PROCESS SEQUENCES OF CHEMICAL PROCESSING

Grey fabric Inspection & Stitching

Shearing & Cropping

SingeingDesizingScouringBleachingDrying

Mercerising

Dyeing Printing

Soaping & Drying

FinishingFolding & Packing

Preparation / Pretreatment Processes used to remove impurities from fibres to make it dye able or printable.Natural fibers and synthetic fibers contain primary impurities that are contained naturally, and secondary impurities that are added during spinning, knitting and weaving processes.Textile pretreatment is the series of cleaning operations .All impurities which causes adverse effect during dyeing and printing is removed in pretreatment process.

Objective of Pretreatment:

To Convert fabric from hydrophobic to hydrophilic state.To remove dust, dirt etc from the fabric.To achieve the degree of desire whiteness.

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Preparation

Following are the steps to be taken before going wet processing :

Grey inspectionStitching

Shearing and croppingSingeing

Desizing

Scouring

Bleaching

Mercerization

Typically a woven cotton fabric would be prepared by sequence of process as shown.In case of knitting sizing step is not involved

Preparation/Pretreatment Singeing

SHEARING & CROPPINGIn shearing ,the fibres are cut in an angular manner on the surface of the fabric itself, resulting in a soft feel Singeing and shearing are almost the same in effect. In singeing the fibres in the interlacement of the fabric are burnt by flames whereas in shearing the fibres are cut in an angular manner on the surface of the fabric itself, resulting in a soft feel.

SingeingBurning of protruding fibres from the surface of fabric or yarn - A mechanical processsingeing refers to the burning-off of Loose fibres not firmly bound into the yarn and/or fabric structure. Textiles materials are most commonly singed in woven or knitted fabric form or in yarn form.Makes fabrics smooth Prevents pilling Improves luster

Advantages of SingeingSingeing of a fabric is done in order to obtain a clean fabric surface which allows the structure of the fabric to be clearly seen.Fabrics, which have been singed, soil less easily than un-singed fabrics.The risk of pilling, especially with synthetics and their blends, is reduced in case of singed fabrics.randomly protruding fibres are removed in singeing which could cause diffused reflection of light.

Necessity of Singeing in Textile

Cotton materials are valued for their smooth appearance. After the formation of fabric it has a fuzzy or hairy appearance due to projecting fibers, thus affecting the luster and smoothness Unsigned fabrics are soiled easilyThe protruding fibers obstruct the subsequent dyeing and printing processGoods which are to be mercerized are signed to maximize the lusterIn fabrics of polyester and cellulosic fiber blends singeing is the best method to control pilling, sometimes double singeing is done to minimize the pilling.

Singeing ProcessSingeing process is as follows:To produce a smooth surface finish on fabrics made from staple fibers first the fabric surfaces are brushed lightly to raise the unwanted fiber ends.Then the fabric is singed with or passed over heated copper plates or open gas flames. The fiber ends burn off.The fabric is moved very rapidly, and only the fiber ends are destroyed.As soon as the fabric leaves the singeing area, it enters a water bath or desizing bath. This stops any singeing afterglow or sparks that might damage the cloth.

TYPES OF SINGEING

Bio polishing It is an enzyme treatment designed to

improve fabric quality and provide following advantages:

Improve pilling resistanceA clear, lint and fuzz-free surface structure

Improved drape and softnessThe effect are durable

Process

Cellulase enzyme is usedEnzyme dosage 1-2% owfpH 4.5-5.5Temperature 40-55oCTime 30-60 min. It is similar to singeing as it also provide similar effect on the fabric.

Fig. 7. Carbomatic® Singeing Line by Bejimac   Fig. 8. DORNIER singeing machine for circular knitted fabrics 

What should control during singeing operation?

Intensity and uniformity of flameWorking speed of the fabric to be singedEffectiveness of singeing : should not add harshness in the fabricShould not damage the synthetic filament yarn in the fabric

Sizing /Slashing Slashing is the process where Size is applied to warp yarns for weaving.The purpose of size is to protect the yarn from the abrasive action of the loom.The process is carried out on a Slasher and the application procedure is called Sizing or Slashing.

