Facile Fabrication of Breathable and Superhydrophobic Fabric Based on Silica Nanoparticles and Amino‐Modified Polydimethylsiloxane

This work attempted to fabricate superhydrophobic fabric via simple immersion technique. Textile fabrics were coated with silica nanoparticles prepared from tetraethoxysilane (TEOS) to obtain sufficient roughness with hydrophobic surface chemistry. Then the coated fabrics were treated with polydimethylsiloxane (PDMS) and aminopropyltriethoxysilane (APTES) to reduce the surface energy. The effects of PDMS concentration on the surface morphology and superhydrophobicity of as-prepared fabric were investigated. The morphology and the composition of superhydrophobic fabric was characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDS) and Fourier transform infrared (FTIR) spectroscopy. The results revealed the formation of spherical silica nanoparticles with average particle size of 250 nm throughout the fabric surface. The possible interactions between silica nanoparticles and APTES, as well as the fabrics were elucidated. Investigating the hydrophobicity of fabrics via water contact angle (WCA) measurement showed that the treated fabric exhibits excellent water repellency with a water contact angle as high as 151° and a very low water sliding angle. It also found that the treated fabric maintained most of its hydrophobicity against repeated washing. The comfort properties of the obtained superhydrophobic fabrics in term of air permeability and bending length did not reveal any significant changes.


Introduction
A surface with a water contact angle (at equilibrium) higher than 150° and contact angle hysteresis lower than 10° is considered as a superhydrophobic surface. Nowadays superhydrophobic coatings have gained increasing attention due to their diverse applications including water repellent and self-cleaning textiles, anti-icing surfaces suitable for power network equipment, anticorrosion devices, biomedical devices, construction industry, antibacterial fabrics, anti-biofouling surfaces in marine industry, and oil-water separation [1]- [3].
A combination of micro-nano roughness and low surface energy can lead to a superhydrophobic surface. So, an intrinsically hydrophobic substrate can be converted to a superhydrophobic structure by creating micro-nano roughness on its surface by means of plasma etching, chemical etching, nanoparticle attachment, etc. The hydrophilic surfaces, superhydropobicity can be obtained by chemically modifying the micro-nano rough surface with low surface energy compounds [2], [4], [5].
One of the widely studied strategies for fabrication of superhydrophobic textiles is based on increasing the surface roughness by coating of the surface with inorganic nanoparticles through the sol-gel process and subsequently lowering the surface energy by attachment of a hydrophobic compound on its surface. Also, using a sol, modified with an appropriate hydrophobic compound, may lead to a superhydrophobic surface coating by a one step process [6]. Hoefnagels et al. [7] turned the hydrophilic cotton to superhydrophobic by a two-step process including the sol-gel based in-situ growing of silica micro-particles on cotton fibers followed by a hydrophobization step using polydimethylsiloxane (PDMS). Xu et al. [8] examined the coating of cotton surface with SiO2 nanoparticles or ZnO nanorods for creation of nano-roughness and modification of the rough surface with n-dodecyltrimethoxysilane (DTMS) to lower the surface energy and prepared superhydrophobic cotton fabrics. Based on their results, cotton fabrics prepared based on SiO2 nanoparticles and ZnO nanorods showed static water contacts angles (WCAs) of 159° and 153° respectively.
Superhydrophobic PET fabric was obtained by a two-step process including coating with SiO2 nanoparticles and PDMS in the first step and subsequent hydrophobization through a sol-gel process using tetraethoxysilane and cetyltrimethoxysilane. The coated textile showed a WCA of 162.5° and was resistant to hydrostatic pressure up to 38.6 KPa [9]. Xue et al. [10] fabricated a self-healing and superhydrophobic PET fabric using polydimethylsiloxane and octadecylamine (ODA) through a dip-coating followed by curing procedure. The required roughness was provided by the self-roughed property of ODA and the obtained superhydropobicity was durable to abrasion (5000 cycles), and washing (120 cycles). TiO2 sol modified with poly(hexafluorobutyl methacrylate) was another approach employed by Yang et al. [11] to prepare superhydrophobic cotton fabrics with WCA of 152.5°. Chauhan et al. reported the preparation of superhydrophobic cotton with self-cleaning and stain resistant properties by simple coating with hexadecyltrimethoxysilane by immersion-drying method. WCA of 157° was obtained. It was concluded that the high contact angle is due to the hierarchical microstructures and presence of long-chain alkyl groups on the modified cotton surface [12].
Herein, we present facile and non-fluorinated approach to construct superhydrophobic and breathable fabric based on hydrolysis and condensation of tetraethylorthosilicate (TEOS) followed by crosslinking with amino-modified polydimethylsiloxane (PDMS). The formation of silica nanoparticles, morphology and structure of treated fabric were investigated by scanning electron microscopy (SEM), energy dispersive X-ray (EDS) and Fourier transform infrared (FTIR) spectroscopy. The treated fabric exhibited excellent eater repellency with durable washing fastness.

