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.