NATURAL SIZING AGENTS

Natural sizing agents are based on natural substances and their derivatives:Starch and starch derivatives; native starch, degradation starch and chemical modified starch products Cellulosic derivatives; carboxymethlycellulose (CMC), methylcellulose and oxyethlycellulose

SYNTHETIC SIZING AGENTS

Polyacrylates Modified polyesters

Polyvinyl alcohols(PVA)

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,

Desizing StarchOnce a starch solution dries, the resulting film will not readily re dissolve in water; therefore, to completely remove starch from a fabric, the polymer must be chemically degraded to make it water soluble. Three chemical methods can be used to degrade starch into water soluble compounds namely, Enzymes, Acid Hydrolysis and Oxidation.

Major Desizing Processes

Rot steeping Enzymatic desizingOxidative desizing

Acid steepimg

Rot steeping Fabric is stored in warm water at 40-60oC

overnightIt remove the starch and water soluble impurities with natural reaction Drawback: Time consumingOne can not certain that in the given duration size will remove completely

Oxidative steeping:Starch and other added impurities are hydrolysed through oxidation process followed by washing process. Oxidizing agents:Sodium Hypochlorite: 2.0-5.0 gm/lit. at room temperature at pH 7.0Hydrogen peroxide:3.0-6.0 gm/lit of H2O2 and 7.0-15.0 gm/lit Sodium Hydroxide, 40oC for 12-16 hoursDrawback: Time consuming ProcessSome time bleaching effect also obtain(If not require in the end product)

Acid steeping Acid hydrolysis lowers the molecular weight and

reduce starch to glucose. Fabric is treated with Sulphuric Acid (0.5% to 1.0 %)

at 40oC It also remove the starch and water soluble impurities

with natural reaction Drawback:

Tendering of cellulosic fibre if precautions are not taken

If the acid remain in the fabric, whole lot may get seriously damage or tendered

Oxidative Desizing of strach

The oxidative desizing methods are relatively unimportant and when using them, damage to fiber can never be completely avoided.

Enzymatic Desizing

Enzyme solution in water with 0.5% to 2.0% on weight of fabric enzymeRequire quantity of Common salt is also required Neutral pHOther method fabric is run continuously in machine having enzyme solution.

ENZYME Enzymes are high molecular weight protein biocatalyst that are very specific in their action.Enzymes are named after the compound they break down, for example: Amylase breaks down amylose and amylopectin, Maltase breaks down maltose andCellulase breaks down cellulose.For desizing starch, amylase and maltase are used.

WORK OF ENZYMES

Amylase will degrade starch into maltose, a water soluble disaccharide and Maltase will convert maltose into

glucose, a simple sugar.

Quality control check

Desizing efficiency testIodine test

Fluidity test

JIGGERA Jig or Jigger is a batch machine which handles fabric in open width. It isa simple machine consisting of let-off and take-up rolls, a trough containing theprocessing liquor and guide rolls which allow the fabric to pass from the let-off to thetake-up roll.

Scouring It is process to remove all undesirable impurities (Natural: like dirt, vegetable matter, grease wax etc and Added: like remaining size material, stains etc.)Natural Impurities: Based on the composition of natural material like cotton, wool, silk etc.

SCOURINGNatural fibers contain oils, fats, waxes, minerals, leafy matter and motes as impurities that interfere with dyeing and finishing. Synthetic fibers contain producer spin finishes, oils and/or knitting oils, grease.These impurities are not soluble in water, they can be removed by Extraction, dissolving the impurities in organic solvents, Emulsification, forming stable suspensions of the impurities in water and Saponification, Converting the contaminates into water soluble components.

Objectives To make the fabric highly hydrophillic.To remove impurities such as oil, wax, gum, husk as nearly as possible.To increase absorbency of fabric or textile material with out physical or chemical change.To produce a clean material by adding alkali to make the fabric ready for next process.To remove non cellulosic substances in case of cotton.

