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3.6.1.4 Requirements of Colored Paper and Board
Depending on the intended purpose of the paper, different fastness properties are required:
3.6.1.4.1 Light Fastness
This is defined as the fastness of a dyed paper to the action of light. It is deter¬mined by both the dye used and the raw materials of the paper. The degree of lightfastness is specified by a test method according to DIN 54 003, which is also used in the textile industry. Originally lightfastness was tested by exposing the dyed material to sunlight under defined conditions. Today artificial light with a radiation spectrum similar to sunlight is used (Xeno test apparatus or Fade-Ome-ter). The lightfastness cannot be given as an absolute value but can only be ex¬pressed in relation to a standard which is exposed simultaneously. In the textile and paper industry, the Blue Wool Scale is used as a standard for comparison. It consists of blue dyeings with lightfastness ratings from 1–8. Rating 1 signifies the lowest, and rating 8 the highest lightfastness. A dyed paper with a lightfastness of 1 will change its shade after one hour “sunlight” exposure. However, the shade will not fade completely until it has been exposed for several hours, depending on the depth of shade and the fibrous material. Since paper is normally not subjected to such severe exposure, a lightfastness of 1 is sufficient for all short-lived paper grades e. g. newsprint, magazines and grades based on mechanical pulps (e. g. for coating base paper, notepad) and/or mixed recovered paper (e. g. for liner, test-liner). A lightfastness of 3 corresponds to a resistance of several days exposure. Dyes with this lightfastness rating can be used for most paper articles, provided that the paper stock, too, has approximately the same lightfastness e. g. for all kinds of high grade printing and writing papers. Dyed paper with a lightfastness rating of 5 does not undergo any change in shade, even on exposure to direct sunlight for several weeks, this rating is needed e. g. for document paper, photo¬graphic paper, laminating base paper.
3.6.1.4.2 Bleed Fastness
This is required for paper and board that are used for food-packaging and tissue papers (napkins and hygienic papers). According to the regulations, tests must be carried out in each case to determine whether and to what extent dye can migrate from a colored paper onto the packed foodstuff or to human skin. The paper may come into contact with water, dilute fruit acid, grease, oil, alcohol or alkali. A test specimen is placed between two uncolored glassfiber papers moistened by dipping into the test solution. This sandwich is then placed between two glass plates of the same size. The whole is then wrapped airtight in a polyethylene film and loaded with a certain weight. The specimen is kept in this condition for 24 h at 20 °C. After drying, the coloration of the glassfiber paper is compared with the Grey Scale to assessing the fastness to bleeding according to DIN 54 002. It should be empha¬sized that the bleed fastness is not only dependent on the dye but also on the fibrous material and the type of dye fixation. A reliable prediction can be made only on the basis of tests with the paper in question. For napkins and hygiene papers a number of direct dyes are suitable and for wood-containing papers and testliner a good bleeding fastness can be obtained with basic dyes.
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