Sustainable and Economical Continuous Vat Dyeing Process

Mazharul Islam
Sr. Executive (Solid Dyeing)
M.Sc. in Textile Engineering (1st Semester)
B.Sc. In Textile Engineering (PTEC Affiliated BUTex)
Email: [email protected]

Introduction to Vat Dyeing:
Vat dye is water insoluble and can’t be applied directly on textile material. Vat Dyeing process is difficult from others dyeing. It involves two process. Here a process Vatting is needed for making the insoluble vat dyes into soluble form. Vat Dye mainly used for cellulose fiber. Compare with other dye, vat dye provide best fastness properties though rubbing fastness is not good. In Bangladesh Vat dye is applied through Exhaust, Semi-Continuous and Continuous Process. Among all the process, Continuous dyeing is preferable & economical particularly where long yardages fabric have to be dyed in the same recipe of color. Vat continuous dyeing saves our time, energy and minimize water effluent.

For Continuous Process:

• Machine Use: Thermosol Dyeing machine & Pad Steam machine.
• Process: Pad Dry Pad Seam Process (PDPS)
• Vat Dye: We used CHT (Switzerland) vat Dyes.
• Fabric: We used 100% Woven Cotton (3/1 “S” Twill), construction: 128×60/20×16.

General phases of continuous vat dyeing:

1. Dispersion by padding.
2. Reduction with solubilizing agent (vatting)
3. Dyes Molecule Absorption.
4. Re-oxidized.
5. Soaping – off, Washing & Neutralization.

Above phases of vat dyeing are described below:

1. Aqueous dispersion by padding: The insoluble vat dyes, in this stage dispersed in water. On PDPS Process dyes are applied as a dispersion by padding.

2. Vatting: Vatted by using Sodium Hydro-sulphite (Hydrose) and Sodium Hydroxide (NaOH). Sodium hydro-sulphite work as a reducing agent. Again salt formation by neutralizing their hydroxide to give a water soluble product. Sodium hydroxide is used as a solubilizing agent.

3. Dye absorption: Dyeing, in which the soluble sodium salt of the “LEUCO vat dye” is absorbed by the Fabric from an alkaline reducing medium.

4. Re-oxidation of vat dye: In Oxidation stage, the soluble form of the vat dye absorbed by the fiber is reconverted into the original insoluble dye by the use of “chemical oxidation.” (E.g. Hydrogen Peroxide).

5. After treatment: After oxidation some insoluble vat dye may be exist on the Fabric Surface. By Soaping – off and Hot water wash we get the optimum fastness, especially rubbing fastness by removing the surface deposited vat dye particles.

For continuous process, general parameter of thermosol machine:

• Machine Speed = 40 m/min
• Temperature = 80°C-120°C-140°C
• Blower = 40°C-70°C-80°C
• IR = 650°C
• Padder Pressure (N/mm) L-M-R = 20-30-20

Let a recipe for olive color:

• Bezathren Yellow 3RT PS = 2.35 g/l
• Bezathren Red FBB EPS= 1.24 g/l
• Bezathren Olive T-EPS = 21.07 g/l
• Heptol SF4 = 2 g/l
• Colorcontin SAN = 2 g/l
• Thermacol MP = 12 g/l
• Antimussol UDF= 0.2 g/l

Process description of continuous vat dyeing (pdps process):
Vat Dye is not soluble in water and so it may be applied to the fabric as a dispersion by padder in Continuous Process. In this process firstly we make Vat Dye Liquid with Dispersing agent & Wetting agent, Sequestering agent and Anti-migrating Agent (according to recipe). Here during mixing of Vat color we must use hot water for uniform mixer of color molecule.

In continuous process the fabric is pass into Dye Bath and Padded then it pass through IR and Hot flue Dryer Chamber according to machine parameter given here then cooling down.

Then the pad dried fabric is introduced into the chemical pad bath of the Pad Steam Machine where it is treated with reducing agent (Sodium Hydro Sulphite), caustic soda (NaOH) and wetting agent – follow the recipe given here. After chemical padding, should be immediately steamed for 1.5 min to1.6 min at 101–102°C. It has been converted to its soluble (LEUCO) form, the vat dye can penetrate into the cotton fibers.

After steaming the fabric is withdrawn from the normal wash and oxidized by an oxidizing agent such as hydrogen peroxide then fabric introduce soaping and washing.

Continuous vat dyeing process flowchart is schematically given below:

Pad Steam Process:

• Machine Name: Goller Pad Steam Machine.
• Machine Speed = 40 m/min
• Padder Pressure (N/mm)L-M-R = 20-30-20
• Steamer Temperature: 101-102°C time around 1.5 min
• Drying Unit temperature: 113°C-115°C

Pad Steam Recipe for Chemical Bath:

• Sodium Hydro Sulphite – (Hydrose) = 40 g/l
• Caustic Soda (NaOH) = 40 g/l
• Colorcontin SAN = 2 g/l

Recipe for Pad Stean Wash Unit:

Wash Unit Temperature = 30°C-70°C-70°C-Over Flow (90°C)-90°C-90°C-90°C-30°C

Recipe:

• Hydrogen Peroxide-50% Conc (H₂O₂): 10 g/l
• Albatex AD: 2-3 g/l
• Acetic Acid (CH₃COOH) = 0.5-1 g/l

Wash Bath:

1. (Bath 01) Normal Temperature (30°C) Wash.
2. (Bath 02) Medium Hot Wash (70°C).
3. (Bath 03) Hydrogen Peroxide (H₂O₂): 5-10 g/l with Medium Hot Wash (70°C).
4. (Bath 04) Hot Wash (90°C) with Water Over flow.
5. (Bath 05) Hot Wash (90°C) with Soaping Chemical (Albatex AD): 3-5 g/l
6. (Bath 06) Hot Wash (90°C).
7. (Bath 07) Hot Wash (90°C).
8. (Last Bath) Neutralization; Acetic Acid (CH3COOH) = 0.5-1 g/l

Reference only for Theory Part:
Chemistry of Dyeing / D.P. Chattopadhyay, in Handbook of Textile and Industrial Dyeing, 2011

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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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