The photostability of wool doped with photocatalytic titanium dioxide nanoparticles
Introduction
The tendency towards photoyellowing of wool is far more serious than other commonly used fibres, including nylon, acrylic, cotton and polyester [1]. Studies on the photostability of wool have demonstrated that wool is yellowed by ultraviolet (UV) light (λ < 380 nm) and bleached by visible light, in particular by blue wavelengths (400–450 nm) [2], [3], [4], [5], [6], [7]. Polypeptide chain scission and hence strength loss is associated with photodegraded wool. Window glass, which attenuates shorter wavelengths, can reduce the phototendering of wool [8].
In order to retain the market position of wool, research into reducing and preventing its photodegradation has been conducted for many years. The effectiveness of some UV absorbers in improving wool photostability has been reported, including the use of sulphonated benzophenones or sulphonated 2-hydroxyphenylbenzotriazole (commercially known as Cibafast W), 2-(2′-benzotriazolyl)-phenol, and the newly emerged inorganic UV absorber nanoparticulate ZnO [9], [10], [11], [12], [13]. Carr and Leaver [14] found that the most effective sulphonated UV absorber also gave the strongest synergistic effect with phenolic antioxidants.
In industry, micron-sized TiO2 is an important pigment and is commonly used as a delustrant in synthetic fibres, in particular for nylon. Micron-sized TiO2 can be used as UV blocker because of its good reflective properties and UV absorption ability. However, the interaction between UV radiation and TiO2 can also produce photoionisation which can subsequently lead to the formation of reactive oxygen species [15], [16] that in wool may ultimately produce unwanted yellow photoproducts. In the photocatalytic reaction, electrons (e−) and holes (h+) are formed when UV radiation <380 nm is absorbed by TiO2. In the presence of atmospheric oxygen and water, the reactive oxygen species superoxide radical anions (O2−), hydroxyl radicals (OH) and hydrogen peroxide can be generated, as shown in Scheme 1.
Titanium dioxide exists in three crystalline forms, brookite, rutile and anatase, although only the two latter structures are normally used as fillers, pigments and UV absorbers in polymers and coatings. It has been generally observed that the anatase form exhibits a higher level of photoactivity than rutile and generates a higher population of hydroxyl free radicals when exposed to UV light. This behaviour has been confirmed in a comparative study on the photooxidation of polyurethane [17] and in a study using the terephthalate anion as a specific fluorescent probe for OH [18]. Both forms are semiconductors, with anatase having a higher energy conduction band (band gap 3.2 eV) than rutile (band gap 3.0 eV) [19]. The primary particle size of nanocrystalline TiO2 controls its photocatalytic reactivity, with smaller particles being the most reactive regardless of their subsequent aggregation into secondary particulates [19]. It has also been reported that anatase-rich mixtures (70–75% anatase, 25–30% rutile) are more effective photocatalysts than pure anatase [20].
The dual role of TiO2 as either a photostabilizer or photocatalyst in synthetic polymers has been extensively studied [17], [21], [22], [23], [24], [25], [26]. However, until now, only limited studies have examined the photoyellowing of wool in the presence of TiO2. Schäfer [27] earlier reported that titanium dioxide, when used as a white pigment, could retard the photoyellowing of bleached and optically brightened wool when applied with fluorescent whitening agents. However, it is not clear whether the anatase or rutile form of TiO2 was used in this study. Deng et al. [28] used TiO2 to improve the mechanical strength of wool powder and PU compound. Daoud et al. [29] demonstrated self-cleaning keratin treated with an anatase colloid. However, little is known on the role of TiO2 as a photostabilizer or a photocatalyst in the process of wool photodegradation.
In this study, the effect of nanocrystalline titanium dioxide (NTD) on the photoyellowing behaviour of wool was examined. Three types of wool sliver, untreated, peroxide-bleached, and bleached followed by application of a fluorescent whitening agent, were mechanically chopped and air-jet milled into fine powders. These powders were then doped with measured amounts of NTD, which allowed even and intimate mixing between wool and NTD particles. Both doped and undoped wool powders were then compressed into uniform discs for photoyellowing examination under different irradiation conditions using simulated sunlight, UV-B and UV-C wavelengths. D1925 yellowness indices were measured to assess the photoyellowing trends. Photo-induced chemiluminescence (PICL) emissions were compared for each type of pure and NTD-doped wool powder to examine the behaviour of free radicals formed in the irradiated samples.
Section snippets
Materials and wool treatments
The mean fibre diameter of clean untreated Merino wool sliver used in these studies was 20.7 μm, with a coefficient of variation of diameter (CVD) of 22.2%.
The bleaching protocol for wool sliver was conducted in an alkaline solution (pH 8.5–9.0) with tetra sodium pyrophosphate (6 g/L) and H2O2 (50% w/v, 15 cm3/L) at 60 °C for 60 min and was completed by a thorough water rinsing. For subsequent fluorescent whitening treatment, formic acid was added to the bath at the end of bleaching stage to reach a
SEM image and UV–vis absorbance spectra
Fig. 1 shows that the titanium dioxide particles in P-25 are aggregates consisting of numerous nanograde crystals. According to the Mie theory [33], for a similar intensity of incident light and wavelength, nanoparticles scatter much less light than micron-scale particles. Hence TiO2 nanoparticles act primarily as a UV absorber rather than as a UV screening material.
Fig. 2 shows UV–vis absorption spectra of aqueous dispersions of P-25 nanoparticles. The absorbance in the visible range is
Conclusions
This study examined the photoyellowing and chemiluminescence properties of powdered wool in the presence of photocatalytic TiO2 nanoparticles. Three types of wool sliver, untreated, bleached, and bleached plus fluorescent whitening agent (FWA) treated, were converted into fine wool powders for doping with photocatalytic TiO2 nanoparticles and compressed into solid discs for photostability testing. Disc samples of all three wool types all experienced a significantly slower rate of photoyellowing
Acknowledgement
The authors would like to thank Mr. Michael Jones from CSIRO for his technical support throughout this work.
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