Superhydrophobic/superoleophilic cotton fabrics treated with hybrid coatings for oil/water separation
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A series of novel superhydrophobic/superoleophilic cotton fabrics with the use of hybrid coatings combined with polyvinylsilsesquioxanes (PVS) polymer and nano-Al2O3 particles were developed successfully by solution immersion. The influences of the concentration of nano-Al2O3 on surface morphology, composition, water-repellent properties, and durability of the treated cotton fabrics were investigated in detail, respectively. Subsequently, oil/water separation performance and self-cleaning ability of the treated cotton fabrics were examined. It is interesting to find that the novel cotton fabrics treated with the hybrid coatings of PVS polymer and nano-Al2O3 exhibited enhanced mechanical properties, excellent water-repellent properties, and durable mechanical stability with the increasing concentration of the nano-Al2O3 particles, compared with cotton fabric treated by the sole PVS without adding nano-Al2O3 particles. Meanwhile, the treated cotton fabrics exhibited high oil/water separation efficiency of 99% and excellent self-cleaning ability when the concentration of nano-Al2O3 particles in composite coatings increases to 2%. The hybrid coatings combined with the PVS polymer and Al2O3 particles are promising in exploitation of superhydrophobic/superoleophilic materials, which can be applied to many fields including oil/water separation, wallpaper and soft tiles, and clothing for oil recovery, water-proof and self-cleaning applications, respectively.
KeywordsCellulose fabrics Hybrid coatings Superhydrophobicity/superoleophilic Mechanical property Oil/water separation
Crude oil, as one of significant natural resources, has greatly promoted social progress of human beings. However, frequent oil leakage and oil spill accidents have led to severe water and soil contaminations during oil exploitation and transportation, which usually bring catastrophic impacts to ocean ecosystem and aquatic environment [1, 2, 3, 4]. The oil-polluted water usually contains harmful and toxic chemicals, which incited extensive attention because of the increasing discharge of industrial oily wastewater as well as the laboratory wastewater. Fortunately, oil/water separation technique affords a promising environmental remediation for oil spills in oceans and rivers. However, conventional techniques, such as centrifugation, absorbents, skimming, and biological treatment, are suffering from the disadvantages of low efficiency, high-energy consumption, secondary pollution, high operation cost, and so on when they are used for the separation of oil/water mixtures [5, 6, 7, 8, 9]. Therefore, advanced materials or techniques with low cost are urgently needed to deal with various oil/water mixture efficiently.
Over the past decades, considerable attention has been paid to exploitations of artificial superhydrophobic/superoleophilic materials for oil recovery from water [10, 11, 12, 13, 14]. It is well known that superhydrophobicity/superoleophilicity is a typical environmental adaption during evolution of diverse creatures . Inspired by many natural surfaces of lotus leaves, butterfly wings, and rose petals, artificial superhydrophobic surfaces with static water contact angle greater than 150° and sliding angle less than 10° can readily be constructed [16, 17]. For instance, some superhydrophobic porous materials in the forms of metal meshes , foams [19, 20], sponges [21, 22], filter paper , and gels  have been successfully developed and widely utilized for oil/water separation. Obviously, most of these porous materials can absorb large amounts of spilled oil into the pores, but these materials also suffer from some deficiencies, such as low oil/water separation efficiency, time-consuming synthetic steps, poor durability, weak mechanical strength, and high cost, which limit their real-life applications . Owing to the intrinsic woven pores, many efforts have been devoted to the exploitations of facile, inexpensive, high separation efficient superhydrophobic fabric-based materials. In comparison with diverse substrates, cotton is eco-friendly, low cost, and abundant in nature; hence, cotton fabric-based materials are considered the optimum candidates for oil/water separation [6, 11].
It is well established that artificial superhydrophobic surfaces can be created low surface energy from chemicals and high roughness on the surface of substrates . However, many hydroxyl groups available on surfaces of cotton fibers make cotton fabric hydrophilic. To transform hydrophilic cotton fabrics into hydrophobic ones, some expensive and toxic fluoride polymers are commonly employed [12, 27, 28, 29]. Due to environment concerns, many scientific researchers are inclined to utilize some environmentally friendly and fluorine-free polymers with low surface energy to develop superhydrophobic cotton fabrics. Recently, polydimethylsiloxanes (PDMS), as typical one class of the most important polysiloxanes, have been widely employed as adhesives to prepare superhydrophobic cotton fabrics [21, 30, 31, 32, 33]. However, the other polysiloxanes with hydroxyl groups have been seldom utilized as coatings to fabricate superhydrophobic cotton fabrics except for liquid methylsiloxane resin . Therefore, these reactive polysiloxanes may have great potential in constructing superhydrophobic cotton fabrics. In our recent work , the reactive polyvinylsilsesquioxane (PVS) has been applied to cotton fabrics and endows them with good thermal stability, but their hydrophobic properties are undesirable due to the limited woven surface roughness. To further increase surface roughness, some inorganic nanoparticles, such as SiO2, ZnO, and TiO2 [36, 37, 38, 39, 40, 41], have been commonly employed to roughen the substrate surfaces to fabricate superhydrophobic cotton fabrics. In recent years, due to high thermal stability and low cost, aluminum oxide particles have been extensively applied on surfaces of various substrates including glass-based materials, aluminum alloy, wool fabric, and glass slides as hybrid coatings for achieving antireflective , anti-wear , superhydrophobic [44, 45], and anticorrosion functionalities . However, no work on superhydrophobic cotton fabrics has been reported using nano-aluminum oxide particles as hybrid coatings. Hence, it is very promising to develop superhydrophobic cotton fabrics using nano-aluminum oxide particle hybrid coatings.
