Dyeing

In short

Dyeing is the uniform, reasonably permanent colouring of a textile throughout its body by transferring dye molecules from a liquid (or vapour) phase into the fibre and holding them there by physical or chemical bonds. It differs from printing (localised colour) and comes after pretreatment, because absorbency, whiteness and pH decide shade depth and levelness. Every dye class is tied to a fibre class by chemistry — reactive and vat on cotton, disperse on polyester, acid on wool and nylon, cationic on acrylic — and the processing recipe follows from that. It is the highest-mark topic in Chemical Processing papers, in GATE TF (dye classes, isotherms, affinity, exhaustion), and in technical rounds at Arvind, Vardhman, Trident, Welspun, KPR, Raymond, Loyal, Nahar, Alok, Shahi and at dye and machinery suppliers (Atul, Kiri, Bodal, Colourtex, Jaysynth, Archroma, Huntsman, Thies, Fong's, Devrekha). It is also where most of a mill's water, salt, energy and effluent load is created, so questions on M:L ratio, right-first-time and ZLD are routine.

Explained

Fundamentals: how a dye gets into a fibre

• Four steps in sequence: (1) dye dissolves/disperses in the bath, (2) diffuses through the aqueous boundary layer to the fibre surface, (3) adsorbs on the surface, (4) diffuses into the amorphous regions and is fixed. Step 4 is the slowest and controls dyeing time.
• Dye enters only amorphous regions through water-swollen pores (pore model, cellulose) or by dissolving in the softened polymer (free-volume model, polyester above Tg). Crystalline regions stay undyed — hence dye uptake falls as crystallinity rises.
• Substantivity: the observed tendency of a dye to leave the bath and go onto the fibre under the given conditions. Affinity (−Δμ°): the thermodynamic driving force, in kJ/mol, independent of liquor ratio.
• Forces of attachment: van der Waals and hydrophobic (disperse–polyester, direct–cotton), hydrogen bonds, ionic/electrostatic (acid–wool, cationic–acrylic), covalent (reactive–cellulose), mechanical entrapment (vat, sulphur, azoic, pigment).
• Zeta potential / Donnan effect: cellulose in water carries a negative surface charge that repels anionic dye anions. Neutral electrolyte (common salt, Glauber's salt) screens this charge and raises exhaustion; it is not a fixing agent.
• Temperature raises diffusion rate and levelness but *lowers* equilibrium exhaustion (dyeing is exothermic). pH controls fibre charge and dye ionisation. Liquor ratio controls exhaustion for low-affinity dyes and controls water, salt and energy cost for all.
• Dyeing stages possible: fibre/loose stock, top, yarn, fabric, garment. Earlier stage = better penetration and blend control; later stage = shorter lead time and less inventory risk (why garment dyeing dominates Tirupur fashion orders).

