Photostability of wool fabrics coated with pure and modified TiO2 colloids
Graphical abstract
Introduction
Wool is considered as one of the most prominent natural fibers widely employed in the textile arena and has attracted great attention to improve its intrinsic features. Wool has a complex configuration in comparison with other types of fiber and is recognized as a natural composite system. The unique physical and chemical structure of wool gives birth to some features such as distinctive hand feel, warmth, and luxurious appearance differentiating wool as a superior fiber. The chemical structure of wool is composed of around 170 different proteins containing 18 amino acids. Wool as a proteinous fiber consists of five fundamental elements of carbon, hydrogen, oxygen, nitrogen and sulfur [1], [2]. It is proven that the presence of interchain cross-links such as disulfide bonds, salt bridges, isopeptide bonds and hydrogen bonds between the wool’s amino acids plays a pivotal role in the quality of fiber [1]. In the main, fine wool is composed of two main parts of cuticle and cortex [3]. The cuticle which covers the outer layer of wool with overlapped scales, in turn, is subdivided into three layers of epicuticle, endocuticle and exocuticle [1]. The properties of wool are greatly affected by its chemical composition and physical configuration. Although wool has some merits compared to other types of fibers, the lack of photostability is recognized as one of the most important drawbacks of wool fibers [4]. It is proven that exposing wool fiber to light with wavelength below 331 nm results in photoyellowing and loss of mechanical strength [1], [2]. The effect of light on wool depends largely on its wavelength. For instance, UVC incident light (200–280 nm) results in a green color in wool, while photoyellowing and photo-bleaching are expected through exposing wool to UVB (280–320) and blue light (400–460 nm), respectively [4], [5]. There are some main contributing factors such as the presence of moisture and oxygen, the finishing history of wool fabrics, and the wavelength of light that affect the rate of photoyellowing [2], [4]. Some treatments such as bleaching and finishing the wool with fluorescent whitening agents are effective in escalating the pace of photoyellowing [6]. Essentially, the photoyellowing mechanism is ascribed to the absorption of light with some aromatic amino acids present in the structure of wool such as tryptophan, tyrosine and phenylalanine, not to mention the natural yellow chromophores [7]. Moreover, the possible role of proline which exists on the outer layer of wool in generating the yellow chromophores has also been reported [8]. Based on the research by Zhang et al., the cuticle layer of wool plays the main role in producing the yellow products during solar illumination [7].
Some UV absorbers have proved effective in dwindling the photoyellowing rate of wool [5]. Of these UV absorbers some compounds such as hydroxyphenylbenzotriazole dispersed in methacrylate polymers [9], thiourea and formaldehyde resin [10], 2-(2′-benzotriazolyl)-phenol/polymer mixture [11], 2-hydroxybenzophenone [12] are noteworthy. More recently, novel approaches have emerged to control the photoyellowing of wool. In particular, employing nanotechnology has led to some promising results in alleviating the photodegradation rate of proteinous fibers. The effectiveness of some inorganic nanostructure materials such as ZnO and TiO2 in improving the UV protection and photostability of fabrics notably wool has been demonstrated [13], [14]. In the main, the UV-blocking behavior of TiO2 results from its electronic structure [15]. It is well-known that TiO2 nanoparticles are activated under UV with energy greater than nanoparticles’ bang gap, which is 3.2 eV and 3.0 eV for anatase and rutile crystallinities, respectively [16]. Under this condition, electrons from the valence band of TiO2 are promoted to the conduction band generating negative electrons and positive holes. Two reaction routes are expected for the generated species at this stage. First, the electrons and holes can react with water and oxygen molecules present in the surrounding of the photocatalyst producing superoxide anions and hydroxyl radicals. These active species play the main role in decomposing the organic contaminants adsorbed on the surface of the photocatalyst. Based on the second path, the generated negative electrons and positive holes react with other electrons and holes bringing about a UV-blocking property for the TiO2 nanoparticles [16], [17]. The crystalline structure of titanium dioxide is also effective in the resultant UV-blocking quality of nanoparticles [16].
