Typical Preparatory Process of Dyeing

Preparatory process of dyeing are necessary for removing impurities from fibers and for improving their aesthetic appearance and processability as fabrics. Removal of impurities increases absorbency and whiteness. If a fabric is absorbent it is easier to dye – which is why need increase absorbency. The main objective of fabric preparation or pre-treatments is to remove any impurities or contaminants from the fabric and make it ready for the subsequent operations such as optical brightening, dyeing, printing or finishing. The overall objectives preparatory process of dyeing include removal of fabric impurities such as protruding fibers, sizing agents, cotton seed husks, fats, oils, waxes, dirt, dust, lubricants, etc.; improved absorbency and / or whiteness of the fabric; minimum fiber damage; uniform residual size, pH, alkalinity, whiteness, absorbency and moisture content. The commonly used preparatory process of dyeing are briefly described in the following sections.

Preparatory Process of Dyeing

Typical Preparatory Process of Dyeing

Heat setting:
Heat-setting process is only used for synthetic fabrics such as those made from polyester or their blends to make them dimensionally stable against subsequent hot processes. Other benefits of heat-setting include less fabric wrinkling, low fabric shrinkage and reduced pilling tendency.

Reasons for heat setting: stabilizing material (shrinkage, distortion, creasing)

  • Pleating and embossing
  • Improve dyeability of fabric

*Heat setting temp is different for each synthetic fiber

What happens during heat setting?

  1. Stress in amorphous regions of thermoplastic fiber removed
  2. Above Tg (glass transition temperate) molecules in amorphous regions move and can form new shapes
  3. Creases can be set permanently
  4. Permanently set twist in yarns

When should heat setting be done? Before dyeing (after wet treatments)

  • Will resist undesirable creasing and wrinkling during dyeing
  • More uniform dyeing
  • If setting temperature not controlled – will result in non-uniform dyeing
  • On greige goods – will set sizes and stains
  • After dying – yellowing and dye sublimation

You may also like: Methods of Heat Setting in Textile Industry

Heat Setting Methods

Dry heat: Polyester


  • Increases dyeing. Rate with acid and disperse dyes
  • Molecular openings increase – dyeing can diffuse more easily into fiber
  • Nylon carpet yarns
  • Temperature: 180 – 2000C (polyester) 190 – 2000C (nylon)
  • Higher temp – better dimensional stability and pilling resistance
  • Saturation value of disperse dye on fabric affect by heat setting

Singeing is a process of passing an open-width fabric over a gas flame at such a distance and speed that it burns only the protruding fibers but does not damage the main fabric. The main objective of the singeing process is to produce a clean fabric surface and reduce fabric pilling tendency by removing the protruding fibers from the fabric surface.

  • Protruding fibers burned to give fabric as smoother surface
  • Pass fabric through burning gas flame at high speed – quench in water
  • Thermoplastic fibers when singed usually dyed by pad-thermosol method

Desizing is a process of removing sizing agents from the fabrics, which are usually applied on the warp yarns before weaving. It is the most important preparatory process of dyeing. Sizing agents mostly comprise macromolecular film-forming and fiber-bonding substances such as starch, PVA and polyacrylates, which are applied on warp yarns to increase their strength and reduce yarn breakages during weaving. Some auxiliaries such as wetting agents, softening agents, lubricating agents or hygroscopic agents may also be included in the sizing recipe. Removal of sizing agents after weaving is necessary in order to make the fabric more absorbent for dyes and other chemical processing agents.

  • Removal of size material from warp yarns in woven fabrics (a lubricant that wasadded to the warp yearns – if it is not desized the dye will not penetrate)
  • Residual size – barrier for yarns and fabrics – will not wet quickly affects dyeabsorption
  • Starch, PVA, CMC (CarboxyMethyl cellulose) *lubricants that were added –these need to be complete removal
  • Removal Methods: water soluble – hot water and alkali

Scouring is a process for removing natural and acquired impurities from fabrics to make them more absorbent and suitable for subsequent processes such as bleaching, dyeing, printing or finishing. Natural cotton contains very small amount of oils, fats and waxes, in addition to acquiring dirt or dust, etc. during transportation and storage, which make is dirty and less absorbent.

