolute ethanol for 4 h to remove unreacted reactants, and dried at 80 • C under vacuum. The fabric was named finished fabric. The effect of different photoreaction times and reactant molar ratios on the hydrophobicity of the fabric was explored. The contact angle and sliding angle were used as evaluation indicators to select the best process conditions.

Characterization
The surface morphology of original cotton fabric, mercapto-modified cotton fabric, and superhydrophobic fabric were sputter-coated with gold and analyzed by scanning electron microscopy (SEM, Hitachi-S4800, Tokyo, Japan,). The SEM images were obtained in vacuum with an accelerating voltage of 3 kV. Elemental analysis was performed by energy-dispersive spectroscopy (EDS, TM3030, Tokyo, Japan) attached to the tabletop scanning electron microscopy system in vacuum with an accelerating voltage of 15 kV. X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi, Haverhill, MA, USA) was performed using an X-ray photoelectron spectroscopy system with an Al Kα X-ray source, and the binding energies were normalized to the C 1s peak at 248.8 eV. Fourier transform infrared-attenuated total reflectance (FTIR-ATR, NICOLET 5700 New York, NY, USA) spectroscopy was used to analyze the surface composition of the original cotton fabric, mercapto-modified cotton fabric, superhydrophobic cotton fabric, and spectra were baseline-corrected and plotted in Origin 9.1. Horiba Jobin Yvon HR800 laser confocal Raman spectrometer collects the Raman spectrum of the sample. The specific parameters are as follows: the wavelength scanning range is 500~3000 cm −1 , the objective lens is 50×, the excitation wavelength is 532 nm, the attenuation power is 25%, and the acquisition is performed 5 times. Static and sliding water contact angle measurements were performed using a Krüss DSA 30 (Krüss Company, Berlin, Germany) instrument. Static sliding contact angles were measured using water droplets (6 µL) placed on the fabric sample surface for 60 s. The sliding contact angles were determined by the minimum tilt angle at which a water droplet (10 µL) rolls off the surface. Each reported test value was the average value of five measurements. Abrasion resistance tests were performed according to a previously reported method [34]. A 500 g weight was placed on the modified fabric sample, and the fabric surface was rubbed against sandpaper (1500 mesh) and moved for 25 cm along a ruler. The scratch tests were conducted for 25 cycles, and the CA and SA were measured after each abrasion cycle. Acid and alkali resistance tests were taken by immersing the finished fabric in solutions of different pH conditions for 48 h adjusted with sodium hydroxide and hydrochloric acid. Laundry tests were carried out according to the AATCC Test Method 61-2006 1A [38] by using a laundry machine at 40 • C in the presence of 10 stainless steel balls and 0.37% soap powder in the washing containers. Figure 2a shows the changes in the static contact angle and sliding angle of cotton fabric under different UV irradiation times. We found that the static contact angle reached 161.8 • and the sliding angle was less than 10 • when the irradiation time was 30 min. The change in the contact angle gradually decreased with increasing irradiation time, which may be due to the completion of the click reaction. The results show that the optimal irradiation time is 30 min.

