Dyeing Technology

The goal of every dyeing is a colored textile in the desire shade homogeneous in hue and depth of shape, produce by an economic process and which exhibit satisfactory fastness properties in the finished take.

There are five steps of dyeing technology:

1. Principle of dyeing

2. Bath dyeing technology

3. Continuous and semicontinuous dyeing

4. Printing

5. Dispensing dyeing and chemical

1. Principle of Dyeing: Basically there are three method of dyeing textile.

a. Mass dyeing: Dyeing of the synthetic polymer before fibre formation.

b. Pigment dyeing: Affixing and insoluble colored on the fibre surface with a binder.

c. Exhaustion dyeing: Exhaustion dyeing from an aqueous bath with dyes that have an affinity for the fibre.

In exhaustion dyeing, the dye which is at least partially soluble in the dye bath is transported to the fibre surface by motion of the dye liquor or the textile. For most dyeing processes demineralized water should be used. It not available complexing agent must be added, where necessary to avoid negative interference of metal ions.

The dyeing rate depends on the factor such as temperature liquor ratio, dye concentration and the chemicals and such the auxiliary products in the dye bath.

The dye producers readily communicable their knowledge on dyeing characteristics of their dyes and on the particular requirements for all textile fibres.

2. Bath Dyeing Technology: Dyeing rates and good leveling properties of the dyes are normally reached at temperature. To achieved short dyeing cycles modern equipment is to be as near as possible to the boiling temperature of the dye liquor. Discontinuous dyeing equipment even permits dyeing under pressure at temperature up to 135^oC.

Bath process is two types. Such as

a. Circulating machines

b. Circulating goods machines

a. Circulating Machines: In which the goods are stationary and the liquors circulates. The goods is flock, card silver, tow, yarn or fabric are loose on cones or perforated beams and the liquor is pumped through the goods.

b. Circulating Goods Machines: In which the textile material and normally also the liquor are in circulates. Traditional dyeing equipment belongs to two groups.

i. Winch becks: Winch becks where fabric is drawn through a dye beck by way of a winch.

ii. Jigger: Jigger, where fabric is mounted in whole width on roll and guided back fixth trough a dye bath.

Process control in bath dyeing: Fibre dyeing is depended in the following parameters. Such as

a. Dyeing time

b. Temperature

c. pH

d. Concentration of liquor

e. filling and emptying the dyeing vessel rate control

f. Automatic metering of dyeing

g. Other chemicals.

3. Continuous and Semicontinuous Dyeing: Continuous dyeing means treating fabric in a processing unit in which application of the dye to the fabric and fixation are carried out continuously.

The following principles applied to all continuous dyeing-

a. Fabric is treated in open width

b. An unevenness in the equipment across the width of the goods leads to unlevel dyeing.

c. The width of the goods and longitudinal tension influence each other (Stretching, shrinking).

d. The running speed determine the dual time in the treatment unit.

e. Interruption cause spoiled fabric.

Application of dyeing: Two types of dye application technique are

a. Direct application of dye liquor by spraying foam application or printing.

b. Continuous immersion of the fabric in the dye a bath and removal of excess liquor by squeezing or suction. The most important technique is the pad process, in which the dry fabric is passed through, goods wetting of the fabric, is important.

Intermediate Dyeing: To assure uniform fixation of dyes the fabric is often dried before the fixing step. During drying the dye liquor migrates to the surface of the fabric, which can increase the visual color yield. This process ends when the liquor content of the fabric is below 30%. The drying equipment normally works with infrared heat or with a hot air stream or a combination of both. Drying should start contact free to avoid smearing of the fabric and soiling of the present.

Fixation: On the dried fabric the dye is only deposited on the fiber surface penetrate into the fibre during a fixation step and be incorporated in the chemical reaction (reactive dyes) aggregation (vat sulfur dyes), ion-pair (disperse dyes). Fixations performed by steaming under various conditions depending on the dyes involved generally saturated strum at 100^oC are applied.

4. Printing: Direct printing is the application of a printing paste containing dyes, thickeners, and auxiliaries directly to the fabric by roller printing (on engraved cylinders) or screen printing (with flat or rotating screens).

 Recently methods were developed for computer-controlled ink-jet printing. Production speed is still low (less than 20m/min). On the product side no problem arises with soluble dyes. Ink-jet processes are already well established in carpet printing.

In transfer printing the design is first printed on paper with dyes sablimable in the range of 190-230^oC.

Two phase printing is the separate application of dyes and fixing agent in reactive and vat printing.

In discharge printing the fabric is dyed with a dischargeable dye and then printed with a discharge paste in the desired pattern.

Special printing techniques were developed e.g. for floor coverings (spray printing space dyeing) and for yarn or wool stubbing.

The most important fibers for printing are cellulosics (70%) and polyester (20%), all other account for less than 10%.

5. Dispensing Dyeing and Chemical: The dyer is obliged to follow precisely the dyeing formulation worked out previously for a particular color. In critical shades like gray or beige, variations of 2-3% in color strength and hue can be seen by the human eye. The amount of dye required for certain shades can vary form Ca 0.111% to 10%, relative to the amount of textile material and for most shades more than one dye is required.