Page 317 - Basic Principles of Textile Coloration

306 DYEING CELLULOSIC FIBRES WITH DIRECT DYES

not necessarily present an appropriate angle to the hydrogen bonding groups in the
dye molecule when the latter is sitting above or below the glucose ring. Cellulose
hydroxyl groups are usually hydrogen bonded together or to water during dyeing.
Many leuco vat dyes, derived from polycyclic quinones and leuco sulphur dyes
(Chapter 17) have high substantivity for cellulose fibres, yet they have neither
linear structures nor many hydrogen bonding groups. Substantivity may therefore
depend more on van der Waals and dispersion forces between the dye and fibre
molecules.

Substantivity may also involve the surface active properties of direct dyes. The
more hydrophobic sections of the dye molecule would prefer to approach the
relatively hydrophobic axially oriented carbon–hydrogen bonds of the glucose rings
in cellulose to avoid interaction with surrounding water molecules. This is the
driving force of surface activity. Since cellulosic fibres are extremely porous, they
have an extended internal surface and surface activity effects could account for a
considerable degree of dye adsorption.

Direct dyes aggregate in solution and in the fibre, another facet of their surface
activity. There is circumstantial evidence that aggregation of the dye molecules in
the cellulose fibre is the cause of the dye’s substantivity. Dye aggregation increases
with increasing dye and salt concentration, and decreases with increasing
temperature. Most evidence suggests that aggregation is not significant in boiling
solutions but the inability of the limited experimental techniques to detect small
aggregates in solution, or in the fibre, could give misleading conclusions. Some
dyes do not appear to aggregate, but are substantive (CI Direct Yellow 12). Others
are so highly aggregated that dyeing is almost impossible at low temperatures
(copper phthalocyanine direct dyes), and others appear to have no substantivity
unless salt is present (CI Direct Blue 1).

Despite the considerable literature on the effects of varying molecular structure
on the substantivity of dyes for cotton, the origins of such substantivity are poorly
understood. Are both dye–fibre and dye–dye interactions as well as surface activity
involved in promoting substantivity of direct dyes for cellulosic fibres? The best
answer one can give is – possibly. How little we understand of this technology.

REFERENCES

1. J. Shore, Cellulosics Dyeing, J. Shore, Ed (Bradford: SDC, 1995).
2. SDC Committee on Direct Dyes, J.S.D.C., 62 (1946) 280; 64 (1948) 145.