Unit 3: Dyes and dye classification
Introduction to Chemical Processing notes · PTU syllabus (BSTD 201-20)
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Unit summary
Colour is added to textiles with dyes and pigments chosen to suit each fibre. This unit covers the definition of colour, dyes and pigments, the classification of dyes and their application to textiles.
After this unit you can
- Explain colour and distinguish dyes from pigments
- Classify dyes by application
- Match dye classes to fibres
- Explain the properties of each dye class
PTU syllabus topics
- Definition of colour
- dyes and pigments
- classification of dyes
- application of dyes on textiles
Reactive
Cotton, viscose
Bright, fast: bonds chemically
Vat
Cotton
Excellent fastness: denim indigo
Acid
Wool, silk, nylon
Applied in an acid bath
Disperse
Polyester
High temperature dyeing
Basic
Acrylic
Very bright colours
Topic 1
Colour, dyes and pigments
- Colour is the sensation produced when light of certain wavelengths reaches the eye; a coloured object absorbs some wavelengths and reflects the rest. A dye molecule has a chromophore (colour-giving group) and auxochromes (groups that deepen colour and help bonding).
Solubility
Soluble or made soluble in application
Insoluble
Affinity
Have affinity; penetrate the fibre
No affinity; held on the surface by a binder
Application
Dyeing and printing
Mostly printing and pigment dyeing
Fastness
Varies by class
Good light fastness; rubbing fastness depends on binder
Topic 2
Classification of dyes by application
Direct dyes
Cotton, viscose
Applied from neutral bath with salt; simple; poor wash fastness
Reactive dyes
Cotton, viscose, wool, silk
Form covalent bonds with fibre; bright shades; excellent wash fastness; applied with salt and alkali — most used for cotton
Vat dyes
Cotton
Insoluble; reduced to soluble leuco form, then oxidised back; best all-round fastness (indigo for denim)
Sulphur dyes
Cotton
Cheap dark shades (black, navy); reduced with sodium sulphide
Azoic (naphthol) dyes
Cotton
Formed inside fibre by coupling; bright reds
Acid dyes
Wool, silk, nylon
Applied in acid bath; ionic bonds
Basic (cationic) dyes
Acrylic, silk
Very bright; poor light fastness on natural fibres
Disperse dyes
Polyester, acetate, nylon
Non-ionic; applied at high temperature (130 °C) or with carriers
Metal-complex and mordant dyes
Wool
Good fastness
Natural dyes
Natural fibres
Indigo, madder, turmeric, lac, cutch with mordants (alum, iron)
- Colour Index (CI) gives each dye a generic name and number, e.g., CI Reactive Red 120.
Topic 3
Application of dyes on textiles
- Dyeing stages apply to all classes: the dye moves through the liquor, is adsorbed on the fibre surface, diffuses into the fibre and is fixed (by ionic bonds, covalent bonds, or insolubilisation).
- Cotton and viscose
- Reactive, direct, vat, sulphur, azoic
- Wool and silk
- Acid, metal-complex, reactive
- Nylon
- Acid, disperse
- Polyester
- Disperse
- Acrylic
- Basic
- Blends
- Two-bath or one-bath methods with both classes (e.g., disperse + reactive for polyester–cotton)
- Fastness properties tested: washing, light, rubbing (crocking), perspiration, sublimation.
Key terms
- Chromophore
- Group giving a dye molecule its colour
- Pigment
- Insoluble colorant fixed with a binder
- Reactive dye
- Dye forming covalent bonds with fibre
- Vat dye
- Insoluble dye applied in reduced soluble form
- Colour Index
- International dye classification system
Quick revision
- Colour theory; chromophore and auxochrome; dyes vs pigments.
- Direct, reactive, vat, sulphur, azoic, acid, basic, disperse, metal-complex, natural dyes.
- Adsorption, diffusion, fixation.
- Dye–fibre matching; fastness tests.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Distinguish a dye and a pigment.
- Q2.Which dye class is most used for cotton?
- Q3.Which dye is used for denim?
- Q4.Name the dye class for polyester.
- Q5.Which dyes are used for acrylic?
- Q6.What is the Colour Index?
Long-answer questions
- Q1.Explain colour and the difference between dyes and pigments.
- Q2.Classify dyes with their properties and fibres.
- Q3.Explain the application of dyes on different textile fibres.
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