fibre dyeing

INTRODUCTION

Dyeing is the process of adding colors to textile products like fibers, yarns and

fabrics. Dyeing is normally done in a special solution containing dyes and particular

chemical material. After dyeing, dye molecules have uncut chemical bond with fiber

molecules. The temperature and time controlling are two key factors in dyeing. There

are mainly two classes of dye, natural and man-made.

The primary source of dye, historically, has generally been nature, with the dyes

being extracted from animals or plants. Since the mid-18th century, however,

humans have produced artificial dyes to achieve a broader range of colors and to

render the dyes more stable to resist washing and general use. Different classes of

dyes are used for different types of fiber and at different stages of the textile

production process, from loose fibers through yarn and cloth to completed garments.

Acrylic fibers are dyed with basic dyes, while nylon and protein fibers such

as wool and silk are dyed with acid dyes, and polyester yarn is dyed with disperse

dyes. Cotton is dyed with a range of dye types, including Vat dyes, and modern

synthetic reactive and direct dyes.

1 NATIONAL INSTITUTE OF FASHION TECHNOLOGY, HYDERABAD

METHODS OF DYEING

Color is applied to fabric by different methods and at different stages of the textile

manufacturing process.

Fibre Dyeing Yarn Dyeing Fabric Dyeing

Stock DyeingSkein(hank)

DyeingBeck Dyeing

Top Dyeing Package Dyeing Jig Dyeing

Dope Dyeing Space Dyeing Pad Dyeing

Tow DyeingWarp-Beam

DyeingJet Dyeing

Union Dyeing

Cross Dyeing


FIBER DYEING

The dye penetration is excellent in fiber dyeing, therefore the amount of dye used to

dye at this stage is also higher. Fiber dyeing is comparatively more costly than yarn,

fabric, and product dyeing. The decision regarding the selection of colors has to be

made early in the manufacturing process. Fiber dyeing is typically used to dye wool

and other fibers that are used to produce yarns with two or more colors. Fibers for

tweeds and fabrics with a “heather” look are often fiber dyed.

Stock dyeing, top dyeing, and tow dyeing are used to dye fibers at various stages of

the manufacturing process.

Stock Dyeing

Stock dyeing is used to dye fibers. In this process, the staple fibers are packed into a

vessel and then dye liquid is forced through them. Although the dye solution is

pumped in large quantities, the dye may not penetrate completely into the fibers and

some areas may be left without dyeing. However, the following blending and

spinning processes mix up the fibers in such a thorough way that it results in an

overall even color. Woolens are usually stock dyed.

A commonly used machine is the conical pan with forced circulation. Loose stock

is packed into the container in which it is retained by a lid which can be held down

with a number of screw-threaded clamps. A flange at the bottom of the container fits

into a seating through which the dye liquor is circulated by a centrifugal pump, and

the direction of the direction of flow can be altered by a two-way valve. The sides of

the container slope inwards so that, as the flow of the liquor pushes the mass

upwards, it becomes mire compressed and automatically closes up channels of least

resistance to the passage of the liquor. A gantry and travelling hoist are necessary to

lift the container in and out.

Top Dyeing

Top dyeing is dyeing worsted wool fibers after they have been combed to straighten

and remove the short fibers. The wool fiber at this stage is known as top. Top dyeing

is preferred for worsted wools as the dye does not have to be wasted on the short

fibers that are removed during the combing process.


In a top dyeing machine, the tops are packed into perforated cans and the lids are

fixed in position. The bottoms of the can rest on a seating through which the dye

liquor can be forced upwards by the centrifugal pump at the side of the machine, or

in the reverse direction by altering the low.

Solution Dyeing or Dope Dyeing

Solution dyeing, also known as dope or spun dyeing, is the process of adding

pigments or insoluble dyes to the spinning solution before the solution is extruded

through the spinneret. Only manufactured fibers can be solution dyed. It is used for

difficult-to-dye fibers such as olefin fibers, and for dyeing fibers for end uses that

require excellent colorfastness properties. Because the color pigments become a

part of the fiber, solution dyed materials have excellent colorfastness to light,

washing, crocking (rubbing), perspiration, and bleach. Dyeing at the solution stage is

more expensive since the equipment has to be cleaned thoroughly each time a

different color is produced. Thus, the variety of colors and shades produced are

limited. In addition, it is difficult to stock the inventory for each color. Decisions

regarding color have to be made very early in the manufacturing process. Thus, this

stage of dyeing is usually not used for apparel fabrics.

