pH, Temperature, Salt and Alkali Management in Reactive Dyeing

1. Purpose / Amaç

This technical publication explains how pH, Temperature / Sıcaklık, Salt–Electrolyte / Tuz–Elektrolit and Alkali / Alkali must be managed as parts of one reaction system during Exhaust Reactive Dyeing / Çektirme Yöntemiyle Reaktif Boyama of cotton and other cellulosic materials.

The main shop-floor rule is:

Salt does not chemically fix the dye to the fibre. It supports the approach and exhaustion of the dye onto cellulose. Covalent fixation is enabled by cellulose activated under alkaline conditions. The same alkali and temperature that accelerate dye–fibre reaction can also accelerate dye hydrolysis in water.

The target is therefore not merely a dark shade. The process must simultaneously deliver Levelness / Düzgünlük, Reproducibility / Tekrarlanabilirlik, Fixation / Fiksaj, Wash Fastness / Yıkama Haslığı, Wet Rubbing Fastness / Yaş Sürtme Haslığı and a low reprocessing rate.

Image note: The current process image is representative. The preferred field photograph should show the dyeing-machine sampling point together with the pH probe or dosing line, from an approximately 45° angle and without company-identifying information.

2. Scope and Technical Boundary / Kapsam ve Teknik Sınır

The publication focuses on:

• Cotton, viscose and similar cellulosic fibres
• Knitted or woven fabrics
• Exhaust reactive dyeing
• Single-bath or controlled multi-stage processes
• Conventional salt–alkali systems

The following applications use related control principles but require separate validation:

• Cationized Cotton / Katyonize Pamuk
• Salt-free or low-salt speciality systems
• Cold Pad-Batch
• Pad–Steam and continuous dyeing
• Reactive printing
• Reactive dyes on protein fibres or polyamide
• Special buffered or neutral fixation systems

Technical warning: The pH and temperature ranges in this publication are general process windows, not universal recipes. The dye supplier’s Technical Data Sheet (TDS) / Teknik Veri Föyü and mill validation always take precedence. A change in reactive-group chemistry can require a different temperature, alkali system, dosing time and fixation time.

3. Reaction Logic / Reaksiyon Mantığı

Three events proceed together in reactive dyeing:

1. Exhaustion / Banyodan Çekim: Transport of the anionic dye from the bath to the cellulose surface.
2. Diffusion / Lif İçine Difüzyon: Movement of dye into accessible fibre regions.
3. Covalent Fixation / Kovalent Bağlanma: Reaction of the reactive group with activated cellulose.

Simplified activation:

Cell-OH + OH- <=> Cell-O- + H2O

Simplified fixation:

Dye-X + Cell-O- -> Dye-O-Cell + X-

Undesired hydrolysis:

Dye-X + OH- / H2O -> Dye-OH

Where:

• Cell-OH: cellulose hydroxyl group
• Cell-O-: cellulose activated under alkaline conditions
• Dye-X: dye containing an active reactive group
• Dye-OH: hydrolysed dye that can no longer form a covalent bond with the fibre

These equations are simplified. The actual mechanism differs for monochlorotriazine, vinyl sulfone, bifunctional and polyfunctional reactive groups.

4. Four Main Process Variables / Dört Ana Proses Değişkeni

4.1 pH Management / pH Yönetimi

pH determines cellulose activation and dye reactivity.

• A neutral or near-neutral phase supports controlled exhaustion and distribution.
• Alkali activates cellulose and starts covalent fixation.
• If pH remains too low, fixation can be inadequate.
• If pH rises too fast or too high, local fixation, unlevelness and hydrolysis risk increase.

Many conventional systems may operate at approximately pH 10.5–11.5 during fixation. This is not a universal recipe and must be validated together with dye class, alkali system, liquor ratio and temperature.

Measurement discipline:

• Calibrate the pH meter daily or at the start of the shift with suitable buffers.
• Do not compare pH readings taken at uncontrolled sample temperatures.
• Keep the mill measurement method constant even when automatic temperature compensation is used.
• Record the pH profile throughout alkali dosing, not only at the beginning and end.

4.2 Temperature Management / Sıcaklık Yönetimi

Temperature simultaneously affects diffusion, reaction rate and hydrolysis rate.

General classification:

Reactive system	General temperature approach	Technical note
Cold-reactive / Soğuk tip	approximately 30–40 °C	High reactivity; alkali and dwell time are critical
Warm-reactive / Ilık tip	approximately 50–60 °C	Many bifunctional systems operate in this region
Hot-reactive / Sıcak tip	approximately 70–80 °C	Lower reactivity may require higher temperature

This table is only a classification. Recommended temperatures can differ even within one commercial dye range.

