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Pretreatment For Fabrics Before Dyeing & Printing

TADM 405: Colouration Technology

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Textile Preparation is the heart of the textile dyeing, printing and finishing processes because fibers, yarns or fabrics have many impurities (i.e. cotton wax, starch, lubricants, silk sericin, wool oil etc.).

More than 60% of the faults on textiles are due to poor preparation.

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WHAT CAUSES IMPURITIES

The raw material in general contains unwanted matter of fibres and the impurities of fibres. This impurities is :

Natural impurities is the dirt is emerge together on cotton.

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WHAT CAUSES IMPURITIES

Impurities from outsideDirt coming from outsider, yarn or fabric , for example : oil, ,dust , parts of leaf, branch, stains of oil from machine and etc.

The addition of impuritiesThe addition material impurities for the fluency of process for example : oil, starch at warp of yarn, etc.

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Typical pre-treatment of 100% cotton involves

•Singeing

•Desizing (only for woven fabrics) by conventional /enzymatic process•Scouring by conventional /enzymatic process•Bleaching (only for pale or bright shades)•Neutralization•Peroxide removal•Bio polishing‐•Mercerizing•Optical Brightening

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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)

Singeing During the spinning process, tiny fibre ends

protrude from the surface of the spun yarns hence fabric woven from these yarns will have these tiny fibre ends protruding from the surface. These fibres interfere with printing of the fabric by taking up parts of fine details of a design.

Singeing therefore is the process of burning out projecting hairy fibres from the surface of the grey fabric by passing the fabric through a gas flame. Both sides of the fabric are burnt out by the flame.

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Singeing It helps in subsequent chemical process done

to the fabric. After dyeing or printing of singed fabric, brightness and luster of colour and fabric becomes better.

Singeing is a process applied to both yarns and fabrics to produce an even surface by burning off projecting fibres, yarn ends, and fuzz. This is accomplished by passing the fibre or yarn over a gas flame or heated copper plates at a speed sufficient to burn away the protruding material without scorching or burning the yarn or fabric.

(hairs) on the fabrics (these fibres normally come from spun yarns).

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Singeing During this process, the fabric is passed over

a flame in a singeing machine at a high speed ( 70 – 100m/min).

When the fibres are singed (burnt off, the fabric is left with a smooth and uniform surface.

This can also be done by using two flames thus both sides of the fabric can be singed by passing the fabric through the machine once.

The fabric is then dipped in water to extinguish any residual sparks, squeezed between two rollers and plaited.

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Singeing The main Objectives for singeing is:

1. To get rid of the small fibres2. To make the fabrics smoother than before3. To help the printing ink or dyestuff to make clearer marks on the fabrics4. To improve rubbing fastness and washing fastness of the dyed or printed fabrics

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De-sizing It is the process of removing starch materials

present in the grey fabric. Generally an enzyme is used to degrade and remove the starch present in the grey fabric.

If the size materials are not removed from the grey fabrics, then the subsequent chemical treatments on the fabric will be irregular, which will cause defective dyeing and printing.

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De-sizing The starch present in the size applied to

the warp yarns before weaving the cloth is removed.

The degradation of the starch can be carried out either by hydrolysis or oxidation.

Hydrolysis is done by using dilute mineral acids or enzyme preparations.

In de-sizing the hydrolysis reaction is carried out up to the stage of soluble dextrin only and not further to a‐glucose.

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De-sizing Acid de-sizing: 0.25% acid

(hydrochloric or sulphuric) is sufficient to solubilize the starch at room temperature in a few hours.

If the concentration of the acid, temperature and duration of the acid de-sizing treatment are increased, the de-sizing will takes place very rapidly, but the cotton attacks the cotton also resulting in the weakening of the fabric.

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De-sizing Enzymatic de-sizing: certain enzymes are used to liquefy (degrade the starch to a soluble state).The singed cloth is passed through hot water then through 0.5 – 2% enzyme solution kept at 60° - 70°C.The enzymes are active over a particular temperature range and pH range beyond which they get inactivated (destroyed).

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De-sizing The enzymes hydrolyses the starch

only (not the cellulose)

On the other hand acid de-sizing attacks cotton cellulose under suitable conditions of time, temperature and acid concentration.

Enzyme de-sizing is therefore safer than acid de-sizing. (enzymes are however more expensive than acids).

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De-sizing Oxidative desizingIn oxidative desizing, the risk of damage to the cellulose fibre is very high, and its use for desizing is increasingly rare. Oxidative desizing uses potassium or sodium persulfate or sodium bromite as an oxidizing agent.

Removal of water-soluble sizesFabrics containing water soluble sizes can be desized by washing using hot water, perhaps containing wetting agents (surfactants) and a mild alkali. The water replaces the size on the outer surface of the fiber, and absorbs within the fiber to remove any fabric residue. 18

Scouring It is the process of removing natural impurities present in

the cotton fibre. The natural impurities are pectin’s, pectose, 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 the main reagent for the scouring of cotton fabric.

The main purpose of scouring is to remove natural and added chemicals of essentially hydrophobic character as completely as possible and leave the fabric in a highly absorptive condition.

