Introduction of Reactive Dyes

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Introduction of Reactive Dyes
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Definition of reactive dyes

• Reactive dyes are a new class of dyes. Molecules
  of reactive dyes contain groups that are capable
  of chemically reacting with the fibers. The
  reactive dye reacts with the fiber to form a
  covalent bond, which improves the washing
  durability and rubbing fastness of the dyed
  fiber. The reactive dye molecules contain
  chemically active groups, which can react with
  cotton, wool and other fibers in aqueous solution
  to form a co-bonded dye with high wash fastness.
  The reactive dye molecules include the parent dye
  and the active group, and the groups that can
  react with the fiber are referred to as reactive
  groups.

3
Classification of reactive dyes

• According to their different active groups,
  reactive dyes can be mainly divided into the
  following two categories
• Symmetric triazole type
• Vinyl sulfone type

4
Symmetric triazole type

• In such reactive dyes, the reactive chlorine
  atoms are more chemically active. The chlorine
  atoms are replaced by cellulose fibers in an
  alkaline medium during dyeing. The reaction
  between the dye and the cellulose fibers is a
  bimolecular nucleophilic substitution reaction.

5
Vinyl sulfone type

• The reactive group of such a reactive dye is a
  sulfate of a vinylsulfone group (D-SO2CHCH2) or
  a ß-hydroxyethylsulfonyl group.
  ß-hydroxyethylsulfonyl sulfate is eliminated in
  an alkaline medium to form a vinylsulfone group
  during dyeing, which is then combined with
  cellulose fibers to form a covalent bond by
  nucleophilic addition reaction.

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• The above two types of reactive dyes are
  currently the most productive reactive dyes in
  the world. In order to increase the fixing rate
  of reactive dyes, two reactive groups have been
  introduced into dye molecules in recent years. In
  addition to the reactive dyes used for dyeing
  cellulose fibers, other reactive dyes for dyeing
  protein fibers such as silk and wool have also
  been developed.

7
Dyeing processes

• The most commonly used dyeing method for reactive
  dyes is dip dyeing. The dip dyeing methods can be
  divided into two categories, including one-bath
  one-step method, one-bath two-step method. The
  one-bath one-step method is to perform dyeing in
  an alkaline bath, that is, to fix the color while
  dyeing. This method is simple in process, short
  in dyeing time, and convenient to operate.
  However, since the adsorption and fixation are
  carried out at the same time, the dye can no
  longer be diffused after fixing, and thus its
  leveling property and the translucency are poor.
  One-bath two-step method means dyeing fiber in a
  neutral bath, then adding an alkali agent in the
  dye bath to adjust the pH to the specified pH
  value of the fixing color (generally 11). At that
  time, the dye and the fiber form a covalent bond
  to obtain a fixed color. The one-bath two-step
  method is a reasonable dyeing method in the dip
  dyeing methods of reactive dyes, and it can not
  only obtain a higher dyeing rate and fixing rate.
  Moreover, it has a good leveling effect, so this
  method is commonly used for dyeing cotton knits.