pretretment of fabric

8/4/2019 Pretretment of Fabric

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TDD-102 Unit-3

Dr. Parul Bhatnagar Dayalbagh Educational Institute1

Dyeing methods and Pre-Treatment Of The Fabric or Fiber before Dyeing

The general sequence of pre-treating the textile goods, gray fabrics when it comes out of the loom is

quiet unattractive and contains natural and added impurities which give undesirable properties to the

textile goods and the subsequent dyeing and printing cannot be carried out successfully.

Different sizes like the starch, CMC are applied to the warp yarns to make them strong before

weaving. These starches if not removed will make the fabric unsuitable for dyeing or printing or

applying any other finishes. Also the natural fatty and waxy material present in the cotton yarn

reduces its absorbency property this making it unsuitable for dyeing.

Therefore Before the fabric is dyed or printed it has to be cleaned as a lot of impurities get collected

during the journey from the fiber to fabric stage. These impurities are in the form of fatty and waxy

materials, grease, dirt, twigs, starch, dust etc. It is important to give the pre- treatment as dyeing and

some other finishes require high degree of absorbency for even application.

1. Singeing: In singeing the protruding fibers are burnt. It is a very skillful job and has to bedone very carefully. There are 2 methods of singeing:

Gas Singeing: The machine is provided with two gas burners which is supplied with a mix

of coal gas and air under pressure. The gray fabric in open width passes over the gas burners

in a slant form and the flame burns the protruding fibers. After singeing the fabric is

immediately dipped in water to extinguish any sparks present in the fabric surface. The gray

fabric is passed in an open width at a very high speed of about 70-100mts per min. By using

two flames both the sides of the fabric can be singed at the same time.

Hot Plate Singeing: In this the 2 copper plates are heated to redness either by means of coal

or electricity. The plates are placed parallel to each other. The gray fabric is passed in open

width in high speed between the two plates without touching the plates. Due to this the

surface hairiness is burnt. The fabric later passes through a water container and is dried and

folded.

2. De-sizing or Steeping: Size means starch. In this process the starch which is present in theyarns or the fabric is attacked along with the dirt, dust, twigs etc. De-sizing can be done by 3

methods-

1.Rot Steeping: It is the simplest method. The fabric is simply soaked in cold water for a

minimum of 24 hours. This is the cheapest method as no chemicals are involved.

2. Acid Steeping: As the name suggests the fabric is treated with .25% HCl for a period of 2

hours. he fabric should be washed thoroughly after the treatment as any remains of the acid

will damage the fabric or the yarn.

3. Enzyme steeping: A solution of 10% common salt with .5% to 1% enzyme is prepared.

The material is soaked for 6 hours or overnight.


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3. Scouring: Scouring is done to remove the greasy and waxy materials which are associatedwith the textile fibers and are present naturally. Scouring, in essence is the treating the yarns

or the fabric in an alkaline which has the property to emulsify the oily and greasy matter by

the process of saponification. The emulsified fat remains suspended in the water solution

leaving the fabric free from the waxy and greasy matter. The fabric is treated with soap

solution along with caustic soda (1.5%) and soda ash (1%) for a period of 2-3 hours. Thetemperature is maintained at boiling point. The material is this further purified. Cotton can

be treated with alkaline conditions while silk and wool which protein fibers are sensitive to

alkaline solutions are treated with dilute acids to remove the impurities.

For thin cottons the concentration of caustic soda can be increased to 3%. For fine cottons as

the impurities are less the caustic soda can even be avoided.

4. Bleaching: Fabrics, yarns and fibers may be bleached to produce white fabrics or forpreparation for dyeing or printing. Bleaching is a chemical finish It is a relatively durable

process when the bleaching method is appropriate to the fiber involved.

Various chemicals are used in bleaching depending on the textile fiber. Cellulosic fibers,

such as cotton are bleached with chlorine compounds (E.g. Sodium Hypo-chloride) or

hydrogen peroxide, which is a more frequently used alternative. Per-borate bleaches are

available for home use, but they are seldom used commercially as the hydrogen peroxide

reacts in much a safe way and is more effective.

Chlorine compounds used for bleaching protein fibers result in the loss of fiber strength and

eventual deterioration of the fabric. Wool and silk may be bleached by the use of sulfur

dioxide gas in moisture. The method is called as STOVING by the industry and produces as

less durable effect on wool as the wool may deoxidize in air and return to its original color.

Hydrogen peroxide is an oxidation reaction type of bleach while sulphur dioxide is reductionbleach.

