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Fastness To Light
Dyed fabrics that are exposed to sunlight will in time fade or change colour. This property is called colorfastness to light. The test for fastness to light: cover half a sample of fabric with opaque paper and expose it to out door light for 20 days. Compare both the halves. If the exposed portion shows perceptible fading, the fabric is not fast to light. As this was very time consuming, accelerated methods using machines that duplicate the fading action of the sun, were later developed.
The machine most often used for this test is fade-o-metre or weather-o-metre, an apparatus having very strong electric lamps or a specific carbon- arc light. If a fabric can with stand an exposure of 40 hours in this with no perceptible loss of colour, it is said to have superlative fatness to light. A fabric has good fastness to light if it withstands an exposure of 30 hours.
When evaluated, the number of hours that the specimens are in the testing machine should be indicated for eg: class 4-40 hours means that the specimen was rated class 4: little colour change, after being tested for 40 hours.
Sunlight Cabinet:
A suitable light exposure rack or cabinet is of wood or metal or other satisfactory material of any convenient size, constructed so as to allow free access of air to all test specimens and covered with window glass to protect the specimens from rain and other elements of the weather. The glass covered must be cleaned at least once each day. The location should be selected so that shadows of surrounding objects will not fall on the exposed specimens. The minimum permissible distance between the glass and the specimen is 3 inch.
Fabrics of cut rectangles with the longer dimension running lengthwise in size not less than 1*2 In and an exposed area of not less than 1*1 In.
Procedure:
Mount a set of standards and the specimens to be tested on the cardboard with an opaque cover made of the same material across one half of each of the standards and specimens where a large number of specimens is to be exposed, they may be mounted in the same manner on as many additional cards as are necessary.
As an aid in rating, each standard and specimen to be compared should have the same area.
Expose the specimen and standards simultaneously to light under the conditions described above. The specimens and standards are only exposed on the cabinet on sunny days between 9 am and 3pm. Specimens and standards are removed from the cabinet after 3pm and are stored in dry room temperature storage area. The specimens are exposed to sunlight in the cabinet for a period of 48 hours. The colour fastness of the exposed fabrics can be assessed by using colour matching spectrophotometer.
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Participants