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Cationic dyes are special dyes for acrylonitrile fibres, and the dyeing method is near-boiling dyeing in a weakly acidic (pH around 4.5) bathtub with acetic acid and sodium acetate as buffers, in the presence of dielectrics and appropriate amounts of auxiliaries. The dyeing mechanism can be divided into three steps:
(1) Dye cations in the dye bath are adsorbed on the fibre surface.
(2) The dye cations diffuse from the fibre surface to the interior.
(3) The dye cations are salt-bonded to the anionic groups contained in the fibres.
Under normal dyeing conditions, the salt formation in the last step is an irreversible reaction, which can lead to high fastness and uneven dyeing. The first and last steps react quickly, the second step diffuses slowly, and it determines the whole dyeing speed. Due to the high affinity between dyestuff and fibre, in order to get even dyeing effect, the temperature of dye bath, pH value, electrolyte, retarding agent and so on must be strictly controlled.
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