Unit 4: Dyeing
Introduction to Chemical Processing notes · PTU syllabus (BSTD 201-20)
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Unit summary
Dyeing must be even, reproducible and fast. This unit covers the stages of dyeing, dyeing machinery for fibre, yarn and fabric, dyeing of blends and dyeing defects.
After this unit you can
- Explain the stages and variables of dyeing
- Describe dyeing at fibre, yarn, fabric and garment stages
- Describe dyeing machines
- Identify dyeing defects and their causes
PTU syllabus topics
- Stages of dyeing
- dyeing machinery
- dyeing of blends
- dyeing defects
Jigger
Open-width woven fabric
Cotton fabrics
Winch
Fabric in rope form
Knits and delicate fabrics
Jet
Fabric in rope, high pressure
Polyester
Package dyeing
Yarn on perforated packages
Yarn for coloured weaves
Topic 1
Stages of dyeing
- 1
Prepare the bath — water, dye, auxiliaries (salt, levelling agent, acid or alkali)
- 2
Enter material; raise temperature gradually
- 3
Exhaustion — dye moves onto the fibre
- 4
Fixation — chemical or physical bonding
- 5
Wash-off and soaping — remove unfixed dye
- 6
After-treatment — fixing agents, softeners
- Material-to-liquor ratio
- 1:5 to 1:20 — lower saves water and energy
- Temperature and time
- Control levelness and penetration
- pH
- Acidic for acid dyes; alkaline for reactive fixation
- Electrolyte
- Salt increases exhaustion of direct and reactive dyes
- Auxiliaries
- Wetting, levelling, sequestering agents
- Continuous dyeing (pad–dry–cure or pad–steam): fabric padded with dye liquor, squeezed, and fixed — for long runs.
Topic 2
Dyeing at different stages
Fibre (stock) dyeing
Loose fibre dyed before spinning
Mélange and heather effects
Top dyeing
Wool tops dyed
Worsted fabrics
Yarn dyeing
Hanks or packages dyed
Checks, stripes, yarn-dyed shirts
Piece dyeing
Fabric dyed
Most solid shades
Garment dyeing
Finished garments dyed
Fashion and quick-response colours
Topic 3
Dyeing machinery
Hank dyeing machine
Hanks hung on rods; liquor circulates
Yarn
Package dyeing machine
Liquor pumped through perforated cones
Yarn
Jigger
Open-width fabric passes back and forth through a small bath
Woven fabrics; low liquor ratio
Winch
Fabric rope moved by a winch reel; little tension
Knits and delicate fabrics
Jet and soft-flow
Fabric moved by liquor jets; high temperature possible
Polyester, knits; fast and level
Beam dyeing
Fabric wound on a perforated beam; liquor pumped through
Lightweight synthetics
Padding mangle and continuous ranges
Pad, then dry or steam
Long runs
Garment dyeing machine
Rotating drum
Garments
Topic 4
Dyeing of blends
- Polyester–cotton: disperse dye for polyester (high temperature, 130 °C) and reactive or vat dye for cotton — in two baths (more reliable) or one bath (saves time). Solid shade — both fibres matched; reserve or cross-dye effects — only one fibre dyed.
- Wool–nylon and polyester–wool: select dyes and temperatures that do not damage wool.
Topic 5
Dyeing defects
Uneven (unlevel) dyeing
Poor preparation, fast dye uptake, poor circulation
Proper scouring, levelling agents, controlled heating
Shade variation (batch to batch)
Wrong recipe, water hardness, liquor ratio changes
Standardise; colour matching
Listing (side-to-centre shade difference)
Uneven pressure or temperature on a jigger
Even tension and temperature
Specks and spots
Undissolved dye, hard water, impurities
Filter and dissolve dyes well; sequestering agents
Crease marks
Fabric folded in rope form
Proper loading, anti-crease agents
Poor fastness
Unfixed dye not washed off
Thorough soaping, fixing agents
Barré (stripes)
Yarn variations
Use yarns from one lot
Key terms
- Exhaustion
- Transfer of dye from bath to fibre
- Liquor ratio
- Weight of material to volume of dye bath
- Jigger
- Open-width fabric dyeing machine
- Jet dyeing machine
- Machine moving fabric with liquor jets
- Listing
- Shade difference between edges and centre
Quick revision
- Bath preparation, exhaustion, fixation, wash-off, after-treatment; variables.
- Fibre, top, yarn, piece and garment dyeing.
- Hank, package, jigger, winch, jet, beam, padding machines.
- Polyester–cotton two-bath and one-bath methods.
- Unlevel dyeing, shade variation, listing, specks, crease marks, poor fastness, barré.
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.What is liquor ratio?
- Q2.Give one use of yarn dyeing.
- Q3.Distinguish a jigger and a winch.
- Q4.Why is polyester dyed at 130 °C?
- Q5.What is listing?
- Q6.Give two causes of uneven dyeing.
Long-answer questions
- Q1.Explain the stages of dyeing.
- Q2.Describe dyeing machines for yarn and fabric.
- Q3.Explain the dyeing of polyester–cotton blends.
- Q4.Describe dyeing defects, their causes and remedies.
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