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| Issue date:01/10/2007 |
| ATA Journal for Asia on Textile & Apparel - Oct 2007 Issue |
| Source:Journal for Asia on Textile & Apparel |
| by Sanjay Gupta |
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| Functionalities being offered in performance fabrics has expanded at a fast pace as fabric technology has evolved, and as fashion innovators have developed newer products to meet the increasing demands of consumers |
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| Sun protective swimwear (Source: Stingray) |
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Ultraviolet (UV) protection is one of the newer innovations gaining momentum in performance apparel for the outdoor and fitness market.
There has been an increasing interest in UV protective apparel products for children, outdoor activities, active sports and aqua sports.
Penetration of ultraviolet radiation (UVR) into the top layer of the skin is known to damage and produce premature ageing of skin and other skin problems including blotches, wrinkles, squamous cells and basal cell cancer.
Australia has among the highest levels of solar UV radiation, mainly because of its geographical position. New Zealand, USA, Switzerland, Norway, Scotland, Britain and Scandinavian countries also have high melanoma rates.
UV protection factor (UPF) describes the level of sun protection provided by fabrics, and it indicates the level that a fabric can block UV rays from reaching the skin. UPF has a scale ranging from 0 to 50+.
For comparison purposes, the Skin Cancer Foundation has rated the fabric in a standard cotton T-shirt to be as low as UPF 7. When the T-shirt gets wet from water or sweat, the UPF level drops by about 50%, to a UPF 3.5. Most UV block fabrics begin at about UPF 30 and go up from there. In addition, most UV blocking performance fabrics provide consistent UV block, wet or dry.
The level of UPF of textiles depends on their construction, spaces between yarns, fiber types, colors and dyes applied, as well as the application of finishing agents such as optical brighteners and ultraviolet absorbers.
Finishing agents
UV protective finishing agents can be incorporated into the fiber matrix, or it can be applied to the surface of the fabric as a finish. Usually sun protection finish is achieved through the use of UV absorbers.
In UV absorbers such as benzotriazole and phenyl benzotriazole, molecules are able to absorb the damaging UV rays of sunlight. UV absorbers are organic or inorganic, colorless compounds with very strong absorption in the UV range of 290-360nm. These compounds convert UV energy into harmless heat energy. Alternatively, isomerisation can occur and the UV absorber may then fragment into non-absorbing isomers. At the same time, UV absorbers can cause discoloration, if used in higher concentrations.
Organic UV absorbers are derivatives of o-hydroxyl benzophenones, o-hydroxyphenyltriazines and o-hydroxy phenyl hydrazines.
The ortho hydroxyl group in the molecule helps in absorption and makes the compound soluble in alkaline solution. Organic compounds like benzotriazole, hydro benzophenone and phenyltriazine can be used by normal padding or coating applications. Ortho hydroxyphenyl and diphenyltriazine derivatives have excellent sublimation fastness and self-dispersing formulation. It can be applied by pad-thermosol process and also in print pastes.
The presence of inorganic pigments in the fibers also allows better diffusion of light from the substrate, thus providing better protection. Titanium dioxide (TiO2), which is used as a delustering agent, and other ceramic materials have an absorption capacity in the UV region of 280-400nm and reflects visible and infrared rays. Incorporation of TiO2 in fiber matrix improves the UV blocking capacity of the fiber. Good skin protection thereby can be achieved by the textile itself, if the fabric is sufficiently dense.
Optical brightening agents (OBAs), or fabric whitening agents, are used at finishing operations, as well as in wash cycles. They are often applied to enhance the whiteness of textiles by UV excitation and visible blue emission.
OBAs can also improve the UPF of cotton and cotton blends, but not of fabrics that are 100% polyester or nylon. The presence of OBAs in the P/C blends (67/33) can improve the UPF though the nature of such finish is semi-permanent, and washing the fabrics will diminish the protection effect.
Dyes
Dyes are also selective absorbers of visible light.
