pre treatments

7/30/2019 Pre Treatments

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Importance Of Pre Treatment For Fabric

Dyeing

Bachelor of Applied Science (Textile Technology)-2012/2013

Lasitha Nawarathna


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Cotton Fabric

Garments are produced using different types of fabrics such as woven fabrics, knitted fabrics and

non-Woven. Basically knitted fabrics can be warp knitted fabrics or weft knitted fabrics. Weft

knitted fabrics can be subgroup as single jersey and double jersey. Further they can group asplain knitted, purl knitted, rib and interlock.

Fabrics

Woven fabric Knitted fabric Non-woven fabricPlain

Twill

Satin/sateen

Weft knitted fabric Warp knitted Fabric

Single jersey Double jersey

- plain knit - rib fabric

- purl knit - interlock fabric

Cotton Woven fabric

Woven fabrics are made of yarns interlaced in a regular order called a binding system, or weave.

Weaving is the process of combining warp and weft components to make a woven structure. Thecomponents need neither be parallel to each other nor cross each other at right angles, but most

woven structures are composed of two sets of components, both flexible and crossing at right

angles

Cotton weaves come in three types:

1. Plain 2. Twill 3. SatinPlain weave fabric is made by taking the weft and threading it over and under each warp.

Gingham and chambray are produced that way.

Twill weave is made by weaving the yarns to form a raised, diagonal pattern. This fabric is

strong and sturdy. Denim, gabardine and herringbone are produced that way

Construction of woven fabric:-


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Cotton Knitted fabrics

Knitted cotton is made by machines with needles that grasp the yarn to create a series of stitches

that form the fabric. Cotton fabric made using this method is soft and stretchable and is used for

T-shirts.

knitted cotton comes in two main types :- weft kitted, warp knitted

construction of knitted fabric

Cotton Non-Woven

Non-woven cotton fabric such as felt is created using chemicals or heat to hold the fibrestogether. This method is used to create cotton pads, bandages, diapers and filters.

Cotton Fibre chemical composition & structure

Natural, Cellulose, Seed, mono-cellular, staple fibre

Repeating unit of polymer-

Cellobiose unit-repeating unit, about 5000 unit make cellulose polymer


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PropertiesDensity- 1.52g/cm3, lengthup to 65mm

Diameter- 11m to 22m length to breadth ratio350 to 6000

Microscopic appearance length wise like a twisted ribbon (convolution)

Cross-section kidney shape with lumen65-70% crystalline, 30-35% amorphous

High strength due to high degree of polymerisation, Wet strength is higher (5%) than wetstrength due to polymer alignment in the amorphous region

Relatively inelasticdue to crystallinity and hydrogen bondsCreasing and wrinkle due to polymer damage in the crystalline region

Contain of cotton fibre

component Percentage

Cellulose 80-90%

Water 6-8%

Waxes and fats 0.5 - 1%

Proteins 0 - 1.5%

Hemicelluloses andpectins 4 - 6%

Ash 1 - 1.8%

characteristics of greige cotton fabric

In normally water absorbency of greige cotton is very poor due to natural wax layer & impurities

as well as synthetic wax has been added when its Knitted & Starch has been added when its

weave. Therefore natural/added wax and impurities must be removed prior to dye the fabric. It

will help to increase absorbency of fabric.


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Why we do pre-treatment

To remove the Natural impurities & waxes

To remove the Added impurities & waxes

To remove the natural colorant matter/pigments.

To remove the Starch in woven fabrics.

Pre-treatment methods for Cotton Fabric

Singeing

Singeing: It is the process of burning out projecting hairy fibres from surface of the grey fabric

by assigns the fabric through a gas flame. Both sides of the fabric are burnt out by the flame. It

helps in subsequent chemical process of the fabric. After dyeing or printing of singed fabric,

brightness and luster of color and fabric becomes better.

Singeing is the process of removing the pills and protruding fibers of the fabric coming from

weaving. This operation may either be done at the beginning of the process or at the end of the

finishing operation.


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Desizing

Desizing is the process of removing the size material from the warp yarns in woven fabrics.

Sizing agents are selected on the basis of type of fabric, environmental friendliness, ease of

removal, cost considerations, effluent treatment, etc.

Desizing, irrespective of what the desizing agent is, involves impregnation of the fabric with the

desizing agent, allowing the desizing agent to degrade or solubilise the size material, and finally

to wash out the degradation products.

Desizingis the second operation after singeing. Desizing of fabric is essential to remove the

sizing materials added during warping to strengthen the warp yarns. This size if present during

subsequent processing will affect the quality of look and finish. There are many ways to remove

the size from the fabric. A few methods are

as below.

a) Acid Desizing - this is an old process of destroying the starch and other size materialsin the presence of acid at elevated temperatures.

b) Oxidative desizing method- desizing with the help of an oxidizing agent such as

Hydrogen peroxide.

c) Enzymatic desizing method - this is a bio degradation method that destroys starch and

other sizing materials in to soluble form that will be washed off during subsequent

washes.


