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Light Fastness Test:

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:

1. American Test Method (AATCC 16E)
2. British Test Method (ISO 105/BO2)

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 light fastness is assessed subjectively with respect  to the blue wool standard. The blue wool scale has grades 1 to 8.  1 being poor and 8 being the best. Rating 5-6 is acceptable for more of the requirements.

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:

1. Treatment of formaldehyde based cationic dye fixing agent tends to lower the light fastness.
2. 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.
3. Generally, dyes based on metal complex tends to have better light fastness.

Reference:  

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