Facile fabrication of asymmetric wettable fabric with weft backed weave for oil/water separation

Abstract

Inspired by the special wettability of lotus leaves with high water repellence, the preparation of superhydrophobic materials has gained much attention. Herein, we report a new method to fabricate the fabric with weft backed weave that gains asymmetric wettability by fabric structure design. The fabric is made up of three types of yarns, in which hydrophobic polyester was applied as warp yarns, and both hydrophobic cotton and hydrophilic cotton served as weft yarns. Due to its asymmetric properties, as one face with a contact angle of ∼150° was superhydrophobic and repelled water and the other one was hydrophilic and absorbed water, the single-faced superhydrophobic fabric has been successfully used for oil–water separation to achieve water-removal and oil-removal. The separation of oil from oil/water mixtures could be achieved by placing the hydrophobic face on the top, and the switched separation of water from oil/water mixtures could be further realized by placing the hydrophilic face on the top after pre-wetting treatment. Moreover, each separation process has a separation efficiency above 95%.

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1. Introduction

Water is the most abundant resource in the natural environment. However, the amount of fresh water available to humans is low for sea water occupies a large part of the total water on the earth. The frequent occurrences of oil spills and industrial waste water cause water pollution and severely affect the ecological environment. How to solve the removal of oil from water is a hot topic urgently required to be settled.¹ However, traditional oil/water separation methods such as gravity separation, air flotation, combustion and biodegradation are often limited by low separation efficiency and high cost.² Hence, the exploration of new materials and design with high efficiency of oil–water separation is necessary.³

Recently, porous materials with special wettability inspired by the natural species, such as lotus that obtains the high water repellence, have attracted much attention for separating oil–water mixtures.^4–6 Superhydrophobic materials^7,8 with a high water contact angle (>150°) allows the penetration of oil through the materials while blocks water, thus achieving the selectively separation of oil–water mixtures.^9–11 However, these materials are easily fouled or even blocked up by oils during the separation process owing to their intrinsic oleophilic property, resulting in a decrease in separation efficiency, which restricts their practical applications and mass production.^12,13 Also, it's difficult for these materials to act as water-removing separators, because oils with light density float on water and the massive water can form a barrier layer which prevents the oil permeation. To conquer these drawbacks, superhydrophilic materials with underwater superoleophobicity inspired by fish skin were developed.^14–16 These materials could permit the flow of water while reject the oil permeation.^17–19 At the same time, water film is formed on the surface of the materials during permeation, which could efficiently prevent the materials from contaminating by oils.^20–22 Therefore, these superhydrophilic materials removing water may act as a complementary for superhydrophobic materials removing oils.²³ So far, various methods were reported to prepare the superhydrophobic or superhydrophilic materials using for oil–water separation, but the materials with asymmetric wettability have been reported less.^24–27 These materials, exhibiting asymmetric wettability on its two faces as one face shows superhydrophobic characterization and the other one is hydrophilic, have potential application values for oil–water separation and microfluidic applications.²⁸ The chemical finishing process is the mainly method to prepare such asymmetric wettable materials.^29–31 For example, Liu et al.³² have developed single-faced superhydrophobic cotton fabric by using the foam finishing process. In the method, the fluoropolymer composite foam with a high viscosity was applied to the cotton fabric surface with a blade-coater, which is resulted in asymmetric wettability on its two faces. Sasaki et al.³³ have reported single-faced superhydrophobicity on cotton fabrics using biodegradable materials via a one-step spraying method. Ethyl-α-cyanoacrylate as adhesive resin and hydrophobized SiO₂ nanoparticles were mixed to fabricate a superhydrophobic coating, single-faced superhydrophobicity was produced by adjusting the distance between the fibers and sprayer and the amount of sprayed solution. These methods had virtues that were of simple process and stable single-faced superhydrophobicity. However, to achieve the single-faced superhydrophobicity, all the methods that reported had the common characteristics of directly chemical or plasma treatment to the fabric.

Here, we report for the first time about optimizing fabric structure design based on weft backed weave to gain single-faced superhydrophobicity of fabric. The fabric with asymmetric wettability was fabricated by using hydrophobic polyester as warp yarns, hydrophobic cotton and hydrophilic cotton as weft yarns. Due to its asymmetric faces as one surface is superhydrophobic that repels water and the other one is hydrophilic that absorbs water; the single-faced superhydrophobic fabric has been successfully used for oil–water separation. What more, the separation of oil from oil/water mixtures could be achieved by placing the hydrophobic face on the top, and the switched separation of water from oil/water mixtures could be further realized by placing the hydrophilic face on the top after pre-wetted treatment. Even after several separation cycles, the fabric remains single-faced hydrophobic and could achieve separation of light oil/water mixtures, heavy oil/water mixtures with separation efficiency of above 95%.

