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UV Resistance

Introduction

UV resistance in textiles refers to a fibre’s or fabric’s ability to resist UV radiation. This can be important for the preservation of the fibre as UV rays? are a cause of degradation to textile fibres, as UV rays? excite the polymer molecules and break  polymer chains thereby resulting in significant damage to the fibre (See Fig.1.) depending on the UV intensity and the duration of exposure. 

Textile items can be used to provide protection to a product or wearer from UV radiation, but to do so effectively the textile requires the ability of resisting UV transmission through the constituent fibres or the penetration of the radiation through the fabric interstices. This means the fibres themselves should be UV resistant and the fabric structure should have good breathability but low optical transparency.

The UV resistance of textiles may therefore looked at from two perspectives;

• That as a functionality to protect the textile article itself from degradation; generally achieved by the inherent characteristics of the fibre, which can be natural or an engineered quality in a synthetic fibre, attained by particular additives to the polymer such as titanium dioxide or carbon black.

•  To protect a wearer or item from UV radiation; achieved through the combination of fibre selection and engineering of the fabric structure, and if higher levels of protection are required special finishes can be applied to the fabric surface.

The level of UV resistance of a textile is therefore dependent upon the inherent characteristics of the fibre used in combination with the fabric structure.

The properties of a fabric structure that greatly influences its UV are: basis weight, density and weave. Generally a closed weave will provide greater resistance to UV penetration, and a heavier weight provides a greater barrier.

Colour can greatly affect the UV resistance of fabrics. Darker colours absorb more radiation than lighter ones; a colour difference in the same type of fabric can provide a significant difference in protection level. However this is a generalisation and dyes will behave differently based on their composition, not necessarily their displayed colour.

Stretching of a fabric reduces its UV resistance, because as the weave or knit structure enlarges so does the fabric interstices, producing small exposed areas within the structure through which UV rays? can pass. For a similar reason the ageing of a product can have a negative impact on its UV protection level. This is because even with modest degradation of a fabric’s structure, during general wear and tear of its  lifecycle,  it can become more permeable to UV radiation.

Wetness or significantly increased Relative humidity? levels can result in the swelling of some fibres, such as cotton, thereby narrowing the interstices of the fabric structure which can increase its UV protection level. However, similar to a lens, the presence of water can focus UV rays? and reduce the effective scattering and Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html by the fabric surface, thus lowering the materials’ UV resistance. 

To increase UV resistance, resistant particles can be applied as a finishing treatment. Often such fabric finishes are part of the dyeing process or an additional stage after dyeing. They provide a lustre and microscopic surface texture to reduce UV penetration by reflecting, absorbing and/ or scattering the radiation.

A further option for imparting UV resistance to a textile made with synthetic fibres is to modify such fibres at the fibre production stage, by incorporating UV resistant particles into the fibre structure, or through the synthesis of polymers with UV resistant chemical structures. This approach will increase protection but due to the wide range of factors that influence textiles’ UV protection, it is difficult to gauge the effectiveness of a given fibre. Therefore assessment of UV protection should be undertaken on the dyed and finished, ready to market product.

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