Hydrophobic Cotton Fabric Surfaces in Various Potential Application Fields

The hydrophobic cotton surface is facilely fabricated by an easy novel method through adsorption of fluorosurfactant on the cotton surface and then polymerization of low-surface-energy fluoro monomer in the presence of an initiator under ambient temperature with a short time. By in situ introducing fluoropolymer on cotton fibers to generate a dual-size surface roughness, followed by hydrophobization with Trifluoroethyl Methacrylate (TFEM), normally hydrophilic cotton has been easily turned hydrophobic, which exhibits a static water contact angle of 132o for a 10 µL droplet and also water droplet can roll off the cotton surface easily. The rough micro/nano-textured surface morphology, after surface fluorination, results in simultaneous hydrophobicity and superoleophilicity. The hydrophobic character was confirmed by a simple drop test and contact angle measurement. Surface composition was evaluated by FT IR and SEM, EDS analysis to confirm the fluoropolymer layer on the cotton surface.

Inspired from the lotus phenomenon construction of such special superhydrophobic (water contact angle is greater than 1500) surfaces are increasingly attractive in various potential application fields both in academic research and practical application such as self-cleaning, anti-contamination and anti-sticking. Superhydrophobicity is an extraordinary wettability with high water contact angle and low sliding angle. Nienhuis et al have elaborated that water drops rolling off on the lotus leave surfaces is due to the presence of a combination of rough micro-nano structure and low surface energy waxy materials on their surfaces. Based on the principle, scientists and researchers have endeavored various methods to fabricate such special hydrophobic and superhydrophobic surfaces by constructing hierarchical micro/nanostructures with low surface energy materials.

Cotton, a soft fluffy fiber has low production cost, low density, good strength in both wet and in dry condition and other unique properties such as comfortability, breathability makes them even more attractive for future applications. It is extremely used raw material to make garments for many years. Cotton is composed of almost pure cellulose which contains hydroxyl groups. In spite of the many advantages of cotton, the hydroxyl groups make them tremendous water-loving adsorbent i.e. hydrophilic. The excessive water absorbability allows the cotton textile to be easily stained and dirtied. Sometimes the cotton textiles are also wetted and contaminated by blood, oily appearance, and even bacteria which are undesired in their use as cloths particularly in hospitality. Thus in recent years, nonwettable cotton textile with high water contact angle value and dirt resistant cotton textile has long been an interesting subject in research.

Modifying textile with hydrophobic chemicals to make surface hydrophobicity is a well-established technology developed in early 1940 (Roach et al. 2008). For example, a patent published by Gao and McCarthy et al (2006) on the basis of hydrophobization with silane. They were successfully fabricated artificial lotus leaf-like polyester fabric. Two factors  surface chemical composition and  surface structure (roughness) promotes the special nonwettable effects on fabrics. A variety of approaches are reported to enhance the surface roughness such as introduction of nanotechnology through electrospinning, plasma treatment and sol-gel technology, chemical vapor deposition.

Silicone compound is also reported to coat on fabric surfaces for many years. Beside nanotechnology, polymer technology plays also an important role to create a surface thin film with high hydrophobic character. Fluorocarbon coating has been employed to get well water repellency as investigated by Shao et al (2004) and others. Recently a new method has been employed to produce a polymeric thin-film coating on a solid substrate via surfactant adsorption is termed as admicellar polymerization. This is a surfactant aided polymerization to coat the cotton fabric by the formation of the ultra-thin film of thickness on the order of 10 nm i.e. in nanoscale finishes without changing the softness, breathability characteristics of cotton fabrics.

