Stages of the Textile Coloration/wet Process
Pre‑Treatment
Pre‑treatment prepares the textile material to receive dye evenly and permanently.
a) Sizing (mainly for woven warp yarns)
- Temporary protective coating (starch, PVA, CMC)
- Prevents yarn breakage during weaving
b) Desizing
- Removes sizing materials
- Essential for uniform wet processing
- Methods: Enzymatic, oxidative, hot washing
c) Scouring
- Removes natural and added impurities:
- Wax, oil, pectin, dirt
- Improves absorbency and dye penetration
- Usually done with alkali + surfactants
d) Bleaching
- Removes natural color from fibres
- Produces required whiteness
- Common agent: Hydrogen peroxide (H₂O₂)
e) Mercerizing (optional – cotton)
- Treatment with strong caustic soda under tension
- Improves:
- Dye uptake
- Color brightness
- Fabric strength and lustre
- Dimensional stability
Core Dyeing Flow
Sizing → Desizing → Scouring → Bleaching → (Mercerizing for cotton) → Dyeing → Finishing → Finished Product
Parallel lines: Wastewater collection → Equalization/Neutralization → ETP (biological/chemical polishing as required).
Sizing (mostly for warp yarns)
Purpose: Add a protective film (starch/synthetic polymers) to reduce breakage during weaving.
Controls: Size add‑on %, viscosity, penetration; use low‑formaldehyde or formaldehyde‑free agents.
Desizing
Goal: Remove size to restore absorbency before wet processing.
Methods: Enzymatic (amylase for starch), oxidative (for PVA blends), hot wash.
KPIs: Desizing efficiency (%), wetting time, absorbency <3 s (drop test).
Scouring
Goal: Remove natural impurities (waxes, pectins) and processing oils.
Chemistry: Caustic soda + wetting/chelating agents at 95–105 °C (cellulosics); milder for synthetics.
Outcomes: Hydrophilicity, uniform dye uptake, minimal barre.
Bleaching
Goal: Achieve target whiteness and remove color bodies.
Agents: Hydrogen peroxide (most common), alternatives like peracetic acid; stabilizers to control decomposition.
KPIs: Whiteness index (CIE), residual peroxide (must be neutralized to avoid dye shade loss).
Mercerizing (cotton/viscose, optional but valuable)
Effect: Caustic treatment under tension causing fibre swelling → higher lustre, strength, dye affinity, dimensional stability.
Windows: 18–22 °Bé NaOH, room temperature, controlled dwell + immediate washing/neutralization.
Dyeing
Select dye class by fibre, end‑use fastness, cost and compliance.
Choose exhaust, pad‑batch, pad‑steam, thermofix or garment dye based on product/machine.
Finishing
Mechanical: Stenter, compacting, sueding/raising, calendaring.
Chemical: Softener (cationic/non‑ionic), resin/anti‑crease, moisture‑management finishes, antimicrobial (compliance‑checked), flame retardant (where applicable).
QA: Hand feel, GSM, shrinkage, skew/spirality, fastness panel.
Wastewater, Equalization & Neutralization
Equalization tank: Buffers pH/temperature/flow, avoiding shock loading of the ETP.
Neutralization: Acid/alkali dosing to pH 6.5–8.5 before biological treatment.
ETP: Primary (coagulation/flocculation), secondary (biological), tertiary (filtration/RO) per local discharge norms and buyer standards (e.g., ZDHC).
Standard Dyeing Curves & Operating Windows
Important: Always align with the dyestuff supplier’s TDS. The curves below reflect typical industrial practice for controlling levelness, fixation and fastness.
Reactive Dyeing on Cellulosics (Exhaust route)
Typical recipe levers:
Electrolyte (NaCl/Na₂SO₄) → promotes exhaustion.
Alkali (Na₂CO₃/NaOH) → triggers fixation.
Temperature: 40–95 °C depending on dye reactivity.
Curve (simplified):
- Wet‑out at 25–30 °C with wetting/levelling, 10–15 min.
- Salt addition in steps (2–3 portions, 10–15 min apart).
- Temperature ramp 1.5–2 °C/min to the chosen shade temp (40/60/90 °C—per dye brand). Hold 30–45 min.
- Alkali addition in steps, hold 30–45 min for fixation.
- Cooling to 60 °C, drain, then wash‑off sequence: rinse → soaping at 95 °C (anionic detergent/chelants) → multiple rinses until conductivity/colour in effluent stabilizes.
- Neutralize (if needed) and finish.
Critical controls: pH 10–12 during fixation, salt profile, full soaping (removes hydrolyzed dye), water quality (hardness).
Disperse Dyeing on Polyester (HT Jet/Winch)
Why: Disperse dyes diffuse into PET at high temperature (≈130 °C).
Curve (typical):
- Start at 45 °C, add dyes (dispersed slurry), dispersant/leveller; run 10–15 min.
- Ramp to 130 °C at ~1–2 °C/min; hold 30–60 min (depth dependent).
- Cool to 70 °C, drain.
- Reduction clearing at 70–80 °C: NaOH + sodium hydrosulfite (or eco alternatives); 15–20 min.
- Rinse, acid wash (pH ~5) if needed → softener and unload.
Critical controls: Temperature profile, dispersant level, oligomer management (filtering/cleaning), sublimation risk (downstream heat).
Acid Dyeing (Wool/Silk/Nylon)
Mechanism: Anionic dyes on cationic fibre sites at pH 2–5.
Curve (leveling‑type):
- Charge bath at 30–40 °C with water, wetting, acetic/formic acid (pH 4–5) and leveller/retarder.
- Add dye; convert to circulation for 10–15 min.
- Ramp to 95–100 °C, hold 20–40 min.
- Bath drop; rinse; adjust pH to neutral; softener as needed.