Most dyes absorb light in the region between 400 and 700 nm, and some also absorb light in the near ultraviolet region. On textiles, dyes often provide tangible UV blocking with the structure of dye molecules playing an important role. Other factors including the type of dye or pigment, absorptive groups present in the dyestuff, depth after dyeing, and the uniformity and additives also influence the UV protection abilities of textile materials. Given the same fabric type, darker colors (e.g. black, navy and dark red) absorb UVR much more strongly than light pastel colors.
Some direct, reactive and vat dyes are capable of giving a UPF of 50+. Some of the direct dyes substantially increase the UPF of bleached cloth, which depends on the relative transmittance of the dyes in the UVB region. Dyes extracted from various natural resources also show the UPF within the range of 15-45, depending on the mordant used.
Besides the transmittance and reflectance of UV radiation, the absorbance of UV radiation by molecules becomes important. Usually azo, anthraquinone, and methine disperse dyes are used for this purpose. While pale orange and blue colored fabrics have UPF values higher than 50, deep dyed fabrics also show excellent protection from UV radiation.
UV absorbers
UV absorbers are compatible with dyes and can be incorporated in the dyeing sequence including normal padding, exhaust, pad thermosol, pad dry cure methods.
UV absorbers are applied between 30-40g/l depending on the type of fiber and its construction. The main limitation is that UV absorbers cannot be applied in a single bath along with other finishing agents.
Ciba Specialty Chemicals offers sun protection through pre-treatment of textiles |
In the market, Clariant offers Rayosan P for disperse dyeing operations on polyester and Rayosan CO for chlorotriazine-based reactive dyeing systems on cellulosics. Apex Chemical Corporation of USA recently developed a sublimation-fast UV absorber (Uvapex CFW) for polyester, which contains a unique dispersion package that inhibits spotting on jet-dyed polyester while offering great lightfastness.
Ciba Specialty Chemicals has developed special UV absorbers for fibers. Ciba Tinofast Cel Liquid is a bi-reactive oxalic acid di-anilide derivative, which can markedly and durably improve the UPF of cotton, viscose, lyocell, polyamide and their various blends, especially if they are printed or dyed in light shades, and are densely constructed. It is best applied on cellulose, together with dyes, preferably by exhaustion. Both exhaustion and fixation rates of this bi-reactive structure are high. The bi-reactive structure ensures stability to repeated washing at up to 95℃.
Application during finishing is not recommended, as the product does not bind to the fiber under acidic conditions.
Nano finishing
With the advent of nano science and technology, a new area has developed in the area of textile finishing called nano finishing.
Coating the surface of textiles and clothing with nano particles is an approach to the production of highly active surfaces for UV blocking properties. Nanoscale titanium gel particles strongly bound to the cotton fabrics can give a UPF≥50 without impairing the tensile properties. Zinc oxide nanoparticles, which have a very narrow size distribution (20-40 nm) and minimal aggregation, can result in higher levels of UV blocking.
Nano zinc oxide (ZnO) particles as UV blocker are more stable as compared to organic UV blocking agents. It enhances the UV blocking property due to their increased surface area and intense absorption in the UV region. Manasorb RB is a UV absorber with a small particle size, making it suitable for spray applications.
UV labeling
UV labeling is an additional requirement on top of other labelling requirements of garments including Permanent Care Labels and Fibre Content labels. Apart from the UPF label, block numbers can also be used based on the UV transmittance value in their respective UVR range.
The UV Standard 801 label |
The "UV Standard 801" label, for example, provides consumers with a reliable guide when purchasing textile products which protective effect against UV radiation has been proven by independent institutes from the International Test Association for Applied Protection.
Private labels such as "Sunsafe" can be attached to the garment if it has at least UPF 40, and fulfills certain design requirements specified in the standard.
Today, recreational exposure accounts for most of the significant UVR exposures of the population, and occupational exposure including those of outdoor workers is also significant. As the issue of global warming takes center stage, continued development of stronger and better UV absorbers, as well as its widespread usage in everyday apparel is gradually expected.
Dr Sanjay Gupta is Professor of Textile Design & Development at the National Institute of Fashion Technology, New Delhi, India
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Copyright © Adsale Publishing Limited. Credit goes to Adsale Industry Portal when used.
We reserve the right to take legal action against any party who reprint any part of this article without acknowledgement. For enquiries, please contact the Editorial Department. |
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