Page 305 - Basic Principles of Textile Coloration

294 DYEING CELLULOSIC FIBRES WITH DIRECT DYES

The major pad dyeing processes are:
(1) pad with dye solution, dry, pad with salt solution, steam;
(2) pad with dye solution, steam;
(3) pad with salt solution, pad wet-on-wet with dye solution, steam.

Cotton/polyester materials can be continuously dyed by padding with
combinations of direct and disperse dyes.

Stripping of dyeings with direct dyes is possible by hypochlorite (NaOCl) or
dithionite (Na2S2O4) bleaching, which destroys the dye on the fibre by oxidation
or reduction, respectively. This is more difficult with aftertreated dyeings. Cationic
aftertreatment agents must be removed from the stripped goods with formic acid
since they influence re-dyeing. Copper complexes can be demetallised by
treatment with EDTA, usually giving a paler shade, and then stripped or re-dyed.

14.3.3 Problems in batch dyeing with direct dyes

A number of problems can complicate the dyeing of cellulosic fibres with direct
dyes. The residual chlorine in the fabric from hypochlorite bleaching may bleach
some dyes and an anti-chlor treatment of the goods, reducing the chlorine with
sodium bisulphite, is then useful before dyeing. Corrosion of the steel in dyeing
machines by sodium chloride is a constant concern. This is worse at higher dyeing
temperatures and then the more expensive Glauber’s salt (Na2SO4.10H2O) or
calcined sodium sulphate (anhydrous) will be preferred. Half the weight of the
anhydrous salt is sufficient.

Longer dyeing times and higher temperatures increase the risk of reduction of
some azo direct dyes by reducing groups in the cellulose. This is more likely to
occur with the lower molecular weight viscose fibres under alkaline conditions.
Ammonium sulphate in the dyebath acts as a slightly acidic buffer to prevent this
type of reduction. Alternatively, azo dye reduction by the cellulose can be
suppressed by addition of a mild oxidising agent such as the sodium salt of m-
nitrobenzene sulphonic acid. Dyeing with direct dyes is carried out in neutral
solution. It is necessary to carefully neutralise the goods after alkaline scouring and
bleaching and to know the pH of the water supply. Some direct dyes are not stable
at high dyeing temperatures and labile groups, such as amide groups as in CI
Direct Red 83, may hydrolyse.

14.3.4 Rates of dyeing with direct dyes and compatibility

The rate of dyeing cotton with direct dyes can vary widely from dye to dye. Times