2. Experimental section

2.1 All materials

Cotton yarns (32 Tex) are purchased from Huarun Textile Group Co., Ltd. Polyester yarns (32 Tex) are purchased from Zhangjiagang Xiang Mei Textile Co., Ltd. Hydrophobic agent (C₈ liquid) was purchased from Shanghai Ding gift industry and Trade Co., Ltd. Hydrogen peroxide (AR), sodium hydroxide (AR), ammonia (AR), tetramethoxysilane (AR), absolute alcohol (AR), sodium silicate (AR) were purchased from Chinese Medicine Group Chemical Reagent Co., Ltd. All of the reagents were directly used without further treatment.

2.2 Hydrophilic treatment to the cotton yarns

Impurities existing on cotton yarns were removed by using alkali (2 g L⁻¹), hydrogen peroxide (2 g L⁻¹), sodium silicate (2 g L⁻¹) and JFC (2 g L⁻¹) mixing water with a bath ratio, 1 : 1. After putting the yarns to the mixtures for 20 minutes at 98 °C, taking out the yarns and flushing the yarns with plenty of water, drying for further usage.

Firstly, adding anhydrous ethanol (25 mL) into the Erlenmeyer flask and dipping the amount of ammonia (14 mL) to the ethanol under stirring. Then the above solution was stirred at 60 °C for 30 min, TEOS (15 mL) and ethanol (15 mL) mixtures were added to the solution, and the silica sol with particle size of about 50 nm was obtained by stirring continuously for 3 hours. Finally, the yarns were dipped to the solution of nano-silicon dioxide and ethanol mixtures with a ratio of 1 : 1 for 10 minutes to receive the hydrophilic yarns.

2.3 Hydrophobic treatment to the polyester and cotton yarns

A low concentration of alkali (20 g L⁻¹) with a temperature of 95 °C was used to treat the yarns for 2 h to obtain the enhanced wetting properties as well as get rid of impurities.

The hydrophobicity of polyester and cotton yarns was improved by the treatment with hydrophobic agent. By dipping the polyester yarns and cotton yarns to the hydrophobic agent (60 g L⁻¹) for 30 minutes, baking at 120 °C for 10 minutes after drying, hydrophobic polyester yarns as warp yarns and cotton yarns as weft yarns were ready to be used.

2.4 The structure of weft backed weave

Our goal is to fabricate a fabric with different wetting properties in two faces; the structure of the fabric with weft backed weave was designed (as in Scheme 2).

The fabric with weft backed weave have three parts, warp yarns (bottom Arabic numerals) and two different weft yarns (left Arabic and roman numerals), the blank of the table are defined as weft interlacing points, the other symbols in the table presented warp interlacing points. One surface of the fabric is made up by the interweaving of warp yarns and weft yarns (A), the other one of the fabric is constituted by the warp yarns and weft yarns (B). Through adjusting the wetting properties of the warp yarns and weft yarns, fabrics with different wettability were obtained. Semi-automatic loom was used to fabricate the fabric with the structure of weft backed weaves.

2.5 Water absorption ability of the fabric

Three kinds of fabrics, superhydrophilic fabric, single-faced superhydrophobic fabric and superhydrophobic fabric were used to test the water absorption ability. The superhydrophilic fabric was made up by the hydrophilic polyester yarns and hydrophilic cotton yarns with the structure of weft backed weave, the superhydrophobic fabric was fabricated by the hydrophobic polyester yarns and hydrophobic cotton yarns with the structure of weft backed weave. Each fabric (1 g) was immersed in pure water keeping 10 min to complete the water absorption. Then taking the fabrics out the water and drying at room temperature for 10 min to remove the residual water. The mass of fabrics before and after water absorption was measured. The water absorption ability was calculated according to the equation below:where w% is the water absorption ability of the fabrics, w_a is the mass of the fabric absorbing water and w₀ is the dried fabric.

2.6 Oil–water separation

Six kinds of oils in different density as chloroform, dichloromethane, carbon tetrachloride, heptane, petroleum ether and dodecane were used to investigate the oil–water separation ability of the fabric with weft backed weave. Oil was stained by oil red, and water was stained by reactive blue. The fabric with weft backed weave was fixed between a glass tube and a flask. Oil–water mixtures were poured into the top glass tube to achieve the separation of oil or water.