This is done without the fabric undergoing significant chemical or physical damage and it prepares the fabric for bleaching. 19

Scouring Scouring is also referred to as kiering,

boiling out or kier boiling. For large scale purposes, scouring is

done in a boiler also called a kier. Scouring is done in a circulating hot

alkaline liquor containing a detergent to a regularly packed column of de-sized cloth usually under pressure for a prolonged period (8 – 24 hrs.)

The composition, temperature of the liquor, pressure in the kier and duration of the boiling depends upon the nature of the cloth being scoured.

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Objectives of scouring To remove natural fat, wax, and oil

materials contained in the fabrics without damaging the fibres

2. To accelerate dye and chemical absorption of the fabrics

3. To improve the handle of the goods (softer)

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Main processes during Scouring

Saponification of fats into water soluble soap and water miscible glycerin under alkaline conditions.

Hydrolysis of protein into water soluble degradation products

Dissolution or hydrolysis to ammonia of simpler amino compounds

Conversion of pectose and pectin into their soluble salts 22

Main processes during Scouring………

Dissolution of mineral matter Emulsification of unsaponifiable

oils and waxes Removal of dirt particles

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Scouring………This can be achieved by usin

g sodium hydroxide and det - ergent at boil for 2 0 3 0

minutes. Syntheticfabricsandot her pr ot ei n f abr i cs ma yuseonlysodi umcar bonat e (weaker alkali) instead.

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Bleaching After the fabric has been de-sized and

scoured it is left in a more absorbent condition. However the natural colouring matter of cotton is still present in the fabric.

In order to obtain a white cloth (to enhance the production of pure or pale shades through printing or dyeing), the normal practice is to discolourise the natural colouring matter present in the cloth.

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Bleaching The discolouring is achieved through bleaching using: Dilute hypochlorite solution (sodium hypochlorite) at room temperature usually under alkaline conditions.

Or hydrogen peroxide under alkaline conditions in the presence of stabilizers like sodium silicate at 80° – 85°C

Or sodium chlorite under acidic conditions at the boil

Or peroxy compounds e.g. peracetic acid 26

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Bleaching Process

• The process is to make the go ods whiter than before. This w

ill help the goods to absorb mo re dyes and chemical and also

make the dye on the goods bri ghter.

• Normally, for cotton fabrics, h ydrogen peroxide in alkali so

lution at boil are the most pop ular bleaching agent.

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Objectives of bleaching

• To whiten the goods

• To make the goods to be suitab le for dyeing and printing with p

ale or bright shade

• To be follow with adding optica l brightening agent process (su

per white)

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Bleaching Of Silk

Natural coloring matters present in silk are associated mainly with sericin and hence are eliminated during degumming. The natural colouring matter of silk can be roughly divided into yellow, green and brown pigments.

However the residual pigments are adsorbed by fibroin and hence silk fabrics made from yellow raw silk after degumming are not white but have a cream colour.

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Silk BleachingThe bleaching process may be based on reducing agents or oxidizing agents.

Reductive Bleaching•Predominant reducing agents used are Sulphur dioxide, sodium hydrosulphite and sodium or zinc sulphoxylate formaldehyde.

•The material bleached with reducing agents tends to re-oxidise and the original color may be restored. Hence oxidizing bleaching is most preferred.

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Oxidative BleachingThe oxidative agents used are

•potassium permanganate,•sodium perborate,•sodium peroxide•Hydrogen peroxide.

The hydrogen peroxide is the most preferred bleaching agent

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Bleaching of Wool

•Wool exhibits by nature a pronounced yellow colour and also on exposure to light, alkali or by microbial degradation. Commercially, wool bleaching is carried out using either an oxidative or a reductive system, or a combined oxidation/reduction process. Oxidative bleaching in the dye bath is also possible.

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In general, oxidative bleaching with hydrogen peroxide gives superior whiteness over reductive methods. Recent research also revealed the use enzymes to enhance the whiteness of bleached wool.

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Mercerizing Process•This process is done to make th

e cotton goods have more luster than before using sodium hydrox

ide solution in cold condition and tension.

• This will lead the cotton fibers to be swollen and increased strengt

h.•It will improve dye absorption of

the fibers. John Mercer was the first chemist in the world who fo und this phenomenon in 1 8 4

4. Therefore, the process was ca lled“ Mercerizing”

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Caustizing ProcessThis is the process nearl

y the same as mercerizi ng but it uses lower con

centration of the hydro xide solution and witho

ut tension.

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Objectives1. To make the goods more luster bec

-ause raw cotton fibers are ribbon like . After this process, the fibers chang e to cylindrical shape resulting more

evenness reflected light from the goods.

Before After 2. To improve dyeability of the goods

-about 5 10%.

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Adding Optical Brightening Agent

• The process is to add special dyestu ff into textile or paper to make them

- brighter when looking at black light l ight bulb or natural sunlight.

• As the dyestuffs are colorless but us ing UV light to excite these dyes. Th

ey will reflect the light at the wavele ngth that human can see (i.e. blue, v

iolet). So the goods look brighter an d whiter.

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