Admicellar polymerization is a helpful method to create the ultra-thin polymer films onto solid surfaces in an aqueous solution. The micellar process is the formation of surfactant bilayer  on a solid surface where adsorption takes place. After addition of monomers into the bilayer monomers will partition into the core of the admicell in a process called adsolubilization. Then in the presence of an initiator, this monomer undergoes polymerization reaction forming a region of high monomer density at the water/substrate interface to form a thick or thin polymeric layer on the substrate of the surface. Finally, the substrate is rinsed to wash away excess surfactant to expose the polymeric layer on the substrate surface . The schematic representation of admicellar polymerization on solid substrate shows in Fig. 1. CMC plays an important role in surfactant aggregation. Lower CMC means low concentration and also less surfactant will be required for adsorption at solid/liquid interface for admicellar polymerization with lower cost. Wu et al. studied the formation of ultra-thin polystyrene films on alumina by this technique using sodium dodecyl sulfate (SDS) as a surfactant. Essumi et al.  also created a surfactant coated alumina with particle size 200 nm by admicellar polymerization technique using a polymerizable surfactant.

Admicellar polymerization has been successfully employed to create various types of polymeric film on different surfaces such as polystyrene on silica, polystyrene on cotton, fluoropolymer on alumina.

Admicellar polymerizations have superior advantages over the above process for its simplicity with low energy consumption when used on textile fabrics (E .A. O’ Rear et al. 2002). Fluorosurfactant contains hydrophilic tail and the hydrophobic head group has specific properties such as low polarizability, low dielectric constant, high vapor pressure, high gas solubility, low surface tension and also low critical micelle concentration [20]. Beside this both fluorocarbon and fluorosurfactant has stronger hydrogen bonding and also larger partition coefficients, higher surface activity compared to the hydrocarbon system minimum amount and smaller concentration are required. Here are approaches to creating a double phase hydrophobic cotton textile by the adsorption of the little amount of fluorosurfactant and solubilization of small quantity of fluoromonomers by admicellar polymerization technique. Little amounts are very important criteria to overcome the high-cost efficiency of fluorochemicals.

Materials Pique cotton fabric was purchased from the local textile shop. The fabric was resized and treated in 10% NaOH solution for 1 hour and then the fabric was washed repeatedly until it was free from any remaining lubricants and other additives. The monomer used 2, 2, 2-trifluoroethylmethacrylate (TFEM) was purchased from Sigma Aldrich. The surfactants used non-ionic fluorosurfactant FS61 was purchased from DuPont India. The initiator potassium persulfate was purchased from Merck. All chemicals were used without further purification. Surface modification of cotton fabrics by Admicellar polymerization The modification was performed by admicellar polymerization method via surfactant adsorption on the surface. A variety of sample formulations were done by trial and error method. We described the best result sample formulation method.

Homopolymerization of 1ml of 3mM TFEM on cotton was carried out in a 30 ml vial containing a 20ml solution of FS61 (1 ml) at the CMC, pH-4 water at temperature 400C. 1%NaCl is used for better surfactant adsorption. At the start of the experiment, the 1g cotton fabric was placed in the vial; the vial was sealed with aluminum foil. The sealed vial was then placed in a thermostated water bath at 400C and shaken at 80 rpm for 1 hour. Then an initiator Potassium persulfate was injected to initiate the polymerization to give an initiator: monomer ratio of 1:1. The vial was resealed and the polymerization was allowed to proceed for an additional 1h at 600C. The excess surfactant was rinsed away with several volumes of water and the sample was dried in an oven at 700C. Determination of hydrophobic properties Drop test Water repellency test is an initial characterization of the treated surface to assess the hydrophobic coating on the cotton surface. Two test methods were employed for assessing water repellency.

An initial characterization of the treated surface was by the drop test. A 10 µL droplet of distilled water was placed on cotton fabric surface carefully with no force from a 20 µL syringe. Time for absorption of water (wetting time) on a fabric surface in the drop test was determined up to a maximum of 120 minutes, at which point the sample passed. A better second method was performed according to AATCC test method 22 (spray test). Contact angle measurement The water contact angles were measured using an automatic video contact-angle testing apparatus optical Tensiometer (TL100 Theta) and software supplied with the instrument at 240C temperature. The contact angle was measured by the sessile drop method.