Effects of UV Irradiation Time and the Ratios of TMTVSi to Octadecyl Mercaptan on the Superhydrophobicity of Cotton Fabric
Coatings 2020, 10, x FOR PEER REVIEW 4 of 13 248.8 eV. Fourier transform infrared-attenuated total reflectance (FTIR-ATR, NICOLET 5700 New York, NY, USA) spectroscopy was used to analyze the surface composition of the original cotton fabric, mercapto-modified cotton fabric, superhydrophobic cotton fabric, and spectra were baselinecorrected and plotted in Origin 9.1. Horiba Jobin Yvon HR800 laser confocal Raman spectrometer collects the Raman spectrum of the sample. The specific parameters are as follows: the wavelength scanning range is 500~3000 cm −1 , the objective lens is 50×, the excitation wavelength is 532 nm, the attenuation power is 25%, and the acquisition is performed 5 times. Static and sliding water contact angle measurements were performed using a Krüss DSA 30 (Krüss Company, Berlin, Germany) instrument. Static sliding contact angles were measured using water droplets (6 μL) placed on the fabric sample surface for 60 s. The sliding contact angles were determined by the minimum tilt angle at which a water droplet (10 μL) rolls off the surface. Each reported test value was the average value of five measurements. Abrasion resistance tests were performed according to a previously reported method [34]. A 500 g weight was placed on the modified fabric sample, and the fabric surface was rubbed against sandpaper (1500 mesh) and moved for 25 cm along a ruler. The scratch tests were conducted for 25 cycles, and the CA and SA were measured after each abrasion cycle. Acid and alkali resistance tests were taken by immersing the finished fabric in solutions of different pH conditions for 48 h adjusted with sodium hydroxide and hydrochloric acid. Laundry tests were carried out according to the AATCC Test Method 61-2006 1A [38] by using a laundry machine at 40 °C in the presence of 10 stainless steel balls and 0.37% soap powder in the washing containers. Figure 2a shows the changes in the static contact angle and sliding angle of cotton fabric under different UV irradiation times. We found that the static contact angle reached 161.8° and the sliding angle was less than 10° when the irradiation time was 30 min. The change in the contact angle gradually decreased with increasing irradiation time, which may be due to the completion of the click reaction. The results show that the optimal irradiation time is 30 min. We also explored the effect of the molar ratio of TMTVSi to octadecylthiol on the superhydrophobic finishing of cotton fabric. From the molecular formula, each mole of tetramethyltetravinylcyclotetrasiloxane contains four moles of double bonds, and each mole of octadecyl mercaptan contains one mole of thiol. According to the principle of click chemistry, one mole of double bonds reacts completely with one mole of mercapto groups. Moreover, 2,4,6,8tetramethyltetravinylcyclotetrasiloxane is a hydrophobic reagent, and octadecyl mercaptan contains hydrophobic groups. Figure 2b shows that the contact angle and sliding angle changed with the molar ratio. When the molar ratios of TMTVSi to Octadecyl mercaptan is 1:4, the contact angle and the rolling angle achieve superhydrophobic performance. We also explored the effect of the molar ratio of TMTVSi to octadecylthiol on the superhydrophobic finishing of cotton fabric.

Effects of UV Irradiation Time and the Ratios of TMTVSi to Octadecyl Mercaptan on the Superhydrophobicity of Cotton Fabric
From the molecular formula, each mole of tetramethyltetravinylcyclotetrasiloxane contains four moles of double bonds, and each mole of octadecyl mercaptan contains one mole of thiol. According to the principle of click chemistry, one mole of double bonds reacts completely with one mole of mercapto groups. Moreover, 2,4,6,8-tetramethyltetravinylcyclotetrasiloxane is a hydrophobic reagent, and octadecyl mercaptan contains hydrophobic groups. Figure 2b shows that the contact angle and sliding angle changed with the molar ratio. When the molar ratios of TMTVSi to Octadecyl mercaptan is 1:4, the contact angle and the rolling angle achieve superhydrophobic performance.  Figure 3c,d shows the mercapto-modified cotton after the MPTES treatment. A slight amount of material is visible on the surface of the mercapto-modified cotton compared with the original cotton surface. This observation may be due to the mercaptan groups of MPTES attached to the surface produced a film on the surface of fiber. After click reaction, the surface of cotton fabric (Figure 3e,f) became rough, and a large amount of block polymer adhered to the surface of fiber, which was bonded together with other fibers. There is no obvious crack boundary, which may be caused by the click reaction. After the reaction, massive particles are formed, and these nanoscale particles form micro/nanoscale rough structures on the microscale fibers of fabric. The structures of finished fabric may contribute to the overall hydrophobicity.

