Color Fastness to light

 

Nowadays the criteria of light fastness is a major concern amongst the dyers.
The stringent requirement of light fastness is getting more and more importance in the European as well as in the American market. It is very much essential to understand the different test methods, grading and factors affecting light fastness.

Generally it is difficult to achieve good grade of light fastness in light, medium, tricky shades viz., khaki, olive, grey, browns etc. A proper combination always helps to arrive at the customer requirement.
Generally two methods of testing are widely accepted by most of the customers.

They are:
American Test Method (AATCC 16E)
British Test Method (ISO 105/BO2)

Some more points that affect the fastness of a printed fabric:
The fastness to light of a print is not governed solely by the colorant. It is also very dependent on the colorant concentration, the thickness of the layer and the binder; other factors such as the spectral composition of the incident light and the atmospheric humidity also play a role. Consequently, it is very difficult to exactly quantify fastness to light. The values are merely given as a guide to formulators in carrying out their own tests.

AMERICAN TEST METHOD (AATCC 16E)

This is an accelerated test method for testing of light fastness. There are different options in this method which are A, B, C, D, E, F, G, H, I, J. These options differ from each other on the basis of light source, panel temperature and humidity. Generally AATCC 16E method is widely used for testing purpose. In this method a test specimen is exposed under the condition specified in various test methods for 20hours, 40 hours or 60 hours and the factors affecting light fastness.

Generally for garment sector the assessment of light fastness is done after 20AFU where as in the case of furnishing fabrics, car upholstery the grading is assessed after 40-60 AFU.

Grading:
Grading of light fastness in this method is given on the basis of grey scale with rating of 1-5. One being poor and the five being the best.
Rating 3 is normally acceptable for most of the requirements.


BRITISH TEST METHOD (ISO 105/BO2)
The light fastness of dyed fabric is evaluated by exposing the fabric samples to xenon ARC. Even though the light sources are same, other conditions are different.
Grading:
The fastness to light is tested in accordance with DIN 16525. The degree of fading is assessed by comparison with the blue scale for wool (DIN EN ISO 105-B01). The fastness to light ratings are as follows:
Rating Property
1 very poor
2 poor
3 moderate
4 fairly good
5 good
6 very good
7 excellent
8 outstanding

There is no direct relation between the ratings of both the above methods. AATCC 16E method (20AFU) is a quick method, while ISO 105/BO2 method takes much longer time where light fastness ratings are high.

Factors affecting the light fastness:

Other than the factors shown above, the treatment of formaldehyde based cationic dye fixing agent tends to lower the light fastness considerably.

Light fastness lowers by half to one rating when dyes are used in combination as compared to rating of individual component for a given depth.

Generally, dyes based on metal complex tends to have better light fastness.

Reference:
AATCC technical Manual. Vol. TI, 1996
Test Methods for color fastness of textiles - SDC.