Critical controls: pH profile, gradual temperature rise, gas‑fading‑sensitive shades on nylon, avoid excessive exhaustion that harms levelness.
2.4 Vat Dyeing (Pad‑Steam/Continuous)
Mechanism: Dye is reduced to a leuco form, padded onto fibre, oxidized back to insoluble state inside fibre.
Continuous pad‑steam route (typical):
- Padder: Apply reduced vat dye liquor (NaOH + sodium dithionite), target pick‑up 70–80%.
- Drying chamber (moderate temp to avoid re‑oxidation before diffusion).
- Steamer at 101–105 °C for 30–60 s (diffusion).
- Oxidation (air/ H₂O₂ / sodium bromate systems).
- Soaping at 95–98 °C to develop brightness and remove unfixed dye.
- Rinsing/neutralizing, then finishing.
Critical controls: ORP (redox), rapid but controlled oxidation, thorough soaping, anti‑backstain agents for darks.
Stages of Dye Application
Color can be added at multiple product stages:
Fibre stage (solution/mass dyeing, dope dyeing) – pigments dispersed in polymer melt (PET/nylon) or tops dyeing for wool.
- Pros: Exceptional batch‐to‐batch shade consistency, excellent light/wet fastness, lowest water use in downstream.
- Cons: High MOQs, less shade flexibility.
Pros: Exceptional batch‐to‐batch shade consistency, excellent light/wet fastness, lowest water use in downstream.
Cons: High MOQs, less shade flexibility.
Yarn stage – hank, package, beam, space dyeing (see Section 4).
- Pros: Stripes, melanges, checks; excellent penetration.
- Cons: Extra process step before fabric; inventory complexity.
Pros: Stripes, melanges, checks; excellent penetration.
Cons: Extra process step before fabric; inventory complexity.
Fabric stage – winch, jet, jigger, pad‑batch/pad‑steam, foam dyeing (see Section 5).
- Pros: Highest flexibility and breadth of methods.
- Cons: Risk of barre/shade variation if greige variations exist.
Pros: Highest flexibility and breadth of methods.
Cons: Risk of barre/shade variation if greige variations exist.
Garment stage – garment dye, pigment overdye, denim garment processes.
- Pros: Fashion effects, small lots, quick response.
- Cons: Dimensional change control, trims compatibility.
Pros: Fashion effects, small lots, quick response.
Cons: Dimensional change control, trims compatibility.
Yarn Dyeing — Types & Best Uses
Hank Dyeing
What: Loose hanks hang on sticks and circulate in liquor.
Advantages: Superior bulk/hand, deep and uniform penetration; ideal for acrylic/wool/cotton yarns for sweaters and scarves.
Considerations: Lower productivity; careful material handling to avoid tangling.
Package Dyeing
What: Yarn wound on perforated tubes; liquor flows inside→out or outside→in under pressure.
Advantages: Scalable and versatile; good for cotton/polyester/nylon; lower labor; reproducible.
Controls: Winding density (cP), flow direction reversals, differential pressure, doffing without collapse.
4.3 Beam Dyeing (Warp Beam)
What: Many ends wound on a perforated beam; liquor flows through under pressure.
Advantages: Large lots, good levelness, efficient for denim, shirting, jacquard, dobby warps.
Considerations: Uniform winding tension critical; designed mainly for woven warps.
Space Dyeing
What: Intermittent application along yarn length to create multicolor/sectional effects.
Methods: Spray/jet, printing or knit‑deknit techniques.
Uses: Fashion yarns, athleisure accents, socks.
Fabric (Piece) Dyeing
Winch Dyeing
How it works: Fabric in rope form circulates over a winch reel through the dye liquor.
Best for: Knits and delicate fabrics (low tension).
Typical M:L: ~1:5 to 1:10 depending on machine.
Pros: Gentle handling, good for open structures.
Cons: Potential creases if loading/rope lengths are not optimized.
Jigger Dyeing
How it works: Open‑width fabric passes back and forth between two rollers over a stationary bath.
Best for: Wovens (e.g., satin, poplin) where open‑width is required.
Pros: Even tension, high levelness; good for Direct, Reactive, Vat.
Cons: Risk of center‑to‑selvedge variation; requires precise tension/edge control.
Foam Dyeing (Low‑liquor application)
How it works: Convert dye liquor to foam with a foaming agent; apply via knife/slot; pass through nip; then dry/cure or fix by steam.
Benefits: Lower water and energy, reduced effluent; suitable for pigment, some reactive/vat continuous processes.
Controls: Foam stability, cell size, pickup uniformity, cure conditions.
QA, Testing & Acceptance (What to Check)
- Shade & ΔE vs standard (D65/10° or buyer‑specified).
- Color fastness panel: Washing, rubbing (dry/wet), perspiration, light, chlorinated water, sublimation (for PET/disperse).
- Physicals after dyeing: GSM, thickness, bursting/tensile, shrinkage and skew/spirality.
- Chemical residues: pH, residual peroxide, free formaldehyde (if resin used), APEO/NPEO (should be ND).
- Appearance: Levelness, barre, crease marks, stains, softener spots.
Troubleshooting Matrix (Quick Wins)
- Uneven shade (knits): Rope length balance, liquor flow, leveller dosage; re‑level at lower temp with additions.
- Back‑staining (vat/sulfur/disperse): Increase anti‑backstain/dispersant; optimize soaping or reduction clearing.
- Low rub fastness (pigment/reactive): Under‑cure or low binder; incomplete wash‑off; add crosslinker/softener balance.
- High shrinkage/spirality: Improve compacting/overfeed; control relaxation drying; yarn twist balance.
- Shade drift post‑heat (PET): Choose high‑sublimation disperse range; manage stenter/transfer parameters.