For the contact angle measurement, a 10 µL drop of distilled, deionized water of surface tension 72.75 mN/m was deposited on fabric using a micropipette from a height of 2 cm. Observations occurred over a 10 min period and the average contact angle was reported by measuring at five different sites of the sample on both site of the cotton textile. The average contact angle was obtained in 1320. Characterization of fluoropolymer coated cotton fabrics For surface morphology of the modified and unmodified cotton fabric was observed in a scanning electron microscope (SEM) Model No. JEOL JSM 5800 scanning microscope. All samples were gold-coated before scanning. SEM images indicate the surface micro/nanostructure. For chemical composition, EDS analysis has also been done using a ZEISS 960A SEM equipped with Oxford Link energy dispersive spectroscopy. FTIR spectra of unmodified and modified cotton fabric were recorded by ATR mode technique using PerkinElmer (L1600300 Spectrum two Lita S.N.96499) FTIR- ATR spectrophotometer. The IR-spectrum was taken over the wave number range of 4,000 cm-1–500 cm-1. This study explains the functionalities present in different untreated and treated cotton fabrics.

Hydrophobic Properties of Coatings Hydrophobicity on cotton surfaces can’t be evaluated by only one method. The drop test and water stay time enable a rapid and simple presentation of water-repellent properties of fabric due to the formation of the continuous, polymeric thin film on the cotton surface. To ascertain the water-repellency characteristics of the fabric samples were assessed for performance using drop test, spray test, and contact angle measurement to obtain a complete understanding of performance. Drops in both surface of the cotton surface in Fig. 3 and rolls of water in Fig. 4 form spheres (also shown in video 1 supporting information) on the cotton surface can demonstrate that hydrophobic film on the surface was created and it prevents water or moisture to penetrate through the surface. The hydrophobicity is related to the surface contact angle. It is the angle formed when a droplet rests on a solid (flat) surface and is surrounded by a gas.

A better contact angle measurement with water droplets was obtained 1320 shown in Fig. 2 and the stay time of water droplets on the cotton surface was 120 minutes. This high contact angle indicates the weak interaction between water and cotton surface exhibiting the conversion of a hydrophilic surface to a hydrophobic surface. On the other hand octane, a liquid with low surface tension (?lv = 21.62 mN/m), spread quickly on the coated fabric within less than 10 seconds indicating the super oleophilicity. This is because the oil has low surface tension than that of water. In addition, the absorption of chloroform was carried out to examine the use of fabric with an organic solvent that had higher densities than water. When the piece of hydrophobic textile was brought into contact with water to approach chloroform, the chloroform droplet could be instantaneously sucked up by the textile underwater .

Additionally, a bright and shiny and transparent surface could be observed underside the water droplet in Fig. 3, which was a remark of trapped air and the establishment of a composite solid-liquid-air interface. All the results mentioned above indicated a stable hydrophobicity on the cotton surface. Surface morphology and chemical composition SEM images are a useful supplement to contact angle to provide the morphology of the surfaces on the modified cotton samples. The SEM imaging reveals the hydrophobic behavior of cotton substrate is a result of the hierarchical rough structure. Inspired by natural surfaces (e.g. lotus leaves, butterfly wings) different types of artificial surfaces have been designed and fabricated. The surface microstructure and composition of lotus leaves has been investigated by Neinhuis and coworkers.

Nienhuis and co-workers investigated the micromorphological characteristics and showed that the water-repellency is based on surface roughness caused by different microstructures (trichomes, cuticular folds and wax crystals). Water on the solid surfaces is primarily in contact with air pockets trapped in the rough surfaces. Wetting behavior can be described when water droplet sits on the cotton surface from the modified equation of Cassie-Baxter follows- cos ?CB = rf f cos ?0 + f – 1 Where ?CB is the observed contact angle (132.630) and ?0 is the intrinsic water contact angle . Where f is the fraction of the projected area of the solid surface wetted by water and rf is the surface roughness of the wetted area. In our study when the fluoropolymer layer appears on the cotton surface through the micellar polymerization surface roughness (rf ) increases in comparison to the smooth wetted area (shown by SEM images). As a result, the droplet rests on the top of solid asperities and the gas is left in the voids below the droplet indicating a shiny, transparent surface underneath the water.