Critical shop-floor points:

• Do not start fixation time before the full load reaches target temperature.
• Define the heating rate together with machine load and circulation.
• Verify actual bath temperature; do not rely only on the controller set value.
• Temperature variation can magnify shade variation, especially when it overlaps with alkali dosing.

4.3 Salt–Electrolyte Management / Tuz–Elektrolit Yönetimi

Cellulose and reactive dye are generally negatively charged in the dyebath. Sodium Chloride / Sodyum Klorür or Sodium Sulfate / Sodyum Sülfat reduces the effect of electrostatic repulsion and supports dye approach and exhaustion onto the fibre.

Salt:

• Increases exhaustion.
• Changes the exhaustion rate.
• Influences the levelness–migration balance.
• Raises ionic strength and conductivity.

Salt does not:

• Create covalent fixation by itself.

Excessive or rapid salt addition can cause:

• Sudden surface exhaustion,
• Head–tail, inside–outside or centre–edge variation,
• Low migration,
• Increased unlevelness risk.

Insufficient salt or low effective ionic strength can cause:

• Low exhaustion,
• Pale shade,
• Excess dye remaining in the bath,
• Low overall colour yield.

Conductivity warning: Conductivity is useful for monitoring electrolyte addition, but salt type, alkali, temperature, water hardness and other ions affect the reading. Do not use a universal “mS/cm = g/L salt” conversion. Establish a mill-specific calibration curve at controlled temperature.

4.4 Alkali Management / Alkali Yönetimi

Common alkali sources include:

• Soda Ash / Sodium Carbonate
• Caustic Soda / Sodium Hydroxide
• Sodium Bicarbonate
• Buffered commercial alkali systems

Alkali selection depends on:

• Reactive-group chemistry
• Shade depth
• Dye combination
• Liquor ratio
• Temperature
• Dosing time
• Machine circulation
• Required fixation profile

A single-point addition of concentrated alkali can create a local pH spike. The result may be rapid local fixation, staining, shade variation or poor reproducibility.

A controlled approach is to:

• Prepare alkali solution at a validated concentration,
• Ensure complete dissolution and filtration,
• Use stepped or linear dosing,
• Relate dosing time to machine rotation and bath turnover time,
• Check backflow and dead volume in the dosing line,
• Verify the pH profile with actual bath samples.

5. Liquor Ratio and Machine Hydrodynamics / Flotte Oranı ve Makine Hidrodinamiği

The same recipe percentage does not create the same g/L value at a different Liquor Ratio (LR) / Flotte Oranı or effective bath volume.

Definition:

LR = Bath volume (L) / Fabric mass (kg)

Approximate bath volume:

V = M x LR

Where:

• V: effective bath volume, L
• M: fabric mass, kg
• LR: L/kg

Actual machine volume can differ because of pipe dead volume, filter, heat exchanger, dosing vessel and liquor retained by the fabric. Use the validated effective machine volume for recipe calculations.

Hydrodynamic controls include:

• Nozzle pressure and diameter
• Fabric speed and cycle time
• Pump flow
• Loading ratio
• Rope opening
• Filter cleanliness
• Foam and air locks
• Mixing time after dosing

6. Recommended Process Control Sequence / Önerilen Proses Kontrol Sırası

Stage 1 — Pre-check / Ön Kontrol

• Verify fabric lot, mass and loading quantity.
• Check pretreatment uniformity, absorbency and residual peroxide.
• Record water pH, hardness and conductivity.
• Verify dye and chemical lots.
• Check machine cleanliness, filter and dosing lines.

Stage 2 — Dye Admission and Homogenization / Boya Alımı ve Homojenizasyon

• Fully dissolve or disperse dye in suitable water.
• Filter according to supplier instructions.
• Admit dye to the bath in a controlled manner.
• Allow one or more validated bath turnovers for homogenization.

Stage 3 — Salt-assisted Exhaustion / Tuzla Çekim

• Dose salt in steps or linearly according to the validated recipe.
• Allow sufficient circulation after each step.
• Monitor conductivity and sample shade together.
• Delay the next step if rapid surface exhaustion is observed.

Stage 4 — Temperature Stabilization / Sıcaklık Stabilizasyonu

• Heat at a controlled rate.
• Do not start alkali fixation until actual bath temperature is stable.
• Use independent measurement if internal temperature uniformity is doubtful.

Stage 5 — Alkali Fixation / Alkaliyle Fiksaj

• Dose alkali in steps or linearly.
• Record pH at the start, intermediate points and end of dosing.
• Start fixation time only after final alkali addition and sufficient bath homogenization.
• Take cycle-based intermediate samples when required.

