Unit 2: Preparatory processes
Introduction to Chemical Processing notes · PTU syllabus (BSTD 201-20)
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Unit summary
Preparation makes fabric clean, white and absorbent so it dyes evenly. This unit covers the preparatory processes for cotton — stitching, singeing, desizing, scouring, bleaching and mercerisation — the preparation of wool and silk, and the preparation of blends.
After this unit you can
- Describe the sequence of cotton preparation
- Explain singeing, desizing, scouring and bleaching
- Explain mercerisation and its effects
- Describe preparation of wool, silk and blends
PTU syllabus topics
- Preparatory process for cotton — stitching
- singeing
- desizing
- scouring
- bleaching and mercerisation
- preparatory process for wool and silk
- preparation of blends
- 1Singeing
Burn off surface fibres
- 2Desizing
Remove starch size
- 3Scouring
Remove wax and oils
- 4Bleaching
Remove natural colour
- 5Mercerisation
Caustic soda: lustre, strength, dye uptake
Topic 1
Preparatory processes for cotton
- 1
Grey inspection and stitching
Pieces sewn end to end into a continuous length
- 2
Singeing
Protruding fibres burnt off by gas flames or hot plates — smooth surface, reduced pilling
- 3
Desizing
Starch size removed by enzymes (amylase), acid or oxidative methods
- 4
Scouring
Boiling with caustic soda (NaOH) and wetting agents removes waxes, pectins, oils — fabric becomes absorbent
- 5
Bleaching
Hydrogen peroxide (most common), sodium hypochlorite or sodium chlorite destroys natural colour
- 6
Mercerisation
Cold concentrated caustic soda (about 20–25%) under tension
Hydrogen peroxide
Alkaline (pH 10.5–11), 85–95 °C, with stabiliser (sodium silicate)
Universal; suits cotton, wool, silk; eco-friendlier
Sodium hypochlorite
Cold, alkaline pH 9–11
Cheap; damages wool and silk; produces chlorinated by-products
Sodium chlorite
Acidic, hot
Good for synthetics; corrosive chlorine dioxide
- Optical brightening agents are added for extra whiteness (fluoresce blue under UV).
Topic 2
Mercerisation
- Mercerisation (John Mercer, 1844; tension added by Lowe, 1889): cotton treated with cold 20–25% NaOH under tension, then washed and neutralised. Fibres swell, become round and smooth.
- Lustre
- Increased — silk-like
- Strength
- Increased by about 20%
- Dye uptake
- Greater and deeper shades
- Absorbency
- Improved
- Dimensional stability
- Better; less shrinkage
- Causticisation: milder treatment (without tension) for absorbency and dye uptake.
Topic 3
Preparation of wool and silk
Main impurity
Grease, suint, dirt, vegetable matter
Sericin (gum)
Process
Scouring with mild alkali (soda ash) and soap or detergent at 40–50 °C; carbonising with dilute sulphuric acid removes vegetable matter
Degumming in boiling soap solution (or enzymes) removes sericin — weight loss 20–30%, lustre and softness develop
Bleaching
Hydrogen peroxide (never hypochlorite)
Hydrogen peroxide
Caution
Strong alkali damages wool; agitation causes felting
Avoid harsh alkali
- Weighting of silk: metallic salts (tin) restore lost weight — reduces durability.
Topic 4
Preparation of blends
- Polyester–cotton: heat-setting of polyester for dimensional stability, then desizing, scouring and peroxide bleaching suited to cotton but without damaging polyester (avoid excessive alkali at high temperature). Polyester–wool and polyester–viscose: milder conditions to protect the natural or regenerated component. Process sequence is chosen to suit the more sensitive fibre.
Key terms
- Singeing
- Burning off surface fibres
- Desizing
- Removing size from warp yarns
- Scouring
- Removing waxes and impurities with alkali
- Mercerisation
- Treating cotton with strong caustic soda under tension
- Degumming
- Removing sericin from silk
Quick revision
- Stitching, singeing, desizing, scouring, bleaching, mercerisation.
- H₂O₂, hypochlorite, chlorite; optical brighteners.
- Mercerisation effects; causticisation.
- Wool scouring and carbonising; silk degumming; bleaching with peroxide only.
- Blends: heat-setting and mild conditions.
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.Why is singeing done?
- Q2.Name an enzyme used for desizing.
- Q3.What is the main chemical used in scouring cotton?
- Q4.Why is hypochlorite unsuitable for wool?
- Q5.List three effects of mercerisation.
- Q6.What is degumming?
Long-answer questions
- Q1.Describe the preparatory processes for cotton.
- Q2.Explain bleaching with different bleaching agents.
- Q3.Explain mercerisation and its effects.
- Q4.Describe the preparation of wool and silk.
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