Saminathan Ratnapandian

Temperature
adsorption and subsequent diffusion most exhaust dyeing processes follow a temperature gradient. Accordingly, dyeing commences at ambient temperature. The temperature is increased gradually to the required value depending on the dye used and held there for the required time period, as shown in Figure 1.4. Based on the dye- material combination, ionic or covalent fixation of dyes may be facilitated by altering the dye bath pH during diffusion [12, 14].

Figure 1.4 Exhaust dyeing line diagram

A traditional practice followed to this day is that exhaust dyeing recipes give the amount of auxiliaries and dyes as a percentage of the mass of the material being dyed. The auxiliaries are added first to the dye bath and allowed to mix thoroughly to obtain uniform concentration in the fibre. This is followed by addition of the dyes, which are allowed to mix completely before increasing the dye bath temperature [14]. These measures are necessary to achieve uniform and level dyeing. In addition to level dyeing, the other desirable aims of dyeing are short dyeing times, shade reproducibility and maximum exhaustion so that a minimum of dye is discharged in the effluent.
Several authors [11, 12, 50] consider that the rate of dye uptake (exhaustion) or dyeing rate depends on:

1. 
   the chemical reactivity of the dye for the fibre
   
2. 
   physical properties of the dye
   
3. 
   the type and construction of the fabric being dyed
   
4. 
   dye concentration
   
5. 
   liquor ratio
   
6. 
   temperature
   
7. 
   auxiliaries used; and
   
8. 
   interchange of dye between liquor and fibre (fabric).
   

The process of dyeing is recognised as complete on reaching equilibrium, wherein there is no significant change in the final concentration of dye in the bath or on the fabric.
Samples are now drawn for comparison against standards and corrections made as required.

In order to achieve short dyeing cycles and thereby higher production, most modern dyeing equipment is enclosed ensuring uniform dye bath temperature with minimum variations. Pressurisation of the equipment allows dyeing at elevated temperatures of up to 130^○C. Circulating liquor and circulating goods (materials) are the two classes of exhaust dyeing machines [12].