Morphology and Composition of the Fabric Samples
Coatings 2020, 10, x FOR PEER REVIEW 5 of 13 Figure 3 presents SEM micrographs of the original cotton fabric, mercapto-modified cotton fabric, and superhydrophobic cotton fabric. Figure 3a,b shows the original cotton. Compared to the Figure 3e,f, it can be found that there are no other substances attached to the surface of the fiber, and the surface of the fiber is relatively smooth, with natural wrinkles and grooves. Figure 3c,d shows the mercapto-modified cotton after the MPTES treatment. A slight amount of material is visible on the surface of the mercapto-modified cotton compared with the original cotton surface. This observation may be due to the mercaptan groups of MPTES attached to the surface produced a film on the surface of fiber. After click reaction, the surface of cotton fabric (Figure 3e,f) became rough, and a large amount of block polymer adhered to the surface of fiber, which was bonded together with other fibers. There is no obvious crack boundary, which may be caused by the click reaction. After the reaction, massive particles are formed, and these nanoscale particles form micro/nanoscale rough structures on the microscale fibers of fabric. The structures of finished fabric may contribute to the overall hydrophobicity. Energy-dispersive spectroscopy spectra of the cotton fabric before and after finishing were investigated. Figure 4 shows that after the cotton fabric is modified by MPTES, the S element accounts for 1.9%, the Si element accounts for 4.9%, and the C element content is 63.2%. After the click reaction on the fabric, the content of S is 3.6%, the content of Si is 3.2%, and the content of C is 66.7%. The content of C and S increased, the content of Si decreased, and the two elements Si and S existed in the S-C, Si-O and Si-C-Si bonds, indicating that the click reaction occurred on the surface of the cotton fabric. Figure 4d,e shows the distribution of C, O, Si, and S on the surface of the modified cotton fabric. It can be seen that the various elements are evenly distributed on the surface. Energy-dispersive spectroscopy spectra of the cotton fabric before and after finishing were investigated. Figure 4 shows that after the cotton fabric is modified by MPTES, the S element accounts for 1.9%, the Si element accounts for 4.9%, and the C element content is 63.2%. After the click reaction on the fabric, the content of S is 3.6%, the content of Si is 3.2%, and the content of C is 66.7%. The content of C and S increased, the content of Si decreased, and the two elements Si and S existed in the S-C, Si-O and Si-C-Si bonds, indicating that the click reaction occurred on the surface of the cotton fabric. Figure 4d,e shows the distribution of C, O, Si, and S on the surface of the modified cotton fabric. It can be seen that the various elements are evenly distributed on the surface.

Morphology and Composition of the Fabric Samples
The original cotton fabric, fabric-SH and finished fabric were measured using X-ray photoelectron spectroscopy (XPS). On the surface of the original fabric, only C and O elements were found. On the surface of the mercapto cotton, observation of the four elements Si, C, O, and S (Figure 5a) showed that MPTES was successfully grafted onto the fiber. In Figure 5b, the C 1s peak of the original fabric shows three different peaks at approximately 284.6 eV, 286.4 eV, and 288.2 eV, which are attributed to C-C or C-H, C-O, and C=O bonds [39,40], respectively. As shown in Figure 5c,d, a new peak at 284.9 eV can be observed, corresponding to C-Si bonds. After the click reaction, the S 2p, S 2s, and C 1s peaks were significantly enhanced (Figure 5a). Furthermore, the C-Si peak of hydrophobic cotton was enhanced compared to that of mercapto cotton (Figure 5d). These results suggested the successful reaction of long-chain thiols with TMTVSi containing carbon-carbon double bonds. The original cotton fabric, fabric-SH and finished fabric were measured using X-ray photoelectron spectroscopy (XPS). On the surface of the original fabric, only C and O elements were found. On the surface of the mercapto cotton, observation of the four elements Si, C, O, and S ( Figure  5a) showed that MPTES was successfully grafted onto the fiber. In Figure 5b, the C 1s peak of the original fabric shows three different peaks at approximately 284.6 eV, 286.4 eV, and 288.2 eV, which are attributed to C-C or C-H, C-O, and C=O bonds [39,40], respectively. As shown in Figure 5c,d, a new peak at 284.9 eV can be observed, corresponding to C-Si bonds. After the click reaction, the S 2p, S 2s, and C 1s peaks were significantly enhanced ( Figure 5a). Furthermore, the C-Si peak of hydrophobic cotton was enhanced compared to that of mercapto cotton (Figure 5d). These results suggested the successful reaction of long-chain thiols with TMTVSi containing carbon-carbon double bonds.

