Flow chart of pre treatment of cotton fabric

 

• Our Presentation Topic Is pretreatment is the heart of wet processing.

 Pretreatment is a heart of processing of textile. In Pretreatment, all these impurities are removed and fabric is brought to a stage where it is more absorbent and white and can be easily processed further. 

 The process which is done to make the textile materials suitable for dyeing and printing. Such as singeing, desizing, scouring, bleaching etc.

Flow chart of pre treatment of cotton fabric:

 

All the processes of this stream are carried out in an aqueous state or aqueous medium. The main processes of this section include :

• singeing

• Desizing

• Scouring

• Bleaching

• Mercerizing

Singeing:

• The verb ‘singe’ literally means ‘to burn superficially’. Technically, singeing refers to the burning-off of. Loose fibers not firmly bound into the yarn and/or fabric structure. Singeing is an important part of pretreatment. This is the burning off of protruding fiber ends from the surface of the fabric.

• Singeing is the process is removing the hairs of fabrics or fibers. In singeing process, we use three technique to applied it. Such as, gas flame technique. At the end of this process, we will find, that the fabrics is smoother than before and the fabrics wet ability is increase.

The technique of singeing:

 

Singeing Process is as follows:

• To produce a smooth surface finish on fabrics made from staple fibres first the fabric surfaces are brushed lightly to raise the unwanted fibre 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.

Figure of singeing M/C:

 

Pahan Mazumder

ID:121-23-2884 Desizing:

• Desizing is the process or removing the starch or size the covering the warp yarn using enzyme. Oxidizing agent, or other chemicals. There are three types of technique in desizing and four method of desizing that has been used.

Objectives Of Desizing:

• To eliminate the water repellent nature of sized cloth.

• To increase the absorbency.

• To reduce the consumption of chemicals in subsequent process. 

Technique of desizing:

 

DesizingMethods:

• Enzymatic Desizing.

• Oxidative Desizing.

• Acid Steeping/Desizing.

• Rot Steein.

• Desizing with hot caustic soda treatment.

• Hot washing with detergent.

• Removable of water soluble sizes.

• Atmospheric Plasma Desizing.

Enzymatic Desizing:

 The hydrolysis of starch using enzymes under particular concentration, temperature and duration is called enzymatic desizing.

 Enzymatic desizing is the most widely experienced method to desize the starch.

Advantages & Disadvantages:

• Advantages:

• No usage of aggressive chemicals: No damage to the fiber.

• Wide variety of application processes.

• Disadvantages:

• Lower additional cleaning effect towards other impurities.

• No effect on certain starches (e.g. tapioca starch).

Oxidative Desizing:

 It is used to remove non-starch sizes that don’t dissolve in water.

 A variety of oxidants can be applied after padding the hydrogen per oxide (H2O2) and steam for 2 to 3 minutes.

 The advantages of oxidative desizing are supplementary cleaning effect, effectiveness for tapioca starches but oxidizing agents may damage to fibres.

Acid Desizing:

• Cold solutions of dilute sulphuric acid (H2SO4) or hydrochloric acid (HCl) are utilized to degrade starch and other sizes.

• However this method has also disadvantage of bad affecting the cellulosic fiber in cotton fabrics.

Lye (Caustic Soda) Desizing:

• In this method fabric containing starch would be expanded under the hot solution of sodium hydroxide (caustic soda -NaOH).

• Caustic soda dissolves starch and form layer, can be separated.

• Padding the fabric in alkali and stacked under 

60 ~80 C for 6 to 

12 hours, the desizing process can be finished.

Some Desizing Agents:

• Ecolase LT

• Finozyme -DX

• Finocon –FBOL

• Enzme GT

Figure of desizing:

 

Md. Khairul Hasan

ID:121-23-2973

Scouring:

• Scouring is the process by which all natural and additive impurities such as oil, wax, fat, hand dust etc. are removed to produce hydrophilic and clean textile material. It is one of the vital processes of wet processing. 

Objects 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.

Scouring process depends on:

 The type of Yarn.

 The color of Yarn.

 The cleanliness of Yarn.

 The twist and count of the yarn.

 The construction of the fabric.

 The shade % and type of shade of finished product.

Chemicals used in scouring process:

Main chemicals Use

Caustic soda Neutralize acidic materials, saponify glycerides (waxes and oil),solubilise silicates

Surfactants Reduce surface tension & minimize interfacial tension.

Detergents Emulsify oil, fats, waxes and remove oilborne stains.

Chelating agent Deactivate metal ions.

Sodium silicate Penetrate & break drown lignin's.

Soda ash Maintain pH.

Solvent Assist emulsification by dissolving oily materials.

