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Colorants

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					                   Colorants — impart color
• Pigments -- insoluble colorants
   – usually inorganic compounds
                                         Malachite and azurite — copper Cu
                                         Vermilion — mercury Hg
                                         Prussian Blue, yellow ochre, red ochre,
                                          magnitite — iron Fe
     Transition Metals                   Cobalt blue, cerulean blue — cobalt Co
                                         Chrome oxide green, chrome yellow —
                                          chromium Cr

 Ideal Properties of Pigments
 •   Lightfast — resists fading
 •   Chemically inert — resists oxidation
 •   Insoluble as possible — resists bleeding
 •   Good brightening or coloring power
 •   Uniform particles which can easily be dispersed in a binder
 •   High opacity or hiding power
           Colorants — impart color
• Pigments -- insoluble colorants
   – usually inorganic compounds

                                    Transition Metals
• Dyestuffs -- soluble colorants
   – usually organic compounds
         Some common organic dyes


                      NH2 OH                                       OH   NH2

         NaO3S                 N   N                     N   N                  SO3Na

                                                                                        O2 N                   N        N           NH 2
                                   CH3O             OCH3

                      SO3Na                                             SO3Na
                                   Chicago Sky Blue 6B                                                   Disperse Orange 3
                                   (Direct Blue 1)                                                    4-(4-nitrophenylazo)aniline




                      N CH3                                                        O
                                                                                   C OCH 2CH3
                                                             CH3                        CH3
   -
       O 3S
                                                                                                Cl-
                 OH
                      C                             CH3CH2 NH               O           NHCH2CH3

                                       CH3                          Rhodamine 6G
                                   N                                (Basic Red 1)
                                   CH3
                                                                                                                    O
Na+ -O3S                                                                                                                        H
                                                                                                                                N
              Lissamine Green B
              (Acid Green 50)                                           N
                                                                                                                N
                                                                                                                H
                                                     N N                                                       indigo       O



                                   Na O3S
                                                     methyl orange
            Colorants — impart color
• Pigments -- insoluble colorants
   – usually inorganic compounds

                                    Transition Metals
• Dyestuffs -- soluble colorants
   – usually organic compounds

• Lakes -- soluble dye attached to inert support
   – Used as pigments

• Fillers -- inexpensive insoluble materials
   – Can be organic or inorganic compounds
    Brief History of Synthetic Colorants

• After the Industrial Revolution…
  – Synthetic pigments — 1704 ―Prussian blue‖
     • Followed rapidly by…
        –   Synthetic ultramarine
        –   Cobalt blue
        –   Cadmium reds and yellows
        –   Chrome oxide green (Army green)
  – Synthetic dyes — 1856 — ―Mauve of Mauvine‖
     • Followed by…
        – Synthetic Indigo
        – Synthetic azo dyes in every color imaginable
                    Artists’ Palettes Through the Ages
                   Red        Yellow      Green           Blue            White          Black

Cave painting    Ochre       Ochre                                    Chalk (CaCO3)   Carbon
10,000 BC        Fe2O3       Fe2O3          —              —          Gypsum          (lampblack)
                                                                      (CaSO4)


Egypt            Red lead    Orpiment   Malachite   Egyptian Blue     Chalk (CaCO3)   Carbon
1300 BC          Pb3O4       As2S3      CuCO3.      CaCuSi4O10        Gypsum          (lampblack)
                                        Cu(OH)2                       (CaSO4)


Pompeii          Vermilion   Orpiment   Malachite   Egyptian Blue     Lime            Carbon
AD 79            HgS         As2S3      CuCO3.      CaCuSi4O10        Ca(OH)2         (lampblack)
                                        Cu(OH)2


Renaissance      Vermilion   Lead/Tin   Green       Ultramarine       Lead White      Carbon
1400 AD          HgS         Yellow     Earth       Complex           PbCO3.Pb(OH)2   (lampblack)
                                        Complex     aluminosilicate
                                        iron        Azurite
                                        silicate    2CuCO3.Cu(OH)2


Impressionists   Vermilion   Chrome     Viridian    Cobalt Blue       Lead White      Carbon
1860 AD          HgS         yellow     Cr2O3       CoO.Al2O3         PbCO3.Pb(OH)2   (lampblack)
                             PbCrO4
                                Dyes
•   Colorant which is homogeneously dispersed in the dye medium
•   Usually soluble
•   Naturally occurring or synthetic organics
•   Categorized by...
    1. Method of Application
    2. Chemical Structure