2.7 Characterizations

Field emission scanning electron microscopy was used at an accelerating voltage of 5 kV to characterize of the surface morphologies of the yarns. And the chemical composition was investigated by using energy-dispersive X-ray spectroscopy. SEM images were used to make sure the structure of weft backed weave. The water contact angle was measured with a 5 μL distilled water droplet at ambient temperature with a contact angle meter. The average contact angle was gained by the measurements at five different points on the surface of the fabric. Magnifier was used to test the warp and weft densities. Venire caliper was served to measure the thickness of the fabric.

3. Results and discussion

3.1 Characterizations of yarns

Fig. 1 showed the SEM images and the EDXS spectrum of the hydrophobic cotton yarns, hydrophobic polyester yarns and hydrophilic cotton yarns respectively. From the SEM images, there is no obvious morphological difference between hydrophobic cotton yarns (Fig. 1a) and hydrophobic polyester yarns (Fig. 1c), while there is obvious grooves on the hydrophilic cotton yarns (Fig. 1b). Then the EDXS spectrum was used to find the difference in the chemical composition of the yarns. From the Fig. 1d and f, we could find that hydrophobic polyester yarns and cotton yarns show a peak for fluorine (F), which is attributed to the fluorine containing compounds (C₈ liquid) coated on the yarns; while hydrophilic cotton yarns showed a peak for silicon (Si), which is attributed to the nano-silica coating on the yarns (Fig. 1e).

Fig. 1 SEM images and EDXS spectrum of the yarns. (a and d) Hydrophobic cotton yarns, (b and e) hydrophilic cotton yarns and (c and f) hydrophobic polyester yarns.

3.2 Preparation of fabric weft backed weave

To prepare the fabric with asymmetric wetting ability, it is important that one face of the fabric is fully coated by the hydrophobic chemicals, while the other one is hardly covered by the hydrophobic chemicals. Hence, we selected weft backed weave to achieve asymmetric structure in which the hydrophobic polyester was applied as warp yarns, hydrophobic cotton and hydrophilic cotton were employed as weft yarns, as shown in Scheme 1.

Scheme 1 Schematic of the fabrication of single-faced superhydrophobic fabric with the structure of weft backed weaves. (a) Hydrophobic treatment to cotton yarns by using fluorine containing compounds. (b) Hydrophobic treatment to polyester yarns by using fluorine containing compounds. (c) Hydrophilic treatment to cotton yarns after the pre-treatment by using nano-silicon. (d) Fabric with the structure of weft backed weaves (hydrophobic and hydrophilic cotton yarns as weft yarns, hydrophobic polyester yarns as warp yarns).

Scheme 2 The structure of weft backed weaves.

Fabric with weft backed weaves obtains rough surface that caused by the random distribution of the micro fibers (Fig. 2), resulting in the superhydrophobic surface of high contact angles with single-step treatment. However, hydrophilic cotton yarns interweave with hydrophobic polyester yarns result in a certain number of hydrophobic pots existed on the hydrophilic surface, which affects the hydrophilic properties. The influence of hydrophobic pots are limited, because hydrophilic cotton taking over nearly whole hydrophilic face as weft yarns are floating on the top of the hydrophobic polyester as warp yarns and the random distribution of the micro fibers of the hydrophilic yarns are well covered the hydrophobic pots.

Fig. 2 Illustration of the structure of weft backed weaves.

3.3 The effect of warp density to the wettability of fabric

As mentioned earlier, fabric with weft backed weave could be realized with asymmetric wettability, while there are a certain numbers of hydrophobic pots on the hydrophilic surface. By changing the reed number which determines the warp density, the wetting ability of the hydrophilic surface could be improved with the decreased number of the hydrophobic pots. The warp density can be defined by the numbers of warp yarns per 10 cm, which was calculated according to the following:

Warp density = reed number × 2 × 2 (2)

With the gradually increased reed number, the warp yarns are arranged more closely resulted in higher density of fabric, while the weft yarns obtain a shorter length of floats. Fig. 3a displays a visual illustration of the fabric with different warp densities.

Fig. 3 The influence of reed numbers to the fabric. (a) Diagram of the effect of reed number to the fabric. (b) Effects of the reed numbers on the wettability of the fabric with the structure of weft backed weaves. (c) Schematic showing the specifications of the fabric with the structure of weft backed weaves.