Tow Dyeing

Tow dyeing is dyeing filament fibers before they are cut into short staple fibers. The

filament fibers at this stage are known as tow.


YARN DYEING

When dyeing is done after the fiber has been spun into yarn, it is called yarn dyeing.

In this method, the dyestuff penetrates the fibers to the core of the yarn. There are

many forms of yarn dyeing- Skein (Hank) Dyeing, Package Dyeing, Space

Dyeing ,and Warp-beam Dyeing.

Skein(hank) Dyeing

Skein dyeing consists of immersing large, loosely wound hanks (skeins) of yarn into

dye vats that are especially designed for this purpose. Soft, lofty yarns, such as hand

knitted yarns are usually skein dyed. Skein dyeing is the most costly yarn-dye

method. High -bulk yarn hanks are immersed into a dye bath for dyeing . Pre-

shrinkage of yarn is done before Dyeing. Lot to Lot variation is a common issue.

Acrylic High Bulk yarn are dyed with small lot size. It is not suitable for bigger Export

order. Good hand feel can be observed in the final fabric.

Package Dyeing

In package dyeing the yarn is wound on a small perforated spool or tube

called a package. Many spools fit into the dyeing machine in which the flow of the

dye bath alternates from the center to the outside, and then from the outside to the

center of the package. Package dyed yarns do not retain the softness and

loftiness that skein-dyed yarns do. They are however satisfactory and very

widely used for most types of yarns that are found in knitted and woven fabrics.

Space Dyeing

In this method, the yarn is dyed at intervals along its length. For these two

procedures- knit- deknit method and OPI Space-Dye Applicator- are adopted. In

the first method, the yarn is knitted on either a circular or flat-bed knitting machine

and the knitted cloth is then dyed and subsequently it is deknitted. Since the dye

does not readily penetrate the areas of the yarn where it crosses itself, alternated

dyed and undyed spaces appear. The OPI Space-Dye Applicator technique

produces multi colored space- dyed yarns. The yarns are dyed intermittently as

they run at high speeds of upto 1000 yards (900 m) per minute through spaced


dyebaths with continuous subjection to shock waves produced by compressed

air assuming supersonic velocities.

The common dyeing process of cotton yarn with reactive dyes at package form is as

follows:

1. The raw yarn is wound on a spring tube to achieve a package suitable for dye

penetration.

2. These softened packages are loaded on a dyeing carrier's spindle one on

another.

3. The packages are pressed up to a desired height to achieve suitable density

of packing.

4. The carrier is loaded on the dyeing machine and the yarn is dyed.

5. After dyeing, the packages are unloaded from the carrier into a trolley.

6. Now the trolley is taken to hydro extractor where water is removed.

7. The packages are hydro extracted to remove the maximum amount of water

leaving the desired color into raw yarn.

8. The packages are then dried to achieve the final dyed package. After this

process, the dyed yarn packages are packed and delivered.

Warp-Beam Dyeing

Beam dyeing is the much larger version of package dyeing. An entire warp beam is

wound on to a perforated cylinder, which is then placed in the beam dyeing

machine, where the flow of the dye bath alternate as in the package dyeing.

Beam dyeing is more economical than skein or package dyeing, but it is only

used in the manufacture of woven fabrics where an entire warp beam is dyed.

Knitted fabrics, which are mostly produced from the cones of the yarn, are not

adaptable to beam dyeing.


FABRIC DYEING

Fabric dyeing, also known as piece dyeing, is dyeing fabric after it has been

constructed. It is economical and the most common method of dyeing solid colored

fabrics. The decision regarding color can be made after the fabric has been

manufactured. Thus, it is suitable for quick response orders. Dye penetration may

not be good in thicker fabrics, so yarn dyeing is sometimes used to dye thick fabrics

in solid colors. Various types of dyeing machines are used for piece dyeing. The

selection of the equipment is based on factors such as dye and fabric characteristics,

cost, and the intended end use. The various methods used for this type of dyeing

include jet dyeing. Jig dyeing, pad dyeing and beam dyeing.