                   Ideal Properties of Dyes
•   Lightfast — resist fading
•   Chemically inert
•   Good brightening power
•   Soluble as possible in order to apply
•   Colorfast — resists washing out of fibers
                         Uses of Dyes

• Coloring fibers and cloth

• Color photography

• Electronics — dye lasers, solar cells, display panels

• Biological and biochemical stains

• Foods and cosmetic

• Lakes used as pigments in modern paints

• Ink jet printer inks
              Sources of Natural Dyes – BUGS
                                          •   Tyrian Purple or ―Royal Blue‖
                                          •   9000 snails to obtain 1 g of dye
                                          •   Used primarily before 8th century
                                              A.D. to dye wool and silk
                                          •   Chemically it is 6,6’-dibromoindigo




•   Kermes — the most ancient dye in
    Europe
    70,000 female oak beetles produce 1
    pound dye
•   Cochineal — Mexico and Central
    America
    Mexican cactus beetle
Kermes, Cochineal — Carmine




   Chemically similar structure, light sensitive
     Dyed wool and silk — “carmine red”
            Sources of Natural Dyes – PLANTS

•   Indigo — used since 2000 B.C.
    Extracted from Indigofera tinctoria
    ―Navy Blue‖ of English sailors
    Blue jeans
•   Insoluble in water
     – Must be chemical reduced to
        soluble leucoindigo to use as dye

                                  O

                                              •   Woad (poor man’s indigo)
                                          H
                                          N


                                                   – Member of the mustard family
                              N
                                                   – Common weed in temperate
                              H
                                      O
                                                     climates
                                                   – Leaves contain same chemical
                                                     as indigo but in lower amounts
                                                   – Celtic war paint and tattoos
                                                                Braveheart
                                                   – Blue robes of priests
         Sources of Natural Dyes – PLANTS


                   •   Madder — ―Turkey Red‖
                        – Root of madder plant found in
                          Europe and Asia
                        – Prepared as a ―lake‖ with Al(OH)3
                        – British ―Redcoats‖
                        – Alexander the Great used it to
                          simulate blood


•   Alizarin
•   Synthetic alizarin prepared in 1875
                                                    O     OH

                                                               OH




                                                    O
     Color Centers in Organic Dyes

• CHROMOPHORES — “color bearer‖ structural
  part of colored molecule which is responsible for
  its color
   – Conjugated double bond systems
   – Aromatic rings
   – Azo groups —N=N—
   – Carbonyl groups      —C=O
   – Quinoid rings    O




                     O
          AUXOCHROMES
• ―color augmenting‖ groups
• functional group with non-bonded
  electrons which acts to strengthen or
  deepen the color and hue

      —NH2      —OH       —OR
                              CARBONYL Dyes
•      Anthroquinone Dyes                                                   O

        – Contain several —C =O
          groups
        – Multiple aromatic rings
•     Originate from plants & animals                                       O

        – Coloring agents in flowers, fruits, vegetables, bugs, etc.


                                                               HO


                                                          HO
       O       OH                     O       OH                      O             O         OH

                    OH                             CO2H                                            CO2H
                                                          HO

                                                                    OH
                         HO                        OH                 HO                           OH

       O                      CO2H    O       OH                           CO2H     O         OH
    Alizarin                         Kermes                                       Cochineal
               Mauveine — first synthetic dye
                         N



         H2N             N             N
                                       H




William Henry Perkin (18 years old), 1856
- Trying to make the antimalarial, quinine                    N
                                                    HO
- patent  dye company
                                             H3CO


                                                          N
                                                    Quinine
                                            AZO Dyes
        First prepared in 1863
        Have widest range of colors of all dyes                    R—N = N—R
        Contain the AZO Chromophore                                 Azo group
        Generally lightfast
        Brilliant colors ranging from reds to blues
                                                              HO



                                                          N

                                    NaO3S             N


                               HO                                                     HO
                                                Orange II
                 OCH3

                          N                                                       N

NaO3S                 N                                   NaO3S               N



             FD&C Red No. 40                                         FD&C Red No. 6
             Allura Red                SO3Na                         Sunset Yellow         SO3Na
        Fiber-Reactive Dye Structure
 • Chromophore
 • Reactive group
 • Leaving group

               SO 3-Na +      OH

                                                          Cl
CH 3O               N=N
                                                   N
                                                         N
                   Na+O 3S                  N
                                                   N
                                            CH 3
                                                         OCH 3


Procion Scarlet H-R, Cibacron Scarlet RP, Chlorine Reactive Red 3
      Methods of Application
• Dye and Substrate can interact through...
    1. ionic forces (+ and — charges)
    2. hydrogen bonding
    3. dispersion forces
    4. covalent bonds

