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.
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?
Stress in amorphous regions of thermoplastic fiber removed
Above Tg (glass transition temperate) molecules in amorphous regions move and can form new shapes
Creases can be set permanently
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
Higher temp – better dimensional stability and pilling resistance
Saturation value of disperse dye on fabric affect by heat setting
Singeing: 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:
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: 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.
Bleaching:
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
Mercerizing: 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
Whiteness:
Reflectance compared against standard white
pH:
Fabric extracted with water pH of extracted measured
Should be as close to 7 as possible – no or very little alkali or acid
Absorbency:
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
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