TiO2 nanoparticles have widely been employed to impart some functionalities such as UV protection [13], self-cleaning [18], [19], [20], antimicrobial activity [21] to textiles. It has been reported that through the application of TiO2 nanoparticles to the surface of textiles, the photostability and UV protection features can be increased [13]. Daoud et al. produced a UV protective cotton fabric through the application of TiO2 colloids [22], [23]. Some other research also showed improved UV protection of cotton through applying TiO2 nanoparticles [24], [25]. The photoyellowing rate of wool fabrics was reduced through stabilizing the TiO2 nanoparticles on fabrics via some cross-linking agents [26]. In a research study conducted by Montazer and Pakdel, the surface of wool fabrics underwent an oxidation pre-treatment and then samples were coated with TiO2 nanoparticles [26]. It was revealed that through increasing the concentration of TiO2 the photoyellowing pace was slowed down. In another research, the same group statistically analyzed the role of TiO2 and cross-linking agent concentrations as well as the finishing duration in the photoyellowing rate of wool fabrics [27]. Increasing the photostability of powdered wool through the application of TiO2 nanoparticles was also demonstrated by Zhang et al. [28]. The generation of free radicals from the illuminated light was monitored through photo-induced chemiluminescence (PICL) spectra [28]. It was observed that TiO2 reduced the population of free radicals generated from wool substrate, hence protecting wool against the rapid photoyellowing [28]. Although TiO2 has been an efficient photocatalyst to impart new properties to textiles, some inherent down sides of TiO2 encouraged scientists to modify its photoefficiency. Therefore, increasing the photocatalytic activity of TiO2, as one of the main objectives, through diverse methods was investigated [29], [30]. Among these methods, combining with other metal oxides, doping with metals, non-metals, and dyes among others have been investigated [29], [30], [31], [32]. Integrating the silica into the TiO2 sol was an efficient way to boost the photocatalytic efficiency of TiO2 nanoparticles applied to wool and cotton fabrics [33], [34]. It has been reported that integrating an optimal amount of noble metals into the synthesis process of TiO2 colloid can result in a higher photocatalytic activity [35], [36].
However, increasing the photocatalytic activity of TiO2 nanoparticles on the surface of fabrics may cause some unfavorable ramifications. There are numerous pieces of research on enhancing the photo-induced features of TiO2 on fabrics particularly wool, but discussion of the potential impacts of this modification on fabric’s photostability is scarce. This research study attempts to shed light on the possible impacts of TiO2 sol modification with silica and noble metals on the photostability and UV protection of wool fabrics. To this end, the surface of wool fabrics was functionalized using TiO2 along with binary and ternary TiO2 nanocomposite colloids. The generation rate of polymer free radicals in the coated samples was detected by PICL spectra, as the main barometer to investigate the photostability of wool fabrics in this research.
Section snippets
Materials
Pure 100% wool fabric with a mass of 225.2 g/m2 woven by Sunshine Group, China, was used as substrate. Titanium tetraisopropoxide (TTIP) 97% and Tetraethyl orthosilicate (TEOS) 98% were utilized as the precursors of TiO2 and SiO2, respectively. Also, silver nitrate (AgNO3), gold (III) chloride trihydrate (HAuCl4·3H2O) and chloroplatinic acid hexahydrate (H2Cl6Pt·6H2O) were utilized as the precursors of Ag, Au, and Pt, respectively. All the chemicals were purchased from Sigma–Aldrich Company.
Synthesizing the colloids
A TiO
PICL
The photostability of wool samples was analyzed through the PICL characterization. In general, the term of chemiluminescence denotes the phenomenon of light emission from decaying process of excited species produced from a chemical stimulation in an organic matter [39]. Therefore, the light emission from the oxidative degradation process of organic materials can be employed as a probe to study the decomposition process [39]. The presence of oxygen is crucial in producing the polymer macroperoxy
Conclusion
The impact of surface functionalization on the photostability of wool fabrics was investigated. Wool fabrics were coated with TiO2, TiO2/SiO2 and metalized TiO2-based nanocomposite sols synthesized through the low-temperature sol–gel method. Three types of tests including photo-induced chemiluminescence (PICL), photoyellowing rate and ultraviolet protection factor (UPF) were carried out to investigate the photostability and UV protection of fabrics. The surface functionalization of wool with TiO
Acknowledgment
We greatly appreciate the assistance of Dr. Keith Millington from CSIRO Manufacturing Flagship for providing the PICL characterization facilities. This research was financially supported by Deakin University, Australia.
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