            Dye         Substrate
Types of Dyes by Application
 •   Acid or Basic Dyes (ionic forces)
 •   Mordant Dyes (ionic)
 •   Direct Dyes (hydrogen bonding)
 •   Vat Dyes
 •   Fiber-reactive Dyes (covalent)
                         Direct Dyes
• Polar dye        Polar Fibers
• Dye applied from a hot water solution

                 H
      DYE        N                   HO
                         Hydrogen Bond
   Polar group       H                    Polar group


• Cotton and cellulose-based fibers
• Synthetic dyes
                      Acidic or Basic Dyes
   • Acidic dye (-)                (+) Basic fiber
   • Basic dye (+)                  (-) Acidic fiber


                DYE         SO3-             NH3+

          Sulfonic acid group             Basic amine group

                DYE        NH3+               -OOC


          Basic amine group             Carboxylic acid group

• Wool, silk, nylon and leather have basic amine groups and acidic carboxylic
  acid groups
              Fiber-Reactive Dyes
• Covalently bonded to fibers
     R—C—O—fiber
     R—C—S—fiber
     R—C—NH—fiber
• Developed in 1950’s
• Used primarily on cellulosic and protein fibers
        Fiber-Reactive Dye Structure
 • Chromophore
 • Reactive group
 • Leaving group

               SO 3-Na +      OH

                                                          Cl
CH 3O               N=N
                                                   N
                                                         N
                   Na+O 3S                  N
                                                   N
                                            CH 3
                                                         OCH 3


Procion Scarlet H-R, Cibacron Scarlet RP, Chlorine Reactive Red 3
                             Vat Dyes
• Usually not soluble in water
• Must be converted to a soluble form to be used as a dye

Indigo: the Classic Vat Dye
• Indigo is insoluble in water
   – Must be chemical reduced to leucoindigo

           O           H                          O- Na+       H
                       N      NaOH/ Na 2 S 2 O4                N

               C   C                                  C    C

           N                                      N
                                Oxidation
                       O
           H                    by Air (O 2 )     H            O- Na+




• Leucoindigo is soluble
• Used to dye cellulose fibers — blue jeans
The Vat Dyeing Process




           Reduction in solublesodium dithionite
              Dyeing in basic textile
      Insoluble indigoto the cloth to reform insoluble indigo
           Oxidation of cotton leucoindigo
      Synthesis of Methyl Orange

                              NH3                         N   N

                                    NaNO2
                                    HCl
       O3 S                                 O3S




                                                                                  N



                      N   N                  N
                                                  NaOH            N   N
                              +

O3S                                                               methyl orange
      (from part A)                                      Na O3S
                   Modern Blueprints
• The Diazo process
   – Sensitized paper
   – Exposed to light
   – Developed with ammonia vapors — NH3(g)
• Developed for large scale operation

Diazo Blueprint Chemistry
 • Paper contains                                HO
    – A diazonium salt and a coupling molecule        R



                                   N2 +X-
                   R
• Diazonium salt is light sensitive — fades to colorless when exposed
  to light

  light
                                           N2 +X-                      X   +   N
                                                                               2(g)
                            R                           R


 • Expose paper to ammonia vapor (basic) to ―develop‖ blueprint
                                HO                                HO
                                           R
                   N2 +X-                                              R
                                                            N=N
      R                                             R


          Diazonium salt
                                Coupler                 Blue diazo dye

                                     NH3
          Fastness in Dyes
• Stability of dyes towards light
• Dyes vary greatly in their lightfastness and
  colorfastness
• Undergo photo-oxidation and photo-
  reduction by light — dyes fade and
  degrade
     What Controls Opacity?
• The more a paint scatters light the more
  opaque it appears to be.
            Light scattering depends on...
• Pigment particle size




         Glass pane              Scratched Glass              Ground Glass




         Transparent              Translucent                   Opaque


        Malachite (dark green)                     Finely Ground (light green)



             40-50m                                       10-12m
       What is the best size?
• Most effective scattering
  – Particles in the 200 - 400nm range
  – 1/2 the wavelength of visible light (400-
    800nm)
• smaller particle size = more scattering
Light scattering also depends
              on..
  • Refractive Index differences of pigment
                         Pigment Particle
    and binder
                                                                        Binder



•If RI of pigment and binder are different = high scattering — opaque

•If RI of pigment and binder are similar = little scattering — transparent

•Higher RI means more bending of light in the medium.

				
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posted:10/27/2011
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