Sustainable and Economical Continuous Vat Dyeing Process

Selim Reza

17 Nov, 2025

Sustainable and Economical Continuous Vat Dyeing Process

Eco-Friendly and Cost-Effective: Continuous Vat Dyeing for a Greener Textile Future

Introduction to Vat Dyeing

Vat dyes are water-insoluble colorants that cannot be applied directly to textile materials in their original form. Because of this insolubility, the vat dyeing process is more complex compared to other dye classes. To make the dye usable, it must first undergo a chemical reduction process known as vatting, which converts the insoluble pigment into a soluble leuco form. Only after this transformation can the dye penetrate the fibers.

Vat dyes are primarily used on cellulose fibers, especially cotton, due to their excellent overall fastness properties. They offer outstanding light fastness, wash fastness, and chemical resistance. However, rubbing fastness is generally lower compared to other reactive or direct dyes.

In Bangladesh, vat dyes are applied using three main dyeing methods:

Exhaust Dyeing
Semi-Continuous Dyeing
Continuous Dyeing

Among these, the continuous process is the most economical and efficient, especially for long yardages of fabric dyed in the same shade. Continuous vat dyeing significantly reduces time, energy consumption, and water effluent, making it a preferred method in large-scale textile production.

Continuous Vat Dyeing Process (PDPS Method)

In continuous dyeing, vat dyes are applied using specialized machinery designed for large-scale production. This method is highly efficient, consistent, and economical, especially when dyeing long yardages of fabric in the same shade.

Machines Used: Thermosol Dyeing Machine. Pad–Steam Machine

Process Type : Pad–Dry–Pad–Steam (PDPS) continuous dyeing method

Dyes Used: CHT (Switzerland) Vat Dyes

Fabric Used: , 100% Woven Cotton (3/1 “S” Twill), Construction: 128 × 60 / 20 × 16

General Phases of Continuous Vat Dyeing

Continuous vat dyeing follows a structured sequence of chemical and mechanical operations that ensure proper dye absorption, fixation, and fastness.

1. Dispersion by Padding

The fabric is passed through a padding mangle containing an aqueous dispersion of vat dye.
This ensures:

Uniform dye distribution
Proper liquor pick-up
Even shade across fabric width and length

2. Reduction (Vatting) with Solubilizing Agent

In this step, a reducing agent—typically sodium hydrosulfite (Na₂S₂O₄) with caustic soda—converts the vat dye into its soluble leuco form.
This transformation allows:

Dye molecules to penetrate the cotton fiber
Improved dye affinity and uptake

3. Dye Molecule Absorption

While passing through controlled temperature zones or steam chambers, the soluble dye is absorbed into the fiber.
Key objectives:

Maximum dye penetration
Uniform fixation
Stable shade development

4. Re-oxidation

After absorption, the dye must be reconverted into its original insoluble form inside the fiber.
Oxidizing agents like:

Hydrogen peroxide
Sodium bromate
Atmospheric oxygen

help lock the dye permanently into the fiber structure.

5. Soaping-off, Washing, and Neutralization

This final stage removes:

Unfixed dye
Reducing agents
Alkali residues

It includes:

Hot wash / boiling wash
Soaping with surfactants
Neutralization (often using acetic acid)
Final cold rinse

The goal is to achieve:

Clean, bright shades
High washing, light, and perspiration fastness
Stable finished fabric

Detailed Description of the Phases in Continuous Vat Dyeing

1. Aqueous Dispersion by Padding

In the first stage, the insoluble vat dyes are dispersed in water to form a stable aqueous dispersion.
During the PDPS (Pad–Dry–Pad–Steam) process, this dye dispersion is applied to the fabric through padding, ensuring:

Uniform liquor pick-up
Even distribution of dye particles across the fabric surface

Proper dispersion is essential for consistent shade and effective vatting in later stages.

2. Vatting (Reduction and Solubilization)

In this phase, the vat dye is converted into its soluble leuco form. This is done using:

Sodium Hydrosulphite (Hydrose) – acts as a reducing agent
Sodium Hydroxide (NaOH) – acts as a solubilizing agent

The reducing agent breaks down the dye molecules, while the alkali converts them into soluble sodium salts.
This reaction allows the dye to become water-soluble, enabling it to penetrate into the cellulose fiber.

