In the recent years, consumers have become increasingly aware of the need for sun protection, which is related to the incidence of sun induced skin damage and its relationship with increased exposures to UV light. UV radiation can lead to acute and chronic reactions and damage, such as acceleration of skin ageing and sunburn. Billions of people live on the earth and each has his or her own color of the skin. In human body the skin color depends on the quantities of melanin, carotene and oxygenated or reduced hemoglobin combined in the skin, as well as the thickness, water content etc. Among other factors, the quantity of melanin that is distributed in the skin determines its fairness or darkness and greatly influences the human complexion, at the same time melanin plays an important role in minimizing the damage that UV rays cause in the skin.

Protection of the skin against the action of solar radiation is a relatively new objective of textile finishing, since the textile does not always guarantee adequate protection. Specific protective functions of textiles against the most diverse influences are attracting more and more attention.
A notable objective of increasing interest in this area is the protection against UV radiation. The reason for this is the tendency for human to suffer increased harmful exposure to UV through natural radiation. The unfinished fabric has the limitation to guarantee adequate protection. Thus, a special additional sun protection finish is applied in the form of UV stabilizers. Electromagnetic radiations of wavelength between 150 and 400 nm are termed as Ultraviolet rays. Approximately 10% of sun’s energy is in the form of ultraviolet radiations. Atmosphere absorbs most of the noxious radiations emitted by the sun, only 5% of the harmful radiations reach to the surface of the earth.

All wave lengths of light including visible light affect fabrics to some extent. There are two critical elements for comprehensive protection and not just UV blocking property. UV absorbers such as Benzotriazole and phenyl benzotriazole, molecules are able to absorb the damaging UV rays of sunlight. UV absorbers convert UV energy in to harmless heat energy. This transformation is regenerative and can be repeated indefinitely. At the same time UV absorbers can cause discoloration, if used in higher concentrations. The construction of woven and knitted fabrics and the fiber types have a great influence on protection from ultraviolet transmittance. The ultraviolet protection factor (UPF) of textiles depends on their construction, the spaces between the yarns, their fiber types, the color, the textile impregnation, and the presence of optical brighteners and ultraviolet absorbers.

UPF also depends on the swelling capacity of the fibers. The UV blocking capacity of a fiber can be improved by incorporating TiO2 into its structure. Good skin protection, which is absolutely essential due to the accumulation of the radiation dose, is achieved by the textile itself with a sufficient weight of the fabric. In other case, e.g., light weight summer garments, a UV absorber can be applied either during fiber manufacture or in the final fabric finish which also offers the same degree of protection. Suitable organic or in organic products are also applied during this process. By using UV absorbers, exposure of the fabric to UV lights is reduced on the one hand as well as the intensity of the transmitted UV light on the other. To indicate the protection from UV radiation the term Sun Protection Factor {SPF} is widely used. SPF is a measure of how much a sun screen protects the skin protection products. SPF is a measure of how a sun screen protects the skin from burning and is measured by timing, as to, how long skin covered with sun screen .The term Ultra Violet Protection Factor { UPF } is widely used by the textile and clothing industry.

UPF is based on a vitro test method and it is a ranking of the sun protective abilities of textile. It is the ratio of an average effective ultraviolet radiation {UVR} irradiance calculated for unprotected skin to the average effective UVR irradiance calculated for skin protected by the test fabric. It is the quotient of the permitted radiation exposure relative to a minimum reddening of the skin in the protected area. The objective is to achieve factors between 30 and 50.

The calculation is based on UV transmission spectra which may be determined with the help of a spectrophotometer. For this, both relevant components, UV-A {wave lengths from 315 to 400 nm and UV-B {280- 315} are detected. Dyes are 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, those dyes often provide a great blocking effect of ultraviolet light transmittance. Disperse dyes are used for dyeing polyester fibers. In addition to conventional high temperature acid dyeing procedures, alkaline high temperature dyeing has become important. Disperse dyes for these applications are usually azo, anthraquinone, and methine. Molecules of disperse dyes have a very low water solubility, and they have polar groups but no ionic groups in their structures. Polyester, an aromatic polyethylene terephthalate, which is highly hydrophobic, is known to have a high protective factor against the influence of disperse dyes on the UPF of fabrics is high. Pale orange and blue colored fabrics have UPF values higher than 50, while a pale red colored fabric does not reach such a value. The structure of dye molecules plays an important role. Besides the transmittance and reflectance of UV radiation, the absorbance of UV radiation by molecules becomes important.

Deep dyed fabrics show excellent protection from UV radiation. The transmittance of UV radiation in the region from 280 to 315 nm is very low, and the transmittance of UV radiation in the region from 315 nm to 400 nm is low as well. This fact and the possibility of UV light absorbing in the short UV region are probably the consequence of the formation of bonds between dye molecules and between dye molecules and fibers.