  • Removal of impurities that interfere with dyeing
  • Synthetic fibers – lubricants
  • Natural fibers – wax, pectin’s, ash, leaves, stems, seed coat fragments
  • Wool – suint, dirt, fat, grease

Scouring Methods

  • Alkali
    • Caustic soda (sodium hydroxide) and surfactants

You may also like: Impurities of Different Natural Fibers with Percentage

The purpose of bleaching is to remove any coloring matter from the fabric and confer it a whiter appearance. In addition to increase in fabric whiteness, the bleaching process may also result in improved fabric absorbency and removal of cotton seed husks and trash from the fabric.

  • Decolorize impurities to restore whiteness in preparation for dying
  • Balance between removal of impurities and damage to fibers

Bleaching methods

  • Hydrogen peroxide – 35% or 50%
  • Sodium hypochlorite – 5.25% *this is the active ingredient in Clorox

*Depending on the fiber you can use either kind of bleaching method

*If you don’t know the fiber is use Hydrogen peroxide because it is a softer bleaching method

*On wool sodium hypochlorite will completely dissolve the fiber

You may also like: Bleaching Process in Textile: Its Purposes and Chemical Requirements

Fluorescent Whitening Agents (Optical Brighteners)

  • Increase whiteness of fabric
  • Emit in blue region of spectrum

Cotton and its blended fabrics are sometimes subjected to a mercerization process to enhance various properties such as increase in dye affinity, chemical reactivity, dimensional stability, tensile strength, luster and fabric smoothness. Mercerization process is performed using caustic soda. Treating cotton with sodium hydroxide. Invented by John Mercer (1850).

Beneficial Effects

  • Increased tensile strength
  • Increased Luster
  • Improved affinity for dyes
  • Improved dyeability of immature fibers
  • Increased water sorption (absorbency)

*Immature is harvested before it is fully grown

*This treatment is mostly for cotton

*Any fiber that is absorbent will dye well

How does concentrated sodium hydroxide work?

  • Amorphous and crystalline regions affected
  • Cell-OH +NaOH Cell-O-Na + H20
  • Soda cellulose neutralized with acetic acid forms hydrated cellulose
  • Dry hydrated cellulose has different crystal structure
    • More amorphous regions
    • Increase in OH groups – increased absorbency

*OH group – hydroxyl groups

*NaOH – Sodium (Na) Hydroxide (OH)

Concentration of caustic soda (sodium hydroxide)

  • 15% untwisting and swelling of fiber begins
  • 20-25% full effects of mercerization observed

Temperature of caustic soda

  • Room temperature satisfactory
  • Higher temperature may improve uniformity of mercerization

Tension during treatment

  • To prevent shrinkage
  • To develop luster
  • Removes convolutions
  • Fiber more parallel = more strength

**Most important treatment for cotton fibers before dying

**A mercerized fabric picks up more color because, mercerization makes the cross section of cotton swell. Which creates a more circular cross section shape than its original kidney bean structure. This allows the fabric to pick up more dye due to a bigger surface area.

Tests Before Dyeing


  • Reflectance compared against standard white


  • Fabric extracted with water pH of extracted measured
  • Should be as close to 7 as possible – no or very little alkali or acid


  • Length of time required for fabric to absorb a droplet of water placed on surface

Spot Dyeing

  • Dye with C.I Direct Blue 1 or C.I direct Red 2
  • Lighter dyeing’s indicate lower degree of mercerization

C.I = Color Index

**this is maintained by 2 data bases one in the US and one in the UK

You may also like:

  1. Singeing and Desizing Process in Textile Industry
  2. Bleaching Process in Textile: Its Purposes and Chemical Requirements
  3. Impurities of Different Natural Fibers with Percentage
  4. Concept Development of Mercerization with Different Parameter and Comparison
  5. Importance and Techniques of Dyes Selection in Textile Dyeing Process
  6. Textile Dyeing Process with Ultrasonic Waves
  7. Salt Free Dyeing of Cotton Fabric with Reactive Dyes
  8. Different Types of Dyes with Chemical Structure
  9. Different Types of Dyestuffs in Relation to the Fiber Substrate
  10. Production Parameter of Yarn Dyeing Section

Share this Article!

Leave a Comment