Form of Scouring:

• Yarn scouring:

 Hank form

 Package form Continuous sheet warp form.

• Fabric scouring:

 Open width form :

• Jigger 

• Pad batch

• Progressive jig

 Rope form

• Kier

• Washer.

Scouring process:

 Batch process.

 Semi – continuous process.

 Discontinuous process.

 Modern process.  

• Scouring methods of Cotton:

 Generally, there are two principle methods of cotton scouring.

 Discontinuous (Kier boiling process or winch dyeing machine)

 Continuous (Scouring in J or L box)

 

 

 

• Main parts of kier boiler:

 Cylindrical vessel

 Mixing tank(liquor prepared according to recipe)

 Multi tubular heat exchanger(heating the liquor)

 Perforated false bottom

 Circular tube(spray liquor on fabric)

• Process: The working process in J-box can be divided into four units. There are

 Impragnaion box Pre – heater.

 J – box.

 Washing unit.

 

 

 

Good scouring

 

Md. Ruhul ID:121-23-3008 Bleaching:

• Bleaching is chemical treatment employed for the removal of natural coloring matter from the substrate. The source of natural color is organic compounds with conjugated double bonds , by doing chemical bleaching the discoloration takes place by the breaking the chromophore, most likely destroying the one or more double bonds with in this conjugated system. The material appears whiter after the bleaching.

The aim of bleaching can be described as following:

• Removal of colored impurities.

• Removal of the seed coats.

• Minimum tendering of fibre.

• Technically reliable & simple mode of operation.

• Low chemical & energy consumption.

• Increasing the degree of whiteness.

• Bleaching Agent

A bleaching agent is a substance that can whiten or decolorize other substances. Bleaching agents essentially destroy chromophores (thereby removing the color), via the oxidation or reduction of these absorbing groups. Thus, bleaches can be classified as either oxidizing agents or reducing agents .

Bleaching agents

 

Type of Bleaching Agents:

 Oxidative Bleaching Agents

 Reductive Bleaching Agents

 Enzymatic Bleaching Agents 

Recipe for Bleaching:

NaOH 17ml/kg

SOAP (DTC) 2ml/kg

STABILIZER 5ml/kg

H2O2 30ml/kg

Reaction time 25min.

speed 50-70m/min

Bleaching agents…

• Oxidative bleaching – the bleaching agent is a chemical reagent which decomposes in alkali solution and produce active oxygen.

• The active oxygen is in fact the intrinsic bleaching agent as it will further destroy partly or completely the coloring matter present in the textile material.

• Reductive bleaching – the bleaching agent will destroy the coloring matter by reductive reaction of SO2.

Bleaching agent:

Auxiliaries used for bleaching:

Stabilizers

Activators

Wetting agents/detergents

Sequestering agents

Anti-corrosion agents

Comparison of Bleaching Efficiency:

pH     Temp.     Time     Wax    Ash Fluidity %Reflect.

NaOCl 11      400C          2h       0.23 0.05 6.2 83

NaClO2 3.8      980C          2h      0.21       0.02 6 86

H2O2 10.5     900C          2h      0.09 0.41 5.1 90

Image of Bleaching:

 

Md. Saiyad Mirajul Haqe

ID:121-23-297

Mercerizing:

• Mercerization is one of the most important finishing processes of cotton with a strong caustic alkaline solution in order to improve the lustre, hand and other properties. It imports gloss to the fiber, increases its hygroscopicity, strength and improves its dye affinity. Mercerizing improves the reactions with a variety of chemicals and elongation of the fibres and also improves the stability of form. Mercerizing process consists in treatment of cellulosic materials with concentrated solutions of caustic soda at a temperature of 15 to 18°C.

Purpose of mercerizing:

1.To improve the lusture.

2.To improve the strength.

3. To improve the dye uptake and moisture regain. 

Effect of Mercerization:

 Improve Luster.

 Increase ability to absorb dye.

 Improve reaction with a verity of chemicals.

 Improve stability of form.

 Improve strength/elongation.

 Improve smoothness.

 It has been shown that the increase in the luster occurs because of an effect.

 The cotton fiber do convoluted.

 The cross-sectional shape changes.

Different mercerizing machinery/technology:

 Knitted fabric mercerizing m/c. Woven fabric mercerizing m/c.

 Automatic hank yarn mercerizing m/c.

 Chainlesss-padless mercerizing range.

 Clip mercerizing m/c.

 Open width fabric mercerizing m/c with caustic recovery unit.

Factors of Mercerizing:

• In mercerizing followings are important: Twaddle

• Temperature

• Tension

• Time

luster of yarn after mercerization:

 

 

 

 

Image of Mercerizing M/C: