DYES FOR PROTEIN FIBERS

Protein  fibers  are  the  most  readily  dyed  fibers due  to  the  numerous reactive   functional  groups   present. They  can be  dyed  with  a  wide range of dyes under acid, neutral, or  slightly  basic conditions. Since the  keratin fibers   are  less  crystalline  and    oriented than  secreted fibers such  as  silk, they  tend   to dye  more  rapidly   and  more  readily to deeper shades.  

Under  acid  conditions  amino  groups  in  the  protein  fibers  are protonated  to  form  NH; groups.  In  this   form,  they  are  able   to  attract   dyes containing    acid  anions   including  acid,  direct, mordant,  and  reactive    dyes. Special    premetallized   acid   dyes  of  sufficient  solubility are used   to  dye protein   fibers   to  fast  colors.  The  functional  groups available in  protein fibers   are  more  reactive  than  hydroxyl   groups  in  cellulosic   fibers.Reactive dyes of  more  1imited  reactivity  have  been   developed  especially  for protein fibers. Protein  fibers  complex  very  readily  with   multivalent metal cations.  Acid  dyes  and  mordant  dyes  may  be rendered  very  fast  by  mordanting  with  metal  salts,  and  chromium  salts  are  especially effective    as  mordants. At  neutral  or  sl ightly  acid  pH,  protein    fibers   may  be  dyed   with cationic  or  basic  dyes;  however,  the  fastness  of  the  dyed   fiber   is  poor without  mordanting  with tannic acid  or  other   mordants    for  cationic  dyes. Azoic and  vat  dyes  find  only  limited use  on  protein   fibers   due  to  the  damaging   effect   that   basic   solutions  of  these  dyes   have   on  protein  fibers. Sulfate  esters   of  reduced    vat  dyes can  be  used   effectively  on  protein fibers, however. Natural  dyes  from   many  sources  have good  affinity for protein    fibers  and   are  used  extensively  in  the crafts area and often in conjunction  with mordants.

DYES FOR VINYL FIBERS

The  vinyl  fibers, with   the  exception  of  vinal  and  vinyon-vinal  matrix fibers, are  extremely  hydrophobic and difficult  to  dye,  and   consequently they  can  be dyed  only  through  pigmentation  of  the  polymer  melt   before fiber formation  or  through  dyeing  with  disperse dyes. Vinal  and  vinyon-vinal matrix   fibers  dye  readily  with dyes   used  on  cellulosics  including direct, mordant, reactive, vat,  and  sulfur dyes.

DYE FOR POLYOLEFIN FIBERS

Polyolefin fibers are  hydrophobic,  and  the  molecular chains  within the fiber   are  tightly packed. Therefore  it  is extremely difficult to dye  polyolefin  fibers or  to  increase their affinity  to  dyes. Colored inorganic salts or  stable organometallic pigments have been  added  to  the  polymer melt prior  to  fiber   spinning  to color   the  fibers. Also,  nonvolatile  acids  or bases or materials such  as  polyethylene  oxides or metal  salts   have  been added to the polymer  prior  to fiber formation   to  increase the  affinity   of  the  fiber   for  disperse,   cationic,    acid,  or  mordant   dyes. Polyolefin   fibers can  be  chemically  grafted  with  appropriate  monomers    after   fiber   formation to  improve  their  dyeability.

DYES FOR POLYESTER FIBERS

Owing  to their high crystallinity  and  hydrophobicity,  the   polyester  fibers are  extremely    difficult to  dye  by  normal    dyeing   techniques    unless  the  fiber   has  been  modified,    as  in the  case   of  modified   terephthalate  polyesters. A  limited  amount   of  polyester    is  solution  dyed  through    incorporation   of  dye   or  pigment   into   the  polymer    melt   prior  to  spinning. It   is more   common   to  use   this   technique  to  incorporate  fluorescent  brightening agents   into  polyester. Only   smaller,   relatively  nonpolar dye  molecules can effectively   penetrate   polyester;    therefore  disperse dyes  have  been the  dye class   of choice  for  the  fiber. 

 Azoic  dyes  and  pigment-binder systems have  also  found  limited   use  on  polyesters. Polyester modified  with  appropriate  comonomers can  be dyed at lower temperatures  or  with  acid  or  basic dyes   depending  on  the  nature of  the  modifying groups.

