Colorants — impart color

1. 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 Transition Metals 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 • Cobalt blue, cerulean blue — cobalt Co • Chrome oxide green, chrome yellow — chromium Cr

2. Transition Metals Colorants — impart color • Pigments -- insoluble colorants • usually inorganic compounds • Dyestuffs -- soluble colorants • usually organic compounds

3. Some common organic dyes

4. Transition Metals Colorants — impart color • Pigments -- insoluble colorants • usually inorganic compounds • 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

5. 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

6. Artists’ Palettes Through the Ages

7. 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

8. 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

9. Sources of Natural Dyes – BUGS • Tyrian Purpleor “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

10. Kermes, Cochineal — Carmine Chemically similar structure, light sensitive Dyed wool and silk — “carmine red”

11. 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 leucoindigoto use as dye • Woad (poor man’s indigo) • Member of the mustard family • Common weed in temperate climates • Leaves contain same chemical as indigo but in lower amounts • Celtic war paint and tattoos Braveheart • Blue robes of priests

12. 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

13. 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 • Carbonyl groups • Quinoid rings —N=N— —C=O

14. AUXOCHROMES • “color augmenting” groups • functional group with non-bonded electrons which acts to strengthen or deepen the color and hue —NH2 —OH —OR

15. CARBONYL Dyes • Anthroquinone Dyes • Contain several —C =O groups • Multiple aromatic rings • Originate from plants & animals • Coloring agents in flowers, fruits, vegetables, bugs, etc.

16. Mauveine — first synthetic dye William Henry Perkin (18 years old), 1856 - Trying to make the antimalarial, quinine - patent  dye company

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

18. Fiber-Reactive Dye Structure • Reactive group • Chromophore • Leaving group Procion Scarlet H-R, Cibacron Scarlet RP, Chlorine Reactive Red 3

19. Methods of Application • Dye and Substrate can interact through... 1. ionic forces (+ and —charges) 2. hydrogen bonding 3. dispersion forces 4. covalent bonds Substrate Dye

20. Types of Dyes by Application • Acid or Basic Dyes (ionic forces) • Mordant Dyes (ionic) • Direct Dyes (hydrogen bonding) • Vat Dyes • Fiber-reactive Dyes (covalent)

21. Fiber DYE HO N H Hydrogen Bond H Polar group Polar group Direct Dyes • Polar dye Polar Fibers • Dye applied from a hot water solution • Cotton and cellulose-based fibers • Synthetic dyes

22. Fiber DYE NH3+ -OOC Fiber DYE SO3- NH3+ Basic amine group Carboxylic acid group Sulfonic acid group Basic amine group Acidic or Basic Dyes • Basic dye (+) (-) Acidic fiber • Acidic dye (-) (+) Basic fiber • Wool, silk, nylon and leather have basic amine groups and acidic carboxylic acid groups

23. 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

24. Fiber-Reactive Dye Structure • Reactive group • Chromophore • Leaving group Procion Scarlet H-R, Cibacron Scarlet RP, Chlorine Reactive Red 3

25. Vat Dyes Indigo: the Classic Vat Dye • Usually not soluble in water • Must be converted to a soluble form to be used as a dye • Indigo is insoluble in water • Must be chemical reduced to leucoindigo • Leucoindigo is soluble • Used to dye cellulose fibers — blue jeans

26. Insoluble indigo in basic sodium dithionite Reduction to soluble leucoindigo Dyeingof cotton textile Oxidation in the cloth to reform insoluble indigo The Vat Dyeing Process

27. Synthesis of Methyl Orange

28. 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 • A diazonium salt and a coupling molecule

29. Diazonium salt light Blue diazo dye Coupler NH3 • Diazonium salt is light sensitive — fades to colorless when exposed to light • Expose paper to ammonia vapor (basic) to “develop” blueprint

30. 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

31. What Controls Opacity? • The more a paint scatters light the more opaque it appears to be.

32. Malachite (dark green) Finely Ground (light green) 40-50m 10-12m Light scattering depends on... • Pigment particle size Glass pane Scratched Glass Ground Glass Transparent Translucent Opaque

33. 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

34. Pigment Particle Binder Light scattering also depends on.. • Refractive Index differences of pigment and binder • IfRI of pigment and binder aredifferent= high scattering— opaque • If RI of pigment and binder are similar = little scattering — transparent • Higher RI means more bending of light in the medium.