Ultraviolet radiations accelerate the physical and chemical deterioration processes of the polymeric substances such as fading of colorants, yellowing of cellulosics, photo-oxidation of polyolefins, embrittlement of coatings etc. It also creates excited states and free radicals, which are capable of initiating a large number of reactions, such as chain scission, cross linking, oxidation, and polymerization. Apart from these detrimental effects, UV rays can also lead to synthesis of new polymers via cross-linking polymerization. Photo-degradation is observed in almost every plastic material upon prolonged exposure to sunlight, thereby restricting their application for outdoor use. The degradation can be minimized by using UV stabilizers, which dissipate the energy acquired from these ultraviolet radiations in the harmless manner thus, protecting the material, and getting themselves destroyed in the process.

UV radiation and textile materials: UV radiation degrades the textile materials, due to exitations in some parts of the polymer molecule. Much depends on the type of fiber and its chemical structure. Due to large surface area available, textile fabrics are more susceptible to attacks by UV radiation. Natural fibers like cotton silk, and wool have lower degree UPF absorption than synthetic fibers. Cotton fabric in a grey state provides a higher UPF because of natural pigments, pectins, and waxes. Dyed cotton fabrics exhibit higher UPF and undyed bleached cotton yields very poor UPF. UVR attacks polyamides the most, by photo oxidation. The fabric looses its strength and its crystalline structure. Polyester too get affected by UV radiation, to the tune of 45-50% after 30 days of exposure. Polyester fibers absorb more in the UVA and UVB regions than aliphatic polyamide fibers. Bleached silk and bleached PAN show very low UPF of 9.4 and 3.9 respectively.

UV Stabilizers /Absorbers

UV absorbers are organic or inorganic colorless compounds with very strong absorption in the uv range of 290 – 360nm. UV absorbers incorporated in to the fibers convert electronic excitation energy in to thermal energy. They function as radical scavengers and oxygen scavengers. The high energy short wave UVR excites the UV absorber to a high energy absorbed may then be dissipated as longer wave radiation. Alternatively, isomerisation can occur and the UV absorber may then fragment in to non- absorbing isomers.

Requirement and selection of a ideal UV absorber:

1. An effective UV absorber has to be absorb through out the spectrum to remain stable against UVR and then to dissipate the absorbed energy to avoid degradation of fabric or loss of color value.
2. Highest absorbency in the ultraviolet region {290-340nm} and no absorbency in the visible region.
3. Should be heat stable and compatible with other additives in the finish formulation
4. Should be non toxic and non- skin irritant.

Types of UV absorbers: Organic UV absorbers are derivatives of o- hydroxyl benzophenones, o – hydroxyphenyltriazes, o – hydroxy phenyl hydrazines. The ortho hydroxyl group in the molecule helps in absorption and to make 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 in organic pigments in the fibers helps in better diffusion of light from the substrate, thus providing better protection. Titanium dioxide and other ceramic materials have an absorption capacity in the UV region of 280-400nm and reflects visible and IR rays.

Technological options:

UV absorbers incorporated in dyeing decreases the dye uptake, except in post treatment application. They are compatible with dyes and are applied by 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 limitations of UV are that they cannot be applied in a single bath along with other finishing agents. Anything in excess will have a detrimental effect on the fabric. With the advent of nano science and technology, a new area has developed in the area of textile finishing called Nano finishing. Growing awareness of health and hygiene has increased the demand for UV protective textiles. Coating the surface of textiles and clothing with nano particles is an approach to the production of highly active surfaces to have UV blocking properties. Zinc oxide {Zno} nano particles embedded in polymer matrices like soluble starch are a good example of functional nanostructures with potential for applications such as UV. Metal oxides like Zno as UV blocker are more stable as compared to organic UV blocking agents. Hence nano Zno will enhance the UV blocking property due to their increase surface area and intense absorption in the UV region.

Sun protection clothing:

Sun protection involves a combination of sun avoidance and the use of protective garments and accessories. Reducing the exposure time to sunlight, using sunscreens and protective clothes are the three ways of protection against the deleterious effects of UV radiation.. The fabric is rated according to an Ultraviolet Protection Factor ("UPF"). UPF is like SPF except UPF rates protection against both UVA and UVB. A garment with a UPF of 50 only allows 1/50th of the UV radiation falling on the surface of the garment to pass through it. In other words, it blocks 98% of the UV radiation. Sun protection clothing is an easy and reliable form of UV protection and so is becoming more and more popular. Garments are tested according to AATCC test method 183. UV protection textiles include various apparels, accessories, such as hats, shoes, shade structures such as umbrellas, etc.

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