Materials
Poly(dimethylsiloxane) mono-glycidyl ether terminated (PDMS) and tetraethylorthosilicate (TEOS, 98%) were purchased from Sigma-Aldrich. Ammonium hydroxide (25% in water), ethanol (C2H6O), toluene (C7H8) and 3-aminopropyltriethoxysilane (APTES) were supplied from Merck Co. (Germany), and used without additional purification. Commercial polyester-viscose fabrics (350 g/m 2 ) was used as the substrate. The fabric was rinsed with detergent and distilled water separately several times and dried at 100 °C for 30 min. Deionized water was used in all the prepared solutions.

Fabrication of silica-coated fabrics
The spherical silica nanoparticles were prepared by the modified Stober method [13]. Briefly, first, a mixture of TEOS (0.39 mol) and ethanol (2.35 mol) was prepared as a solution A.
Similarly, 2.94 mol distilled water, 2.35 mol ethanol, and NH4OH were mixed to form solution B. Then, the solutions A and B were mixed together at 40 C for 40 min. Finally, the washed fabric immersed in a solution of silica nanoparticles for 20 min and after dip-coating, the fabrics were left at room temperature to remove the solvent and cured at 80 °C for 10 min.

Fabrication of superhydrophobic fabric
In this procedure, different amounts of PDMS were first added into 50 mL toluene to find the optimum value. After stirring about 60 min, 0.5 mL APTES solution was added. Then, the mixture was stirred (500 rpm) for 3 h at room temperature. Finally, the silica-coated fabrics were dip-coated in APTES-PDMS solution for 20 min and cured at 120 °C for 60 min. The schematic representation of the superhydrophobic coating of fabric is shown in Figure 1.

Characterization
The surface morphology of pristine and treated fabrics as well as the composition of materials were studied after gold coating of samples by scanning electron microscopy (Hitachi su3500) and energy-dispersive X-ray spectroscopy (EDS). Fourier transformed infrared (FTIR) spectra of samples were recorded on AVATAR FTIR instrument (Thermo Nicolet, USA) to study the functional group analysis and possible reactions.
Water contact angle (CA) measurement was carried out at room temperature using home-made instrumentation including microscope equipped with a CCD camera and PCTV vision software.

Structural analysis
Amino-modification of mono-glycidyl ether terminated PDMS with APTES has been carried out and the possible reactions was investigated by FTIR ( Figure 2a). Diminishing the characteristic absorption band of epoxide ring at 913 cm -1 as well as increasing the hydroxyl and amine groups at 3420 and 1495 cm -1 , respectively, confirm the successful amine-modification of PDMS [14].
As can be seen in spectrum of modified PDMS, the absorption band of oxirane group was completely disappeared, revealing complete reaction between APTES and PDMS. The possible chemical reaction process between silica nanoparticles and modified PDMS is represented in

Superhydrophobic property
The hydrophobicity of fabrics was assessed using water contact angle (CA) measurement. The pristine fabric, due to the presence of hydrophilic hydroxyl and carboxyl groups, was completely concentration of PDMS. The high water contact angle obtained here is due to the formation of micro-nano roughness by silica nanoparticles as well as the low surface energy provided by PDMS. Table 1 summarized the hydrophobic properties of different substrate in term of fabrication approach, contact angle and washing fastness. As shown in Table 1, with respect to reported literature, the proposed fluorine-free coating formulation exhibited robust superhydrophobic with water contact angle higher than 150.

Physical properties
The breathability and physical properties of pristine and superhydrophobic fabric were measured in terms of air permeability, crease recovery angle and bending length and the obtained results summarized at Table 2 Table 2. There were increases in bending length of superhydrophobic fabric for both the warp and weft directions. It could be attributed to the formation of hydrogen bonds between the hydroxyl group of fibers and the hydroxyl group of superhydrophobic coating, making the fiber a bit difficult to bend.

Durability
To study the durability of treated fabric against laundering, they were subjected to several washing cycles. The CA of treated fabric as a function of washing cycles is shown in Figure 5. The obtained results imply that the CA of superhydrophobic fabric after 25 domestic washing cycles with water and 0.23% detergent solution decreased from 150 to 143.

Conclusion
We have successfully fabricated superhydrophobic fabric by modifying the pristine fabric with silica nanoparticles followed by amino-modified PDMS through a dip coating process. The morphological study of treated fabric exhibits spherical silica nanoparticles distributed throughout the fabric surface, providing necessary nanoscale roughness for hydrophobicity. The prepared fabric shows outstanding superhydrophobicity with the contact angle of 151º and long-term stability toward washing cycles. The employed compounds provided micro-nano roughness as well as the suitable functional groups favoring the low surface energy provided by PDMS.
Considering the air permeability, bending length and crease recovery angle of superhydrophobic fabrics revealed that the treatment did not significantly affect the comfort properties of fabric.