Stage 6 — Rinsing, Neutralization and Washing-off / Durulama, Nötralizasyon ve Yıkama

• Remove free electrolyte and alkali by controlled rinsing.
• Neutralize at the validated stage.
• Remove hydrolysed and unfixed dye with an adequate time–temperature–chemical combination.
• Verify final bath pH and fabric-extract pH against customer requirements.

7. Indicative Process Window / Gösterge Proses Penceresi

This table is a control logic, not a recipe:

Process stage	Main variable	General target
Dye admission	pH, dissolution, filtration, temperature	Homogeneous distribution before fixation
Salt dosing	Conductivity, dosing time, bath turnovers	Controlled and level exhaustion
Heating	Actual temperature, heating rate	Uniform temperature throughout the load
Alkali dosing	pH profile, dosing curve, cycle time	Fixation without local pH spikes
Fixation	pH, temperature, time, intermediate shade	Target colour yield and reproducibility
Washing-off	pH, temperature, time, bath clarity	Removal of hydrolysed dye
Final control	ΔE, K/S, wash and rubbing fastness	Conformance to specification

8. Dosing Calculations / Dozaj Hesapları

8.1 Salt Quantity / Tuz Miktarı

Salt (kg) = Target concentration (g/L) x Effective bath volume (L) / 1000

8.2 Commercial Alkali Quantity / Ticari Alkali Miktarı

Commercial alkali (kg)
= Pure active target (g/L) x Bath volume (L)
  / [1000 x Product active fraction]

Use the active fraction as a decimal. For 99% purity, use 0.99.

8.3 Worked Example / Hesaplama Örneği

Data:

• Fabric: 400 kg
• Liquor ratio: 1:6
• Effective bath volume: 2,400 L
• Salt target: 50 g/L
• Pure soda target: 12 g/L
• Commercial soda purity: 99%

Bath volume:

V = 400 x 6 = 2,400 L

Salt:

Salt = 50 x 2,400 / 1000
Salt = 120.0 kg

Commercial soda ash:

Commercial soda = 12 x 2,400 / (1000 x 0.99)
Commercial soda = 29.1 kg

Technical warning: The 50 g/L and 12 g/L values are used only to demonstrate the calculation. They are not dyeing recommendations. Actual targets must be based on dye combination, shade depth, reactive group, LR and TDS.

9. Exhaustion, Colour Yield and Fixation Indicators / Çekim, Renk Verimi ve Fiksaj Göstergeleri

Approximate exhaustion by dyebath absorbance:

E (%) = [(A0 - At) / A0] x 100

Where:

• A0: initial dyebath absorbance
• At: absorbance at time t
• E: approximate exhaustion percentage

Keep dilution ratio, wavelength, cuvette, temperature and turbidity procedure constant.

Colour-strength retention before and after washing-off:

Colour-strength retention (%)
= (K/S_after washing-off / K/S_before washing-off) x 100

This is not an absolute chemical fixation percentage. It is an operational comparison indicator for the same fabric, dye combination and measurement conditions.

Evaluate colour difference as:

ΔE_CMC(l:c)

under identical measurement geometry and conditions, using the ISO 105-J03 approach. Define acceptance limits from the customer specification.

10. Measurement and Sampling Plan / Ölçüm ve Numune Alma Planı

Sampling point	Measurement
Fresh water	pH, hardness, conductivity, temperature
After dye admission	pH, temperature, visual homogeneity
Before salt	Initial absorbance or reference sample
After each salt step	Conductivity, cycle count, intermediate shade
Start of alkali dosing	pH, temperature, cycle time
Alkali intermediate points	pH, conductivity, percentage dosed
End of alkali dosing	Final pH, temperature, homogenization time
Mid and end fixation	Intermediate shade, bath absorbance, process time
End of washing-off	Bath clarity, pH, conductivity
Finished fabric	ΔE, K/S, extract pH, wash and rubbing fastness

11. Defect–Root Cause–Corrective Action / Hata–Kök Neden–Düzeltici Faaliyet

Observed defect	Possible root cause	Verification	Corrective action
Unlevel dyeing	Salt or alkali too fast; poor circulation; local pH; temperature difference	Dosing graph, cycle time, pH trend, machine flow	Extend dosing; redefine steps; verify circulation and mixing
Pale shade / low K/S	Low exhaustion; insufficient alkali; inadequate fixation temperature/time; hydrolysis	Bath absorbance, pH, temperature and time trend	Correct the salt–alkali–temperature profile by single-variable trials and TDS
Dark shade	Excess exhaustion; effective-volume error; weighing or dye-strength variation	Bath volume, weighing, dye lot, conductivity	Calibrate machine volume; verify weighing and recipe transfer
Batch-to-batch variation	Water quality, LR, dosing time, pH or dye-lot variation	Batch comparison graph	Apply SPC limits; record water and chemical lots
Poor wet rubbing	Inadequate washing-off; redeposition of hydrolysed dye; surface dyeing	Wash bath, surface K/S, wet crock test	Optimize rinse–neutralize–soap sequence and dispersing system
Poor wash fastness	Unfixed dye; inadequate fixation or washing-off	ISO 105-C06 and process samples	Verify fixation; improve washing-off temperature, time and bath count
Dye spots	Incomplete dissolution; missing filtration; hard water; local alkali	Filter residue, particles, hardness	Correct dissolution, filtration, sequestering and dosing lines
Head–tail variation	Rapid exhaustion; dosing-line dead volume; sampling timing	First/last cycle samples, dosing-line check	Redesign dosing entry and mixing time
High fabric pH	Inadequate neutralization or rinsing	ISO 3071 extract pH	Correct neutralization quantity and rinse sequence

