Superhydrophobicity of cotton fabrics treated with silica nanoparticles and water-repellent agent

Abstract

To obtain the superhydrophobic water-repellent cotton fabrics, cotton fabrics were treated with silica nanoparticles and/or a cost-effective water-repellent agent (WR agent). Two different silica nanoparticles were synthesized via a sol–gel process and their shapes, sizes, and compositions were characterized. It was found that silica particles are spherical and have diameters of 143 and 378 nm. For the cotton fabrics treated with the WR agent alone, the water contact angles on the fabric surface remained lower than 20° at the WR agent concentration of 0.3 wt% or less. Silica nanoparticle treatment itself did not change the hydrophilic surface of cotton fabric, indicating that water drops were adsorbed into fabrics due to the hydroxyl groups on both cotton and silica nanoparticle surfaces. However, for the cotton fabrics treated with both silica nanoparticles and the WR agent, a contact angle above 130° can be obtained even at the very low WR agent concentration of 0.1 wt%. Therefore, superhydrophobic cotton fabrics could be obtained via the combined treatment of silica nanoparticle and WR agent, which is cost effective compared with fluorinate silane treatment.

Introduction

Since the discovery of the self-cleaning effect of such things as lotus leaves [1], [2], [3] and insect wings found in nature [4], superhydrophobic surfaces have received great attention because of their wide range of applications such as frictionless flow water pipes [5], hydrophobic treatment on vehicle windshields [6], and superhydrophobic fabrics [7], etc.

It is well-known that superhydrophobic surfaces with self-cleaning properties are caused by the water-repellent double structures of the surfaces [2]. These hierarchical double structures are achieved by the two physical characteristics: surface roughness due to the micro- or nano-structures and hydrophobicity because of waxy materials on top of the rough structures. Thus, to attain superhydrophobic surfaces, modification of surface chemistry is always needed [8], [9] in conjunction with the surface roughness.

Recently, roughened surfaces have been commonly obtained by introducing nano-size particles onto the surface. The nano-size particles can be synthesized easily via the sol–gel method [10], [11]. There are several kinds of inorganic nano-size particles such as SiO₂, TiO₂, and ZnO [12], [13], [14]. Surface-modifying chemicals of fluorinated silanes [15], [16], [17], [18], [19] or alkylated silanes [20], [21], [22], [23], [24] have been used to transform the hydrophilic surface of the particles into hydrophobic ones.

For practical applications of superhydrophobic surfaces, economical and environmental aspects in treatments of nano-size materials and hydrophobic chemicals have become important parameters. For instance, fluorinated silane used to create a hydrophobic surface of the nano-size particles is too expensive to be applied in general purpose. Furthermore, most fluorinated materials may often cause serious risks for the human health in case of skin contact and for the environment in case of emissions of fluorine during and after the treatment process [25], [26], [27]. Therefore, it is necessary to minimize the usage of fluorinated materials.

Recently, as markets in leisure and outdoor sporting textiles have been expanded, the needs for superhydrophobic fabrics have increased. There have been some reports on the improvement of hydrophobic properties of several kinds of fabrics using nano-structures achieved by nanotechnology [7], [14], [28], [29]. Cotton has always been the principal clothing fabric due to its attractive characteristics such as softness, comfort, warmness, biodegradation, and low cost. However the abundant water-absorbing hydroxyl groups on cotton surfaces make the fabrics absorbent and easily stained by liquids. Therefore, additional finishes are required to impart superhydrophobicity and self-cleaning properties on cotton fabrics [14].

In the present study, we have introduced superhydrophobicity to the hydrophilic cotton fabric by the combined treatment of silica nanoparticles and a cost-effective commercial WR agent. Two different silica nanoparticles were prepared via the typical sol–gel process and treated onto the cotton fabrics with/without the WR agent. Surface structures and compositions of the cotton fabrics treated with silica nanoparticles and/or the WR agent were characterized by using scanning electron microscopy, particle size analyzer, energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and scanning prove microscopy. The hydrophobicity of cotton fabrics was evaluated by measuring the contact angles of sessile water drops on the fabrics.