Reactive Dyes on Wool and Silk

  Reactive Dyes on Wool and Silk. Reactive dyes produce brilliant shades on wool with good fastness. They differ from reactive dyes for cellulose fibers because the reactivity of the chemoactive groups in wool is considerably higher than that of the  hydroxyl  groups  in  cellulose.  To  achieve  level  dyeing  on  wool,  dyes with reduced reactivity must be used, and an auxiliary agent added. Among the reactive groups in wool the most important are amino, imino, and hydroxyl  groups.  Reactions  occur  in  a  weakly  acidic  medium  (pH  3±5)  and include nucleophilic substitution of leaving groups (usually Cl, F, and, rarely, sulfonate  or  ammonium  groups)  or  addition  reactions  to  polar  aliphatic  double bonds. Anchor  groups  such  as  N-methyltaurine  ethylsulfone  and  b-sulfatoethylsulfone have the advantage that their functional groups are masked at the start of the process; consequently, no premature reaction occurs below boiling temperature .  At  the  same  time,  level  dyeing  is  made  possible  by  the  increased solubility. Other dyes are bifunctional and can have a cross-linking effect on wool .
   In the acidic range, ionic bonds are formed between dye and fiber, and the dye is still capable of migration. At pH 5, covalent binding to the fiber predominates. In the dyeing process for wool, a nonionic or weakly cationic leveling agent is added to the liquor, and the pH adjusted to 3±4 with formic or acetic acid. The process is started at 40 DC, and after some time the pH is adjusted to 5±6 with sodiumdihydrogenphosphate. Dyeing is conducted at boiling temperature for 1 h.
   To eliminate hydrolyzed dye, an aftertreatment is performed at 80 DC with ammonia (pH 8.5±9.0). The last rinsing bath is weakly acidified. On silk, reactive dyes are only applied when brilliant shades are needed and when the colorfastness produced with acid dyes does not meet the required level . Dyeing is performed on silk with the addition of sodium sulfate. The system is heated up to 50±70 DC, and soda is added. Dyeing is completed at 70DC. An afterwash at 80 DC increases wetfastness.