Nanoparticle Coatings for UV Protective Textiles

RJTA Vol. 14 No. 2 2010 Nanoparticle Coatings for UV Protective Textiles Takuya Tsuzuki* and Xungai Wang Centre for Material and Fibre Innovation, Institute for Research Technology and Innovation Deakin University, Geelong Technology Precinct, Victoria, 3217 Australia ABSTRACT As the intensity of ultraviolet (UV) radiation increases every year, effective methods to block UV rays to protect human skin, plastics, timber and other polymer materials are urgently sought. Textiles serve as important materials for UV protection in many applications. The utilisation of nanoparticles to textile materials has been the object of several studies aimed at producing finished fabrics with different performances. This article reviews the recent advancements in the field of UV blocking textiles and fibers that are functionalised with nanostructured surface coatings. Different types of UV blocking agents are discussed and various examples of UV blocking textiles that utilise zinc oxide (ZnO) and titanium dioxide (TiO2) are presented. Future challenges, such as wash-fastness and photocatalysis, are also discussed. Keywords: UV Blocking, Nanoparticles, UPF, Functional Coatings, ZnO, TiO 2 1. Introduction mechanical properties. This situation is aggravated every year due to the recent ozone depletion caused by the increased generation of man-made free radical catalyst gas molecules, such as nitric oxide, nitrous oxide and organo-halogen compounds. Therefore, the development of effective UV-shielding materials is of great importance to our health, society and environment. The detrimental effects of ultraviolet (UV) radiation are having an increased impact on our society (Ramirez & Schneider, 2003). UV light has wavelengths that range from 40 to 400 nm. Hence, its energy is sufficiently high enough to cause various damages to organic substances. The primary source of UV rays in nature is the sun. Since the stratospheric ozone layer shields most of the UV radiation from the sun, only the UV rays with relatively long wavelengths, i.e., UVB (290-320 nm) and UVA (320-400 nm) radiation, reach the Earth’s surface. Yet, these UVA and UVB rays can still cause adverse effects onto our body and other organic materials around us. For example, UVA and UVB rays are responsible for the development of various pathologies, such as skin cancer, suppression of the immune system, cataracts, premature aging of the skin, Alzheimer’s disease and inflammatory disorders (Ansel, 1985; Downes, 1993). Textiles have been used in sun-shielding applications for centuries, owing to their unique characteristics, such as flexibility, good mechanical strength, soft feel, fashion appeal and so on. The World Health Organisation recommends that loose-fitting full-length clothing be used for outdoor activities to protect our skin from UV rays (World Health Organization, 2002). The number of other UV-blocking applications of textiles, such as hats, awnings, and cover sheetings, has increased drastically in the last decades. Outdoor professionals and outdoor-sports practitioners have increased their awareness toward the importance of UV protection with textiles (Moehrle, 2000). UV radiation can also cause severe damage in textiles, plastics, paints and timber products in the forms of discoloration, chalking and reduced The UV blocking properties of textiles can be improved by changing the parameters, such as thickness, fabric opening, fiber types and colours (Dulęba-Majek, 2009). * Corresponding author. Tel.: 61-3-52273205; Fax: 61-3-52272539 E-mail address: takuya@deakin.edu.au (Takuya Tsuzuki) 9 RJTA Vol. 14 No. 2 2010 S(λ) are defined by the CIE International Commission on Illumination (CIE Research Note, 1987). UPF ratings indicate how much the textile material reduces UV radiation that causes skin reddening. For example, UPF 50 indicates that the UV rays in the wavelength range of E(λ) is reduced by 50 times. However, UV blocking provided by untreated textiles is generally insufficient to protect the skin (Hoffmann, 2001). This is particularly true for light summer clothing that is worn in the season when UV light is stronger (Adam, 1998). In the past, it was considered that UVB rays are more harmful than UVA rays, as UVB rays cause sunburns and suntans. For many years, sunscreen lotions blocked only UVB rays and science was not clear on the body's reaction to UVA. However, recent research shows that UVA is the primary cause of premature skin aging (photo-aging), and exacerbates UVB's carcinogenic effects (Gasparro, 2000). Such findings help increase the awareness of the importance of broad spectrum UV protection. Moreover, textiles need to protect their own fibers and dyes from harsh UV radiation in addition to the skin and other materials that lie beneath the textiles (Katangur, 2006; Sun, 2008). This “self-protection” is of particular importance when the exterior textiles require good mechanical properties, such as in applications as parachute, tent, awnings, etc. Self-protection of textiles cannot be achieved by altering the fabrication parameters, such as thickness, fabric opening, fiber types and colours. Hence, in order to protect both textiles themselves and the materials under the textiles, it is necessary to provide UV-blocking treatment to the textiles. This article reviews the recent advancements in the field of UV blocking textiles and fibers that are functionalised by using nanostructured surface coating technologies. 2. UPF tests are normally carried out across both the UVA and UVB regions. However, E(λ) has very small values in the UVA range and is significantly high in the UVB range, as shown in Figure 1. Hence, UPF rating mostly reflects UVB protection instead of a broad spectrum UVA-UVB protection. Ultraviolet Protection Factor (UPF) For the assessment of UV blocking properties of textiles, “ultraviolet protection factor (UPF)” is normally used (AS/NZS 4399:1996; AATCC 183-2004; EN 13758-1:2002). Like “sun protection factor (SPF)” which is used to evaluate the UV blocking effects of sunscreens, UPF is based on the erythemal action spectrum, i.e., the spectral range which causes skin redness. The definition of UPF is pretty much the same as that for SPF as shown in Equation (1): λ2 UPF = ∫ E (λ ) ⋅ S (λ )dλ λ λ2 Fig. 1. Relative erythemal action spectrum, E(λ), standard solar spectral irradiance, S(λ), and E(λ)•S(λ) in the UV light region. 1 ∫ E (λ ) ⋅ S (λ ) ⋅ T (λ )dλ λ1 Currently, the textile industry does not use ratings for UVA protection. The personal care industry has started introducing new rating systems to assess broad spectrum UV absorption in the forms of “UVA/UVB ratio” and “critical wavelength” (Tadros, 2008). (1) where E(λ) is the relative erythemal action spectrum, S(λ) is the standard solar spectral irradiance, and T(λ) is the optical total transmittance of the textile specimen. E(λ) and 10 RJTA Vol. 14 No. 2 2010 It may be helpful to adopt similar parameters in the textile industry to assess the overall UV protection properties of fabrics. which may cause health problems once they contaminate food and drinks (Mahltig, 2005). In contrast to organic UV absorbers, inorganic UV absorbers, such as zinc oxide (ZnO), titanium dioxide (TiO2) and cerium oxide (CeO2), have excellent light-fastness. It is expected that the inherent stability of inorganic UV absorbers would provide a UV protective effect over a much longer period than organic UV absorbers. The mechanism of UV absorption in these semiconductor inorganic materials involves the use of photon energy to excite electrons from the valence band to the conduction band. For example, ZnO has a bandgap energy of ~3.3 eV that corresponds to the wavelengths of ~375 nm. Light below these wavelengths has sufficient energy to excite electrons, and hence, is absorbed by ZnO. On the other hand, light with a wavelength longer than the bandgap wavelength will not be absorbed. The absorbed light does not destroy the inorganic semiconductors, but is generally converted to heat which is negligible at room temperature. Therefore, such wide bandgap inorganic semiconductors absorb UV light, and if small enough, give good transparency to visible light (Dodd, 2008). The UPF values of a textile depends on the production conditions, such as fiber content and weave, fabric colour, finishing processes and the presence of additives. UPF values also vary depending on the post-production conditions, such as stretch, wear and moisture content. It is recommended that the minimum UPF values for UV-protective clothing should be 40 to 50+ (Adam, 1998). However, an average weight cotton T-shirt that is often worn in the summer has an SPF value of only ~ 7, which offers a much weaker UV protection than any commercial sunscreen on the market (Adam, 1998). As such, untreated textiles provide inadequate skin-protection (Palacin, 1997). 3. Nanoparticle UV Absorbers 3.1 UV Absorbers The use of UV absorbers on textiles to improve UV protection of the skin, dyestuff and textile itself is a relatively new field of research (Hoffmann, 2001; Eckhardt & Rohwer, 2000; Palacin, 1997; Reinert, 1997; Hilfiker, 1996). Inorganic UV absorbers have other superior properties over organic UV absorbers. For example, ZnO has a long history of safe topical use and is known as an anti-irritant, astringent and skin-healing agent (Baldwin, 2001). Nonetheless, many types of organic UV absorbers have been tested (Riva, 2006). In addition, ZnO has broad UV absorption spectra while typical organic UV absorbers have UV absorption peaks only at a specific wavelength range due to the typical absorption bands of molecular systems (Figure 2) (Scalia, 2006, Mahltig, 2005). Organic UV absorbers are relatively cheap and generally transparent so that they can be used in many coloured fabrics and textiles (Ibrahim & El-Zairy, 2009; Scalia, 2006). In fact, ZnO is said to have the broadest spectrum absorption range among many inorganic UV absorbers (Figure 2) (Innes, 2002). However, the majority of the organic absorbers are ultimately destroyed by the UV radiation they absorb and hence, the UV-absorption efficiency decreases over time. In addition, upon photo-decomposition of organic UV absorbers, free radicals are often generated, which contribute to the degradation of other organic molecules (Scalia, 2006). However, ZnO suffers from poor chemical stability. It can dissolve under both high and low pH conditions. TiO2 has excellent chemical stability, but the UV-absorption range is narrower than ZnO so that it often relies on light scattering effects in addition to light absorption effects to block UV light. Furthermore, organic UV absorbers are small sized molecules and can leach out from textiles, 11 RJTA Vol. 14 No. 2 2010 particle, λ is the wavelength of incident light, and m is the relative refractive index defined as (refractive index of particles) /(refractive index of matrix). As shown in Equation 2, the intensity of scattered light is proportional to the sixth power of the particle diameter, and therefore, the particle size has a dominant effect on the strength of light scattering. Reducing the particle size will dramatically decrease the scattering. For a particle suspension with a constant weight percent of particles, Nd3 is fixed in Equation 2 and the scattering intensity is proportional to the particle diameter to the power of 3. Fig. 2. Typical transmission spectrum of a TiO2 nanoparticle suspension, ZnO nanoparticle suspension and phenylacrylate-based organic absorber solution. Note that the relative transmission values between the curves depend on the concentration of the UV blocking agents in the system. Therefore, for decreasing the whitening effect, it is important to use particles as small as possible. Figure 3 shows UV-Vis transmittance spectra of ZnO nanoparticles with different average particle sizes. It is evident that smaller particles give higher transparency in the visible light region while the UV blocking property remains the same. 3.2 As shown in Equation 2, the refractive index is another important parameter which governs the intensity of scattered light. Most of the organic materials have refractive indices of around 1.4-1.6. TiO2 (rutile) has a refractive index of 2.7. ZnO, however, has a refractive index of only 2.0. As such, the scattering intensity of ZnO is significantly smaller than that of TiO2, provided that the particle size is the same, and hence, gives higher transparency than TiO2. Nanoparticles Since inorganic UV absorbers are normally provided in particulate forms, it is difficult to obtain high UV-shielding properties while maintaining high visibility. Light scattering by small particles leads to an opaque whitening effect, much like the clouds in the sky that appear white due to light scattering by small water droplets, which may result in an unacceptable hazy appearance of textiles. This is of particular importance for applications on coloured fabrics. This problem can be overcome by reducing the particle size down below 50 nm, which is explained as follows. The intensity of scattered light by a single particle is a function of the particle size as well as the refractive indices of the particles and the media. According to the Mie theory, when the particle size is small in comparison to the wavelength of light, the intensity of scattered light by a particle, Is, is given as (Kerker, 1969): 2 Nd 6 m 2 − 1 IS ∝ 4 Ii λ m2 + 2 Fig. 3. Typical transmission spectrum of ZnO nanoparticle suspension in water at a particle loading level of 0.01 wt%. (2) where N is the number of particles, Ii is the intensity of incident light, d is the diameter of the 12 RJTA Vol. 14 No. 2 2010 By using nanoscale UV-blocking particles, it is possible to achieve high transparency while retaining good UV-absorption properties. The nanoscale diameters can also realize a uniform distribution of the particles in host matrices, which results in a significant increase in the effectiveness of blocking UV radiation (Dodd, 2008; Innes, 2002; Tsuzuki, 2002). nanoparticles and nanorods covered the entire surface of the fibers, and there was evidently no agglomeration. A promising bactericidal photocatalytic activity and excellent protection against UV radiation were achieved (Fei, 2006). Daoud et al. dipped knitted cotton substrates into a titanium tetraisopropoxide solution in absolute ethanol at a pH of 1–2 using a dip-pad-cure process. The process resulted in the formation of a uniform coating of TiO2 nanoparticles with a near spherical grain morphology and 15–20 nm in diameter. The UPF value increased from 10 to 50+. Twenty repeated washings induced no deterioration in the UPF rating (Daoud, 2005). High surface area and high surface energy of nanoparticles are advantageous for better affinity to fabric surfaces which increases the durability of the coatings. 4. UV Blocking Nanoparticles Treatment by The synthesis of TiO2 nanoparticles often require solvent or expensive raw materials that leads to costly processes. Hence, the use of commercial TiO2 nanoparticles has been explored. Nanoparticles that contain functional coatings to achieve UV-blocking properties have been developed during the last decade. Such coatings have been applied in commercial products as sunscreens and paint systems (Spanhel, 2006). Yu et al. (2008) developed a method where UV-blocking treatment is applied onto cotton fabrics by dyeing and finishing in one bath and two steps. Commercial nano-TiO2 of ~ 50 nm in diameter was used as an inorganic anti-UV agent and an adhesive; poly vinyl pyrrolidone (PVP) was used to improve the wet fastness. Nano-TiO2 and PVP were added into the dyestuff solution and applied onto fabrics at 70°C followed by washing, soaping, washing and drying. It was found that when the TiO2 content was increased from 0% to 1.5% in the treatment solution, the UPF values increase from 15 to 35. They found that the treatment of nano-TiO2 in this approach has little effect on the wearability of the cotton fabric. In the textile industry, the last five years have seen the rapid development of UV-blocking treatment of textiles by means of nanotechnology. In most cases, TiO2 or ZnO nanoparticles are used. 4.1 TiO2 Nanoparticles Xin et al. (2004) studied the improvements of UV blocking properties in cotton fabrics with TiO2. The coating was prepared using a sol-gel method in ethanol. A thin layer of TiO2 with a thickness of ~ 100 nm formed on the fiber surfaces. The films were not made of dispersed nanoparticulates, but were nanograined continuous films (Daoud & Xin, 2004). Sójka-Ledakowicz et al. (2009) applied commercial nano-TiO2 and aminosilane-modified nano-TiO2 onto polyester using common techniques, such as padding, spraying and sol–gel coating. The average particle sizes were as large as 300 – 460 nm. UPF values of 50+ were obtained. The treated fabrics also showed high efficiency in the degradation of red wine stains. Nonetheless, the UPF value of the cotton fabrics increased from 10 to 50+. The high UPF value was maintained even after 55 home launderings. Direct synthesis of TiO2 nanoparticles onto fabrics have been investigated by Fei et al. (2006) and Daoud et al. (2005). Fei et al. first synthesised nanosized TiO2 particles which consisted of granular anatase and brookite, and rod-like rutile. Then, the peptized TiO2 sols were adsorbed onto cellulose fabrics using a dip-pad-cure process. As a result, a homogeneous film of nanocrystallites that consisted of nanoparticles and nanorods of less than 100 nm, formed on the cotton fibers. The Many of the commercial TiO2 nanoparticles are sold as dry powders that mostly consist of agglomerated nanoparticles. This poses practical challenges in making well-dispersed nanoparticles in the coating system to fully utilise the advantages of nanosized particles. 13 RJTA Vol. 14 No. 2 2010 4.2 transmittance at 350 nm was reduced from ~ 80% to ~ 20%. ZnO Nanoparticles The preparation of ZnO in pH-neutral conditions is relatively easier and cheaper than the synthesis of TiO2. In addition, the UV absorption spectrum of ZnO is broader than TiO2 (Figure 2). ZnO has additional functionality, including antifungal (Sawaji, 2004) and antistatic (Zhang & Yang, 2009) properties that TiO2 cannot offer. Hence, the use of ZnO nanoparticles for UV-blocking functionalization has been investigated more widely than TiO2. Lu et al. (2006) encapsulated commercial ZnO of 20–100 nm in diameter with polystyrene cells using an emulsion method. First, ZnO nanoparticles were dispersed in an oil monomer phase and then the oil phase was dispersed in water as nanoemulsion droplets. The monomers were subsequently polymerized using an oil-soluble initiator. The size of the prepared nanohybrids spheres was 50–200 nm in shell diameters and 20–100 nm in core diameters. In order to increase the affinity of nanospheres on cotton fibers via ester bonding, methacrylic acid was selected as the co-monomer. After the formation of nanoemulsion, cotton fabrics were dipped into the ZnO suspension for 12 h at 68oC, followed by polymerisation, drying and curing at 120oC for 5 min. It was found that almost all fibers are coated with a grainy layer. A UPF of more than 900 and a wide UV-blocking range (380–280 nm) was achieved. However, the UPF value decreased rapidly from 86.6 to 15.3 after only 10 home launderings. Wang at al. (2004, 2005) studied sol-gel derived ZnO crystallites for the UV protection of textiles. Transparent solutions of zinc acetate and triethenamine in 2-methoxyethanol, equivalent to 3 wt% ZnO, was applied onto cotton fabrics twice, using a dip-pad-cure process, and followed by curing at up to 400oC. Dumbbell-shaped ZnO crystallites that were larger than 500 nm were grown on cotton fabrics. When a curing temperature of at least 150oC was employed, a UPF of more than 400 was achieved. The UV-blocking range was 352-280 nm. This finishing process provided a constant wash-fastness after a washing process that is equivalent to 5 cycles of home laundering. Yadav et al. (2006) studied the improvements of UV blocking properties in cotton fabrics using ZnO nanoparticles. In their research, a simple method was used to coat fabrics with ZnO nanoparticles. ZnO was prepared by a wet chemical method using zinc nitrate and sodium hydroxide as precursors and soluble starch as a stabilizing agent. These nanoparticles, which have an average size of 40 nm, were coated onto bleached cotton fabrics (plain weave, 30 s count) using an acrylic binder. The nano-ZnO (2%) coated cotton fabric blocked about 75% of the incident UV light. Becheri et al. (2008) produced peptised ZnO nanoparticles from zinc chloride and sodium hydroxide solutions and applied them onto cotton and wool fabrics. The wool and cotton samples were soaked for 10 min in a 2-propanol dispersion of ZnO nanoparticles (5% w/w), under gentle magnetic stirring. The clothes were then squeezed to remove the excess dispersion, and dried in an oven at 130oC for 15 min at atmospheric pressure. The UV transmittance of the fabrics was reduced from ~ 90% to ~ 20% at ~ 350 nm. After 5 launderings, the large agglomerates were removed from the textile surface and more than 50% of their initial amount remained on the fabric, despite the fact that the nanoparticles were not covalently grafted to the fabric materials. Vigneshwaran et al. (2006) developed a method to impregnate ZnO–soluble starch nanocomposites. ZnO nanoparticles of ~ 38 nm in diameter were synthesized using water as a solvent and soluble starch as a stabilizer from zinc nitrate hexahydrate and sodium hydroxide in water. Nano-ZnO was impregnated into cotton fabrics using a pad-dry-cure method. The coated fabrics showed enhanced protection against UV radiation; the Mao et al. (2009) reported direct growth of ZnO nanoparticles on silicon dioxide (SiO2) coated cotton fabrics. After hydrothermal processing, the cotton fabric was covered with needle-shaped ZnO nanorods that were ~ 24 nm in diameter. The coated fabrics had an excellent UV-blocking property with a UPF value over 50. However, the UPF was reduced to 50% of the original value after laundering 5 times. 14 RJTA Vol. 14 No. 2 2010 4.3 Other Nanoparticles nano-ZnO coated UV-blocking fabrics is significantly higher than the control sample, and hence, increased breathability (Yadav, 2006). Grancarić et al. (2009) applied natural zeolite nanoparticles of ~ 200 nm in diameter onto white cotton and polyester fabrics. Zeolites are porous aluminosilicate hydrates. They do not possess UV absorption properties, but can scatter or diffuse UV light as other white mineral particles. The textiles coated with zeolilte nanoparticles showed increased UPF values up to ~160. Although natural zeolites are yellowish, the coatings did not significantly affect the fabric whiteness. 4.4 They also found that friction is significantly lower for nano-ZnO coated fabrics than bulk-ZnO coated fabrics. ZnO has higher electric conductivity than TiO2 and hence, gives antistatic properties (Zhang & Yang, 2009) and electromagnetic-filed-shielding properties (Li, 2007) to textiles. In addition, self-cleaning properties can be achieved with TiO2 nanoparticle coatings on fibers as widely documented. By combining different types of nanoparticles and nanotreatments on the surface of fabrics, multifunctional textiles will be realised. Recently, Beringer and Hofer demonstrated that by combining the nanoparticles of hydroxyapatite, TiO2, ZnO and Fe2O3 with other organic and inorganic substances, the surfaces of the textile fabrics can be appreciably modified to achieve considerably greater abrasion resistance, water repellency, UV resistance, and electromagneticand infrared-protection properties (Beringer, 2004). Ash-Fastness and Adhesion Wash-fastness is a particular requirement for textiles and strongly correlated with nanoparticle adhesion to the fibers (Becheri, 2008). High wash-fastness is achieved by forming continuous coating layers of TiO2 and ZnO (Yu & Shen, 2008). For the use of nanoparticles, increased wash-fastness can be achieved by mixing nanoparticles with binders (Yadav, 2006; Vigneshwaran, 2006), or by forming covalent bonding between nanoparticles and fibers (Lu, 2006). 5. Otherwise, the UV protection properties decrease rapidly through laundering (Mao, 2009; Becheri, 2008). Most of the above examples concern only UPF values where the protection of underlying materials, such as human skin, plastics and timber products, are considered. However, as mentioned earlier, textiles also have to protect fibers and dyes in themselves from harsh UV radiation. Colour-fading and deterioration of mechanical strength are serious problems in exterior textile applications. It is expected that coatings that contain inorganic semiconductor nanoparticles would significantly prolong the lifetime of outdoor textile products by effectively blocking UV rays on the fiber surfaces. Improvement of wash-fastness still remains as a great challenge for achieving practical UV blocking textiles by using nanocoatings. 4.5 UV Protection of Fibers and Dyes in Textiles Additional Functionality Applications of TiO2 and ZnO nanoparticles onto textiles provide additional functions apart from UV blocking abilities. TiO2 and ZnO show antibacterial properties under UV irradiation. Vigneshwaran et al. reported that UV blocking textiles made of coatings that contain ZnO nanoparticles show excellent antibacterial activity against two representative bacteria, Staphylococcus aureus (Gram positive) and Klebsiella pneumoniae (Gram negative) (Vigneshwaran, 2006). Katangur et al. (2006) used commercial ZnO and TiO2 nanoparticles of 15 - 70 nm in diameter to protect Kevlar fabrics from sun-induced degradation. Kevlar fabrics were coated with 5 wt% nanoparticle-embedded acrylic coatings that were 20 m in thickness. After a week of UV exposure, the coated fabrics retained 95% of their tear and tensile strengths, whereas unprotected Yadav et al. found that the air permeability of 15 RJTA Vol. 14 No. 2 2010 fabrics retained only 67% of their original strengths. as SiO2 or aluminium oxide (Al2O3) (Wang, 2009). This is based on the assumption that if the coating layer is thicker than the diffusion length of photo-exited electrons and holes, photo-generated excitons would be trapped within the coating layer so that free radical generation would be inhibited. Sun et al. (2008) investigated the use of ZnO nanoparticles dispersed in an acrylic polymer to reduce the colour fading of dyed polyester fabrics under simulated sunlight. Highly transparent films of polymer and ZnO mixture were produced and applied onto dyed polyester fabrics. The results showed strong dependence on the type of dyes. The coating treatment reduced the fading rate of a dyestuff that contains a benzopyran chromophore. However, it did not provide protection to dyes that contain anthraquinone or azo chromophores. Furthermore, the fabric treatment increases the rate of colour fading of anthraquinone based dyes. Recently, we successfully produced ZnO@SiO2 core-shell structures using the Stöber method (Wang, 2009) and a microemulsion technique. The SiO2-coated ZnO showed significantly reduced photoactivity in comparison to uncoated ZnO while maintaining high transparency and strong UV blocking properties (Figure 4a). It was also shown that the yellowing of wool fabrics is drastically reduced by the coating of SiO2-coated ZnO nanoparticles. Sun et al. made a finding which indicated that the inherent photocatalytic activities of TiO2 and ZnO can be detrimental to the fabrics and dyes that TiO2 and ZnO are supposed to protect (Dodd, 2008). When ZnO absorbs UV radiation, its band gap energy of around 3.2-3.3 eV allows electrons to be excited from the valence to the conducting band. The consequence of this is the generation of positive charged holes in the valence band. The excited electrons in the conduction band can either recombine with the holes in the valence band, or both can react with water or oxygen on the surface of the ZnO particle to produce reactive superoxide and hydroxyl free radicals. Due to their photocatalytic properties, TiO2 and ZnO are used to decompose dyes in effluent or organic contaminants in water using natural sunlight. Direct treatment of fabrics with TiO2 and ZnO nanoparticles may allow the free radicals to migrate from the coating film into the fibers, which would result in the degradation of dyes and fibers. So far, the degradation of fibers by the photocatalytic activities of TiO2 and ZnO has not been fully investigated. Hence, the novel ZnO-core@SiO2-shell hybrid nanoparticles offer promising nanocoatings for the UV-blocking treatment of textiles without colour-fading of dye-stuff on the textiles. A silica-layer also helps to overcome the poor chemical stability of ZnO. Another approach is to dope nanoparticles with impurity ions. It is generally considered that dopant ions create chemical, and in some cases, physical defects in the crystal lattice that act as trapping sites of the photo-generated electrons and holes, which promote the recombination of the charged particles, and result in reduced photocatalytic activity. Recently, the authors and collaborators investigated the photoactivity of manganese and cobalt doped ZnO nanoparticles synthesised by using various techniques. The photocatalytic activity of powders produced by mechano-chemical processing was evaluated using the spin-trapping technique with electron paramagnetic resonance spectroscopy (Dodd, 2009). It was found that the photocatalytic activity of CoxZn1−xO nanoparticles progressively decreases with the doping level. In contrast, the activity of MnxZn1−xO nanoparticles initially increases. Currently, our research group is investigating the methods to reduce photocatalytic activities of TiO2 and ZnO to fully utilise the superior properties of these inorganic nanoparticles as effective UV blocking agents. In general, two different approaches have been taken. One approach is to coat nanoparticles with inorganic materials, such 16 RJTA Vol. 14 No. 2 2010 Fig. 4. Photo-degradation of Rhodamine-B dye molecules under simulated sunlight in the presence of (a) uncoated and SiO2-coated ZnO nanoparticles and (b) undoped, Mn 5wt% doped and Co 5wt% doped ZnO nanoparticles. with doping, up to a level of 2 mol% and thereafter declined. The effect of dopants on the photocatalysis of ZnO was found to be dependent on the synthesis methods. When MnxZn1−xO and CoxZn1−xO nanoparticles were synthesised by co-precipitation methods, the nanoparticles show reduced photoactivity (Figure 4b). Organic UV absorbing agents have attractive features, such as low cost, high transparency and ease of application. However, they suffer from critical drawbacks, such as poor light fastness and toxicity. Inorganic UV absorbing agents have advantages over organic agents in terms of photostability and safety. High transparency can be obtained by reducing the size of particles to nanoscale. Nanosize particles offer additional advantages, such as enhanced UV shielding effect and higher affinity towards fiber surfaces. We also studied the colour-fading of dyed polystyrene fabrics coated with undoped and manganese-doped ZnO nanoparticles (Sun, 2009). It was shown that manganese doped ZnO, which were produced using a co-precipitation method, provide improved UV protection against colour-fading as compared to undoped ZnO. However, the level of improvement is different for the two different types of dyes. For a benzopyran dye, the colour fading is greatly reduced, whereas much less improvement is found for an anthraquinone based dye. Further research on the control of photocatalysis in ZnO is on-going at Deakin University for application in UV blocking textiles. 