DYES FOR POLYAMIDE FIBERS

With the  exception  of  the aramid fibers, the polyamides  dye  readily with   a  wide  variety    of  dyes.    Since   the  polyamides   contain   both   acid   carboxyl ic   and   basic  amino   end groups   and   have   a  reasonably  high   moisture  regain,   the  fibers   tend   to  dye  like   protein fibers such  as  wool   and  silk. Since  the   molecular    structure    is  somewhat   more   hydrophobic, more   regular, and more densely   packed   in  the  polyamides    than  in  protein   fibers, they also exhibit   to  some  degree  the dyeing   characteristics  of  other  synthetic fibers such  as   polyesters  and   acrylics. Due to  their highly regular  molecular structure and  dense chain  packing,  the  aramid   fibers   resemble   polyester    and are  dyed only by  small   dye   molecules    such   as  disperse    dyes. Polyamides such  as  nylon  6,  6,6,   and  Qiana   can  be   readily   dyed   with  dyes   containing anionic   groups,   such   as  acid,   metallized    acid,   mordant   dyes,   and   reactive dyes  and   with   dyes   containing    cationic  groups   such   as  basic   dyes. Acid dyes  on   nylon   can  be  mordanted    effectively     for  additional     fastness;    however,   the   colorfastness  of   basic   dyes   is   poorer and   more difficult  to stabilize   by  mordanting. Vat   and  azoic   dyes   can  be  applied    to  nylons   by modified   techniques,    and  polyamides can  be  readily  dyed   by  disperse   dyes  at temperatures  above   80°C.    Aramids  can   only  be   dyed  effectively     with   disperse   dyes  under   rigorous  dyeing  conditions. The   biconstituent   fiber   of nylon  and  polyester  can  be  effectively dyed   by several   dye   types  due  to  the nylon  component,    but  for  deep   dyeings  disperse   dyes   are  preferred.  Nylon  6 and  6,6  are  produced    in  modifications  that  are  light,   medium, or deep   dyeable  by  acid  dyes or  specially  dyeable  by  cationic   dyes.

DYE FOR ACRYLIC FIBERS

The nature and  distribution  of  acrylonitrile  and  co monomer   or  comonomers   in the  acrylic   fibers affect   the  overall  dye ability   and  the  classes   of  dyes   that  may  be  used  in  dyeing   these   fibers. Both  acrylic   and  mod acrylic fibers   can  be  dyed  using   disperse   dyes,   with  the  more   hydrophobic and  less crystalline    modacrylic    being   more dyeable  with  this   dye  class.    The   polar cyanide  groups   in  the  acrylonitrile  unit  of  these   fibers  have   some  affinity for   acid   dyes   and   particularly  mordanted  systems   containing   copper  or chromium  ions. Addition   of  an  acid  or  basic comonomer    such  as  acrylic   acid or  vinyl   pyridine  as  comonomer   imparts    improved   dye ability    with  basic  and acid   dyes,  respectively,    for  these   fibers. Vat  dyes  can  be  used  on  acrylic fibers   to  a limited   extent.

DYES FOR MINERAL AND METALLIC FIBERS

The mineral and metallic fibers are essentially undyeable, and special techniques must be used to impart color to the fibers. Thermally stable ceramic pigments can be added to molten glass prior to fiber formation, or pigment-binder systems may be appl ied to the surface of the mineral and metallic fibers. Glass fibers can also be sized with a protein which then can be insolubilized and dyed with conventional protein dyes. Glass fibers

are colored by coronizing, which involves preheating of the glass substrate to high temperatures to remove all organic materials followed by coloration with a pigment-binder system. The metallic fibers may also be colored through anodizing the metal (often aluminum) filament present or through pigmentation of the plastic layer coating the metal. The nature of the metal in the organometallic fibers determines their ultimate color.

DYES FOR CELLULOSE ESTER FIBERS

Acetate and triacetate fibers can  be effectively  dyed  using   disperse  dyes. The   rate   of  dyeing   is  more rapid  with   the more  hydrophobic triacetate  fibers   than  with   acetate. Under   special  conditions, azoic   and  vat dyes  may  be  used  to  dye   these  fibers. Acetate fibers  also  have   affinity for  selected   acid  and   direct   dyes.   Since  acetate   loses   its   luster  above 85°C,  dyeings   must  be  carried   out   at  or below   this  temperature. Addition of  pigments   or solventsoluble  dyes to the  acetate   or  triacetate    spinning "dope"   prior to  fiber  spinning leads   to colored   fibers   possessing   excellent colorfastness, although   the  colors  available are  limited.

DYES FOR ELASTOMERIC FIBERS

Since   the  elastomeric fibers   are  often   a   component    in  the   core  of blended   yarns,  coloration    is  not   important    in  all   applications. Rubber fibers  cannot  be  dyed  readily  and  are  colored  through  mixing of  pigments into the  rubber prior   to  extrusion    into  fibers. Spandex   fibers are more dyeable and can be dyed with acid, reactive, basic, or vat dyes. Anidex can be dyed with disperse or basic dyes.The nylon component of spandexnylon fibers can readily be dyed with acid, basic, disperse, or vat dyes.

