Page 40 - Basic Principles of Textile Coloration

PREPARATION FOR DYEING 29

component gives tensile strength, abrasion resistance and dimensional stability to
the fabric, while the cotton provides water absorbency, comfort and reduced
pilling (Section 4.3.2).

2.4 PREPARATION FOR DYEING

Newly constructed textile materials are called grey or greige goods. At this stage, the
overall manufacturing sequence is far from complete. Natural or synthetic chemicals
in fabrics can interfere with wetting and dyeing. Preparation is the series of processes
used to remove them in readiness for dyeing and finishing. The objective of
preparation is to remove as much of the unwanted impurities as possible from the
fibres to produce a fabric that will uniformly absorb solutions of dyes and chemicals.
Uniform water absorbency is more critical than complete removal of contaminants.
Penetration of solutions into very compact fabrics, made from fine, high-twist yarns
packed tightly together, is not necessarily rapid and is even more difficult for poorly
prepared goods. Good preparation is vital for successful dyeing and printing since
non-uniform absorption of dyes is immediately visible.

The unwanted chemicals are of a very diverse nature. They are often processing
aids such as lubricants or warp size. Fabrics containing wool and cotton, however,
may contain substantial amounts of natural impurities that impede uniform
wetting. These include hydrophobic waxes that are difficult to emulsify. The
preparation of wool or cotton fabrics can involve much of a textile finishing mill’s
resources. Some of the processes that are used in the preparation of materials
made from natural fibres are discussed below.

Raw wool fibres may contain 30–50% of impurities including wool wax (often
called grease), and inorganic salts (suint), as well as varying amounts of dirt, straw,
excrement, and burrs. Before textile manufacturing begins, gentle scouring at 40–
50 °C with a weakly alkaline detergent solution removes the bulk of these
(Section 7.2.3). Mild alkaline scouring of wool fabric before dyeing eliminates
lubricating oil that was added to facilitate carding and spinning. Residues of
vegetable material are often apparent in grey wool fabrics. This cellulosic material
does not absorb the dyes used for colouring the wool and shows up as pale patches
after dyeing. Impregnating the wool with sulphuric acid solution and drying has no
effect on the wool but causes hydrolysis of all cellulosic materials. Beating the
dried fabric pulverises and removes the brittle cellulose hydrolysis products. The
process is called carbonising (Section 7.3.3). The acid remaining in the wool
serves for subsequent dyeing operations.