12. Mandatory Process Record Fields / Proses Kayıt Formunda Zorunlu Alanlar

• Order, shade, fabric and lot code
• Machine number and nominal load
• Actual fabric mass
• Effective bath volume and LR
• Water pH, hardness and conductivity
• Dye name, batch number and weighed quantity
• Salt type, batch, quantity and dosing time
• Alkali type, activity, quantity and dosing time
• Heating rate and actual temperature trend
• pH trend
• Conductivity trend
• Fabric cycle time
• Intermediate sample times and ΔE/K/S results
• Washing-off steps
• Final fastness and extract-pH results
• Deviation, intervention and reprocessing record

13. Safety and Chemical Preparation / Güvenlik ve Kimyasal Hazırlama

• Prepare alkali solutions with suitable personal protective equipment.
• Add concentrated caustic soda to water slowly and under control; account for splashing and exothermic heat.
• Prepare dye and chemical solutions at validated temperatures.
• Do not mix incompatible chemicals in one preparation vessel.
• Clean dosing tanks, lines and valves during product change.
• The Safety Data Sheet (SDS) and mill safety procedure take precedence.

14. Technical Warning on Acceptance Criteria / Kabul Kriteri Hakkında Teknik İkaz

This publication does not define a universal:

• Salt g/L,
• Alkali g/L,
• pH tolerance,
• Fixation time,
• Heating rate,
• ΔE limit,
• Fastness grade.

Acceptance criteria must be based on dye range, customer specification, shade depth, fibre blend, machine type, LR, water quality and mill process capability.

Especially important: Two dye combinations operating at the same pH and temperature may not accept the same dosing profile because their reactive-group compatibility and exhaustion kinetics differ.

15. Sustainability and Right-First-Time Production / Sürdürülebilirlik ve İlk Seferde Doğru Üretim

Environmental improvement in reactive dyeing is not limited to lowering salt or water. A major gain is Right First Time (RFT) / İlk Seferde Doğru production.

Monitor:

• kg salt / kg fabric
• kg alkali / kg fabric
• L water / kg fabric
• kWh / kg fabric
• Steam / kg fabric
• Right-first-time rate
• Correction and redye rate
• Number of washing-off baths
• Effluent COD, colour and conductivity load

When applying low-salt or high-fixation dye systems, do not transfer the conventional recipe directly. Validate through laboratory, pilot and production trials.

16. Shop-Floor Control Summary / Saha Kontrol Özeti

1. Verify pretreatment and residual peroxide.
2. Verify effective bath volume and LR.
3. Fully dissolve, filter and homogenize dye.
4. Dose salt under control; monitor conductivity with a mill calibration.
5. Stabilize target temperature throughout the load.
6. Dose alkali in steps or linearly according to bath turnover time.
7. Record pH throughout dosing, not only at the end.
8. Start fixation timing from the actual homogenization point.
9. Remove hydrolysed dye by the correct rinse–neutralize–wash-off sequence.
10. Close colour, fastness and fabric-extract pH in the same batch record.

17. References / Kaynaklar

1. CottonWorks — Dyeing Basics
2. CottonWorks — Textile Dyeing Booklet
3. DyStar — Cadira Reactive Brochure
4. DyStar — Sera ECO WASH
5. ISO 3071:2020 — Determination of pH of Aqueous Extract
6. ISO 105-C06:2010 — Colour Fastness to Domestic and Commercial Laundering
7. ISO 105-X12:2016 — Colour Fastness to Rubbing
8. ISO 105-E04:2013 — Colour Fastness to Perspiration
9. ISO 105-J03:2009 — Calculation of Colour Differences
10. AATCC TM61 — Colorfastness to Laundering: Accelerated
11. AATCC TM8 — Colorfastness to Crocking: Crockmeter
12. Eco-friendly Salt/Alkali-free Exhaustion Dyeing of Cotton Fabric with Reactive Dyes