NATURAL DYE

Natural  dyes  and  pigments  are derived   from  mineral,  animal, or  plant  sources  and  generally  are complex mixtures  of materials. A number of  natural   dye s can  be classified   as  acid  or  vat  dyes  and  can  be readily mordanted. In general, the  natural dyes  give  more  muted  tones than synthetic dyes  and  are useful   only on natural  or  regenerated   fibers.

PIGMENTS

Pigments as a class  are colored  materials  that  are  insoluble    in their  medium  of  application. As a  result,  they cannot penetrate or become readily   fixed to a  fiber  and  must  be  "locked"  onto  the  fiber  surface by use of a  polymeric adhesive binder that  encapsulates  and  holds   the pigment  onto the  fibers. Pigments  include inorganic salts,  insoluble azo  or  vat  dyes, toners, lakes, metallic  complexes,  and  organ metallic  complexes. Some binders are  preformed  water-insoluble  polymers   applied  from  solvents or  as emulsions,   whereas  others are  water-soluble or emulsifiable polymers which can  be chemically crosslinked  and  insolubilized  after  application  by  drying and  heating. The  pigment binder systems   tend  to  stiffen textiles,  and  have moderate   to  poor  fastness,  since  they  are  surface  treatments.

DYE AND DYE CLASSIFICATION

Dyes as  colored  unsaturated organic molecules must  have affinity for fibers  to  be  effectively applied. The   dye s on  fibers  are  physically bound to the  fiber by one or more  physical  forces  including hydrogen   bonding, vander  Waals, or  ionic forces   and   in certain cases chemically bound  by covalent  bonds. Dyes  may  be  classified  in a number of  ways,   including color,  intended use,  trade  name, chemical constitution, and  basis   of  application. Of  these classification  methods, chemical constitution  and  basis of  application  have been  mo st  widely used. Chemical constitution  indicates the  major  chromophoreS  present  in the dye but  does not indicate more  than  such structural aspects of  the  dye. A classification scheme for dyes   has  been  developed and  evolved  for  use by dyers which  is  based on the method  of application and to a lesser degree  on  the  chemical constitution  of  the  dye  class. The classification  scheme  and  major dye classes are outline below.

Dyes Containing Anionic Functional Groups	Dyes  Requiring    Chemical Reaction   before   Application
Acid  dyes	Vat  dyes
Direct   dyes	Azoic   dyes
Direct   dyes	Sulfur  dyes
Mordant   dyes
Reactive    dyes

                                                  

Dyes Containing Cationic Groups	Special  Colorant  Classes
Basic  dyes	Disperse    dyes
	Solvent   dyes
	Pigments
	Natural    dyes

      Dyes  classified  by this scheme  are assigned  standard  designations according   to  dye  class,   color, and  overall constitution   by  the  Society of

Dyers and  Colorists in the Color Index   (e.g"  Acid  Blue 141, Vat Green 17,

Disperse   Red  17).

DISPERSE DYE<>

Disperse dyes were   formulated and  introduced  to  permit  dyeing  of   hydrophobic  thermoplastic fibers   including  acetate,  triacetate, nylon,   polyester, acrylic, and other synthetics. The  disperse  dyes are small  polar  molecules,   usually  containing   anthraquinone  or  azo  groups, which  do not have  charged  cationic or anionic groups  within  the structure. The disperse  dyes  are sparingly  soluble in water and must be dispersed  with aid  of a surfactant  in the dyebath.   As  the small  amount of  dissolved disperse  dye diffuses  into the  fiber, additional  dye  dispersed  in solution  is dissolved, until  the disperse  dye  is nearly  completely  exhausted  onto  the fiber.  A  special class of  reactive disperse  dyes  has been  introduced that can  react  with  fibers  1ike  acetate  and  nylon  after  diffusion   into the fiber.   The  light-  and washfastness of  these  dyes is  generally  good, but difficulty  has  been encountered  with  fume fading with  certain of  the disperse  dyes.   Many disperse  dyes  have  appreciable vapor pressures  at elevated  temperatures  and can  be "dyed" onto thermoplastic  fibers  by subl imation, which  involves  diffusion of  the dye vapors  into the fiber.

AZOIC DYE

Azoic or naphthol  dyes are formed in situ on the fabric through  a coupling reaction of  an aromatic alcohol or amine such as naphthol (the couplingcomponent) with a diazonium salt (the diazo component). Thedye formed contains an azo group:

 

The fabric is usually impregnated first with  the aromatic coupling component followed by immersion of the fabric in a solution  containing the diazo component,with the azoic  dye being formed  instantaneously. Owing  to the instability of  the components, the dyeings are carried out near room temperature. The dyes have moderate fastness, and dyeings may be mordanted to

increase fastness.

SULFUR DYE

Sulfur dyes  are inexpensive complex reaction mixtures of

selected  aromatic compounds with sodium polysulfide.The  sulfur dyes are

chemically reduced  in the presence of  base prior  to appl ication to  the

fiber,  and  are reoxidized after  dyeing  on the  fiber  by  oxygen in the  air  or

by application of  a mild oxidizing agent such as hydrogen  peroxide.

The sulfur dyes are similar in application and fiber affinity to vat dyes,

and also are available in a solubilized  leuco form:

Sulfur  dyes are inexpensive, and have adequate colorfastness properties for

most applications.

BASIC DYE

Basic or cationic dyes are colored cationic  salts  of  amine   derivatives. Basic dye  cations will  migrate toward negative charges  inside   the

fiber. The  dyes  may  be  applied to cellulosic,    protein,   nylon,  acrylic, and

specially  modified  synthetic  fibers. Although the dyes  generally  are  of

striking  brilliance  and  intensity,  the colorfastness    of  the  dyes  on cellulosic,   protein, and  nylon   fibers  is  generally poor. Colorfastness can  be

improved through   mordanting with tannins or  other  complexing  agents. Since

the  insoluble parent  amine is  regenerated  from basic dyes  at  basic  pH,  the

basic  dyes  are applied   from  mildly  acid   or neutral   solutions. The  dyes  are

also  important in "one-shot  dyeing" of paper products.

REACTIVE DYE

Reactive   dye s are dye s which   usually  have  the  basic  struc-

ture  of acid,  direct ,  or  mordant  dyes but  which  in  addition  have  a reactive

group  capable  of  covalent  bond  formation  with   the fiber. Since  the  fiber

must  have  reasonable  reactivity   toward  the  dye  reactive group,  application

of  these  dyes has  been  limited  to  cellulosic ,  protein,  and  nylon  fibers  for

the  mos t pa rt.

The  fastness  of  reactive dyes  covalently   bound   to  the  fiber  is  excellent . Reactive   functional groups  have  been   selected   for  incorporation  into  reactive  dyes  which   will  react  readily   with  the  fiber   after  diffusion   into  the

structure  but  which   will  not   hydrolyze  (decompose)   in  the   water  solvent

used   in  dye  application.  Acidic   or  basic  conditions  are   necessary  for

successful  and  rapid   reaction   of  the  reactive dye  with   the  fiber,   so  dye

appl ication   is  carried   out  at  either   sl ightly  acid   or  basic  pH  (hydrogenion  concentration). Procion  dyes  are  the  best  known  of  the  reactive   dyes.

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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