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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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Market Sectors

The UV resistant property is a requirement for a diverse range of textile applications across many sectors, such as sail cloths in Sports and Leisure, seat coverings in Automotives, and as an essential characteristic for maintaining strength in applications such as Construction, Geotextiles and General Engineering. As UV light can be a contributing factor to colour fading, preventing this is a challenge faced by the Interiors sector.

For apparel applications the intrinsic UV resistance of fabrics used in clothing is enough to provide the general protection required from regular wear garments. However as an added value function, UV finishes have been applied to provide extra protection. A UV protection rating is displayed on many apparel on sale in Australia and New Zealand, where the climatic conditions make the functionality of UV protection in clothing much more relevant than, say, in Europe.  A UV protection rating is displayed on many garments on sale in countries where the climatic conditions make the functionality of UV protection in clothing an important requirement, in particular Australia and New Zealand.  The market demand is more prevalent for Children’s wear and is driven by safety advice around UV exposure.

Advanced Links:

Defined UV protection by Apparel Textiles

Role of Clothes in Sun Protection

Garments as Solar Ultraviolet Radiation screening materials

In Personal Protective Equipment UV resistance becomes more relevant in terms of wearer protection. In certain industrial applications such as Welding and Plasma cutting there is a risk of Erythemahttp://en.wikipedia.org/wiki/Erythema and even skin burns; therefore a UV resistant quality is of importance. In this application it may also be of importance to provide Heat Resistance as a functionality of these personal protective garments.

In specialist technical apparel the application of UV resistance can be vital to maintain wearer health. The space suit is an example of this; the functionality is of particular importance, as UVC rays are filtered by the Earths atmosphere this natural protection is not provided outside of it, in space, so an enhanced level of resistance is required.

Spacesuit fabric engineering

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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.

 

 

 

The Effects of Ultraviolet Light on Polymeric Materials

UV light contributes to the development of skin cancer, the aging of plastics, and the discolouration of dyes and artwork. Typical property changes in a material include reduced ductility (increased brittleness), chalking, colour changes, a reduction in toughness, and cracking. Recently, polymers have been developed that are designed to degrade under light, and are used in products like Biodegradable? plastic bags.

The effect of UV light on a polymeric material depends on the flux (power) of the light, its wavelength, and the chemical structure of the material. Ultraviolet light interacts primarily with a structure’s pi electrons, meaning that double bonds and aromatic groups in a structure interact most strongly with it. For example, nylon 6 absorbs UV light in its amide bonds:

Picture 1

Structure of nylon 6 

Polymers containing no pi electron clouds, e.g. polyethylene, are relatively unaffected by UV light.

The effect of UV light on a material can be determined using a number of analytical methods; typically:-

• Chemical structure analyses by employing UVand infrared spectroscopy and/or NMR (nuclear magnetic resonance);
• Surface analyses by scanning electron microscopy (SEM),
• Detection of free radicals by EPR (Electron Paramagnetic Resonance);
• measurement of molecular weight by viscosity measurements or end group analysis; or by
• changes in mechanical properties. 

A major factor for UV resistance is the bulk or thickness of the material: the thicker the material, the more UV resistant it is because less UV will penetrate through to the centre of the material.

The UV resistance of materials may be determined by outdoor or laboratory testing methods. Outdoor testing is more expensive and time consuming than laboratory testing, but it remains the most appropriate by which other methods are compared. Laboratory test methods are not easily correlated with outdoor exposures because the wavelengths of the light sources do not always match those of sunlight.

Sunlight intensity varies considerably according to location, time of day, season, and atmospheric conditions. In the UK the light flux reaching sea level is greatest in July at noon. However, the overall flux is far less than that in, for example, a desert at the Equator, or elsewhere at high altitude. Thus, the testing of a material’s resistance to UV degradation must be carefully considered according to location of use. Common testing sites are Arizona, Florida and Japan. These areas have high ambient temperatures and levels of ultraviolet radiation.

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Terms and Definitions

UPF is the abbreviation for Ultraviolet Protection Factor and an accompanying numbered system defines a fractional value of how much UV radiation can penetrate the product, for example a material of UPF 30 will allow a 1/30th of radiation to pass through it; the higher the UPF rating, the higher the protection level.

The abbreviation SPF (Sun Protection factor ) and accompanying number denotes the sun protection in terms of maximum exposure time provided by a product. For example, if say 10 minutes was the maximum unprotected exposure time for skin burn to occur. A product with an SPF 10 rating will increase the maximum exposure by a multiple of 10 to 100 minutes;  a individual suffering skin burn after 1 hour of unprotected exposure, would be provided with 10 hours protection  when wearing a SPF 10 garment.


 

Regulations, Legislation and Test Standards

The Australian/New Zealand standard board were the first agency to introduce standardised testing and classification of UPF rating, since this standardised test methods have been introduced worldwide. These Standards outline the labelling system to be used in accordance to that particular standard.

The Swiss standard 801 tests fabrics through simulated ageing conditions such as wear, stretching and washing, and is widely used. Centres approved for undertaking this test method are outlined by the International Test Association for Applied UV Protection, of which the Hohenstein Institute  is a founding member. 

USA
AATCC Test Method 183 - Transmittance or blocking of Erythemally weighted Ultraviolet radiation through Fabrics

ASTM D 6544  - Standard Practise for Preparation of Textiles Prior to Ultraviolet Transmission Testing

ASTM D 6603 - Standard Guide for Labelling of UV protective Textiles

British Standards
BS EN 13758-1:2002  - Textiles. Solar UV protective properties. Method of test for apparel fabrics

BS EN 13758-2:2003  - Textiles. Solar UV protective properties. Classification and marking of apparel

BS EN 12224:2000   - Geotextiles. Determination of the resistance to weathering

Australasian

AS/NZS (Australasian Standard 4399:1996)

International

Test Standard 801 is the only standard which tests textiles after experiencing conditions close to wear. 

International Testing Association for Applied UV Protection: Standard 801 

ISO 10640:2011 - Plastics. Methodology for assessing polymer photoageing by FTIR and UV/visible spectroscopy

The consumer claims directive 2005/20, outlines how products must be presented to the consumer.

Developments in the Field

  1. Sun Guard is a laundry additive for domestic use which can be applied to garments to provide a UPF of 30. https://sunguardsunprotection.com/index.php
  2. Advances in finishing techniques are enabling the enhanced efficiency of adding UV absorbers or blockers at the finishing stage. Lowering the use of water and energy and providing greater durability, Plasma technology is being explored as a means of providing this functionality.
  3. Nano finishes are being increasingly explored for their ability to add a range of durable functional finishes, including that of UV resistance.
  4. Nano-diamond structures are being explored as UV resistance finishes on fibres, http://pubs.acs.org/doi/abs/10.1021/nn800445z
  5. Schoeller® Technology recently launched Coldblack® a finishing technology that allows dark colours to not absorb heat while exposed to UV rays?, also providing a protective function. http://www.coldblack.ch/index.php?id=49

Recommended Reading:

A quite general introduction to UV and fabrics is given in “Engineering apparel fabrics and garments”, J. Fan, Woodhead Publishing, CRC Press, 2009.

Handbook of Polymer Degradation, S. H. Hamid, Marcel Dekker Inc, 2000, 573-727.

Polymer Photodegradation, J. F. Rabek, 1995, 296-303.