Dye classes and the fibres they belong to

• Direct (substantive) dyes — anionic, long planar azo molecules, applied to cotton/viscose near the boil with salt; cheap and simple but poor wash fastness; after-treated with cationic fixing agents or copper salts. Used for linings, low-cost shades.
• Reactive dyes — form a covalent bond with cellulose –OH (ether bond with vinyl sulphone/halotriazine, under alkali) or with wool/nylon –NH₂. Two reaction mechanisms: nucleophilic substitution (monochlorotriazine MCT, dichlorotriazine DCT, difluorochloropyrimidine) and nucleophilic addition (vinyl sulphone, from β-sulphatoethyl sulphone). Bifunctional dyes (e.g. MCT + VS) give higher fixation.
  • Sequence: exhaustion with salt at neutral pH → fixation with alkali (soda ash, bicarbonate, caustic) → wash off. Hydrolysis of dye by water competes with fixation, so 10–35% of dye is lost as hydrolysed dye and must be washed out; this is the main cause of coloured effluent.
  • Classes by temperature: cold brand (DCT, 30–40 °C), medium (VS, 50–60 °C), hot brand (MCT, 80–85 °C).
• Vat dyes — water-insoluble anthraquinone/indigoid pigments. Vatting: NaOH + sodium hydrosulphite (sodium dithionite) reduces them to the soluble leuco form, which is substantive; after absorption, oxidation (air, H₂O₂, perborate) regenerates the insoluble pigment inside the fibre, then soaping at the boil develops the final shade. Best all-round fastness; costly. IK / IW / IN methods differ in caustic and salt concentration according to the dye's substantivity. Indigo is the special case — ring dyeing of warp yarn for denim.
• Sulphur dyes — reduced with sodium sulphide/glucose, cheap deep blacks and navies for khaki and denim; moderate light fastness, risk of acid tendering on storage; sulphide in effluent is a pollution issue (now largely replaced by pre-reduced liquid sulphur blacks).
• Azoic (naphthol) dyes — the dye is *built inside* the fibre: impregnate with Naphthol AS (as sodium naphtholate), then develop with a diazotised fast base / stabilised fast salt. Brilliant reds and oranges; some bases are now restricted because they release banned aromatic amines.
• Disperse dyes — non-ionic, sparingly soluble, the only class for polyester (also acetate, nylon). Need a dispersing agent (lignosulphonate, naphthalene sulphonate condensate) and pH 4.5–5 (acetic acid / ammonium sulphate).
  • Three methods: carrier dyeing at ~100 °C with a swelling carrier (o-phenylphenol, methylnaphthalene); HTHP at 125–135 °C for 30–60 min in a pressurised jet — the standard mill route; thermosol (pad–dry–bake 190–215 °C for 60–90 s) for continuous polyester/cotton fabric.
  • Classified low/medium/high energy (A/B/C–D) by molecular size and sublimation fastness — high-energy types for HT and thermosol; low-energy for carrier and for polyester–wool blends.
  • Reduction clearing (caustic + hydrosulphite + dispersing agent, 70–80 °C) removes surface dye from dark shades to restore rubbing and wash fastness.
• Acid dyes — anionic, for wool, silk and nylon; bond ionically to protonated –NH₃⁺ groups. Levelling (strong acid, pH 2.5–3.5, good migration, poor wet fastness), milling (pH 4–6), super-milling / metal-complex (1:2 premetallised) at pH 6–7 with better fastness. On nylon, uptake is limited by the number of amine end groups — the cause of barré streaks.
• Basic (cationic) dyes — for acrylic and modacrylic, bonding to –SO₃⁻ groups of the co-monomer. Dyeing starts only above the critical dyeing temperature (near the fibre Tg, ~75–85 °C); strike is violent, so heat at 0.5–1 °C/min and use cationic retarders. The fibre has a fixed saturation value (SF); recipes must not exceed it.
• Pigments — not dyes; no affinity, held by a binder (pad–dry–cure). Used for garment/fabric dyeing in pale fashion shades; weak rubbing fastness.
• Natural dyes — madder, indigo, catechu, turmeric, lac; need metal mordants (alum, iron, tannin). Niche/handloom (khadi, Kalamkari, Ajrakh, GI handloom clusters) and governed by GOTS-type certification rather than fastness alone.

Machinery and methods of application

• By method: batch/exhaust (dye slowly exhausts from a finite bath — flexible, most Indian mills), semi-continuous (pad–batch, pad–roll, pad–jig: pad the dye on, store/rotate for fixation), continuous (pad–steam for vat/reactive, thermosol for disperse — only economical above ~5,000–10,000 m per shade).
• Material circulates (winch, jet, soft-flow, airflow), liquor circulates (package, beam, loose stock), or both (modern soft-flow, high-temperature package).
• Jigger: open-width fabric passes between two rollers through a small bath; M:L about 1:2–1:3, low water, no rope marks — used for viscose, satin and lining fabrics; tension-sensitive.
• Winch (beck): rope form, M:L 1:20–1:30; cheapest, used for knits in small units; creasing and high water use.
• Jet / soft-flow: fabric rope carried by a liquor jet at 1:5–1:8; HTHP versions to 140 °C for polyester. Airflow machines carry the fabric on an air stream at M:L 1:3–1:4, cutting water, salt and steam sharply.
• Package/cheese dyeing: yarn wound on perforated spools, liquor pumped in-to-out and out-to-in alternately; needs correct winding density or you get inner/outer shade variation. Hank dyeing for handloom and carpet yarn; beam dyeing for warp sheets and fine fabric.
• Garment dyeing: paddle and rotary drum machines; very short lead time (Tirupur, Bengaluru), but shrinkage and seam-puckering risk.
• Cold pad–batch (CPB) reactive dyeing: pad fabric with dye + alkali, batch on a rolling A-frame for 4–12 h at room temperature, then wash off. Highest fixation, least energy and salt; needs good padder control and a covered batch to avoid edge drying.
• Auxiliaries: wetting agent, sequestering agent (for Ca²⁺/Mg²⁺ hardness and iron), levelling/migrating agent, dispersing agent, antifoam, buffer (acetic acid–acetate, bicarbonate), after-soaping agent. Soft water (ideally below 50 ppm hardness) is a precondition for reproducible dyeing.

