FASTNESS PROPERTIES OF DISPERSE DYES

The fastness to washing and light of dyeings with disperse dyes on synthetic and acetate fibres is usually moderate to good. The washing fastness on nylon, however, is only fair, particularly for deep shades. The results of washing fastness tests on deep polyester dyeings often depend upon how well residual disperse dye particles on the fibre surface have been cleared after dyeing. When disperse dyes have migrated from inside the polyester fibre to the surface during thermal treatments such as heat setting or drying, the dyeings may have reduced fastness to washing, dry cleaning and rubbing (crocking). This effect is enhanced when the dyes are soluble in hydrophobic surface finishes such as softeners. The fastness to wet treatments of dyeings on secondary cellulose diacetate is inferior to that on the more hydrophobic triacetate. Dyeings of artificially-made fibres with disperse dyes generally have good fastness properties. For any fibre, however, a particular fastness property will vary considerably from dye to dye.

Typical fastness properties of disperse dyes on all synthetic fibres

Fastness property	Acetate	Triacetate	Nylon	Polyester	Acrylic
Washing	Moderate	Good	Poor to fair	Good	Very good
Light	Good	Good	Moderate to good	Good	Good
Crocking	Good	Good	Good	Moderate to good	Moderate
Gas fume fading	Fair	Moderate	Fair	Moderate	Good

The light fastness of disperse dyes may be very good in standard shades but is less so for pale shades, and lower still for tests conducted using a carbon arc light source. Non-ionic UV absorbers increase the light fastness for dyed fabrics such as those used for automobile upholstery. Certain blue and violet anthraquinone disperse dyes with basic amino groups are very sensitive to fading by nitrogen dioxide in polluted air. High temperature combustion processes produce low concentrations of nitrogen oxides. They are most abundant in industrial or city environments. Their effect on dyeings is called gas fume fading. This type of fading is usually worst for dyeings with sensitive dyes on cellulose acetate fibres. It is less severe on nylon and polyester fibres, but still poses a problem when the highest fastness is required. Nitrogen dioxide will nitrosate a relatively nucleophilic primary amino group of the dye, converting it into a hydroxyl group. This reaction usually reddens the shade. Colourless fading inhibitors protect sensitive dyes. These are readily nitrosated amines that preferentially react with the nitrogen dioxide and thus protect the dye. The fading inhibitor is added to the dyebath towards the end of dyeing, or is applied in an aftertreatment. Dyes that are more resistant to nitrogen dioxide fading have less nucleophilic phenylamino groups.

Ozone is a major air pollutant in metropolitan centres. It causes oxidation of many types of dyes, close to the fibre surface. Ozone fading of dyed nylon carpets and fabrics in automobile interiors is particularly serious for pale shades when the dye has not adequately penetrated into the fibres. Aftertreatment of the dyeing with amine or phenol anti-oxidants, similar to gas fume fading inhibitors, improves the fastness to ozone fading of sensitive blue anthraquinone disperse dyes.