Mechanism of Scouring

The change occurring of cotton fibres during scouring

Chemical used in scouring process

Form of scouring Yarn scouring :Hank Package Fabric scouring : open width Jigger Pad batch Rope form Kier

Methods of scouring

There are two principal methods of scouring

Discontinuous method (winch , jigger kier)

Continuous method (scouring in j box )

J BOX

J Box Process

J box

Kier boiler

Difference between scouring and souring

SOLVENT SCOURING Certain organic solvents will readily dissolve oils fats and waxes and these solvents can be used to purify textiles. Removal of impurities by dissolution is called Extraction. There are commercial processes where textiles are cleaned with organic solvents. Fabrics processed this way are said to be "Dry Cleaned".

Desizing and Scouring

Bleaching This is a process of whitening-fibers, yarns, or fabrics having natural colour.

It is carried out by using oxidising agents like

Sodiumhypochlorite (NaOCl)

Hydrogen peroxide (H2O2)

Sodium Chlorite (NaClO2)

The color producing agents in natural fibers are often organic compounds containing conjugated double bonds. It is known in dye chemistry that conjugation is necessary for an organic molecule to perform as a dyestuff.Decoloration can occur by breaking up the chromophore, most likely destroying one or more of the double bonds within the conjugated system.

Bleaching Oxidative bleaches oxidize color bodies into colorless compounds. For example, double bonds are known to be oxidize into epoxides which easily hydrolyze into diols. The major bleaching agents used in textile preparation are sodium hypochlorite, hydrogen peroxide and sodium chlorite.

Colour – Colourless

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Sodium Hypochlorite (NaOCl)

Sodium hypochlorite is made by bubbling chlorine into a solution of sodium hydroxide.

when acid is added to a hypochlorite solution, chlorine gas is liberated.

HOCl is active bleaching agent (hypochlorous acid )

Antichlor

Fabrics bleached with hypochlorite will develop a distinctive chlorine odor.This odor can easily be removed with an aftertreatment consisting of sodium bisulfite and acetic acid.

Souring

After the sodium hypochlorite bleaching some CaCl2 , MgCl2 and other miniral matter live in fabric. After that the fabric

feels harsh and stiff.To reduce these chlorides fabric treat with

Hcl or sulphuric acid.

H202 Bleaching Hydrogen peroxide was first used to bleach cotton in the 1920's.By the invention of the J-box which lead to continuous processing. Today, it is estimated that 90 to 95 % of all cotton and cotton/synthetic blends are bleached with hydrogen peroxide.

MechanismHydrogen peroxide is a weak acid and ionizes in water to form a hydrogen ion and a perhydroxyl ion.

The perhydroxyl ion is the active Hydrogen peroxide can also decompose. This reaction is catalyzed by metal ions e.g. Cu++, Fe+++. This reaction is not desired in bleaching because it is an ineffective use of hydrogen peroxide and causes fiber damage.

Effect of pHAt pH < 10, hydrogen peroxide is the major specie so it is inactive as a bleach. pH 10.2 to 10.7 is optimum for controlled bleaching. Sodium hydroxide is used to obtain the proper pH. At pH > 11, there is a rapid generation of perhydroxyl ions. When the pH reaches 11.8, all of the hydrogen peroxide is converted to perhydroxyl ions and bleaching is out of control.

Stabilizers

Stabilizers must be added to the bleach solution to control the decomposition of hydrogen peroxide. Stabilizers function by providing buffering action to control the pH at the optimum level and to complex with trace metals which catalyze the degradation of the fibers. Stabilizers include sodium silicate, organic compounds and phosphates.

Sodium SilicatesSodium silicates are the most commonly used and most effective hydrogen peroxide bleach stabilizers.silicates have a natural affinity for ferrous ions and ferrous ions are naturally present in cotton.The silicates are adsorbed onto the ferrous ions in the fiber, producing a species that catalytically enhances bleaching while reducing bleach decomposition and fiber damage.

Test For Degree of Bleaching

Whiteness Index-CCM

WhitenessThe standard ceramic tile is measured

and set to equal 100. The other specimen are rated against this

standard. Unbleached fabrics will give values in

the 50 to 60 range.Well breached fabrics will rank 95 or

better.

OPTICAL BRIGHTENERSCertain organic compounds possess the property of fluorescence which means that they can absorb shorter wave-length light and re-emit it at longer wave-lengths.A substance can adsorb invisible ultra-violet rays and re-emit them within the visible spectrum. Therefore a surface containing a fluorescent compound can emit more than the total amount of daylight that falls on it, giving an intensely brilliant white. Compounds that possess these properties are called Optical Brighteners or OBA's.

