Page 194 - Basic Principles of Textile Coloration

TERMS USED IN DIRECT EXHAUST DYEING 183

migration is impossible. In the early stages of the process, the dyeing conditions do
not promote reaction between the dyes and fibre and the dyes are allowed to
migrate from fibre to fibre. The reaction of the dye with the cotton starts as soon
as the pH of the dyebath is increased by addition of sodium carbonate.

10.4.2 Amounts of dye and colour yield
The amounts of dyes used to produce the colour desired are usually expressed as a
percentage of the weight of the dry material, and abbreviated as % owf, or % dye
on the weight of fibre. Thus, 1.00 kg of dye used to colour 100 kg of cloth
corresponds to a 1.00% dyeing. Colour reproducibility depends upon accurate
weighing of both the goods and the dyes. This may be difficult when hydrophilic
fibres such as cotton, viscose or wool are present. These absorb varying amounts of
water from the air depending upon the relative humidity and temperature. In such
cases, the regain of the material must be known and the amount of dye calculated
on the basis of the dry goods.

With increasing amounts of absorbed dyes, the colour of the goods obviously
becomes deeper, but also usually duller, and often with a slight change in hue.
Deep shades frequently have lower fastness to wet treatments and rubbing than
pale shades produced with the same dyes. On the other hand, deeper dyeings have
higher light fastness. The colour yield is the depth of colour of a dyeing per unit
amount of dye in the material. Quantitative assessment is possible from
measurement of the reflection spectrum of a sample of the dyeing and the per-
centage of the dye (Chapter 24). Dyehouse laboratories often determine the
relative colour yields of comparable dyes to evaluate their cost effectiveness.

10.4.3 Liquor-to-goods ratio
The liquor-to-goods ratio, or simply the liquor ratio, gives the weight of the bath
solution relative to the weight of the dry material being dyed. Since the density of
the solution is usually very close to 1.00 kg l–1, the liquor-to-goods ratio is usually
given in l kg–1. Many newer dyeing machines operate with lower liquor ratios in
order to minimise the consumption of energy for heating the water. An increase in
the liquor ratio causes a decrease in the degree of exhaustion, under the given
conditions, and therefore a decrease in the depth of colour of the dyeing. If dyeing
is described by a simple equilibrium constant K in the form of the ratio of the