FTIR-ATR and Raman Spectroscopy
FTIR spectroscopy was used to evaluate the chemical structure of the modified fabric. The three curves in Figure 6a are the absorbance spectra of original cotton fabric, mercapto cotton and superhydrophobic cotton fabric. Compared to that of the original fabric, the absorbance spectrum of mercapto cotton has a weaker peak at approximately 2540 cm -1 responding to -SH, clearly showing the successful attachment of thiol groups of MPTES to cotton fabric. The peak at 720-830 cm -1 on the

FTIR-ATR and Raman Spectroscopy
FTIR spectroscopy was used to evaluate the chemical structure of the modified fabric. The three curves in Figure 6a are the absorbance spectra of original cotton fabric, mercapto cotton and superhydrophobic cotton fabric. Compared to that of the original fabric, the absorbance spectrum of mercapto cotton has a weaker peak at approximately 2540 cm -1 responding to -SH, clearly showing the successful attachment of thiol groups of MPTES to cotton fabric. The peak at 720-830 cm -1 on the superhydrophobic cotton fabric is attributed to the stretching mode of Si-O-Si and the asymmetric stretching mode of the Si-C bond [41]. The peak at 1470 cm -1 is due to the asymmetric and asymmetric deformation vibrations of methyl C-H bonds, the absorption peaks at 2917 cm -1 and 2850 cm -1 correspond to the symmetrical and asymmetric vibrations of the -CH 2 group in the alkyl chain and a characteristic stretching vibration peak of -CH 3 appears at 2972 cm -1 [35]. After finishing, a bending vibration peak of Si-C appeared at 1260 cm -1 [42]. The above characteristic peaks indicate that click chemistry reaction successfully occurred on the MPTES-modified cotton fabric under ultraviolet light. Figure 6b shows the Raman spectrum of the modified and unmodified fabrics. A characteristic peak was found near 2578 cm -1 on the cotton fabric modified by MPTES [43], but no such peak was found in the finished fabric, which indicated that the click response occurred successfully.