COLOR THEORY

Color is defined a s the net response of an observer to visual physical

phenomena involving visible radiant energy of varying intensities over the

wavelength range 400 to 700 nanometers (nm). The net color seen by the

observer is dependent on integration of three factors:

 (l) the nature of the light source,

(2) the light absorption properties of the object observed, and

(3) the response of the eye to the 1i ght refl ected from the object.

 The relative intensities of the various wavelengths of visible

1ight observed by the eye are translated by the mind of the observer

resulting in the perception of color. In color measurement, the human eye

is replaced by a photocell which detects the light energy present at various visible wavelengths. Visible 1ight is a narrow band of electromagnetic radiation from 400 to 700 nm (1 nm equals 10-9 meters) detected by the human eye. Radiation falling below 400 nm is ultraviolet radiation, and that falling above 700 nm is infrared radiation; both are unseen by the human eye. If pure lightof a given wavelength is observed, it will have a color corresponding to that wavelength. Pure wavelengths of light are seen when white light is refracted by a prism into a "rainbow" spectrum of continuous color. Light sources such as sunlight, incandescent light, and fluorescent light are continuums of various wavelengths of light with the relative amounts of the various wavelengths of light being dependent on the overall intensity and type of light source. Sunlight at noon has very nearly the same intensityof each wavelength of 1ight throughout the visible spectrum, whereas at dusk sunlight is of lower intensity and has greater quantities of the longer, red wavelengths than of shorter, blue wavelengths. Fluorescent lights generally contain large amounts of shorter, blue wavelengths, while incandescent tungsten lights contain a large component of longer, red wavelengths compared to noon sunlight. Differences in intensity and wavelength distribution between light sources has a profound effect on the color observed for a dyed textile, since the textile can absorb and reflect only that 1ight available to it from the source. When a dyed fabric appears different in color or shade under two different light sources, the phenomenon is referred to as "flare." When two fabrics dyed with different dyes or dye combinations match under one light source but not under another, the

effect is called "metamerism." When 1ight from a source strikes a dyed textile surface, different portions of the light of the various wavelengths are absorbed by the dye, depending of the structure and light absorption characteristics of the dye. Light not absorbed by the dye on the textile is reflected from the surface as diffuse 1ight, and the observer sees the colors shown in Table 17-1. The color seen is a composite of all the wavelengths reflected from the fabric. If significant direct reflectance of light from the fabric occurs, the fabric exhibits a degree of a gloss. If little or no light throughout the visible range is absorbed by the fabric and the majority of 1ight is reflected, the fabric appears white. If the fabric absorbs all of the light striking it, the fabric is black. If uniform light absorption and reflectance across the visible wavelengths occurs at some intermediate level, the fabric will be a shade of grey.

Table 17-1. Colors After Absorption/Reflectance

Wavelength of Light Absorbed (nm)	Light Absorbed by Dyed Textile	Color Seen by the Observer
400-435	Violet	Yellow-green
435-480	Blue	Yellow
480-490	Green-blue	Orange
490-500	Blue-green	Red
500-560	Green	Purple
560-580	Yellow-green	Violet
580-595	Yellow	Blue
595-605	Orange	Green-blue
605-700	Red	Blue-green

The dye absorbs di screte packages or quanta of 1i ght, and the dye

molecule is excited to a higher energy state. This energy is normally

harmlessly dissipated through increased vibration within the dye molecule

as heat, and the dye is then ready to absorb another quantum of light. If

the dye cannot effectively dissipate this energy, the dye will undergo

chemical attack and color fading or color change will occur or the energy

will be transferred to the fiber causing chemical damage.

DYES FOR CELLULOSIC FIBERS

Cellulosic fibers   can   be  dyed  readily using a number of dye classes. The  less crystalline regenerated cellulosics (rayon)   dye  more readily than  the  more crystalline natural cellulosic fibers  such   as  cotton. Cellulosics can  be  dyed  or printed using  direct, reactive, basic, vat, sulfur, or  azoic  dyes.  Cellulosics can  also be colored using pigment binder systems.  Mordanting of  direct and  basic dyes  on  cellulosics  improves   overall fastness. Since the hydroxyl   groups   in  cellulosics     are  less  reactive    than  many   functional   groups   found  in  protein    fibers,   reactive   dyes   such  as  the   dichloro  triazine    (Procion   M)  or  dichloroquinoxaline    (Levefix)    dyes   are  often   used on  cellulosic    fibers  due  to  their  higher    reactivity.     The  general    stability of  cellulosic   fibers   to  basic  solutions   permits   them  to  be  effectively   dyed by  vat,  sulfur,   and  azoic  dyes  without   significant  damage.