3. Dye Absorption

In this stage, dyeing takes place.
The soluble leuco-vat dye, now present as a sodium salt, is absorbed into the cotton fiber from the alkaline reducing medium.
Key points:

Proper temperature and pH help maximize dye uptake
Longer dwell time ensures deeper penetration
Uniform absorption ensures shade consistency

4. Re-oxidation of Vat Dye

Once absorption is complete, the dye inside the fiber must be reconverted into its original insoluble pigment form.
This is done through oxidation, commonly using:

Hydrogen Peroxide (H₂O₂)
Or atmospheric oxygen

During oxidation, the soluble leuco dye becomes insoluble again inside the cotton fiber, ensuring:

Excellent wash fastness
High light and perspiration fastness
Permanent color fixation

5. After-Treatment (Soaping, Washing & Neutralization)

Even after oxidation, small amounts of insoluble dye particles may remain on the fabric surface.
These must be removed to achieve the best fastness properties.

After-treatments include:

Soaping-off with detergents/surfactants
Hot water wash to remove unfixed dye
Neutralization (typically with acetic acid)
Final rinsing

Proper after-treatment ensures:

Improved shade brightness
Higher rubbing fastness
Clean, uniform appearance

General Parameters for Thermosol Machine in Continuous Vat Dyeing (PDPS Process)

Below are the typical operational parameters used in the Thermosol range during the continuous vat dyeing process. These values may vary slightly depending on fabric construction, dye class, shade depth, and machine condition.

Machine Parameters

✔ Machine Speed:

40 m/min

✔ Temperature Zones: (Thermosol machine generally has multiple drying/heating chambers)

1st Zone: 80°C
2nd Zone: 120°C
3rd Zone: 140°C

These stepped temperatures ensure:

Proper moisture evaporation
Controlled dye migration
Prevention of shade variation and streakiness

✔ Blower Temperature:

40°C → 70°C → 80°C
Blower heat helps maintain uniform drying and stabilizes fabric temperature before steaming.

✔ IR (Infrared) Heating:

650°C
IR heating improves:
Surface temperature control
Quick moisture evaporation
Better preparation for the padding/steaming stage

✔ Padder Pressure (Left–Middle–Right):

L-M-R = 20 – 30 – 20 N/mm

This balanced pressure ensures:

Even liquor pick-up
Consistent shade across fabric width
Reduced risk of side-center-side variation

Olive Color — Continuous (PDPS) Recipe (per litre of pad liquor)

Component (trade name)	Dose (g/L)	Role
Bezathren Olive T-EPS	21.07	Main vat pigment (olive shade)
Bezathren Yellow 3RT PS	2.35	Shade modifier — yellow contribution
Bezathren Red FBB EPS	1.24	Shade modifier — red contribution
Heptol SF4	2.00	Wetting / leveling aid (improves penetration & wetting)
Colorcontin SAN	2.00	Dispersant / anti-redeposit (keeps dye dispersed)
Thermacol MP	12.00	Penetration & leveling / carriers or process assistant (as supplied)
Antimussol UDF	0.20	Anti-musseling / lubricant to improve fabric movement

Recommended Pad Liquor & Application Parameters

Pad liquor preparation: Mix dyes & auxiliaries into warm water with thorough agitation to give a homogenous dispersion. Use the dispersant (Colorcontin SAN) and Heptol SF4 to stabilize the dispersion before padding.
Pad pickup (target): 70–80% wet pick-up (adjust to machine & fabric to obtain uniform shade).
Padder pressure (L–M–R): 20 – 30 – 20 N/mm (as your machine settings).
Pad → Dry → Pad → Steam (PDPS) sequence for continuous run.

Vatting (Reduction) — practical notes

Vatting agents are sodium hydrosulphite (hydrose) and sodium hydroxide (NaOH) to convert the vat dye into the soluble leuco-form.
Typical approach: prepare a reducing/alkali bath or apply in a controlled on-machine step (follow CHT supplier guidance for exact dosages).
Control points: pH, temperature and freshness of reducing solution are critical to avoid re-oxidation/spotting. Work under inert conditions as far as possible and avoid excess aeration prior to fiber penetration.

Supplier note: Exact hydrosulphite & caustic doses depend on dye batch, shade depth and fabric — always confirm with dye supplier (CHT) and run lab trials.

Steaming / Thermosol & Oxidation

Machine speed: 40 m/min (as provided).
Thermosol zones (typical): 80°C → 120°C → 140°C (follow your machine programme).
Blower temps: 40°C → 70°C → 80°C; IR: 650°C (as machine spec).
Oxidation: after dye absorption, re-oxidise the leuco dye to the insoluble form. Oxidation can be achieved by controlled chemical oxidant or controlled aeration/steam + mild oxidant. Use the oxidant and conditions recommended by the vat dye supplier to avoid yellowing or surface deposits.