6. In the past, UV blocking treatment of textiles by nanocoatings have been investigated mostly using TiO2 and ZnO. Various techniques were studied to synthesise and apply nanoparticle coatings on cotton, polystyrene and Kelver fabrics. It was found that nanocoatings of ZnO and TiO2 significantly increase the UV blocking properties. However, improvement of wash-fastness and control of the photoactivity of nanoparticles still remain as great challenges in achieving practical UV blocking textiles. We have made some progress in the development of the methods to reduce the photoactivity of ZnO nanoparticles using impurity doping and particle-surface coatings. Summary Textiles are important materials for protecting human skin and other materials from harsh UV radiation. However, untreated textiles have limited abilities to block UV light. Moreover, textiles lack the inherent ability to protect themselves against UV rays to avoid colour-fading and fiber-deterioration. Hence, it is important for textiles to be treated with UV absorbing agents. As the development of nanotechnology innovation progresses, more choices in commercial nanoparticles will be available on the market. By combining different types of nanoparticles and nanotreatments on the surface of fabrics, UV blocking textiles with multifunctionality may be realised. 17 RJTA Vol. 14 No. 2 2010 REFERENCES Synthesised by Mechanochemical Processing’, Journal of Nanoparticle Research, vol. 10, pp. 243-248. [12] Dodd, A.C., McKinley, A.J., Tsuzuki, T. & Saunders, M. 2009, ‘Tailoring the Photocatalytic Activity of Nanoparticulate Zinc Oxide by Transition Metal Oxide Doping’, Materials Chemistry and Physics, vol. 114, pp. 382-386. [13] Downes, J.E., Swann, P.G. & Holmes, R.S. 1993, ‘Ultraviolet Light-Induced Pathology in the Eye: Associated Changes in Ocular Aldehyde Dehydrogenase and Alcohol Dehydrogenase Activities’, Cornea, vol. 12, no. 3, pp. 241-248. [14] Dulęba-Majek, M. 2009, ‘Transmission of UV radiation through woven fabrics in dependence on the liter-thread spaces’, Fibers & Textiles in Eastern Europe, vol. 17, no. 2, pp. 34-38. [15] Eckhardt, C. & Rohwer, H. 2000, ‘UV protector for Cotton fabrics’, Textile Chemist and Colorist & American Dyestuff Reporter, vol. 32, no. 4, pp. 21-23. [16] European Standard EN 13758-1:2002, ‘Textiles – Solar UV protective properties’. [17] Fei, B., Deng, Z., Xin, J.H, Zhang, Y. & Pang, G. 2006, ‘Room temperature synthesis of rutile nanorods and their applications on cloth’, Nanotechnology, vol. 17, pp. 1927–1931. [18] Gasparro, F.P. 2000, ‘Sunscreens, Skin Photobiology, and Skin Cancer: The Need for UVA Protection and Evaluation of Efficacy’, Environmental Health Perspectives, vol. 108, suppl. 1, pp. 71-78. [19] Grancarić, A.M., Tarbuk, A. & Kovaček, I. 2009, ‘Nanoparticles of Activated Natural Zeolite on Textiles for Protection and Therapy’, Chemical Industry & Chemical Engineering Quarterly, vol. 15, in press. [20] Hilfiker, R., Kaufmann, W., Reinert, G. & Schmidt, E. 1996, ‘Improving Sun protection factors of fabrics by applying UV absorbers’, Textile Research Journal, vol. 66, no. 2, pp. 61-70. [21] Hoffmann, K., Laperre, J., Avermaete, A., Altmeyer, P. & Gambichler, T. 2001, ‘Defined UV Protection by Apparel Textiles’, Arch Dermatol, vol. 137, pp. 1089-1094. [22] Ibrahim, N.A. & El-Zairy, E.M.R. 2009, ‘Union disperse printing and UV-protecting of wool/polyester blend using a reactive [1] AATCC Test Method 183-2004, ‘Transmittance or Blocking of Erythemally Weighted Ultraviolet Radiation through Fabrics’. [2] Adam, J. 1998, ‘Sun-protective clothing’, Journal of Cutaneous Medicine and Surgery, vol. 3, no. 1, pp. 50-53. [3] Ansel, J.C., Mountz, J., Steinberg, A.D., DeFabo, E. & Green, I. 1985, ‘Effects of UV radiation on autoimmune strains of mice: increased mortality and accelerated autoimmunity in BXSB male mice’, The Journal Of Investigative Dermatology, vol. 85, no. 3, pp. 181-186. [4] Australian /New Zealand Standard AS/NZS 4399:1996, ‘Sun protective clothing Evaluation and classification’. [5] Baldwin, S., Odio, M.R., Haines, S.L., O'Connor, R.J., Englehart, J.S. & Lane, A.T. 2001, ‘Skin benefits from continuous topical administration of a zinc oxide/petrolatum formulation by a novel disposable diaper’, Journal of the European Acaddemy of Dermatology and Venereology, vol. 15 suppl. 1, pp. 5-11. [6] Becheri, A., Dürr, M., Lo Nostro, P. & Baglioni, P. 2008, ‘Synthesis and characterization of zinc oxide nanoparticles: application to textiles as UV-absorbers’, Journal of Nanoparticle Research, vol. 10, pp. 679–689. [7] Beringer, J. & Hofer, D. 2004, ‘Nanotechnology and its application’, Melliand International, vol. 10, no. 4, pp. 295-296. [8] CIE research Note, 1987, ‘A Reference Action Spectrum for Ultraviolet Induced Erythema in Human Skin’, CIE Journal, vol. 6, pp. 17-22. [9] Daoud, W.A. & Xin, J.H. 2004, ‘Low temperature sol-gel processed photocatalytic titania coating’, Journal of Sol-Gel Science and Technology, vol. 29, no. 1, pp. 25-29. [10] Daoud, W.A., Xin, J.H., Zhang, Y.H., Qi, K. 2005, ‘Surface characterization of thin titania films prepared at low temperatures’ Journal of Non-Crystalline Solids, vol. 351, pp. 1486-1490. [11] Dodd, A.C., McKinley, A.J., Tsuzuki, T. & Saunders, M. 2008, ‘A Comparative Evaluation of the Photocatalytic and Optical Properties of Nanoparticulate ZnO 18 RJTA Vol. 14 No. 2 2010 b-cyclodextrin’, Carbohydrate Polymers, vol. 76, pp. 244–249. [23] Innes, B., Tsuzuki, T., Dawkins, H., Dunlop, J., Trotter, G., Nearn, M.R. & McCormick, P.G. 2002, ‘Nanotechnology and the cosmetic chemist’, Cosmetics, Aerosols and Toiletries in Australia, vol. 15, no. 10-12, pp. 21-24. [24] Katangur, P., Patra, P.K. & Warner, S.B. 2006, ‘Nanostructured ultraviolet resistant polymer coatings’, Polymer Degradation and Stability, vol. 91, no. 10, pp. 2437-2442. [25] Katangur, P., Patra, P.K., Warner, S.B. 2006, ‘Nanostructured ultraviolet resistant polymer