**1.1 Dyeing auxiliaries**

Auxiliaries are compounds which are not an integral part of the dyeing process but by adding them to the dye bath the dyeing can be improved. The main functions of auxiliaries are to prepare or improve the substrate in readiness for colorants (by wetting, providing sorption sites, improving or resisting the migration of dyes), to stabilize the application media (by improving dye solubility, stabilizing a dispersion or solution), to protect or modify the substrate (by creating or resisting dyeability, protecting against the effects of temperature and other processing conditions), to improve the dyes fastness (by various after-treatments) and to enhance the properties of laundering formulation. The main ranges of dyeing auxiliaries are: crease inhibitors, wetting agents, defoamers, diffusion accelerants, carriers, dispersing agents, dye-protective agents, fibre-protective agents, fixing agents, levelling agents, migration-inhibiting agents, pH regulators, buffers, sequestering agents, UV absorbers, fibre stabilizers, UV protective agents and wash-off agents.

#### **1.2 Levelling agents**

For all dye-fibre systems, level dyeing problems can be divided into either *gross unlevelness throughout the substrate* which is related to the dyeing process or *localized unlevelness* which is related to non-uniformity of the substrate (Burkinshaw, 1995). The receptivity of different regions of a fibre or of different fibres in a mixed yarn may not be the same for a given dyestuff, thus causing uneven dyeing. These undesired effects can be eliminated or diminished by the use of levelling agents. Levelling agents usually contain functional groups which are similar to those by which the dyestuffs are attached to the fibre. There are two mechanisms involved in the activities of levelling agents: there is a competition for sites on the fibre between dyestaff and levelling agent; or the agents can slow down the migration of dye by forming complex micelles with the dyestuff molecules which are released slowly to the fibre. A very large number of levelling agents have been developed in attempts to get the right balance of properties for particular types of dyestuff and fibres, cyclodextrins could be one of the most promising.
