Reactive Dyes for Cotton Printing: Chemistry, Fixation, and Wash Fastness
Why Reactive Dyes Dominate Cotton Printing
Cotton textile printing demands colourants that survive repeated laundering at 40–60°C with detergent, perspiration exposure, rubbing, and chlorine bleach in some markets. Reactive dyes meet this challenge uniquely: they form covalent bonds with cellulose hydroxyl groups, creating a dye-fibre union that behaves chemically like a single molecule.
The bond strength of a covalent ether or vinyl sulphone linkage (160–250 kJ/mol) far exceeds the hydrogen bonding and van der Waals interactions holding vat or direct dyes on fibre. This translates to wash fastness ratings of 4–5 on the ISO 105-C06 scale, compared to 2–3 for direct dyes and 3–4 for vat dyes at equivalent difficulty.
Reactive Dye Chemistry
Reactive Groups
The reactive group is the electrophilic moiety that bonds with cellulose nucleophiles under alkaline conditions. The main commercial types:
| Reactive Group | Bond Type | Fixation pH | Temp (°C) | Hydrolysis Rate |
|---|---|---|---|---|
| Monochlorotriazine (MCT) | C-O ether | 11–12 | 100–103 (steam) | Low |
| Dichlorotriazine (DCT) | C-O ether | 10–11 | 60–80 | Medium |
| Vinyl sulphone (VS) | C-O ether | 11–12 | 60–80 | Medium |
| Bifunctional (MCT+VS) | C-O ether (×2) | 10–11 | 60–80 | Low |
| Monofluorotriazine (MFT) | C-O ether | 8–9 | 100–103 | Very low |
Monochlorotriazine (MCT) dyes (e.g., Procion H-type) require high-temperature steaming (100–103°C) for fixation. They are the most stable during printing paste storage — pot life exceeds 8 hours at 20°C. Used predominantly in rotary screen and flat-bed screen printing.
Vinyl sulphone dyes (e.g., Remazol-type) fix at lower temperatures (60°C alkali pad-dry-steam) and offer higher colour yield. However, vinyl sulphone groups are more sensitive to moisture during storage — printing pastes must be used within 4–6 hours.
Bifunctional dyes (MCT + VS) combine the storage stability of MCT with improved fixation efficiency. At pH 11.5 and 102°C steam, fixation degrees of 80–90% are achievable, versus 65–75% for monofunctional dyes. This is the fastest-growing segment.
Fixation Chemistry
The fixation reaction is a nucleophilic substitution (MCT/DCT, MFT) or addition (VS):
- MCT: Cl is displaced by cellulose -O⁻; hydroxide competes (hydrolysis side reaction)
- VS: -CH=CH-SO₂- attacks cellulose -O⁻ via Michael addition
The competing hydrolysis reaction is unavoidable: water also reacts with the reactive group, producing a non-fibre-reactive dye that must be washed off. Degree of fixation (DOF) = (dye bonded to fibre) / (total dye applied) × 100%.
Typical DOF values:
- MCT monofunctional: 60–75%
- VS monofunctional: 65–80%
- Bifunctional: 80–90%
The hydrolysed (unfixed) fraction is the primary source of wash-off difficulty and effluent colour load in textile printing.
Printing Paste Formulation
A standard reactive dye printing paste consists of:
| Component | Function | Typical Level |
|---|---|---|
| Reactive dye | Colourant | 10–80 g/kg |
| Sodium alginate thickener | Viscosity, definition | 20–60 g/kg (5–8% stock) |
| Urea | Hygroscopic agent, dye migration | 50–150 g/kg |
| Sodium bicarbonate | Alkali source (mild, for storage) | 10–20 g/kg |
| Sodium carbonate (soda ash) | Fixation alkali | 15–25 g/kg (added just before printing) |
| Resist salt L | Mild oxidant, prevents reduction | 10 g/kg |
| Water | Balance | to 1000 g |
Urea is essential for cellulose swelling during steaming, facilitating dye migration to fibre and improving dye solubility. Reducing urea below 50 g/kg reduces colour yield by 15–25%, particularly in deep shades. In digital (inkjet) printing, urea is replaced by functional pre-treatment of the fabric.