Quality, colour measurement and fastness

• Levelness (uniformity across the lot), penetration (dye through the yarn cross-section, not ring-dyed), right-first-time (RFT) percentage, and batch-to-batch shade are the four mill KPIs. Causes of unlevel dyeing: uneven pretreatment, hardness, too fast strike, poor liquor circulation, temperature gradient, immature cotton (neps), tension variation.
• Colour measurement: spectrophotometer, illuminant D65/10° observer, CIELAB L*a*b*; shade passed on ΔE*ab or the tighter CMC(2:1) tolerance (commonly ΔE ≤ 1.0 for apparel). Metamerism = a match under one illuminant that fails under another; check under D65, TL84 and incandescent (A).
• K/S from Kubelka–Munk is linear with dye concentration and is the basis of computer colour matching (CCM) and recipe prediction/correction.
• Fastness testing follows the ISO 105 series (BIS publishes IS/ISO equivalents): wash (C06), light with xenon arc (B02), rubbing/crocking (X12), perspiration (E04), water (E01), hot pressing (X11), chlorinated water, and dry-cleaning. Rated on the grey scales — 1–5 for change in shade, 1–5 for staining of adjacent fabrics (5 = best; 1–8 blue wool scale for light fastness).
• Compliance: azo dyes that can release the listed carcinogenic aromatic amines are prohibited in textiles in India under notification made under the Environment (Protection) Act, 1986, and in the EU under REACH; buyers add Oeko-Tex Standard 100, ZDHC MRSL and GOTS (for organic) limits on heavy metals, formaldehyde, APEO and chlorinated carriers. Exact banned-amine lists and limits are revised periodically — verify against the current notification and the buyer's RSL.
• Environment: dyehouses are a CPCB Red category activity needing Consent to Establish/Operate under the Water (Prevention and Control of Pollution) Act, 1974 and the Air Act, 1981. Tirupur's dyeing and bleaching units run on Zero Liquid Discharge (CETP/IETP with RO + multiple-effect evaporator and salt recovery) following Madras High Court orders; similar ZLD conditions apply in parts of Gujarat and Rajasthan. Worker safety, chemical handling and welfare in the dyehouse now come under the four Labour Codes in force from 21 November 2025, the OSH&WC Code having replaced the Factories Act, 1948.

Key points

• Dyeing – uniform colouring throughout the material, as against printing, which is localised.
• Rate-controlling step – diffusion of dye inside the fibre, not adsorption at the surface.
• Substantivity vs affinity – substantivity is the observed uptake under set conditions; affinity (−Δμ°) is the thermodynamic driving force in kJ/mol.
• Salt in cotton dyeing – screens the fibre's negative zeta potential to raise exhaustion; it does not fix the dye.
• Alkali in reactive dyeing – ionises cellulose –OH to cellulosate, which attacks the reactive group and forms the covalent bond.
• Hydrolysed dye – reactive dye killed by water; has no affinity after wash-off and causes coloured effluent and poor wet fastness if left in.
• Vat dyeing cycle – vatting (reduce) → dyeing (leuco form) → oxidation → soaping.
• Disperse dye – non-ionic and sparingly soluble; the bath is a dispersion, so dispersing agent and pH 4.5–5 are mandatory.
• HTHP vs carrier – 130 °C pressure dyeing is cleaner and cheaper than adding a toxic, odorous carrier at 100 °C.
• Reduction clearing – caustic + hydrosulphite after dark polyester shades to strip surface dye and recover rubbing fastness.
• Critical dyeing temperature – temperature near fibre Tg at which cationic dye uptake on acrylic suddenly begins; control the heating rate.
• Barré in nylon – streaks from variation in amine end-group content between yarn lots.
• Isotherms – Nernst (disperse/polyester), Freundlich (direct/cellulose), Langmuir (acid/wool) — saturation only in Langmuir.
• Half-dyeing time t½ – time to reach half of equilibrium uptake; a direct measure of dyeing rate.
• M:L ratio – kg material : litres liquor; lower M:L means less water, salt, steam and effluent but tighter machine control.
• K/S – Kubelka–Munk function, linear with dye concentration; the basis of CCM recipe prediction.
• Grey scale – 1–5 visual rating for change in shade and staining; light fastness uses the 1–8 blue wool scale.
• ZLD – zero liquid discharge (RO + MEE + salt recovery), mandatory for Tirupur dyeing units.