The large water contact angles on the cotton surface imply that less amount of surface is in direct contact with water. Fig. 6 presents the representative FESEM image of the cotton fabric possessing uniformly oriented three-dimensional microfibers with an average diameter of 5 µm. The surface of the individual fiber is smooth and no polymeric aggregation on the cotton surface as shown in Figure 6a. After the complete polymerization and drying the sample at 70oC, a compact coating with unusual roughness was observed with a number of nanoscale polymeric aggregations along with bumpy appearances that are uniformly distributed on the fiber surface in Figure 6b. This recommends the achievement of fluoromethacrylate polymerization on the cotton fabric through admicellar polymerization. This hierarchical micro- and nanoscale bumpy appearances of the coated fabric surface provided roughness in the cotton fabric. The chemical composition of fabric surface was analyzed by EDS studies. Energy dispersive spectroscopy (EDS) analysis has been performed in the upper part (vertical) section of cotton fabrics in Fig. 7. to observe the chemical composition of the textile and also to check where polymerization of a fluorinated monomer in cotton takes place (surface or inside cotton fabric). EDS spectrum of the upper part of the plane untreated cotton fabrics in Fig. 7a reveals an absence of fluorine and only peaks corresponding to cellulose (carbon and oxygen) appears. A slight peak appears for silicon for impurity.

A characteristic peak is observed suggesting the uniform coverage of fluoropolymer coating on the treated cotton fabric. These results proposed that admicellar polymerization of fluorinated monomers takes place preferentially at the surface of the cotton fabric. This supports along with SEM images that a fluoropolymer layer covers the surface to create surface hydrophobicity. EDS study shows that about 11.979% (Shown in supporting information S1) Fluorine elements are deposited on the cotton surface after modification indicating hydrophobicity. Infrared studies FTIR spectra of modified cotton and unmodified cotton are shown in Fig. 8. In infrared spectra the minor changes are observed, indicating in the micellar process the internal bonds of in cotton fabric are not destroyed. FT-IR ATR spectra of unmodified fabric and fluoromonomer treated modified fabric in Fig. 8 showed characteristic cellulose peaks around 1100-1200cm-1.

Other characteristic bands related to the chemical structure of cellulose were the hydrogen-bonded OH stretching at 3350-3200 cm-1, the C-H stretching at 2900 cm-1, and the C-H wagging at1314 cm-1 . The OH bending of absorbed water was also observed in 1642cm-1. Figure 8 shows a transmittance at around 1751 cm-1 in the FT-IR ATR spectrum of fluorinated cotton indicating the presence of the carbonyl stretching frequency of the COF group . The frequency at 1010 cm-1 in the modified cotton fabric is a characteristic frequency of the C-F bond. Shih Hsien Yang et al. have prepared super-hydrophobic films using pulsed hexafluorobenzene plasma and showed a characteristic peak around 999 cm-1.The C-F stretching frequency is absent in case of untreated fabric but appears in the treated fabric both in sample C indicating polar C-F bond between the cotton fabric and fluoropolymer. This data indicates that the hydrophobic cotton surface was achieved through homopolymerization of the monomer and fluorine is attached to the cotton surface which affects the water repellence behavior of the modified cotton fabric. This surface polymerization clearly implies that hydrophobicity is strictly related to the quantities of the attached polymer to the cotton surface rather than their chemical composition.

A thin film of poly (2, 2, 2- trifluoroethylmethacrylate) has been created and deposited on cotton substrates by the admicellar polymerization process. It was found after admicellar polymerization process cotton fabric shows a high water contact angle i.e. the surface turned in to hydrophobic. SEM and EDS analysis clearly indicate the surface roughness and chemical composition after fluoropolymeric layer formation.