The wettability of fabric with weft backed weave was determined by weft yarns. By decreasing the reed number, hydrophobic pots on the hydrophilic surface reduced and weft yarns obtained a higher length of floats, the fabric had a better wettability on the hydrophilic surface. The wetting behaviors on the fabric with different warp density were evaluated on the basis of contact angle measurements. Fig. 3b showed the effects of the reed number on the wettability of the fabric with weft backed weave. It can be found that changing the reed number has no effect on the hydrophobic surface, which remains contact angles higher than 150°. However, with the decreased of the reed number, the spreading time, ranging from 4 s to 2 s, of the water droplet on the hydrophilic surface reduces, indicating the improvement of the hydrophilic property on the hydrophilic surface. As for the optimized fabric with weft backed weaves, magnifier was used to test the warp and weft densities, venire caliper was served to measure the thickness of the fabric. From Fig. 3c, we can find that the fabric with weft backed weave gains warp density of 320 yarns per 10 cm, weft density of 400 yarns per 10 cm, and thickness of 0.5 mm.

3.4 Surface wettability of the fabric

The surface wettability was evaluated by observing the behavior of water droplet on the surface of the fabric. As shown in Fig. 4a, water droplets that colored by blue reactive dyes were placed on both side of the fabric. On the hydrophobic surface, the water droplets were found to have a contact angle of 152.8° and do not spread out (Fig. 4b), which indicate the excellent superhydrophobicity of the fabric. However, on the hydrophilic surface, water droplets were quickly absorbed once placed on the surface of the fabric, as shown in Fig. 4c. The process of water droplets on the different surfaces of the fabric could be seen in Video S1.† This result demonstrates the fabric with weft backed weave exhibits asymmetric wetting ability on its two surfaces: one face is hydrophobic that repels water, the other one is hydrophilic that absorbs water.

Fig. 4 Photographs of water droplets on the fabric with the structure of weft backed weaves. (a) Water droplets that colored by the blue dye on the hydrophobic side of the fabric are spherical, while those placed on the hydrophilic side are absorbed by the fabric. (b) Water droplets on the hydrophobic side having CA more than 150°. (c) Water droplets on the hydrophilic side having CA about 0°.

The wetting ability of the fabric with weft backed weave was further studied by measuring the water contact angle, rolling angle and oil contact angle, as illustrated in Fig. 5. Fig. 5a shows a water droplet on the hydrophobic with a water contact angle of ∼150°, Fig. 5b shows the rolling angle of the fabric of ∼14°, indicating a good water repellence. The oil contact angle was found to be ∼0°, as shown in Fig. 5c. On the hydrophilic surface, a water droplet absorbed into the fabric within 2 s (Fig. 5d–f) showing the good hydrophilic property of hydrophilic face.

Fig. 5 Contact angles test of fabric. Water droplet on a (a and b) super-hydrophobic face and (d–f) a hydrophilic face of the fabric with the structure of weft backed weaves. (a) Measurement of water contact angle (∼150°) on the hydrophobic surface of the fabric. (b) Measurement of an inclination angle (∼14°) for rolling of. (c) Measurement of an oil contact angle (∼0°) on the hydrophobic surface of the fabric. (d–f) Water droplet on the hydrophilic surface was absorbed into the fabric within 2 s.

The underwater oil wettability was examined by immersing the fabric in aqueous solutions (as shown in Fig. 6). Fig. 6a shows the image of oil droplets (chloroform) on the hydrophilic surface of the fabric underwater. When the hydrophilic surface of the fabric with weft backed weaves is pre-wetted in aqueous solution, it becomes oil-repulsive for different kinds of oils, such as chloroform (high density) with a contact angle of ∼154° (Fig. 6b), heptane (low density) with a contact angle of ∼152° (Fig. 6c). The results showed hydrophilic face of the fabric was oil-repulsive underwater, which provided the possibility for separating water from oil.

Fig. 6 Underwater contact angles test of the hydrophilic surface. (a) Image of the oil droplets (chloroform) on the hydrophilic surface of the fabric underwater. (b) Oil (heptane) with a CA of ∼152° in water. (c) Oil (chloroform) with a CA of ∼154° in water.

3.5 Asymmetric wetting ability of fabric

The asymmetric wetting ability of the fabric with the structure of weft backed weave was tested under water. Fig. 7 shows the water absorption ability of the single-faced superhydrophobic fabric (ii) compared to that of superhydrophilic fabric (i) and superhydrophobic fabric (iii). The original mass of the three kinds of fabrics was 1 g, the weight of the hydrophilic fabric that having been immersed in water for 10 min was 2.6 g, showing excellent water absorption ability of 160%. The immersion test of the single faced superhydrophobic fabric was measured to be a weight of 2.2 g, showing lower water absorption ability of 120% than that of the hydrophilic fabric. As for the hydrophobic fabric, no mass changes after being immersed in water for 10 min. The results above indicate that the fabric with weft backed weave obtains good water absorption ability on the hydrophilic face while repels water on the hydrophobic one.