Meanwhile, to further reduce the cost of materials, the nano-aluminum oxide was utilized to increase the surface roughness of cotton fabrics. To the best of our knowledge, there are no further reports on cotton fabrics functionalized with hybrid coatings consisting of the PVS polymer and nano-aluminum oxide. This work is mainly focused on the effects of concentration of nano-aluminum oxide on hydrophobic properties, surface morphology, and durability of the cotton fabrics treated with the PVS/nano-aluminum oxide coatings. In the meantime, oil/water separation performance and self-cleaning ability of the treated cotton fabrics were examined. This work provides a one-step method to fabricate superhydrophobic cotton fabrics with high oil/water separation efficiency and self-cleaning ability, and these functional fabrics exhibit numerous potential in oil/water separation and industry oil wastewater purification.
2 Experimental section
Desized, scoured, and bleached cotton fabric (200 g/m2) was obtained from Jinqiu Textile Company, Shaoxing, China. Vinyltrimethoxysilane (97.9%) was kindly supplied by Nanjing Chengong Silicon Co., Ltd., Nanjing, China. Concentrated hydrochloric acid (37%), methylene blue (AR), orange G and acid red 112, oil red O, anhydrous ethanol (AR), and α phase nano-aluminum oxide (about 30 nm) were purchased from Shanghai Reagent Plant, China. Toluene (AR) and silicone fluid (PMX-200, 50 cSt) were purchased from Tianjin BoDi Chemical Reagent Co., Ltd. All the chemicals were used as received. Polyvinylsilsesquioxanes (PVS) was prepared according to our previous work [35, 47].
2.2 Preparation of finished cotton fabric samples
The recipes of cotton fabrics treated with hybrid coatings of the PVS polymer and nano-Al2O3
Weight of PVS (g)
2.3 Laundering procedure of the functional cotton fabrics
The washing experiment was carried out according to a modified literature method . In brief, the washing procedure was performed in a 500-mL beaker containing 200 mL distilled water and 1.0 g detergent (main ingredient: sodium dodecyl benzene sulfonate) by magnetically stirring at 300 rpm for 30 min at 40 °C. Subsequently, the washed fabrics were moved from the detergent solution and rinsed within 200 mL distilled water with stirring for 15 min. Finally, the laundered cotton fabrics were obtained by drying the rinsed cotton fabrics in the air oven at 120 °C for 1 h. The entire procedures are defined as one washing cycle.
Zeta potential was measured by dynamic light scattering (DLS) using a Malvern Zetasizer Nano ZS (Worcestershire, UK). For these measurements, 0.01 wt% of particles was added in aqueous media and subsequently dispersed by ultrasonication for 30 min. X-ray diffraction (XRD) analysis was carried out on an UItima IV (Rigaku Corporation) X-ray diffractometer with Cu Ka radiation; the steps size and scan rate were 0.02° and 5 °C/min, respectively. Morphologies of the as-prepared cotton fabrics were conducted on a JSM-IT300 (JEOL Ltd., Japan) scanning electron microscope (SEM) at voltages of 20 kV, respectively, after the samples were sputtered with thin platinum layer. The surface elemental analysis of cotton fabrics coated by composite coatings was characterized by energy dispersive X-ray spectroscopy (EDX) attached to the SEM. Surface roughness was tested by using an atomic force microscope (AFM, Dimension Loon, Bruker Company). The tensile strength, elongation at break, and modulus of the treated cotton fabrics were measured according to the GB/T3923-1997 standard test method. At least three measurements for each sample (25 cm × 5 cm) were recorded. Static contact angles and rolling angles were measured utilizing deionized water droplets (i.e., 5 μL for static contact angle, 15 μL for rolling angle) with a KRŰSS contact angle instrument (DSA 100, Germany) at room temperature (25 °C). Average values of water contact angle (WCA) and sliding angle were obtained by measuring at least five different positions on the same sample.
