Carpets fading in the sun

ISO 105-B02 – 'Colour fastness to artificial light: xenon arc fading lamp test' is frequently required in many flooring specifications.

Environmental factors, such as sunlight (including ultraviolet radiation), high temperatures and water, can dramatically affect carpets and other flooring materials.

Lightfastness

Light visible to the human eye forms part of a much larger spectrum of radiation that includes X-rays, microwaves and radio waves. Different forms of radiation are distinguished by their wavelengths. For example, X-rays have a much shorter wavelength than radio waves. The shorter the wavelength, the more penetrating and generally more damaging the radiation tends to be. The part of the spectrum visible to the human eye is bound by ultraviolet light at shorter wavelengths and infrared at longer wavelengths.

It is a common misbelief that ultraviolet light is the sole cause of discolouration and degradation of plastics, textiles and other materials. While UV radiation does play its part, it is by no means the only cause. Visible light energies can be more damaging than the shorter UV wavelengths.

Lightfastness testing predicts a material’s resistance to fading, yellowing, darkening or physical degradation, such as cracking or shrinkage, when exposed to light. The light source can be natural daylight or artificial. For consistency of exposure and acceptable test times, intense artificial light sources are generally used. Elevated temperatures of 40ºC and above are also used to accelerate the light-ageing mechanisms.

There are three main laboratory sources of artificial daylight: carbon arc, mercury vapour and xenon arc lamps. Xenon arc lamps are the preferred method for predicting performance to light exposure as they most accurately reproduce the natural daylight spectrum. SATRA specifies this source in its own test method, as do BS, EN, ISO, IUF and AATCC methods.

Control of exposure

To measure how much radiation the samples have been exposed to, they are compared to eight Blue Wool Standards (BWS) – woollen fabrics with specific blue dyes known to fade at certain rates, which have been exposed simultaneously to the test sample (see above photograph). After testing, a sample is compared to the BWS and the closest match is recorded. BWS are numbered from one (fades very rapidly) to eight (takes hundreds of hours to fade). A lower figure therefore indicates a poor lightfastness. Each standard fades in roughly twice the time taken for the proceeding sample – so BWS6 is twice as resistant to fading as BWS5 and so on. Values below BWS4 are rarely considered good enough for general use, with BWS5 and above being preferred.

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