Durability and Stability
In practical applications, it is essential to enhance the durability of superhydrophobic surfaces in complex surroundings, such as laundering, strong light exposure, abrasion, strong acid and alkali conditions. To test the UV irradiation durability of the superhydrophobic cotton fabric, the samples were exposed to ultraviolet light irradiation for different times. The CAs and SAs of the fabric were recorded every 4 h to evaluate the effects of ultraviolet irradiation (Figure 7a). It can be seen from the figure that with increasing ultraviolet irradiation time, the static contact angle of the cotton fabric gradually decreases, and the sliding contact angle also slightly decreases but remains above 149°, still exhibiting a good hydrophobic property. Therefore, the hydrophobic properties of the fabric prepared by this method have a certain resistance to ultraviolet aging. Abrasion resistance tests were also performed, whereby a 500 g weight was placed on the modified fabric sample, and the fabric surface was rubbed against sandpaper (1500 mesh) and moved for 25 cm along a ruler [29]. As the number of abrasion cycles increases, the contact angle increases slightly and then decreases ( Figure  7b), but the contact angle is kept above 140°. This result may be because the fiber became rough when the number of abrasions was small, thereby increasing the contact angle. Finally, as the number of abrasion cycles increased, the fiber surface was gradually destroyed. SEM image of Figure 8a shows the state of the fiber surface after 25 abrasions. Compared with Figure 3e,f, the micro-nano structure of the fiber surface was destroyed, and the surface of the fabric after abrasion has obvious cracks, the roughness of fiber surface was damaged. Figure 7c shows the changes in the CAs and SAs of the superhydrophobic cotton samples dipped in a solution with different pH values (pH = 1, 3, 5, 7, 9, 11, and 13) for 48 h. The CA is significantly reduced, but it can still be maintained above 140°, from 161.8° ± 1.5° to 140.5° ± 1.8°, and the SA increases to 9.9°. Figure 8b,c are SEM images of finished fabric immersed in acid and alkali. The surface of the fabric after impregnation became smooth compared to the finished fabric. This verifies that the hydrophobicity of the fabric is owing to the rough surface. These results indicate that the acid-base treatment has a certain effect on the superhydrophobic performance of cotton fabric, but the cotton fabric still maintains a high hydrophobic performance. Therefore, the superhydrophobic cotton fabric has a certain acid and alkali resistance. Laundry tests were carried out according to the AATCC Test Method 61-2006 1A [38] by using a laundry machine at 40 °C in the presence of 10 stainless steel balls and 0.37% soap powder in the washing containers. In this work, the modified cotton fabric was tested after washing with water for 0 min, 45 min, 90 min, 135 min, 180 min, 225 min, and 270 min. The CA of the cotton fabric gradually decreased with increasing washing time, but after 270 min washing time, remained above 145°. Thus, the superhydrophobic fabric exhibits excellent resistance to UV exposure, laundering, abrasion, and

Durability and Stability
In practical applications, it is essential to enhance the durability of superhydrophobic surfaces in complex surroundings, such as laundering, strong light exposure, abrasion, strong acid and alkali conditions. To test the UV irradiation durability of the superhydrophobic cotton fabric, the samples were exposed to ultraviolet light irradiation for different times. The CAs and SAs of the fabric were recorded every 4 h to evaluate the effects of ultraviolet irradiation (Figure 7a). It can be seen from the figure that with increasing ultraviolet irradiation time, the static contact angle of the cotton fabric gradually decreases, and the sliding contact angle also slightly decreases but remains above 149 • , still exhibiting a good hydrophobic property. Therefore, the hydrophobic properties of the fabric prepared by this method have a certain resistance to ultraviolet aging. Abrasion resistance tests were also performed, whereby a 500 g weight was placed on the modified fabric sample, and the fabric surface was rubbed against sandpaper (1500 mesh) and moved for 25 cm along a ruler [29]. As the number of abrasion cycles increases, the contact angle increases slightly and then decreases (Figure 7b), but the contact angle is kept above 140 • . This result may be because the fiber became rough when the number of abrasions was small, thereby increasing the contact angle. Finally, as the number of abrasion cycles increased, the fiber surface was gradually destroyed. SEM image of Figure 8a shows the state of the fiber surface after 25 abrasions. Compared with Figure 3e,f, the micro-nano structure of the fiber surface was destroyed, and the surface of the fabric after abrasion has obvious cracks, the roughness of fiber surface was damaged. Figure 7c shows the changes in the CAs and SAs of the superhydrophobic cotton samples dipped in a solution with different pH values (pH = 1, 3, 5, 7, 9, 11, and 13) for 48 h. The CA is significantly reduced, but it can still be maintained above 140 • , from 161.8 • ± 1.5 • to 140.5 • ± 1.8 • , and the SA increases to 9.9 • . Figure 8b,c are SEM images of finished fabric immersed in acid and alkali. The surface of the fabric after impregnation became smooth compared to the finished fabric. This verifies that the hydrophobicity of the fabric is owing to the rough surface. These results indicate that the acid-base treatment has a certain effect on the superhydrophobic performance of cotton fabric, but the cotton fabric still maintains a high hydrophobic performance. Therefore, the superhydrophobic cotton fabric has a certain acid and alkali resistance. Laundry tests were carried out according to the AATCC Test Method 61-2006 1A [38] by using a laundry machine at 40 • C in the presence of 10 stainless steel balls and 0.37% soap powder in the washing containers. In this work, the modified cotton fabric was tested after washing with water for 0 min, 45 min, 90 min, 135 min, 180 min, 225 min, and 270 min. The CA of the cotton fabric gradually decreased with increasing washing time, but after 270 min washing time, remained above 145 • . Thus, the superhydrophobic fabric exhibits excellent resistance to UV exposure, laundering, abrasion, and strong acid and alkali conditions, which is an advantage in complex environments.