3 Results and discussion
3.1 Zeta potential of Al2O3 colloid particles
3.2 XRD analysis
3.3 Morphologies and chemical composition of the treated cotton fabrics
3.4 Mechanical properties of the treated cotton fabrics
3.5 Water-repellent properties and durability of the novel cotton fabrics
In order to estimate the superhydrophobic/superoleophilic property, some different liquid droplets (both toluene and silicone oil were dyed with oil red O, water was dyed with methylene blue) were dropped on the surface of the representative cotton fabric treated by the PVS/nano-Al2O3 hybrid coatings; both the toluene and silicone oil droplets were spread out and absorbed immediately on its surface, but the water droplet without shape changes was kept on its surface. These liquid droplet experiments clearly demonstrate that the cotton fabrics possess excellent superhydrophobic/superoleophilic property, as exhibited by optical photos in Scheme 1.
To evaluate the washing durability of the treated cotton fabrics, the static water contact angle was measured again after the representative sample CPAl-2 was washed different times. Figure 8b clearly demonstrates that no remarkable changes have been found in the static water contact angle value of the representative cotton fabric, confirming that the cotton fabrics treated with the hybrid coatings consisting of the PVS polymer and nano-Al2O3 particles possess excellent washing durability. To examine superhydrophobic stability of the cotton fabrics treated with the hybrid coatings, water droplets dyed with different colors (0.1% dye) on the surface of the treated cotton fabrics (CPAl-3) can be kept for several hours (from 0.5 to 1.5 h), or cannot be assimilated into cellulose fabric by capillary action; only some dye stains were left on the surface of the treated cotton fabrics after the water evaporated completely, verifying that the hybrid coatings consisting of the PVS polymer and nano-Al2O3 particles can impart the treated cotton fabrics excellent water repellency stability, as displayed in Fig. 8c.
To further examine the mechanical durability of the treated cotton fabrics, the sand paper abrasion experiment was carried out. In this test, sand paper (1200 mesh) was used as abrasion surface; the representative cotton fabric was placed faced-down onto sand paper under loading of 100 g and dragged forward along one direction (about 20 cm); this whole process is defined one abrasion cycle. The static water contact angle value of the representative sample (CPAl-2) was measured after its surface was abraded. After the surface of the representative cotton fabric (CPAl-2) was rubbed 50 times, its static water contact angle value decrease to 152.2 ± 1.9°, which is lower than that of the unrubbed representative sample (161.1 ± 2.5°). This reducing value of static water contact angle should be attributable to loss of surface roughness of cotton fibers after abrasion. It is noteworthy that the static water contact angle value of the rubbed cotton fabric slightly decreases to 150.7 ± 1.9° after 500 times of abrasion. But no remarkable changes can be observable in the static water contact angle value of the representative cotton fabric with an increase in abrasion times, verifying that the hybrid coatings consisting of the PVS polymer and nano-Al2O3 particles endow cotton fabrics with robust mechanical durability. Collectively, the hybrid coatings of the PVS polymer and nano-Al2O3 particles are promising water-repellent materials, which can be applied to many water-proof fields such as wallpaper, soft tiles, and clothing.
3.6 Oil/water separation and self-cleaning ability of the superhydrophobic cotton fabrics
At the same time, the self-cleaning experiment was simulated. Methylene blue powder was sprinkled on surfaces of the pristine cotton fabric, CPAl-0 and CPAl-2, and this dye was flushed with water. After several minutes, most methylene blue powder on surfaces of the cotton fabrics was carried away by the rolling water droplets; some dye stains were left on the surface of the reference materials (CPAl-0 and the pristine cotton fabric) by capillary action; only the CPAl-2 surface keeps clean. This neat surface demonstrates that the cotton fabrics treated with the hybrid coatings have excellent self-cleaning ability, as clearly exhibited in Fig. 9c. To sum up, these results mentioned above suggest that the hybrid coatings combined with the PVS polymer and Al2O3 particles impart cotton fabrics excellent oil/water separation and self-cleaning ability, which are promising potentials in oil recovery, water-proof and self-cleaning applications.
Novel hydrophobic cotton fabrics are successfully prepared using hybrid coatings combined with the PVS polymer and nano-Al2O3 for the first time. The influences of concentration of nano-Al2O3 particles in hybrid coatings on surface morphologies, mechanical properties, and hydrophobic properties of the treated cotton fabrics are discussed in detail. And the oil/water separation performance and self-cleaning ability of the treated cotton fabrics are examined. Results show that the novel cotton fabrics treated with the hybrid coatings of PVS polymer and nano-Al2O3 exhibited enhanced mechanical properties, excellent water-repellent properties, and durable mechanical stability with the increasing concentration of the nano-Al2O3 particles, compared with cotton fabric treated by the sole PVS without adding nano-Al2O3 particles. Meanwhile, the treated cotton fabrics exhibit high oil/water separation efficiency of 99% and excellent self-cleaning ability when the concentration of nano-Al2O3 particles in composite coatings increases to 2%. Therefore, the hybrid coatings combined with the PVS polymer and Al2O3 particles are promising in fabricating superhydrophobic/superoleophilic materials, which can be applied to many fields including oil recovery, water-proof and self-cleaning applications.
This work was financially supported by the National Natural Science Foundation of China (NO.51503161), the Foundation of Wuhan Textile University (173006), and the National Key Research and Development Program of China (NO.2016YFA0101102).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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