Self-cleaning and superhydrophilic wool by TiO₂/SiO₂ nanocomposite

Abstract

Wool fabrics were functionalised using TiO₂ and TiO₂/SiO₂ nanocomposites through a low-temperature sol–gel method. Titanium terta isopropoxide (TTIP) and tetra ethylorthosilicate (TEOS) were employed as precursors of TiO₂ and SiO₂, respectively. Nanocomposite sols were devised based on three molar ratio percentages of TiO₂/SiO₂ 70:30, 50:50, and 30:70 to investigate the role of each component. The self-cleaning and hydrophilicity of wool fabrics were analysed based on the removal of coffee stain under UV and water droplet contact angle measurements, respectively. It was observed that applying TiO₂/SiO₂ 50:50 and 30:70 sols to wool rendered the fabric superhydrophilic. Fabrics functionalised with TiO₂/SiO₂ 30:70 showed the highest efficiency in stain removal, followed by samples functionalised with TiO₂/SiO₂ 50:50.

Introduction

Among various types of natural fibres, wool has been the subject of numerous scientific research in order to modify its intrinsic features using nanotechnology [1]. Dwindling the photo-degradation and shrinkage of wool, and imparting new features such as self-cleaning and antimicrobial activity to wool have attracted a great deal of attention [1], [2]. Wool is well known for its outstanding resilience, breathability and softness, which have made it a superior fibre [3]. Wool has a composite structure consisting of fibrils with α-keratin structure embedded into an intermacrofibrillar matrix [4], [5]. The outer layer of wool with a thickness of 2.5 nm is called epicuticle and is covered with a fatty acid layer [4]. This covalently bonded layer protects the interior parts of the fibre against alkali and oxidising media as well as proteolytic enzymes [3], [4]. The scales which cover the outmost layer of wool play the main role in quality of some features such as friction, hand feel, shrinkage, dyeability and water uptake of fibre [6]. The hydrophobicity of textiles can result in unfavourable static charge on garments and outfits, causing discomfort to consumers [4]. Also, this feature can be an obstacle for an effective wet textile chemical processing such as dyeing and anti-shrinkage finishing [7].

Titanium dioxide, thanks to its photocatalytic properties, has been used in functionalising textiles [8]. UV illumination plays the main role in triggering the photocatalytic activities of titanium dioxide. Titanium dioxide is excited under UV ray with energy equal to or greater than its band gap (3.2 eV anatase TiO₂) [9]. Subsequently, pairs of negative electrons and positive holes are generated in conduction and valence bands of titanium dioxide, respectively [10], [11]. These active species in turn react with water and oxygen molecules producing hydroxyl radicals and super oxide anions. Contaminants adsorbed on the photocatalyst surface undertake some reactions with the generated active species forming water and carbon dioxide as the main by-products of this process [2]. Although the functionality of pure titanium dioxide on textiles has been proved, integrating silica as a metal oxide into the surface coating formulation has been put forth to augment the efficacy of titanium dioxide photocatalyst [12], [13]. The presence of silica plays dual main roles of increasing the surface area in the vicinity of titanium dioxide and surface acidity of the photocatalyst [12], [13].

Self-cleaning cotton was introduced through the growth of anatase nano titanium dioxide crystallite onto the fabric surface by Daoud and Xin [14]. Based on their approach, cotton and wool fabrics were treated with nano titanium dioxide colloids through a low temperature sol–gel method [10], [15], [16]. Stain removal capability of cotton and wool fabrics treated with titanium dioxide was assessed based on the colour fading of stains such as coffee and red-wine after UV illumination [15], [16], [17]. Similarly, Montazer and Pakdel treated wool fabrics with nano titanium dioxide powder (Degussa P-25) sonicating in an aqueous solution in the presence of cross-linking agents namely butane tetra carboxylic acid (BTCA) and citric acid (CA) [18]. It was confirmed that nanoparticles were anchored to the functional groups such as NH₂, COOH and SH on the surface of wool. The self-cleaning function was assessed based on the colour degradation amount of fruit juice and Acid Blue 113 under UV [18]. There are other reports on the self-cleaning function of nano titanium dioxide [15], [19], [20], [21], [22]. Along with pure TiO₂ application, TiO₂/SiO₂ has also shown some promising results in functionalising the textiles. Enhanced UV-blocking and self-cleaning functions have been reported after applying the TiO₂/SiO₂ systems to textiles [23], [24], [25]. Pakdel and Daoud have successfully enhanced the self-cleaning function of nano titanium dioxide on cotton fabrics through integrating the silica nanoparticles into the coating system [26].

Changing the water absorption characteristics of various substrates using nanotechnology has also been an attractive research arena for scientists. Fujishima et al. demonstrated that the contact angle on TiO₂ surface could be reduced to zero after UV illumination [27]. Adding the hydrophilicity function to some natural textiles such as wool would bring about increased comfort for end users [28]. Montazer et al. treated wool fabrics with titanium dioxide nanoparticles to improve its hydrophilicity with UV illumination [29]. Likewise, antimicrobial and superhydrophilic wool was reported after a mixture of silica and silver nanoparticles was applied to wool fabrics [30]. Hydrophilicity of TiO₂/SiO₂ systems on some substrates such as glass and tile has also been studied [31], [32]. Corona discharge and enzymatic treatment have also been reported to increase the water absorption capability of wool [33]. Removing the fatty acid layer of wool cuticle by helium plasma treatment, wool fabrics became more hydrophilic [34], [35].

In this investigation, wool fabrics were treated with colloids of TiO₂/SiO₂ through a low temperature sol–gel method using the dip-pad-dry-cure process. This study was set out to investigate the synergistic role of silica in enhancing the functionality of TiO₂ on wool fabric, and to elucidate the impacts of silica addition on self-cleaning and hydrophilicity of wool fabrics. Wool fabrics were stained with coffee and exposed to UV to assess the self-cleaning property. The water absorption behaviour of treated samples was analysed based on the water droplet contact angle.