Colour fastness to light
The final use of a textile product is an important and determining factor in the colouring process, because the colour must be colourfast under the conditions in which it will be used.
The colour on a textile product can be affected by UV from sunlight whether or not in combination with moisture and heat. The dye oxidizes and changes hue in the process. The degree to which a fabric is resistant to this form of discolouration is called lightfastness. A fabric with good lightfastness will retain its colour better and longer than a fabric with poor lightfastness.
Execution of the test
The most commonly used test for determining light fastness is EN ISO 105-B02. This test uses an artificial light lamp that imitates exposure to sunlight in a very intensive manner and allows the test to be carried out relatively quickly and under controlled conditions. The degree of discolouration is assessed by simultaneous measurement of standardized blue dyed wool samples with increasing lightness. The scale runs from 1 to 8. The test is also suitable for white bleached or optically whitened textiles.
Application of the lightfastness test according to EN ISO 105-B02
This test method is mainly used for the assessment of textile materials that are exposed to prolonged sunlight such as tent fabric, garden furniture upholstery but also summer clothing and workwear.
Blue clothing fabric discolours to purple/violet
A special case of discolouration due to the effect of sunlight that we regularly see at VTC is the discolouration of blue-coloured clothing to a purple/violet colour. This discolouration is only visible at specific places on the garment such as the top of the back, the chest and the top of the upper legs. The discolouration usually occurred within a few weeks to months after the garment was put into use.
If a lightfastness test is carried out on these garments, the result is often reasonable to good and of a level that does not explain the discolouration that was observed.
In these cases the problem is often caused or exacerbated by the combination of sweat and sunlight. Blue dyes in particular can be sensitive to this. That precisely this plays a role can be demonstrated by a special variant of the lightfastness test (EN ISO 105-B07).