Basics of dyeing

1. National Textile University
Department of Design
Textile Processing
Usman Zubair, PhD
01/10/2019

2. Course Outline
 Introduction to textile processing
 Preparation processes
 Singeing, Desizing, Scouring, Bleaching, Mercerizing
 Dyeing Fundamentals
 Colour science
 Printing Fundamentals
 Printing techniques
 Finishing Fundamentals
 Physical Finishes (shearing, suede, calendaring)
 Chemical Finishes (easy care, antimicrobial, silicon etc.)

3. Textile Dyeing
 Two major processes used for coloration of textiles are dyeing and printing.
 Dyeing comprises the application of colorant to the entire body of a textile substrate
with a reasonable degree of fastness.
 Dyeing can be defined as a process during which a textile substrate is brought in
contact with the solution or dispersion of a colorant.
 Dyeing could be exhaust or pad dyeing.
 Exhaust dyeing is a batch process while pad dyeing is a continuous application of
dyes.
 Dyes are either soluble in the dyeing medium (e. g. water) or can dissolve into the
textile substrate.

4. Exhaust Dyeing
 In exhaust dyeing, a finite amount of textile materials (in the form of fibers, yarn or
fabric) is placed in the dye liquor and remains in its contact throughout the dyeing
time, during which the dye molecules gradually move (or exhaust) from the liquor
toward the fabric, for absorption and fixation in the textile material.
 The general phases in exhaust dyeing include the following:
 Disaggregation of dye particles in aqueous solution or dispersion
 Exhaustion or movement of the dye molecules from the solution / dispersion towards the textile
substrate
 Adsorption of the dye molecules on the surface of the textile substrate
 Absorption, penetration or diffusion of the dye molecules into the fibers of the textile substrate
 Fixation of the diffused dye in the fibers through chemical bonding or by some other mechanism

5. Pad Dyeing
 In pad dyeing method, a continuous batch of fabric in open width, passes through an
impregnator (or padding trough) containing dye liquor, followed by a passage
between a pair of squeeze rollers. The pressure of the squeeze rollers can be
adjusted to obtain a desired wet pick-up.
 After passing through the squeeze rollers, the fixation of the dye on the fabric may be
accomplished by variety of means including:
 making a batch of fabric and keep rolling the batch for a specific period (pad-batch dyeing method);
 passing the fabric through a drying and fixation unit (pad-dry-fix dyeing method);
 passing the fabric through a drying and steaming unit (pad-dry-steam dyeing method);
 passing the fabric through a steaming unit (pad-steam dyeing method).
 After both the exhaust and pad dyeing methods, the dyed fabric is usually subjected
to a washing / rinsing step to remove any unfixed dye from the fabric.

6. Dyeing of Various Textile Substrates
 Textile materials can be dyed in fiber, yarn, fabric or garment form.
 Dyeing of fibers is known as ‘stock dyeing’.
 Addition of colorant to the polymer melt o solution prior to their extrusion is called
‘dope dyeing’ or ‘solution dyeing’.
 Dyeing of yarns in the form of wound packages, skeins or beams is known as
‘package dyeing’, ‘skein dyeing’ or ‘beam dyeing’ respectively.
 Fabric dyeing is also known as ‘piece dyeing’.

7. Colorants - Dyes and Pigments
 There are two main types of colorants: pigments and dyes.
 Dyes are either soluble in the dyeing medium (e. g. water) or can dissolve into the
textile substrate.
 Pigments are neither soluble in the dyeing medium nor can dissolve into the
substrate.
 Colorant source could be natural or synthetic.
 From the application point of view, dyes have been classified into different groups,
each group being suitable for certain types of textile substrates.
 The most commonly used type of dye for cotton, polyester and acrylic are reactive
dyes, disperse dyes and basic dyes, respectively.

8. Dyeing Machinery
Commonly used dyeing machines are as follows:
 Exhaust Dyeing Machines
 Package dyeing machine: mainly used for dyeing yarn in package form
 Winch or beck dyeing machine: mainly used for dyeing knitted fabrics (in rope form) at atmospheric pressure but
may also be used for woven fabrics
 Jet dyeing machine: mainly used for dyeing knitted fabrics (in rope form) at atmospheric or higher pressure but may
also be used for woven fabrics
 Jigger dyeing machine: mainly used for dyeing woven fabrics (in open-width form)
 Pad Dyeing Machines
 Pad-batch dyeing machine: used for dyeing fabrics in open-width form in semi-continuous manner
 Pad-steam dyeing range: mainly used for dyeing cotton fabrics in open-width form in full-continuous manner
 Pad-thermosol dyeing range: mainly used for dyeing polyester and polyester / cotton blended fabrics in open-width
form in full-continuous manner
 Stenter: mainly used for simultaneous finishing and dyeing of fabrics with pigments

9. Types of Dyes - Application
On the basis of applications dyes can be categorized into the following classes.
 Acid Dyes
 Basic Dyes
 Direct Dyes
 Disperse Dyes
 Reactive Dyes
 Sulphur Dyes
 Vat Dyes

10. Acid Dyes
 The acid dyes are usually applied under acidic conditions.
 They are commonly used for dyeing protein fibers (e. g. wool and silk) and nylon fibers.
 Acid dyes are anionic in nature, and their negatively charged anions are attracted by
positively charged amino groups in wool under acidic conditions.
 The application of acid dyes on wool or nylon results in ionic bond or salt links between
the anionic dye and the positively charged groups in the fiber under acidic conditions.
 Because of opposite charge, there is a rush of dye to fabric that can result in unlevelling
of the dyeing.
 To avoid unlevel dyeing, some retardation in the dyeing rate is obtained by making use of
sodium sulphate.

