In order to clarify what fastness means, we shall examine various fastness properties that a thread or yarn may be required to have. We shall study how grades of fastness are established and what they signify and finally we should take a look at what fastness you can expect from various classes of dyestuff on the major substrates of interest to us.
The most desirable fastness properties in any thread or yarn is arguably of wash fastness.
In general, the dyers used to carry out two main wash fastness tests, viz.,
Test-1: A length of coloured thread is plaited with a white partner and treated at 95°C in an alkaline soap solution for 30 minutes. The degree of staining on to the adjacent white thread ( which can be of one or more white substrates) is assessed as in the change of shade of the original colour. The test is commonly known as ISO4.
Test-2: As in Test-1 above but treatment is only at 60°C. The test is called ISO3.
Another popular fastness demand is to rubbing both wet and dry with the sample being hand or machine rubbed. Only the staining on the white calico test fabric is recorded.
Fastness to light is either carried out in sunlight ( a low method) or in an artificial Xenon lamp tester (much faster). Along with the test sample are eight blues of known light fastness which fades to the same degree as the sample gives the light fastness grading of the sample, only change of the shade is recorded.
Fastness to bleach, either peroxide or hypochlorite are severe tests where normally only change of shade need be recorded.
A less severe bleach type test is fastness to chlorinated water which is meant to represent the effect of swimming pool waters on textiles; usually for swim and beach wear garments.
Fastness to hot pressing at a wide range of temperatures with both change of shade and staining being relevant. Disperse dyes on polyester can sublime ( literally evaporate) on some severe permanent pleating processes and even at low iron heats many classes of dye will stain ( but may not do so on ISO3 wash tests) white fabric on contact.
There are some exotic fastness requirements like fastness to vulcanizing , a process used to cure rubber footwear or fastness to stone washing, a fickle process used to fade cotton denim jeans.
Nearly all fastness properties are assessed on a scale of 1 to 5 with 5 being the best rating and 1 the worst. The Grey Scale I can be used to assess the change in shade. Staining scales are slightly different but the same usage principle applies.
Exceptionally, light fastness is measured on a scale of 1 to 8 with 8 being the best and 1 the worst.