After-treatment (soaping / washing / neutralisation)

Soaping / hot wash: Remove surface-deposited insoluble particles to improve rubbing fastness and brightness.
Neutralisation: Remove residual alkali (use dilute acetic acid or recommended neutraliser).
Final rinse: Cold rinse to remove any traces of auxiliaries.

Quality control & targets

Check: Shade match (spectrophotometer), wet rub, dry rub, wash fastness, and pH of final fabric.
Common defects to watch: side/centre shade variation, oxidation marks, surface specks (incomplete soaping), and poor rub fastness (insufficient removal of surface dye).

Safety & handling

Follow standard chemical safety: PPE (gloves, goggles, apron), good ventilation, avoid skin contact with NaOH and hydrosulphite.
Prepare fresh reducing baths and discard/neutralise wastes per local disposal regulations.
Store dyes and auxiliaries according to MSDS instructions.

Process Description of Continuous Vat Dyeing (PDPS Process)

Vat dyes are insoluble in water, so in continuous dyeing they must first be dispersed and later converted into their soluble leuco form through chemical reduction. The PDPS method ensures uniform dye penetration, high color fastness, and efficient large-scale production.

1. Preparation of Vat Dye Liquor (First Pad Bath)

In continuous vat dyeing, the dye is applied to the fabric as a dispersion. Since vat dyes are not water-soluble, proper dispersion is critical.

Key Steps:

Prepare the vat dye solution using:
- Dispersing agent
- Wetting agent
- Sequestering agent
- Anti-migrating agent
- (All chemicals added according to the recipe)
Hot water must be used during mixing to ensure uniform dissolution and dispersion of dye particles.
The mixture is stirred properly to form a stable dye dispersion.

2. Fabric Padding with Vat Dye Dispersion

The fabric is passed through the dye bath and squeezed by the padder to maintain the required wet pickup.

Machine parameters used:

Thermosol Machine Speed: 40 m/min
Temperature Profile: 80°C → 120°C → 140°C
Blower Temp: 40°C → 70°C → 80°C
IR Temperature: 650°C
Padder Pressure (L/M/R): 20 / 30 / 20 N/mm

After padding, the fabric contains a uniform amount of dispersed vat dye.

3. Drying (IR + Hot Flue Dryer)

The padded fabric passes through IR heaters followed by hot flue drying chambers.

Purpose:

Remove surface moisture
Fix the dispersion uniformly
Prevent migration of dyes
Prepare the fabric for chemical reduction in the next stage

Once dried, the fabric is cooled before entering the chemical pad bath.

4. Chemical Padding (Reduction & Alkali Pad Bath)

The dried fabric is treated with a reducing and alkaline solution in the Pad Steam machine.

Chemical bath contains:

Sodium Hydrosulphite (Reducing agent)
Caustic Soda (NaOH) (Alkalinity creator)
Wetting agent
Anti-migrant (if required)
(Exact dosage based on recipe)

Function:

Sodium hydrosulphite reduces the insoluble Vat Dye into soluble leuco vat dye.
Caustic soda provides the required pH (approx. 11.5–12) for reduction reaction.
Wetting agents help the leuco dye penetrate more deeply into the cotton fiber.

5. Steaming (Leuco Formation & Dye Fixation)

After chemical padding, the fabric is immediately steamed.

Steaming Conditions:

Temperature: 101–102°C
Time: 1.5–1.6 minutes

Purpose:

Convert vat dye into fully soluble leuco form
Swell cotton fibers for deeper dye penetration
Fix the dye molecules inside the fiber structure

6. Oxidation

Once dye penetration is complete, the fabric is oxidized to convert the leuco form back into its original insoluble vat dye form inside the fiber.

Oxidation agents may include:

Hydrogen peroxide
Sodium bromate
Atmospheric oxidation (rare in continuous process)

7. Soaping-Off, Washing & Neutralization

To remove unfixed dye particles and improve fastness:

High-temperature soaping is performed (90–95°C).
Multiple washings remove alkali and unfixed dye.
Neutralization using acetic acid ensures removal of residual alkali.

This final finishing ensures excellent wash fastness, rubbing fastness, and shade stability.

