A Practical Dyer's Guide to Reactive Dyeing of Cotton - Part-1

Introduction:
Reactive dyes are probably the most popular class of dyes to produce 'fast dyestings' on piece goods. These were first introduced a little over 40 years based on a principle which has not been used before. These dyes react with fibre forming a direct chemical linkage which os not easily broken.
Their low cost, ease of application, the bright shades produced by them coupled with good wash fastness make them very popular with piece good dyers. Even in thereads these classes are gaining in popularity for cotton sewings.

Chemistry of reactive dyes:
Reactive dyes differ from other colouring matters in that they enter in to chemical reaction with the fibre during the dyeing process and so become a part of the fibre substances.
A reactive dye may be represented as
R - B - X
where R - Chromogen
B - Bridging group
X - Reactive system
When this reacts with the fibre, F, it forms
R - B - X - F
The wet fastness of the dyesings produced depends on the stability of the true covalent bond X-F.

Reactive Systems:
Some of the popupar reactive systems is use today:
Reactive dyes are based on Cyanuryl chloride. The cold brand dyes (M brand) are based on dichloro triazinyl derivatives whereas the "H" brands are monochloro triazinyle derivatives.
The reactivity of the chlorine atoms decreases greatly as they are successively substituted. Thus the dichloride derivative (M) is more reactive than the mono chloro reactive (H) dyes. This is shown by the fact that "M" dyes will react readily with cellulose at room temperatire in the presence of mild alkalies such as sodium carbonate, whwere as "H" dyes need to be heated at least to 60°C and require more strongly alkalines before reaction will take place at a reasonable rate.
The other popular systems are based on Vinyl suplhones (Remazols) and trichloro pyrimidyl. The Remazols are very popular for discharge printing and can given excellant white discharge from a dark base.
Reference: Textile Processing Guide