Page 163 - Basic Principles of Textile Coloration

152 AUXILIARY CHEMICALS FOR WET PROCESSING AND DYEING

CHAPTER 9

Auxiliary chemicals for wet processing and
dyeing

Besides fibres, water and dyes, and specific chemicals for preparation and finishing
processes, the textile industry consumes vast quantities of auxiliary chemicals,
other than simple acids, bases and salts. These auxiliary products include many
kinds of complex organic chemicals used as detergents, wetting agents, dyeing
assistants, dispersing and emulsifying agents, dyeing aftertreatment chemicals,
lubricants and softeners. They serve to improve the wet processes used in textile
finishing. Their consumption in the textile industry exceeds that of dyestuffs and
many are used in the manufacture of other types of products such as paper, paints
and household detergents.

Unfortunately, like dyestuffs, the brand names and identifying codes of these
auxiliary products offer little or no information about their composition.
Commercial auxiliary products are frequently mixtures, often containing much
water. Any given product sold for a specific purpose can have multiple functions
such as wetting, detergent, dispersing and dyeing control capability. For these
reasons, the role of auxiliary chemicals in textile wet processing and dyeing is
confusing. A chapter on this subject, however, is essential for understanding
modern dyeing processes.

Many of these auxiliary chemicals are surfactants; they are chemicals exhibiting
surface activity. This means that their molecules tend to accumulate at the surface
separating two immiscible materials such as water and a fibre, or water and oil.
The concentration of the surfactant at the interface may be more than 500 times
greater than in the bulk of the aqueous solution. Surfactant molecules have two
key structural features. One part of the molecule is a hydrophobic ‘tail’, such as a
long alkyl chain (C15H31–); the other part is a hydrophilic polar or ionic group, or
‘head’, such as a carboxylate ion (–CO2–). Substances such as C15H31CO2– Na+
are slightly soluble in water, strongly adsorbed at phase boundaries, and have a
pronounced tendency to aggregate in solution. The aim of this chapter is to
introduce these surface-active chemicals and to examine how they function in
processes such as scouring and dyeing.

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