Sodium alginate at 4–6% in stock solution, used at 3–5% in paste, gives printing viscosity of 4,000–8,000 mPa·s (Brookfield, 20°C, 20 rpm) for screen printing. Alginate is preferred over synthetic thickeners because it does not react with reactive dyes and washes off completely, leaving no stiffness.
Alkali must be added immediately before printing — never to the stock paste for storage. Pot life of alkalised paste is 4–8 hours (MCT) or 3–5 hours (VS) at 20°C.
Fixation Conditions
Steaming (Screen Printing)
MCT dyes require saturated steam at 100–103°C for 5–12 minutes. Superheated or dry steam causes poor fixation. A rapid ager or festoon steamer provides the controlled moisture environment needed.
Critical parameters for steaming:
| Parameter | Target | Effect of Deviation |
|---|---|---|
| Temperature | 102 ± 1°C | ↓ temp: lower DOF, paler shade |
| Time | 8–10 min (MCT) | Under-time: hydrolysed dye not fixed |
| Steam saturation | 100% RH | Dry steam: severe fixation loss |
| Fabric moisture | 10–15% | Too dry: poor dye migration |
Pad-Steam (Continuous Fixation)
For VS and bifunctional dyes, continuous pad-steam processes (J-box or loop steamer) at 102°C for 60–90 seconds are viable. Pad liquor pH is adjusted to 12–12.5 with NaOH; fabric enters the steamer within 30 seconds of padding to minimise pre-fixation issues.
Cold Pad Batch (CPB)
CPB is energy-efficient for long runs: fabric is padded with dye + alkali solution (NaOH, sodium silicate), batched on a roll at 20°C for 12–24 hours, then washed. Bifunctional dyes give DOF of 85–92% by CPB, with excellent level dyeing. Print applications of CPB are limited by definition requirements.
Washing-Off Protocol
The washing-off stage removes unfixed hydrolysed dye, print paste chemicals, and auxiliaries from the fabric surface. Incomplete washing causes:
- Bleeding in subsequent wash
- Reduced wet fastness rating
- Staining of adjacent light fabrics
Standard washing sequence for reactive printed cotton:
- Cold rinse (2× overflows, 20°C) — removes print paste and loosely associated dye
- Hot rinse (80°C) — removes urea, alginate, and partially fixed dye
- Soap wash (95°C, 2 g/L non-ionic detergent, 10 min) — removes surface dye deposits
- Hot rinse (80°C) — removes soap and detergent
- Warm rinse (40°C) — reduces thermal shock before drying
- Cold rinse — final
Water volume: 10–15 L/kg fabric minimum for each stage. Inadequate washing is the single biggest cause of poor wash fastness in reactive printing — a washing fastness rating of 3 typically improves to 4–5 simply by adding one additional hot rinse step.
Wash Fastness Standards
| Test | Standard | Target Rating | Method |
|---|---|---|---|
| Wash fastness | ISO 105-C06 | 4–5 | Multi-fibre adjacent fabric |
| Wet rubbing | ISO 105-X12 | 3–4 | Wet rubbing crock tester |
| Dry rubbing | ISO 105-X12 | 4–5 | Dry rubbing |
| Perspiration | ISO 105-E04 | 4–5 | Acid and alkaline |
| Light fastness | ISO 105-B02 | 5–7 | Blue wool scale |
Summary
Reactive dyes for cotton printing achieve industry-leading wash fastness through covalent bonding with cellulose under alkaline conditions. Bifunctional dyes (MCT + vinyl sulphone) represent the current performance optimum with DOF of 80–90%, pot life of 6–8 hours, and wash fastness of 4–5 on ISO 105-C06. Optimising the steaming stage (102°C saturated steam, 8–10 min) and implementing a thorough five-step washing sequence are equally important as dye selection. Chemzip supplies a full range of reactive dyes for cotton printing — from economical monofunctional grades to high-fixation bifunctional systems — together with sodium alginate thickeners and print paste auxiliaries.