11. Basic Dyes
 The basic dyes are most commonly used for dyeing polyacryonitrile or acrylic
materials.
 They are also known as cationic dyes because of the presence of positive charge in
the dye molecules under dyeing conditions.
 During dye application, the negatively charged acrylic fiber attracts the positively
charged dye cations for ionic bonding.
 Due to high attraction between the oppositely charged fiber and dye molecules, there
is risk of unlevel dyeing because of high rate of dyeing.
 This risk may be reduced by careful control of dyeing temperature and used of
suitable retarding agents.

12. Direct Dyes
 The direct dyes are one of the cheapest groups of dyes used for dyeing cotton and other
cellulosic materials.
 They are water soluble and can be applied relatively easily using a variety of methods.
 These dyes are anionic in nature and have negative charge in aqueous solution, as do
the cellulosic fibers.
 The addition of common salt (sodium chloride) or Glauber’s salt (sodium sulphate) is
usually necessary during dyeing to overcome repulsion between the negatively charged
dye and the substrate.
 After absorption into the fiber, these dyes are held to the fiber by hydrogen bonding and /
or Van der Waals forces.
 Direct dyes usually do not have very good wash fastness properties and tend to fade
away from the fabric on repeated washings.

13. Disperse Dyes
 The disperse dyes are mainly used for dyeing polyester.
 Disperse dyes have extremely low water solubility and usually used in the form of
aqueous dispersions.
 The disperse dyes are usually applied in acidic pH, in the presence of a dispersing
agent.
 Other dyeing auxiliaries may include wetting agent, levelling agent and dyeing
carrier.

14. Reactive Dyes
 The reactive dyes constitute the most commonly used class of dyes for dyeing cellulosic textiles,
because of their good all-round properties,
 such as water solubility, ease of application, variety of application methods, availability of
different shades, brightness of colour shades, good to excellent wash and light fastness and
moderate price.
 Reactive dyes may have poor fastness to chlorine bleach.
 The reactive dyes are further classified according to the type of their reactive groups, giving
them different degrees of reactivity.
 The important process variables for dyeing with reactive dyes by exhaust method, include:
dyeing temperature, type and amount of electrolyte (e. g. common salt or Glauber’s salt), dyeing
pH (controlled by type and amount of alkali used), liquor to material ratio and dyeing time.
 The fixation of reactive dyes on cellulosic fibers takes place through formation of covalent bonds
under alkaline conditions (pH 9−11).

15. Vat Dyes
 Vat dyes are among the most expensive dyes used for dyeing cellulosic materials with best overall fastness properties, including
washing fastness, light fastness and chlorine fastness.
 They are preferred for dyeing workwear or uniforms, or where the textiles and apparels are expected to undergo repeated
industrial launderings.
 The vat dyes are generally not soluble in water.
 However, solubilized vat dyes are also available but are usually more expensive as compared to generally available insoluble vat
dyes.
 Indigo blue vat dyes are commonly used for producing indigo denim, with different wash-down and worn-out looks.
 General phases in dyeing with vat dyes are as follows:
 Reduction: conversion of insoluble vat dye into soluble sodium leuco vat anions, with the help of a reducing agent (sodium dithionite) and alkali (sodium
hydroxide)
 Diffusion: penetration of the reduced / solubilized sodium leuco-vat anions into fibers
 Rinsing: removal of excess alkali and reducing agent from the dyed material
 Oxidation: conversion of vat dye absorbed in the fibers back into insoluble form
 Soaping: Complete removal of surface dye molecules
 The vat dyes have excellent light fastness due to stable electron arrangement in the chromophore (colour-bearing group) of the
dye molecule and presence of numerous benzene rings.
 Vat dyes have excellent wash fastness owing to the aqueous insolubility of the oxidized dye absorbed in the fiber and due to
large vat dye molecules trapped within the polymer system.
 However, vat dyes are usually very expensive and need more expertise for their application because of greater number of steps
involved in dyeing.

16. Sulfur Dyes
 The sulphur dyes are named so because of the presence of sulphur atoms in their molecules.
 Like direct dyes, sulphur dyes are also quite cheap for dyeing cellulosic textiles with limited
colour fastness properties.
 The commonly used sulphur dyes are not soluble in water and need to be converted into
soluble form by reduction with the help of a reducing agent and an alkali.
 Sulphur dyes are usually easier to reduce and more difficult to re-oxidize as compared to vat
dyes.
 General phases in the dyeing of cellulosic materials with sulphur dyes include:
 Reduction: conversion of the water-insoluble dye into soluble form, in the presence of a reducing agent and alkali
 Application: absorption of solubilized sulphur dye by the textile substrate
 Rinsing: removal of any loose colour from the substrate before oxidation
 Oxidation: conversion of the dye absorbed by the textile substrate back into the insoluble form
 Soaping: for increased colour brightness and fastness of the final shade.
 Unlike direct dyes, the colour range of sulphur dyes is quite limited to black, brown, olive and
blue shades. Moreover, sulphur dyes are not available in as bright colours as are available in
other class of dyes.

17. Presentations/Assignment
Each presentation will span 30 mins.
 Dyeing Techniques (Fashion based)
 Printing Techniques (Novel and Fashion based)
 Denim Washing Techniques
 Garment (Knitwear) Washing Techniques
 Chemicals Finishing Possibilities (Functional)
 Physical Finishing Possibilities
 Denim Dry Process
 Coating of Textiles and applications
 Chemicals Finishing Possibilities (Protection)