Man- made textiles can be bleached by saturating the fabric with bleach solution; the

temperature is raised according to the fiber involved for the prescribed time. Then the fabric

is thoroughly washed and dried. The current trend of bleaching is the use of optical

brighteners. Generally the manmade fabrics do not require bleaching as they are white if the

manufacturing process has been carried out properly.

Dyeing: The main objective of dyeing is to produce uniform and reproducible shades with

maximum exhaustion of the dye in a short time. The colour may be added to the textile goods at any

stage.

a. The colour is added before the fabric is extruded from the spinnerets. This is called as Doupdyeing and is applicable on the synthetic dyes. This is also called as solution dyeing orspun

dyeing. The fabrics made from the yarns with this by this method of dyeing have very good

washing fastness as the dye becomes an integral part of the fiber.


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b. Dye can be applied on the fibers. When the fibers are dyed before being spun in to yarnsthen the method is called as Stock dyeing. In case of wool it is called as TOP dyeing.

c. Dyes applied on the yarn staged. Dye application in the fabric stagee. Dye application in the Garment stage.The basic steps involved in dyeing are as follows:

A Impregnation on the colour: Soak the sample in the colour.

B Development or Fixation of the colour

C. After TreatmentSoaping and washing

D Drying

Dyeing Of Loose fibers or Stock Dyeing: The simplest equipment used for dyeing is a rectangular

beck made of wood or a cement beck. Loose fibers are dumped in which are occasionally stirred

with a long rod. An improved form of equipment consists of a lid. The loose fibers are packed in

this container and the dye is circulated with a pump. The direction of the flow can be reversed with

the help of a control valve. There is also arrangement to regulate the liquor temperature and theprovision for entering the loose fibers and for removing the dyed fibers.

Dyeing of Yarns: When dyeing is done after the fiber has been spun into yarns, it is described as

yarn dyeing.The purpose is to have the dye stuff penetrate to the fibers in the core of the yarn. The

yarns can be dyed either in the form of a hank or they can be wound on a suitable package like a

spool or a cone or bobbins. The most common method is to use a perforated metal spool on which

the yarn is wound and dyed. The yarns can also be beam dyed. The primary reason for dyeing in the

yarn form is to create interesting check, stripes with coloured yarns in the weaving process.

Chambrays, seersuckers, plaid, gigham are woven with different coloured threads. Yarns dyed are

usually deeper and richer in colour.

1. Hank or skein dyeing: The yarn may be prepared in a skein or hank form and is then dyed.The loose arrangement of the yarn allows easy allows for excellent dye penetration. The simplest

equipment consists of a rectangular tank which is provided with a steam pipe at the base for

heating dye solution. Water inlet and outlet pipes are also fixed. Number of hanks each weighting

about 200gms are suspended from the top of the tank on a rod and almost of the tank is filled

with dye solution. 2 dyers standing on either side of the tank rotate the hanks from time to time.

Dyeing is started at room temperature and gradually is increased by letting the steam in. This is the

most expensive method of yarn dyeing but the colour penetration is best and the yarns retain a

softer texture.

2. Package dyeing: Yarn wound on spools, cones, or similar units and then dyed is referred as package-dyed yarn. The packages of yarn are stacked on perforated rods in a rack and then

immersed in a tank wherein the dye is forced outward from the rods under pressure through the

spools and then back through the package toward the centre to penetrate the entire yarn as

thoroughly as possible. Most cotton used for knitted outerwear is package dyed.


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3. Warp-Beam Dyeing: The method is similar to package dyeing but is more economical. Yarnis wound onto perforated warp beam, immersed in tank, and dyed under pressure or the dye liquor

is circulated through the vessel. It is used when fabrics are to be woven with dyed warp yarns.

4. Space Dyeing: Yarn that is space dyes is dyed at interval along its length. One procedure is theknit de-knit method in which the yarn is knitted on a knitting machine. The knitted cloth is thendyed and then it is de-knitted. The dye penetrates the loops of the yarns but since it does not readily

penetrates the areas on the yarn where it crosses itself, alternating dyed and un-dyed space appears.

Variations in hue are possible in the fabrics subsequently produced.

5. Piece or garment dyeing: The great bulk of dyed fabrics in the market are piece dyed. Thismethod allows the producers to have small lots of fabrics of all finds of fibers in batches. It is a

thoroughly satisfactory as regards to evenness in dyeing, and over fastness if proper dye selection is

made. Fabrics can be piece dyes whether it is composed of only one kind of fiber or yarn or blends

of different fibers or combination of different yarns. When the fabric is made of one kind of firber

then the dyeing is relatively uncomplicated because one dye is used. However, when the fabric

contains a blend of fibers the a special procedure is required that are specific to one particular fiberused. These procedure are called as union dyeing and cross dyeing.

pretretment of fabric

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