Colour and protection: dyes, pigments, paints and coatings: Key terms, quiz and sources

Key terms

Chromophore

A group of atoms, such as the azo group -N=N- or a carbonyl group, that creates an absorption band in the visible spectrum.

Auxochrome

A salt-forming group such as -SO3H, -OH or -NH2 that controls a dye's solubility and its attachment to the substrate.

Conjugated system

A chain of alternating single and double bonds along which electrons spread; longer chains absorb longer wavelengths.

Dye

A coloured compound that dissolves in the medium in which it is used and enters the substrate molecule by molecule.

Pigment

An insoluble coloured or white particle that stays solid in use and needs a binder to hold it to a surface.

Lake

An organic colour made insoluble by precipitating it with a metal salt, usually onto an inorganic base such as clay or alumina.

Dye intermediate

A substituted aromatic compound, such as aniline or beta-naphthol, from which dyes are assembled.

Diazotisation

Conversion of a primary aromatic amine into a diazonium salt with nitrous acid, usually at 0 to 10 C.

Coupling

Reaction of a diazonium salt with a phenol, naphthol or amine to form an azo dye.

Leuco form

The reduced, water-soluble form of a vat or sulphur dye, which is reoxidised to the insoluble colour inside the fibre.

Exhaustion

The movement of dye from the dyebath onto the fibre, the first of the four steps of dyeing.

Fixation

The final step of dyeing, in which dye is bonded or locked into the fibre so that it resists washing.

Hiding power

A pigment's ability to obscure the surface beneath, set mainly by refractive index and particle size.

Extender

A cheap, low-refractive-index mineral such as talc, whiting or china clay that cuts paint cost and controls sheen and texture.

Binder

The film-forming resin or oil in a paint that holds pigment together and to the surface.

Pigment volume concentration

The volume of pigment as a share of pigment plus non-volatile binder; it controls gloss and durability.

Volatile organic compound (VOC)

Solvent that evaporates from a coating and helps form ground-level ozone; limited by regulation in g/L or lb/gal.

Electrodeposition (e-coat)

Applying a water-borne primer by making the metal part an electrode, so paint deposits until its own insulation stops the current.

Let-down

The paint-making step in which the concentrated pigment grind is thinned with the rest of the binder and solvent.

Try it in the table

• Open the Dyes tile and look at how it splits demand by dye class or fibre. Compare it with the 2002 world figures here: reactive and disperse dyes at 28% each, sulphur 15% and indigo 6%. Which two fibres does that tell you drive world dye demand?
• Open the Benzene tile and find the share that goes to nitrobenzene and aniline. Using 0.655 t of benzene per tonne of nitrobenzene and 1.39 t of nitrobenzene per tonne of aniline, work out how much benzene a 15,000-tonne-a-year aniline plant needs (about 0.91 t per tonne of aniline).
• Open the Titanium dioxide tile and look for the split between the sulphate and chloride routes. For a 50,000-tonne sulphate plant on ilmenite, estimate the copperas and minimum sulphuric acid from this module's stoichiometry (up to 3.5 t and about 2.5 t per tonne of pigment).
• Open the Paints and coatings tile and compare its split between decorative and industrial paint with Dryden's 70:30 for India in the 1990s and Kent's 56% decorative for the United States in 2010. Has India moved towards the industrial-heavy pattern Dryden expected?
• Open the Carbon black tile and note the share going to tyres versus pigments, then open the Organic pigments tile. Compare price per kilogram with Shreve's 70 to 84 cents for carbon black and about $12.90 for organic pigments, and think about what that gap says about batch versus continuous production.
• Open the Polyester fibre tile. Polyester absorbs only 0.3 to 0.5% moisture, so it takes disperse dyes at 130 C or by thermosol at about 205 C. Consider how growth in polyester shifts demand from reactive to disperse dyes, and the energy use of the dyehouse.

Final quiz

Beta-carotene absorbs at 466 and 497 nm while vitamin A absorbs at 325 nm. What explains the difference?

Shreve's nitrobenzene plant uses 655 kg of benzene per tonne. The theoretical minimum is 634 kg. Roughly what share of benzene ends up in product?

Why is the Bechamp iron route to aniline being replaced?

Which pairing of dye class and fibre is correct?

What is the main source of wasted dye and coloured effluent in reactive dyeing?

In the chloride route to titanium dioxide, what happens to the chlorine?

What does the pigment volume concentration mainly control?

Why did Dryden expect India's paint market to shift towards industrial coatings?

Sources for this module

• Kent, J.A. (ed.), Handbook of Industrial Chemistry and Biotechnology, 12th ed., Springer, 2012. Ch 13 Dye Application, Manufacture of Dye Intermediates and Dyes (Freeman and Mock); Ch 14 The Chemistry of Structural Adhesives (Abbey and Zalucha); Ch 29 Pigments, Paints, Polymer Coatings, Lacquers, and Printing Inks (Webster and Ryntz) · pdf pp. 484-556 (Ch 13); 558-559 (Ch 14); 1119-1131 (Ch 29)
• Austin, G.T., Shreve's Chemical Process Industries, 5th ed., McGraw-Hill, 1984. Ch 8 Industrial Carbon; Ch 24 Surface-Coating Industries; Ch 39 Cyclic Intermediates and Dyes · pdf pp. 149-153 (Ch 8); 442-461 (Ch 24); 790-812 (Ch 39)
• Gopala Rao, M. and Sittig, M., Dryden's Outlines of Chemical Technology for the 21st Century, 3rd ed., East-West Press, 1997. IB Indian chemical industry; IIIC Paints and Varnishes (including Pigments); IVF Chemicals from Aromatics (including Dyes) · pdf pp. 15, 25, 319-325, 548-553