OBA

If brilliant white finish is required in textile, then bleaching is not sufficient. For this purpose Fluorescent brightening agents are used. For synthetic fibres-OBA –Optical Brightening Agents are mostly used.

• Fluorescence is the ability of a substance to absorb radiation of light and re-radiate it at a longer wavelength.

Incident light

Visible

U.V

Fibre without FBA Fibre with FBA

Eye detect visible only

Reflected UV & Visible

Eye detect visible & UV both

Reflected Visible Plus UV converted to visible

Incident light

Fluorescent brightening agent(FBA)

OBA /FBA /BA

Optical brightening agents are based on (DAST) diamino stilebenedisulfonic acid Oba absorb ultraviolet light to within the range of 350 to 360nm and reemit the absorbed energy at 400 t0 500 nm with a maximum wavelength at 430nm.Due to presence of sulphonic acid group present in OBA structure contribute to water solubility.

Properties of OBA

Mostly they are anionic in charge (negetive )

pH typically between 8.5 to 11.0.OBA addition will impart

BrightnessWhiteness

b* value(+b-yellow,-b blue )Lvalue

a* value (lower green), (-a red)

Types of OBA Basic classes of chemicals used in OBAs  include:Triazine-stilbenes (di-, tetra- or hexa-sulfonated)Disulphonated (cellulose)Tetrasulphonated (universal)HexasulphonatedCoumarinsImidazolines

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The Examples of preparation processes of cotton and its blends fabrics

100% Cotton fabrics

1. Singeing

2. Desizing (only for woven fabrics)

3. Scouring

4. Bleaching (only for pale or bright shades)

5. Mercerizing

6. Adding of Optical Brightening Agent

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100% Silk Fibers

1. Degumming (to get rid of non-fibrous materials (sericin))

2. Bleaching (to make the goods whiter)

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100% Wool Fibers

1. Carbonising (to remove leave, bark and other parts of the plants from the wool fibers)

2. Scouring (to get rid of fat and oil from the fibers)

3. Bleaching (to make the fibers whiter)

Scouring of Wool• Wool is a protein fibre

and sensitive to bases. Wool can be scoured by using detergent at pH upto 7 at 60oC

• To remove vegetable material it is carbonised by using Dilute Sulphuric acid.

• It has tendency of felting

Carbonising of wool

• Carbonising is a chemical treatment of wool with strong acid (H2SO4)designed to remove any remaining vegetable impurity.

Felting and milling• Felting is the tendency of the fibres to form

matted entanglements such that a woven fabric, for example, can become so matted that the warp and weft yarns are no longer visible.

• The result is a fabric which is apparently composed entirely of a entangled web of fibres and the fabric is referred to a felt.

• Milling is the process of deliberately felting wool goods to achieve the effects of felting.

Natural Impurities-Silk• Silk contain 22 to

25% gum

Scouring of SilkDe-gumming: • It is scoured by using

Soap and soda ash• Treatment for 1 hour at

50oC (repeat this process three times)

• Bleaching is carried out with hydrogen peroxide (No NaOH) With sodium silicate

Mercerization

This is the process applicable only on cellulosic fibres especially cotton.The main purpose of mercerization is to alter the chemical and physical properties of the fibre.

Change in cross section

1.Cross section of cotton before mercerisation

2-5 swelling process in 18% NaOH66 Rinsing process after swelling

7 Final state

Change in cross section

Mercirization /CausticizationBoth Mercerization and causticization require cotton to be treated with concentrated solutions of sodium hydroxide (caustic soda). Mercerization requires higher concentrations of caustic soda (19 to 26 % solutions) whereas causticization is done with concentrations ranging between 10 to 16%.One major difference between the two is that causticizing improves the dyeing uniformity and dye affinity of cotton without improving luster.