Fig. 7 Testing of water absorption ability. Water absorption ability of (i) hydrophilic fabric, (ii) single faced hydrophobic fabric and (iii) hydrophobic fabric.

3.6 Oil–water separation of the fabric

Superhydrophilic and underwater superoleophobic materials can be used for oil/water separation. The fabric with weft backed weave (320 yarns/10 cm) obtains asymmetric wettability on its two face as one face is superhydrophobic that repels water and the other one is oil-repulsive underwater. The oil–water separation capability of the fabric was tested by two types of oil–water mixtures (chloroform/water, heptane/water). A schematic diagram of the oil–water separation process is shown in Fig. 8. Gravity was the only force for the separation. A mixture of chloroform dyed with red dye and water dyed with blue dye (1/2 v/v) was poured into the hydrophobic surface of the fabric that fixed between two glass tubes (Fig. 8a), the oil immediately permeated through the fabric (Fig. 8b), while water remained on the fabric (Fig. 8c). The oil/water separation can be seen in Video S2.† By contrast, switching the hydrophilic surface of the fabric on the top and pre-wetting the fabric by water within 1 minute to keep the hydrophilic surface wettable,³⁴ a mixture of heptane dyed with red dye and water dyed with blue dye (1/2 v/v) was poured into the hydrophilic surface (Fig. 8d), water permeated the fabric with a certain liquid pressure (Fig. 8e), whereas oil was still stayed on the fabric (Fig. 8f). The process of separation can be seen in Video S3.†

Fig. 8 Photographs of oil–water separation of Janus fabric with the structure of weft backed weave. The fabric was fixed between two glass vessels. Putting the hydrophobic surface of the fabric on the top, mixtures of chloroform and water was poured into the upper glass tube (a), oil permeated through the fabric (b), while water stayed in the upper glass tube (c). Pre-wetting the hydrophilic surface of the fabric and putting the hydrophilic surface on the top, mixtures of heptane and water was poured into the upper glass tube (d), water permeated through the fabric with a certain liquid pressure (e), while oil stayed in the upper glass tube (f).

To further study the separation efficiency of a variety of oil–water mixtures, chloroform, dichloromethane, carbon tetrachloride, heptane, petroleum ether, and dodecane were mixed with water, the mass (water or oil) before and after the separation was measured, respectively. The separation efficiency was calculated by m after/m before × 100%, the mixtures of light oil and water were separated by putting the hydrophilic surface on the top, while the heavy oil and water were separated by placing the hydrophobic surface on the top. The separation efficiency of the fabric with weft backed weaves was measured up to above 95% for all the light and heavy oil–water mixtures, as shown in Fig. 9a. In addition, the oil–water separation performance of the fabric with weft backed weave after several separation times under the same experimental condition was measured to test its reusability. The fabric retains single-faced superhydrophobicity as hydrophobic surface obtaining a contact angle higher than 150° while the hydrophilic surface gaining a contact angle of 0° (Fig. 9b). In addition, the separation efficiency for the light oil (heptane) and water mixtures, heavy oil (chloroform) and water mixtures still remain at above 95%. Even after several cycles (Fig. 9c), the separation efficiency had no obvious change, which indicate the good usability and recyclability of the fabric with weft backed weave.

Fig. 9 Testing of oil–water separation efficiency. (a) Separation efficiency of different kinds of oil–water mixtures. (b) Water contact angle of the fabric on hydrophobic surface and hydrophilic surface after several cycles. (c) Separation efficiency of the fabric for light oil (heptane)/water mixtures and heavy oil (chloroform)/water mixtures after several cycles.

4. Conclusions

In summary, we have fabricated single-faced hydrophobic fabric with weft backed weave and further demonstrated its application for oil/water separation. Three types of yarns were applied as warp/weft yarns to fabricate the asymmetric wettable fabric as one surface is superhydrophobic that repels water and the other one is hydrophilic that absorbs water. The hydrophobic face of the fabric has a water contact angle more than 150°, while the hydrophilic face has a water contact angle of 0°. The fabric can separate the oil from oil/water mixtures with a high separation efficiency of up to 95% by putting the hydrophobic surface on the top; meanwhile, the separation of water from oil/water mixtures could also achieved with a separation efficiency of up to 95% by reversing the hydrophilic surface on the top. The results reported herein provide a new strategy for the fabrication of oil/water separation material.

Acknowledgements

The paper is supported by Ph.D. Programs Foundation of Ministry of Education of China, 20130075130002. Authors are also thankful to National support project of Manufacture and Application Technology of Super Softener Dyeable Co-polyester, 2009BAE75B02.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24515c

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