Summary of Key Stages in PDPS Process

Dye dispersion preparation
Padding with vat dye dispersion
IR + hot flue drying
Chemical padding (Sodium hydrosulphite + NaOH)
Steaming (Reduction & dye penetration)
Oxidation
Soaping, washing & neutralization

Continuous Vat Dyeing (PDPS) Process Flowchart — Pad Steam Section

Machine & Process Parameters

Machine Name: Goller Pad Steam Machine
Machine Speed: 40 m/min
Padder Pressure (L–M–R): 20 – 30 – 20 N/mm
Steamer Temperature: 101–102°C
Steaming Time: 1.5 min (approx.)
Drying Unit Temperature: 113–115°C

Pad Steam Chemical Bath Recipe

Component	Dose (g/L)	Function
Sodium Hydrosulphite (Hydrose)	40	Reducing agent — converts insoluble vat dye into soluble leuco form
Caustic Soda (NaOH)	40	Alkali — provides high pH to assist reduction and solubilization
Colorcontin SAN	2	Dispersant / anti-redeposit agent — ensures uniform dye penetration

Process Description (Pad Steam Section)

Fabric Entry: The pad-dried fabric from the first padding and drying stage enters the Pad Steam machine.
Chemical Padding: Fabric is immersed in the chemical pad bath containing hydrosulphite, NaOH, and dispersant.
Padding & Pressure Application: The padder applies controlled pressure (L–M–R = 20–30–20 N/mm) to ensure uniform chemical pick-up.
Steaming: Immediately after padding, the fabric passes through the steamer at 101–102°C for ~1.5 min, allowing the vat dye to be converted into its soluble leuco form and penetrate the cotton fibers.
Drying: After steaming, the fabric passes through the drying unit at 113–115°C to remove excess moisture and stabilize the fabric for further processing (oxidation and soaping).

This can be illustrated in a simple schematic flowchart as:

Fabric → Chemical Pad Bath (Hydrose + NaOH + Dispersant) → Padder → Steamer (101–102°C, 1.5 min) → Drying Unit (113–115°C) → Cooling → Oxidation & Soaping

Pad Steam Wash Unit — Continuous Vat Dyeing

Wash Unit Temperature Profile

Bath / Stage	Temperature (°C)
Bath 01	30 (Normal temperature wash)
Bath 02	70 (Medium hot wash)
Bath 03	70 (Medium hot wash with H₂O₂)
Bath 04	90 (Hot wash with water overflow)
Bath 05	90 (Hot wash with soaping chemical)
Bath 06	90 (Hot wash)
Bath 07	90 (Hot wash)
Last Bath	30 (Neutralization)

Chemical Recipe for Wash Unit

Chemical	Dose	Purpose
Hydrogen Peroxide (H₂O₂, 50% conc.)	10 g/L	Oxidation — re-oxidizes leuco vat dye to insoluble form
Albatex AD	2–3 g/L (Soaping bath 3–5 g/L)	Soaping / wetting agent — removes unfixed dye, improves rubbing fastness
Acetic Acid (CH₃COOH)	0.5–1 g/L	Neutralization — removes residual alkali from previous stages

Stepwise Wash Unit Process

Bath 01 – Normal Wash (30°C): Removes surface dust, loose particles, and water-soluble impurities.
Bath 02 – Medium Hot Wash (70°C): Removes water-soluble auxiliaries and improves penetration for oxidation.
Bath 03 – Hydrogen Peroxide Medium Hot Wash (70°C): Dose H₂O₂: 5–10 g/L,
- Oxidizes leuco vat dye back to insoluble form inside cotton fiber.
Bath 04 – Hot Wash with Overflow (90°C): Removes residual H₂O₂ and other water-soluble chemicals. Overflow ensures continuous fresh water replacement.
Bath 05 – Hot Wash with Soaping Chemical (90°C): Add Albatex AD: 3–5 g/L, Removes surface-deposited dye particles for optimum rubbing fastness.
Bath 06 & 07 – Hot Wash (90°C): Ensures complete removal of unfixed dye and soaping chemicals.
Last Bath – Neutralization (30°C): Add Acetic Acid: 0.5–1 g/L, Neutralizes residual alkali, stabilizes fabric pH, and prevents yellowing.

Notes / Key Points

All temperatures and chemical doses may be adjusted slightly depending on fabric construction, dye class, and depth of shade.
Proper control of H₂O₂ concentration and pH is essential to avoid over-oxidation or fabric damage.
Continuous water overflow in bath 04 improves washing efficiency and reduces dye effluent concentration.
This wash sequence ensures: Maximum fastness properties (washing, rubbing, light). Complete removal of surface-deposited dye. Neutral pH fabric ready for finishing

Reference (Theory Only):

Chemistry of Dyeing / D.P. Chattopadhyay, in Handbook of Textile and Industrial Dyeing, 2011