Self-cleaning Performance of the Modified Fabric
Compared to the original cotton, the superhydrophobic cotton fabric has good self-cleaning ability. It can be seen from Figure 9 that the untreated cotton fabric is contaminated with dyes and does not have self-cleaning ability, while the superhydrophobic cotton fabric remains clean ( Figure  9), with the water drops removing the dye on the fabric. This result shows that the superhydrophobic cotton fabric has a self-cleaning ability. This self-cleaning function can be applied in daily life. Additionally, the original cotton and superhydrophobic cotton were placed in a beaker filled with water, as shown in Figure 10a. It was found that the original cotton sank to the bottom of the beaker, and the superhydrophobic cotton floated on the water surface, which is because the original cotton has hydrophilic groups, and capillary action causes the fabric to absorb water. For the modified cotton fabric, air remains on the surface of fabric, and a solid-liquid-air interface is established [33]. In practical applications, the environment is complex, sometimes the fabric will come into contact

Self-cleaning Performance of the Modified Fabric
Compared to the original cotton, the superhydrophobic cotton fabric has good self-cleaning ability. It can be seen from Figure 9 that the untreated cotton fabric is contaminated with dyes and does not have self-cleaning ability, while the superhydrophobic cotton fabric remains clean ( Figure  9), with the water drops removing the dye on the fabric. This result shows that the superhydrophobic cotton fabric has a self-cleaning ability. This self-cleaning function can be applied in daily life. Additionally, the original cotton and superhydrophobic cotton were placed in a beaker filled with water, as shown in Figure 10a. It was found that the original cotton sank to the bottom of the beaker,