Formulas and examples

Example 1 — Reactive dyeing recipe (exhaust method). 300 kg bleached cotton knit, 2% shade, M:L 1:8, salt 60 g/L, soda ash 20 g/L.

• Liquor = 300 × 8 = 2,400 L. Dye = 2 × 300/100 = 6.0 kg.
• Dye concentration = 6,000 g / 2,400 L = 2.5 g/L.
• Salt = 60 × 2,400/1,000 = 144 kg; soda ash = 20 × 2,400/1,000 = 48 kg.
• Note the salt is 24 times the dye weight — this is why low-M:L airflow machines and CPB are pushed.

Example 2 — Exhaustion, fixation and wash-off load. Same bath. Residual dye after fixation measures 0.30 g/L; soaping liquor accounts for 0.63 kg of hydrolysed dye.

• E = (2.5 − 0.30)/2.5 × 100 = 88.0% → dye on fibre = 0.88 × 6.0 = 5.28 kg.
• Fixed dye = 5.28 − 0.63 = 4.65 kg → fixation on absorbed dye = 4.65/5.28 = 88.1%.
• Total fixation = 4.65/6.0 = 77.5%; dye lost to effluent = 0.30 × 2.4 + 0.63 = 1.35 kg (22.5%).

Example 3 — Partition coefficient and standard affinity. A disperse dye on polyester at 130 °C: 24.0 g dye in the fibre per kg polyester, 0.055 g/L left in the bath.

• K = 24.0/0.055 = 436.
• T = 403 K; −Δμ° = 8.314 × 403 × ln 436 = 3,350.6 × 6.078 = 20,364 J/mol ≈ 20.4 kJ/mol.
• The positive affinity confirms strong partition into the fibre; disperse dyeing follows a Nernst isotherm because the dye simply dissolves in the polymer (no fixed sites, no saturation until the solubility limit).

Example 4 — Effect of temperature on dyeing rate. A dye has Ea = 80 kJ/mol and t½ = 20 min at 60 °C. Find t½ at 80 °C.

• 1/333 − 1/353 = 0.0030030 − 0.0028329 = 1.701 × 10⁻⁴ K⁻¹; Ea/R = 80,000/8.314 = 9,622.8 K.
• k₂/k₁ = exp(9,622.8 × 1.701 × 10⁻⁴) = exp(1.637) = 5.14.
• t½ (80 °C) = 20/5.14 = 3.9 min — a 20 °C rise cuts dyeing time five-fold, which is also why strike control and levelling agents matter.

Example 5 — Saving from a lower liquor ratio. Same 300 kg lot moved from a jet at 1:8 to an airflow machine at 1:5, salt held at 60 g/L.

• Liquor: 2,400 L → 1,500 L; water saved 900 L per batch.
• Salt: 144 kg → 90 kg; 54 kg saved. At ₹8/kg salt = ₹432; at ₹60 per m³ for water plus ZLD treatment = 0.9 × 60 = ₹54. Total ≈ ₹486 per batch, plus the steam for heating 900 L less. (Utility and salt rates vary by cluster — use the mill's own costs.)

Where it is used

• Cotton knit processing and garment export — Tirupur, Erode and Bengaluru: reactive exhaust and garment dyeing for KPR, Shahi, Gokaldas and their dyehouses, under ZLD discharge conditions.
• Polyester and blended fabric — Surat and Bhilwara: HTHP jet dyeing with disperse dyes, thermosol continuous ranges for suiting (Raymond, Siyaram, Sangam, Donear).
• Yarn and denim — Vardhman, Nahar, Arvind, Trident: package and hank dyeing of cotton and acrylic yarn, indigo rope/slasher dyeing for denim warp.
• Home textiles — Welspun, Trident, Alok (Panipat, Karur, Solapur): vat and reactive dyeing for towels, bed linen and made-ups with high wash-fastness requirements.
• Dyes and auxiliary manufacture — Atul, Kiri, Bodal, Colourtex, Jaysynth in Gujarat and Maharashtra; application labs, technical service and shade-development roles.
• Job roles — dyeing shift in-charge, lab/CCM colourist, process development and RFT engineer, quality and fastness lab, ETP/ZLD chemist, buyer-side compliance (ZDHC/Oeko-Tex) auditor.