Beck Dyeing

It is used for dyeing long yards of fabric. The fabric is passed in rope form through

the dyebath. This rope of the fabric moves over a rail onto a reel which immerses it

into the dye and then draws the fabric up and forward and brings it to the front of the

machine. This process is repeated many times until the desired color intensity is

obtained.

Jig Dyeing

It is similar to the process of beck dyeing with a slight variation. The fabric in jig

dyeing is held on rollers at full width rather than in rope form as it is passed through

the dyebath.

Pad dyeing

Padding is also done while holding the fabric at full width. The fabric is passed

through a trough having dye in it. Then it is passed between two heavy rollers which

force the dye into the cloth and squeeze out the excess dye. Then it is passed

through a heat chamber for letting the dye to set. After that it is passed through

washer, rinser and dryer for completing the process.

Jet dyeing


Fabric is placed in a heated tube where jets of dye solution are forced through it at

high pressures. The fabric too moves along the tube. The solution moves faster than

the cloth while coloring it thoroughly.

Union Dyeing

Union dyeing is “a method of dyeing a fabric containing two or more types of fibers or

yarns to the same shade so as to achieve the appearance of a solid colored fabric”.

Fabrics can be dyed using a single or multiple step process. Union dyeing is used to

dye solid colored blends and combination fabrics commonly used for apparel and

home furnishings.

Cross Dyeing

Cross dyeing is “a method of dyeing blend or combination fabrics to two or more

shades by the use of dyes with different affinities for the different fibers”. The cross

dyeing process can be used to create heather effects, and plaid, check, or striped

fabrics. Cross dyed fabrics may be mistaken for fiber or yarn dyed materials as the

fabric is not a solid color, a characteristic considered typical of piece dyed fabrics. It

is not possible to visually differentiate between cross dyed fabrics and those dyed at

the fiber or yarn stage. An example is cross dyeing blue worsted wool fabric with

polyester pin stripes. When dyed, the wool yarns are dyed blue, whereas the

polyester yarns remain white.

Cross dyeing is commonly used with piece or fabric dyed materials. However, the

same concept is applicable to yarn and product dyeing. For example, silk fabric

embroidered with white yarn can be embroidered prior to dyeing and product dyed

when an order is placed.


PRODUCT DYEING

Product dyeing, also known as garment dyeing, is the process of dyeing products

such as hosiery, sweaters, and carpet after they have been produced. This stage of

dyeing is suitable when all components dye the same shade (including threads). This

method is used to dye sheer hosiery since it is knitted using tubular knitting

machines and then stitched prior to dyeing. Tufted carpets, with the exception of

carpets produced using solution dyed fibers, are often dyed after they have been

tufted. This method is not suitable for apparel with many components such as lining,

zippers, and sewing thread, as each component may dye differently. The exception

is tinting jeans with pigments for a “vintage” look. In tinting, color is used, whereas in

other treatments such as acid-wash and stone-wash, chemical or mechanical

processes are used. After garment construction, these products are given the

"faded" or "used" look by finishing methods as opposed to dyeing.

Dyeing at this stage is ideal for quick response. Many T-shirts, sweaters, and other

types of casual clothing are product dyed for maximum response to fashion’s

demand for certain popular colors. Thousands of garments are constructed from

prepared-for-dye (PFD) fabric, and then dyed to colors that sell best.


REMOVAL OF DYES

If things go wrong in the dyeing process, the dyer may be forced to remove the dye

already applied by a process called "stripping". This normally means destroying the

dye with powerful reducing agents such as sodium hydrosulfite or oxidizing agents

such as hydrogen peroxide or sodium hypochlorite. The process often risks

damaging the substrate (fiber). Where possible, it is often less risky to dye the

material a darker shade, with black often being the easiest or last option.


fibre dyeing

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