MORDANT DYE

Mordant  dyes  are acid  dyes that  have special sites other

than acid salt  anion  groups   that  can  react  with  a  metal  salt  mordant.  Mordant   dyes  are "tailor-made"    to   chelate   with  metal   ions   to  form  a  strong

organometallic complex of  limited  solubility  and   greater colorfastness .

The  fiber   may  be  dyed   initially and  then   mordanted  (postmordanting),  dyed

and   mordanted  simultaneously (comordanting), or  mordanted  and   then  dyed

(premordanting) .    Of  the three  methods, postmordanting  is preferred. Salts

of  chromium,   aluminum,   copper,  iron, tin,  and  cobalt  are commonly used  as

mordants. Since  the  mordant   affects  the  electron  distribution  and  density

within  the  dye,  the  color of the  dyed  fabric  tends  to change.

DIRECT DYES

Direct or substantive  dyes are a special class  of dyes which

penetrate cellulosic  fibers readily and have good affinity  for these fibers

due to their size  and shape.  Whereas acid dyes are  large and bulky, direct

dyes are  long, narrow, and  flat in molecular  structure, which  allows them

to readily enter  the cellulose structure and interact  with the cellulose in

such a  way as  to provide good  fiber affinity.   Direct dyes often  contain

one or more  azo groups connecting  aromatic chromophores,  thereby providing

a straight  chain dye molecule.   Since charge development  is not  a primary

consideration  in diffusion  of direct  dyes onto cellulosics,  the dyes  are

usually applied  from basic  solutions in which  cellulosics are more  stable

and more likely to  swell.

Direct   dye s  fall   into   three   main   categories.     Class   A  direct    dyes

level  well   even  in  the  presence   of  high  salt  concentrations,     while   Class  B

direct   dyes  have   poor  leveling    properties   which   can  be  improved   by  proper

adjustment    of  sal t  concentration.      Class  C  dyes   have  poor   level ing   properties,   but  exhaust   well   on  raising   the  temperature    of  the  dyebath   in  the

absence   of   salt.   The   direct   dyes  are   reasonably    colorfast   on  cellulosic

fabrics,   but  fastness    can  be  improved   by mordanting    with  metal   salts,   cationic   fixing   agents,    or  formaldehyde    or   through   reaction    with   diazonium

compounds    or  diazotization     of  the  dye   and  reaction    with  a  coupl ing   com-

pound.

VAT DYE

Vat dyes are usually water-insoluble dyes that can be chemically reduced in the presence of base to form a water-soluble and colorless leuco form of the dye, which is then appl ied to the fiber. Vat dyes can be readily applied to cellulosic fibers and most synthetic fibers, but care must be taken in applying the dyes to protein fibers due to the high basicity of the leuco dye solution which can damage protein fibers.

The dyes are usually indigoids (such as indigo) or anthraquinone derivatives and applied at low (30 °_60 °) temperatures. After application of the leuco form of the vat dye, the dye is reoxidized on the fabric by oxygen in the air or through treatment of the dyed fabric with a mild oxidizing agent. The vat dyes are reasonably colorfast if poorly held surface dye has been removed.

Stable sulfate esters of reduced leuco forms of vat dyes are available which do not require prior chemical reduction before application.

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