Self-cleaning Performance of the Modified Fabric
Compared to the original cotton, the superhydrophobic cotton fabric has good self-cleaning ability. It can be seen from Figure 9 that the untreated cotton fabric is contaminated with dyes and does not have self-cleaning ability, while the superhydrophobic cotton fabric remains clean (Figure 9), with the water drops removing the dye on the fabric. This result shows that the superhydrophobic cotton fabric has a self-cleaning ability. This self-cleaning function can be applied in daily life. Additionally, the original cotton and superhydrophobic cotton were placed in a beaker filled with water, as shown in Figure 10a. It was found that the original cotton sank to the bottom of the beaker, and the superhydrophobic cotton floated on the water surface, which is because the original cotton has hydrophilic groups, and capillary action causes the fabric to absorb water. For the modified cotton fabric, air remains on the surface of fabric, and a solid-liquid-air interface is established [33]. In practical applications, the environment is complex, sometimes the fabric will come into contact with common liquids used in daily life, here, we tested saline, tea, dyed water, cola, milk, coffee. It can be seen that the raw cotton is soaked by these liquids, and the surface of the superhydrophobic cotton exhibits spherical droplets. These droplets did not wet the modified fabric. These results show that the modified cotton fabric has antipollution ability and can be applied to related fields in daily life.
Coatings 2020, 10, x FOR PEER REVIEW 10 of 13 with common liquids used in daily life, here, we tested saline, tea, dyed water, cola, milk, coffee. It can be seen that the raw cotton is soaked by these liquids, and the surface of the superhydrophobic cotton exhibits spherical droplets. These droplets did not wet the modified fabric. These results show that the modified cotton fabric has antipollution ability and can be applied to related fields in daily life.   Figure 11a,b presents a schematic diagram of the effect of the oil-water separation test using the superhydrophobic cotton fabric. Carbon tetrachloride was dyed red by oil red O, and water was dyed blue with methyl blue. Some activated blue-dyed water was mixed with oil red O-dyed carbon tetrachloride and poured into an oil-water separation device, as shown in Figure 11. The carbon tetrachloride in the red part was filtered through the treated fabric, and the water in the blue part ultimately remained in the upper container without being filtered by the fabric. Therefore, the superhydrophobic cotton fabric prepared by this method has a certain oil-water separation ability. Figure 11c,d show carbon tetrachloride and cyclohexane dyed with oil red O, respectively. It can be with common liquids used in daily life, here, we tested saline, tea, dyed water, cola, milk, coffee. It can be seen that the raw cotton is soaked by these liquids, and the surface of the superhydrophobic cotton exhibits spherical droplets. These droplets did not wet the modified fabric. These results show that the modified cotton fabric has antipollution ability and can be applied to related fields in daily life.   Figure 11a,b presents a schematic diagram of the effect of the oil-water separation test using the superhydrophobic cotton fabric. Carbon tetrachloride was dyed red by oil red O, and water was dyed blue with methyl blue. Some activated blue-dyed water was mixed with oil red O-dyed carbon tetrachloride and poured into an oil-water separation device, as shown in Figure 11. The carbon tetrachloride in the red part was filtered through the treated fabric, and the water in the blue part ultimately remained in the upper container without being filtered by the fabric. Therefore, the superhydrophobic cotton fabric prepared by this method has a certain oil-water separation ability. Figure 11c,d show carbon tetrachloride and cyclohexane dyed with oil red O, respectively. It can be 3.6. Oil/water Separation Property of the Modified Fabric Figure 11a,b presents a schematic diagram of the effect of the oil-water separation test using the superhydrophobic cotton fabric. Carbon tetrachloride was dyed red by oil red O, and water was dyed blue with methyl blue. Some activated blue-dyed water was mixed with oil red O-dyed carbon tetrachloride and poured into an oil-water separation device, as shown in Figure 11. The carbon tetrachloride in the red part was filtered through the treated fabric, and the water in the blue part ultimately remained in the upper container without being filtered by the fabric. Therefore, the superhydrophobic cotton fabric prepared by this method has a certain oil-water separation ability. Figure 11c,d show carbon tetrachloride and cyclohexane dyed with oil red O, respectively. It can be seen that the superhydrophobic cotton fabric can absorb carbon tetrachloride and cyclohexane. These phenomena prove that the superhydrophobic cotton fabric possesses a certain ability to remove oil pollution.

Conclusion
In summary, we have successfully prepared a superhydrophobic cotton fabric by click chemistry. The click reaction occurred on the modified cotton fabric under ultraviolet light irradiation. It was demonstrated that the cotton fabric was endowed with resistance to mechanical laundering, UV irradiation, abrasion, and strong acid and alkali solutions with pH values ranging from 1 to 13. Importantly, the cotton fabric not only exhibited superhydrophobicity with a WCA of 161.8°, but also showed good self-cleaning and antipollution properties. In addition, the finished fabric demonstrated an oil-water separation and self-cleaning ability, and thus could be used in practical applications.

Conclusions
In summary, we have successfully prepared a superhydrophobic cotton fabric by click chemistry. The click reaction occurred on the modified cotton fabric under ultraviolet light irradiation. It was demonstrated that the cotton fabric was endowed with resistance to mechanical laundering, UV irradiation, abrasion, and strong acid and alkali solutions with pH values ranging from 1 to 13. Importantly, the cotton fabric not only exhibited superhydrophobicity with a WCA of 161.8 • , but also showed good self-cleaning and antipollution properties. In addition, the finished fabric demonstrated an oil-water separation and self-cleaning ability, and thus could be used in practical applications.
Author Contributions: Methodology, X.C. and B.W.; Formal analysis, X.C. and R.C.; Project administration, G.C. and T.X.; Writing -original draft, X.C. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.