Colour fastness is a material’s resistance to colour fading when exposed to the environment. In order to ensure the quality of a material, you have to test the material against real-life conditions in which it is likely to face during its expected service life. The way to do this is to test the endurance of the material against specific conditions, such as accelerated weathering. Once the accelerated weathering test programme has been completed, physical and mechanical assessments need to be carried out to determine any effects of the testing. This is usually carried out using grey scale testing, under a light.
Grey scale testing is the evaluation of differences in colour shading of a material’s appearance. Grey scale is used in the assessment of change in colour, occurring in colour fastness testing, as described in ISO 105-A02 (BS EN 20105-A02) and when using lightfastness standards. A tool, known as a grey scale, is used to visually assess and compare the loss of colour of a specimen by identifying a half-step rating of 5, 4-5, 4, 3-4, 3, 2-3, 2, 1-2, and 1. The half-step scale consists of pairings of grey colour swatches, from 5 being good to 1 being poor. One half of the pair is always the same shade of grey with the second half getting lighter as you move down the ratings. Each pairing illustrates the difference in shade between a sample which has undergone testing and a control sample, which corresponds to a numbered rating.
A colour fastness cabinet is used to help the observer identify the severity of the surface changes of the exposed sample material using daylight lights, or any other type of lights like fluorescence etc. The exposed test specimen is placed alongside an untreated, control specimen of the same product. The difference in colour between the two specimens is measured against the sections on a grey scale and a corresponding grade is given. A grey scale ranges from 5 being no colour change at all to 1 being the lightest and biggest change in colour.
The specimens are only given a rating and the pass or fail criteria is usually specified by specific industry standards or by the material suppliers and OEMs. This is a method to help determine how well the material will perform within its intended service life.
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