Direct dyes are characterised by their affinity for cellulose (arising from their chemical
structure) and their solubility in water (the result of the presence of sulpho groups).
This means it is possible to dye them easily and with some degree of fastness, even if
limited. In printing practice, the direct dye in a thickened solution is printed on to the
fabric and then dried.
During subsequent steaming in saturated steam the thickened paste and the fibres
swell, and the dye dissolves again and diffuses from the film of paste into the fibres.
Here the dye is held fast on the fibril surfaces by van der Waals forces and hydrogen
bonds. After an adequate steaming time an equilibrium is established between dye in
the fibre and dye in the paste. During the final washing of the print, however, the
unfixed dye together with the thickening paste are rinsed off and can stain the undyed
parts of the cellulose.
At the same time the equilibrium, which has been established at this stage, can be disturbed by the dye migrating from the fibre into the washing liquor. For both these reasons, textile printing using direct dyes gives good results only if a special fixing agent is employed. A quaternary ammonium (cation-active) compound may be applied after printing and steaming to interact with the dye anions. The larger, less soluble, complexes which are formed in the fibre as a result show considerably lower mobility, and the print is noticeably ‘faster’. In many cases, however, the usually very good fastness to light deteriorates noticeably after such treatment. Alternatively, dyes suitable for aftertreatment with copper may be used.
These are azo direct dyes which contain one or two hydroxyl groups and one carboxyl group in positions ortho to the azo groups. The result is an increase in the size of the molecule around the central copper(II) ion, and a consequent noticeable improvement in light fastness properties. It is only the need for the relatively complex and lengthy process of aftertreatment with copper in a bath of acetic acid (10–30 min at 60–70 °C) that has prevented this group, which lies somewhere between direct and mordant dyes, from becoming significant in textile printing.
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| Direct dyes |
At the same time the equilibrium, which has been established at this stage, can be disturbed by the dye migrating from the fibre into the washing liquor. For both these reasons, textile printing using direct dyes gives good results only if a special fixing agent is employed. A quaternary ammonium (cation-active) compound may be applied after printing and steaming to interact with the dye anions. The larger, less soluble, complexes which are formed in the fibre as a result show considerably lower mobility, and the print is noticeably ‘faster’. In many cases, however, the usually very good fastness to light deteriorates noticeably after such treatment. Alternatively, dyes suitable for aftertreatment with copper may be used.
These are azo direct dyes which contain one or two hydroxyl groups and one carboxyl group in positions ortho to the azo groups. The result is an increase in the size of the molecule around the central copper(II) ion, and a consequent noticeable improvement in light fastness properties. It is only the need for the relatively complex and lengthy process of aftertreatment with copper in a bath of acetic acid (10–30 min at 60–70 °C) that has prevented this group, which lies somewhere between direct and mordant dyes, from becoming significant in textile printing.
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