Dyeing Process | Process of Dyeing | The Chemistry of the Dyeing Process

The dyeing of a textile fiber is carried out in a solution, generally aqueous, known as the dye liquor or dye bath. For true dyeing to have taken place, coloration of fabric and absorption are important determinants. 

Coloration: 

The coloration must be relatively permanent: that is not readily removed by rinsing in water or by normal washing procedures. Moreover, the dyeing must not fade rapidly on exposure to light. 

Absorption: 

The process of attachment of the dye molecule to the fiber is one of absorption: that is the dye molecules concentrate on the fiber surface. There are four kinds of forces by which dye molecules are bound to the fiber: 

1) Ionic forces 2) Hydrogen bonding 3) Vander Wals’ forces and 4) Covalent chemical linkages

Dyeing of Wool: 

In the dyeing of wool which is a complex protein containing about 20 different amino acids, the sulfuric acid added to the dye bath forms ionic linkages with the amino groups of the protein. In the process of dyeing, the sulfate anion (negative ion) is replaced by a dye anion. In the dyeing of wool, silk and synthetic fibers, hydrogen bonds are probably set up between the azo, amino, alkyl amino and other groups and the amino Co-NH-groups. Covalent chemical links are brought about in the dye-bath by chemical reaction between a fiber-reactive dye molecule, one containing a chemically reactive center and a hydro-oxy group of a cotton fiber, in the presence of alkali. 

The Chemistry of the Dyeing Process : 

Exhaustion In any dyeing process, whatever the chemical class of dye being used, heat must be supplied to the dye bath; energy is used in transferring dye molecules from the solution to the fiber as well as in swelling the fiber to render it more receptive. The technical term for this process is exhaustion. Levelness: An Important Quality 

Evenness of dyeing, known as levelness is an important quality in the dyeing of all forms of natural and synthetic fibers. It may be attained by the control of dyeing conditions viz. 

• By agitation to ensure proper contact between dye liquor and substance being dyed and by use of restraining agents to control rate of dyeing or strike. Solvent Dyeing Serious consideration has recently been given to the methods of dyeing in which water as the medium is replaced by solvents such as the chlorinated hydrocarbons used in dry cleaning. The technological advantages in solvent dyeing are: 1. Rapid wetting of textiles 
• Less swelling 
• Increased speed of dyeing per given amount of material 
• Savings in energy, as less heat is required to heat or evaporate per-chloro-ethylene. Thus it eliminates the effluent (pollution) problems associated with the conventional methods of dyeing and finishing.