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Basic Principles

Basic Principles

Ultra Violet (UV) light is electromagnetic radiation with a wavelength between 10-400 nano meters. See Fig.2. UV rays? are emitted in sunlight and from certain industrial applications, such as plasma cutting or electric arcs used in welding, but sunlight is the prime energy source of such radiation. 

UV constitutes around 5% of the total incident sunlight on the earth’s surface (visible light 50% and IR radiation 45%). Although the amount of UV in sunlight is proportionally much lower than the other types of radiation, it has the highest quantum energy which is greater than the bonding strength of many organic molecules, and so can rupture molecular chains.  UV radiation has therefore detrimental effect on many organic materials (e.g. polymers, fibres, etc) and human skin; overexposure to UV is widely associated with the incidence of skin cancer.

The UV wavelength range is usually divided into three sections according to the associated energy and harmful effects caused.

The long wavelength ultraviolet rays (abbreviated: UV-A) are within the band 320 nm-400 nm. These cause transformation of melanin in the skin (the dermis), leading to rapid pigmentation (i.e. sun tanning) which occurs within a period of a few hours of exposure, but for such short duration gives minimal immediate damage. However, these UV rays? penetrate deeply into the skin and repeated short exposure will cause premature ageing, resulting in loss of skin Elasticity? accompanied by lines and wrinkles.

Shorter wavelengths  from 290 nm to 320 nm (abbreviated: UV-B) are higher in energy and can penetrate to a depth of a few millimeters into the skin, causing acute chronic reactions and damage; such as skin reddening (Erythemahttp://en.wikipedia.org/wiki/Erythema) or sunburn.

The shortest wavelengths of 10 nm-290nm (abbreviated: UV-C), are severely damaging, but are absorbed by the ozone layer so do not reach the earth’s surface.

The UV –A and UV-B which are not filtered by the atmosphere can provide vitamin D (peak production occurring between 295 and 297 nm), and therefore are both beneficial and damaging to human health.  Consequently, it is necessary to make textile materials resistant to UV so that they can be used to reduce skin exposure, in order to gain the beneficial effects, and to be exploited in technical applications requiring longevity. 

All electromagnetic radiation shares the characteristics of Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html, Refractionhttp://missionscience.nasa.gov/ems/03_behaviors.html, Diffractionhttp://missionscience.nasa.gov/ems/03_behaviors.html, scatter and absorption on contact with an object.  Therefore UV resistant materials must utilise these characteristics in order to provide a suitable protection level. How the radiation interacts with textiles largely depends on the properties of the polymeric materials from which the textiles are made.

Absorption, Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html and scattering are currently the methods most commonly adopted to provide protection against UV, and there are a range of different compounds having chemical structures that can perform these reactions to UV rays?. With the absorptive reaction, the UV rays? hit the surface of the textile but the radiation is absorbed into the energy states of the compound that do not lead to polymer degradation; instead, the energy is dispersed as heat. The reflective reaction bounces the UV radiation back to its source, therefore preventing damage occurring. Scattering disperses the UV radiation, which reduces the potency for it to carry out damage. In imparting UV resistance to textiles these methods may be used in combination.