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Scouring

Scouring:It is the process of removing natural impurities present in the cotton fibre. The natural

impurities are pectins, pectos, ash, wax, mineral compounds, etc. if those impurities present in

the cotton fibres are not removed, then it will be difficult to dye or print the fabric uniformly.

Normally caustic soda ash is used as main reagent from scouring of cotton fabric.

1.Saponification:

The vegetable oil, which is immiscible with water, is glyceride of fatty acids. When such oils are

heated with a solution of sodium hydroxide in water, the oil splits up into its constituents-fatty

acid and glycerine. Glycerine is miscible with water easily and the fatty acids reacts with sodium

hydroxide present in the solution forming its sodium salt( i.e. Soap which is also soluble in

water). Thus oil is removed.

2.Emulsification:

Wax and non saponifiable oils are removed by emulsification as they are immiscible in water.

Normal washing soap is used as a emulsifying agent

which makes emulsion of them.

A surfactant is defined as a material that can greatly reduce the surface tension of water when

used in very low concentrations. A particular type of molecular structure performs as a

surfactant. This molecule is made up of a water soluble (hydrophilic) and water insoluble

(hydrophobic) component.


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3. Detergency : After removal of fats by saponification and waxes by emulsification, the

remaining constituents, dust and dirt particles have to be removed by a good detergent. Detergent

not only removes the dirt particles but also keep them in dispersed or suspended form in the

scouring solution and does not allow

them to settle on other parts of the

fabric.

Thus in the final stage of scouring i.e.

detergency, the hydrophilic end of the

surfactant is strongly attracted to the

water molecules and the force of

attraction between the hydrophobe and water is very less. As a result the surfactant molecules

align themselves at the surface and internally so that the hydrophile end is towards the water and

hydrophobe is squeezed away from the water. This internal group of surfactant molecules is

called Micelles.

Apart from removing these impurities, the major technical aspect is to improve the absorbency.

Achieving good absorbency necessitates the thorough removal of cotton wax, which is located

mainly in the fibre surface. Not all good detergents possess good wetting/re-wetting properties;

hence a combination of surfactants to provide good wetting, emulsifying and re-wetting property

is preferable.

The changes occurring of cotton fibers during scouring:

Saponifiable oils and free fatty acids are converted into soaps.

Pectins and pectoses are converted into soluble salts of pectic acid.

Proteins are degraded to simple soluble amino acids or ammonia.

Mineral matters are mostly dissolved.


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Non-saponifiable oils are emulsified by the soluble soaps generated from the saponifiable

oils.

Additive dirts are removed.

Residual sizing materials are broken down into soluble product

Objective of Scouring

To make the fabric highly hydrophilic.

To remove impurities such as oils, waxes, gum, husks as nearly as possible.

To increase absorbency of fabric or textile materials without physical and chemical

damage.

To produce a clean material by adding alkali.

To make the fabric ready for next process.

To remove non-cellulosic substance in case of cotton.

Bleaching

It is the process of removing natural coloring matters present in the cotton fibre. For his purpose,

hydrogen peroxide, bleaching powder or other bleaching agents are used. Hydrogen peroxide is

the best and most commonly used bleaching agent. If this bleaching treatment is not carried out

before dyeing or printing hen the color yield and shade matching may be a great problem.

Bleaching Process: Bleaching is a process to make the fabric or yarn look brighter and whiter.

This is achieved by oxidizing or reducing the coloring matters in to colorless form.

Mostly widely used textile bleaching method is Hydrogen Peroxide bleaching.


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Mercerization

It is a special chemical pre-treatment on cotton fabric t improves the properties and performance

of cotton fabric. The cottonfabric is treated under tension in the 20% caustic soda solution which

is called mercerization many properties of cotton fabric is improved, some of them are

mentioned below:

Changes during mercerizing process

A. Fiber level

Swelling

Cross sectional morphology changes from beam shape to round shape

Increases fabric strength

Increases absorbency power

B. Molecular level

Hydrogen bond readjustment

Orientation (parallelization) of molecular chains in amorphous region along the direction

of fiber length.

Orientation of the crystallinity in the direction of the fiber length.

Increased crystallinity

C. Chemical Changes

.Increased rate of reaction on hydrolysis and oxidation

.Liberation of heat during the caustic treatment.(heat of sorption and heat of reaction)

Increase in the alkali absorption.

Increase in the absorption of iodine.


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Main important factors of pre-treatments

Good Hydrophilic fabric for dyeing.

Fabric with good wetting properties

Increases fabric softness and handle property.

Reduces dye consumption

Reduces chemical consumption in dyeing

Fabric with good dyeability.

Improve tearing strength.

Improve dimension stability.

Improve lusture.

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