Interview questions

• Q: Why is common salt added when dyeing cotton with reactive or direct dyes? A: Cellulose in water carries a negative surface charge that repels the anionic dye; electrolyte screens this charge, lowers the Donnan barrier and increases exhaustion. It plays no part in fixation.
• Q: What exactly does the alkali do in reactive dyeing? A: It ionises cellulose hydroxyls to cellulosate anions, which are strong nucleophiles and attack the dye's reactive group to form the covalent bond.
• Q: Why can reactive dyeing never reach 100% fixation? A: Water competes with cellulosate for the same reactive group, hydrolysing part of the dye; the hydrolysed dye has no reactivity and must be washed off, so practical total fixation is about 60–90% depending on the reactive system and method.
• Q: Why are disperse dyes the only choice for polyester? A: Polyester is highly crystalline, hydrophobic and has no ionic sites, so only small, non-ionic, sparingly water-soluble molecules can dissolve into the polymer above its glass transition.
• Q: HTHP or carrier dyeing — which would you choose and why? A: HTHP at 130 °C, because it gives faster, more level dyeing without the cost, odour, effluent load and light-fastness loss that carriers bring; carriers are used only when no pressure machine is available or for polyester blended with wool.
• Q: What is reduction clearing and when is it skipped? A: Treatment with caustic and sodium hydrosulphite after dark disperse shades to destroy surface dye and restore rubbing and wash fastness; it is normally skipped for pale shades.
• Q: Describe the vat dyeing sequence in one line each. A: Vat the insoluble dye with caustic and hydrosulphite to the soluble leuco form, let it exhaust onto cellulose, oxidise it back to the insoluble pigment in the fibre, then soap at the boil to develop the final shade and fastness.
• Q: Why does equilibrium exhaustion fall as temperature rises, when dyeing rate increases? A: Dye adsorption is exothermic, so the equilibrium shifts back towards the bath at higher temperature; the rate, governed by diffusion, still increases with temperature.
• Q: Which isotherm applies to acid dyes on wool and why? A: Langmuir, because the dye binds to a finite number of protonated amino groups, so uptake saturates when the sites are filled.
• Q: What causes barré in nylon dyeing? A: Differences in amine end-group content, draw ratio or heat history between yarn packages, which change acid-dye uptake and show up as streaks; corrected with levelling agents, blended yarn lots or disperse dyes.
• Q: Why must acrylic be heated slowly through 75–85 °C? A: That is the critical dyeing temperature near Tg, where cationic dye strike is sudden and almost irreversible; a slow rise and a retarder are needed for levelness because these dyes do not migrate afterwards.
• Q: How does cold pad–batch reduce cost and effluent? A: Dye and alkali are padded on and fixed during a 4–12 h cold batch, so there is no heated bath, far less salt and water, higher fixation and much lower colour in the effluent.
• Q: Two batches match on the spectrophotometer but the buyer rejects them. Why? A: Metamerism — the recipes differ in dye combination, so they match under one illuminant and differ under another; check under D65, TL84 and illuminant A and re-match with the same dyes as the standard.

Common mistakes

• Treating salt as a fixing agent and alkali as an exhausting agent — the roles are the opposite way round.
• Reporting fixation as exhaustion: a 95% exhausted reactive bath may still have only 75% total fixation after wash-off.
• Writing "dissolve disperse dye" — it is dispersed, not dissolved; forgetting the dispersing agent and pH 4.5–5 is a classic lab failure.
• Skipping sequestering agent in hard water, then blaming the dye for dull shades, specks or precipitation.
• Dyeing on poorly pretreated fabric — residual size, wax, seed coat or alkali makes level dyeing impossible, and no dyeing recipe can fix it.
• Mixing units in recipe sums: shade % is on weight of material, while auxiliaries are in g/L of liquor, so a change in liquor ratio changes auxiliary weight but not dye weight.
• Omitting oxidation or soaping in vat dyeing, or reduction clearing in dark polyester, then recording poor rubbing fastness as a dye defect.
• Quoting light fastness on the 1–5 grey scale — change in shade and staining use 1–5, but light fastness uses the 1–8 blue wool scale.

Related topics

Pretreatment: desizing, scouring and bleaching · Textile printing · Textile finishing · Colour measurement and computer colour matching · Textile fibre structure and properties · Textile effluent treatment and ZLD