Mercirization / CausticizationCaustic soda solution swells cotton fibers breaking hydrogen bonds and weak van der Waal forces between cellulose chains. The expanded, chains rearrange and re-orient and when the caustic soda is removed , the chains form new bonds in the reorganized state. When done tensionless, the cotton fiber swells, the cross section becomes thicker and the length is shortened. Because of fiber thickening, the fabric becomes denser, stronger and more elastic. Under tension, The fibers become permanently round and rod like in cross section and the fiber surface is smoother, adding to fiber luster.

Objectives

improved luster increased ability to absorb dye

improved reactions with a variety of chemicals

improved strength/elongation improved smoothness

improved hand

Procedure

If cotton is dipped into a strong alkaline solution such as lithium hydroxide, caustic soda, or potassium hydroxide,The fibers will swell and shrink. If the fibers are placed under tension while in this swollen state and then rinsed with water, the alkali will be removed and a permanent silk-like luster will result.

Effect of alkali on cotton fibres

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PHYSICAL CHANGE

Improved in strength Improved in lusture (under tension)Change in cross section from bean shape to almost circular change in longitudinal view convoluted to cylinderical

Chemical change Improvement in moisture regain

improvement in dye uptake Formation of celluloseII

Mercerising can be carried out on yarn in hanks, woven and knitted fabric

through one of the following different treatments:

· mercerising with tension· caustification (without tension)

· ammonia mercerising.

Ammonia mercerising

Cotton yarn and fabric can be treated with anhydrous liquid ammonia as an alternative to caustic soda. Effects similar to mercerising are obtained, although the lustre grade is inferior to caustic soda mercerising. Traces of ammonia have to be removed, preferably with dry heat treatment followed by steaming. This method is not widely used. Only a few plants are reported to use ammonia mercerising in Europe.

Mercerization methods

Tension Mercerization (treatment under tension)

Slack Mercerization (treatment without tension)

Mercerization machines

Yarn mercerizationFabric mercerizationFabric mercerization:

Pad chain type mercerizationPad chainless type mercerization

Padless chainless type mercerization

Test for MercerizationBarium Activity Number

AATCC Test Method 89 is a common test used for quantifying the degree of Mercerization. It is based on the fabric‘s ability to absorb barium hydroxide. The procedure is carried out on the fabric both before and after Mercerizing and the barium number is calculated as shown below.

Test for MercerizationUnmercerized fabric will give a barium number of 100 to 105. Completely mercerized fabric will give a barium number of 150. Commercially treated fabrics fall in a range between 115 to 130.

\Yarns are Mercerized as continuous sheets in a fashion similar to woven fabrics. Greige

yarns enter the caustic saturator and are held so they do not shrink until the caustic has been washed out.

Afterwards, the yarns are dried continuously and wound onto bobbins or spools. These conditions will develop satisfactory luster, elasticity and dyeing properties; however, if

higher luster and strength is desired, the yarns should be stretched beyond the greige length during the process. Increase stretch,

however, reduces dye affinity and elasticity. The reasons for mercerizing yarns are: 1. To produce lustrous, strong 100 % cotton sewing thread.

2. To produce yarns for constructions that are difficult to mercerize. 3. To produce yarns for special construction effects.

4. To produce yarns for blends containing fibers too sensitive formercerizing. 5. To provide more complete mercerization for luster and strength.

Work Procedure In natural condition the cotton fiber is a flat, twisted, ribbon-like filament. When immersed in caustic solution it swells out and takes on a round and a hair like appearance, and becomes plump instead of flat.

How it is done The cotton is soaked in strong caustic soda or caustic potash solution for a few moments under stretch and then washing in pure water to remove the caustic.

How it is doneThe cotton is soaked in strong caustic soda or caustic potash solution for a few moments under stretch and then washing in pure water to remove the caustic.

The cellulose is changed into hydro-cellulose or cellulose-hydrate.Cellulose cannot be dyed so easily. Hydro-cellulose on the other hand, absorbs almost any kind of dye readily. Mercerised cotton takes dyes so fast, that chemicals are added in the dye bath to check the process in order that the dyes may not enter so rapidly as to render the shading uneven.

Woven fabric

Knitted

Taking a cotton blend, and then mercerising will produce an effect called

as crepon effect. Similarly, mercerisation can be used to produce seersucker effect by Mercerising only

certain stripes on the warp direction by covering the rest of the cloth by suitable

means.