coatings’, Polymer Degradation and Stability, vol. 91, pp. 2437-2442. [26] Kerker, M. 1969, The Scattering of Light; Academic Press: New York. [27] Li, Y., Wu, D.X., Hu, J.Y. & Wang, S.X. 2007, ‘Novel infrared radiation properties of cotton fabric coated with nano Zn/ZnO particles’, Colloids and Surfaces, vol. A300, pp. 140-144. [28] Lu, H., Fei, B., Xin, J.H., Wang, R. & Li, L. 2006, ‘Fabrication of UV-blocking nanohybrid coating via miniemulsion polymerization’, Journal of Colloids and Interface Science, vol. 300, pp. 111-116. [29] Mahltig, B., Böttcher, H., Rauch, K., Dieckmann, U., Nitsche, R. & Fritz, T. 2005, ‘Optimized UV protecting coatings by combination of organic and inorganic UV absorbers’, Thin Solid Films, vol. 485, no. 1-2, pp. 108-114. [30] Mao, Z., Shi, Q., Zhang, L. & Cao, H. 2009, ‘The formation and UV-blocking property of needle-shaped ZnO nanorod on cotton fabric’, Thin Solid Films, vol. 517, pp. 2681–2686. [31] Moehrle, M., Heinrich, L., Schmid, A. & Garbe, C. 2000, ‘Extreme UV exposure of Professional Cyclists’, Dermatology, vol. 201, pp. 44-45. [32] Palacin, F. 1997, ‘Textile finish protects against UV radiation’, Melliand International, vol. 3, pp. 169-172. [33] Ramirez, R. & Schneider, J. 2003, ‘Practical guide to sun protection’, Surgical Clinics of North America, vol. 83, no 1, pp. 97-107. [34] Reinert, G., Fuso, F., Hilfiker, R. & Schmidt, E. 1997, ‘UV-protecting properties of textile fabrics and their improvement’, Textile Chemist and Colourist, vol. 29, no. 12, pp. 36-43. [35] Riva, A., Algaba, I.M. & Pepió, M. 2006, ‘Action of a finishing product in the improvement of the ultraviolet protection provided by cotton fabrics: Modelisation of the effect’, Cellulose, vol. 13, pp. 697–704. [36] Sawaji, J. & Yoshikawa, T. 2004, ‘Quantitative evaluation of antifungal activity of metallic oxide powders (MgO, CaO and ZnO) by an indirect conductimetric assay’, Journal of Applied Microbiology, vol. 96, no. 4, pp. 803-809. [37] Scalia, S., Tursilli, R., Bianchi, A., Lo-Nostro, P., Bocci, E., Ridi, F. & Baglioni, P. 2006, ‘Incorporation of the sunscreen agent, octyl methocycinnamate in a cellulosic fabric grafted with b-cyclodextrin’. International Journal of Pharmaceutics, vol. 308, pp. 155–159. [38] Sójka-Ledakowicz, J., Lewartowska, J., Kudzin, M., Leonowicz, M., Jesionowski, T., Siwińska-Stefańska, K. & Krysztafkiewicz, A. 2009, ‘Functionalization of textile materials by alkoxysilane-grafted titanium dioxide’, Journal of Materials Science, vol. 44, pp. 3852–3860. [39] Spanhel, L. 2006, ‘Colloidal ZnO nanostructures and functional coatings: a survey’, Journal of Sol-Gel Science and Technology, vol. 39, pp. 7–24. [40] Sun, L., Rippon, J.A, Cookson, P.G., Wang, X., King, K., Koulaeva, O. & Beltrame, R. 2008, ‘Nano zinc oxide for UV protection of textiles’, International Journal of Technology Transfer and Commercialisation, vol. 7, no. 2-3, pp. 224 – 235. [41] Sun, L., Rippon, J.A., Cookson, P.G., Koulaeva, O. & Wang, X. 2009, ‘Effects of undoped and manganese-doped zinc oxide nanoparticles on the colour fading of dyed polyester fabrics’, Chemical Engineering Journal, vol. 147, no. 2-3, pp. 391-398. [42] Tadros, T.F. 2008, Colloids in Cosmetics and Personal Care, Wiley-VCH, Weinheim, Germany, pp. 297-300. [43] Tsuzuki, T., Robinson, J.S. & McCormick, P.G. 2002, ‘UV-Shielding Ceramic Nanoparticles Synthesised by Mechanochemical Processing’, Journal of the Australian Ceramic Society, vol. 38, no. 1, pp. 15-19. [44] Vigneshwaran, N., Kumar, S., Kathe, A.A., Varadarajan, P.V. & Prasad, V. 2006, ‘Functional finishing of cotton fabrics using 19 RJTA Vol. 14 No. 2 2010 zinc oxide–soluble starch nanocomposites’, Nanotechnology, vol. 17, pp. 5087–5095. [45] Wang, J., Tsuzuki, T., Sun, L. & Wang, X. 2009, ‘Reducing the photocatalytic Activity of Zinc Oxide Quantum Dots by Surface Modification’, Journal of the American Ceramic Society, vol. 92, no. 9, pp. 2083–2088. [46] Wang, R.H., Xin, J.H. & Tao, X.M. 2005, ‘UV-blocking property of dumbbell-shaped ZnO crystallites on cotton fabrics’, Inorganic Chemistry, vol. 44, pp. 3926–3930. [47] Wang, R.H., Xin, J.H., Tao, X.M. & Daoud, W.A. 2004, ‘ZnO nanorods grown on cotton fabrics at low temperature’, Chemical Physics Letters, vol. 398, pp. 250–255. [48] World Health Organization. 2002, Intersun: The Global UV Project. Sun protection: simple precautions in the sun, available at: http://www.who.int/peh-uv/index.html, Accessed September 10, 2009. [49] Xin, J.H., Daoud, W.A. & Kong, Y.Y. 2004, ‘A new approach to UV blocking treatment for cotton fabrics’, Textile Research Journal, vol. 74, pp. 97-100. [50] Yadav, A., Peasad, V., Kathe, A.A., Raj, S., Yadav, D., Sundaramoorthy, C. & Vigneshwaran, N. 2006, ‘Functional finishing in cotton fabrics using zinc oxide nanoparticles’, Bulletin of Materials Science, vol. 29, no. 6, pp. 641–645. [51] Yu, Q. & Shen, A. 2008, ‘Anti-ultraviolet Treatment for Cotton Fabrics by Dyeing and Finishing in one Bath and Two Steps’, Journal of Fiber Bioengineering and Informatics, vol. 1 no. 1, pp. 65-72. [52] Zhang, F. & Yang, J. 2009, ‘Preparation of Nano-ZnO and Its Application to the Textile on Antistatic Finishing’, International Journal of Chemistry, vol, 1, no. 1, pp. 18-22. 20 Copyright of Research Journal of Textile & Apparel is the property of Hong Kong Institution of Textile & Apparel and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.