Continuous process of dyeing:

When there is a need of high production of one shade of fabric especially for uniform cloth for military, paramilitary, school etc, continuous dyeing process is very useful. In the continuous dyeing, fabric is dyed in open width. In the continuous dyeing process numbers of machines are placed in sequence so the dyed fabric can be obtained in one go. The textile substrates are allowed to enter continuously into a dye range. The speeds can vary between 10 to 100 meters per minute. Continuous dyeing process is very useful for polyester-cotton blends. In this process dye utilization is excellent i.e low wastage of dyes. Even the energy utilization is lower than batch wise process.

Importance of blending Polyester and cotton:

Blends of polyester and cotton fibres have become very important to the textile industry. Cotton gives the aesthetic and comfort properties demanded by consumers, while the polyester component adds to performance properties.

How to dye p/c blend in continuous dyeing process?

As both the fibres are having different dyeing behavior, both the fibres  of the two fibres, dyeing blends of these two fibres is fairly straightforward. Each fibre may be dyed the same colour, or they may be dyed different hues. Polyester has no affinity for most of the classes of dyes used to colour cotton, the cellulose being only stained by disperse dyes. The “Thermosol” process for dyeing polyester was developed by the DuPont company for the continuous dyeing of polyester fabrics. This single development allowed the rapid growth of polyester fabrics in the early to mid-fifties.

The process involves padding on the disperse dye together with auxiliaries that minimize migration, drying, For cotton/polyester, the fabric must be absolutely clean, since residual oils or dirt will be set into the fabric during the Thermosol treatment. The fabric must also wet out instantaneously and uniformly to insure adequate absorption during the padding operation.

The thermosol Continuous dyeing of polyester process typically consists the following:

Dye application: The dye solution for padding should be homogeneous. It should be thick. To make solution thick, a thickener, usually, sodium alginate at 0.15-0.3 gm/l is used. The viscous/thicker dye solution improves wet pick up and minimize migration during drying, thus eliminating side to side and two sided dyeings (i.e. when the back of the fabric is a different depth compared to the face) affects. Wetting agent may be added for rapid wetting of fabric. In this solution, fabric is padded by passing in to saturated dye solution and the excess solution is squeezed by passing between two squeeze rollers using padding mangle. The expression may be maintained around 80% (wet pick up). After that fabric is dried before fixation of dye at higher temperature.

The cotton dye may be applied from the same bath as the disperse dye. Vat dyes, Sulphur dyes or a Reactive dye can be used depending on the hue and fastness required. In this experiment, a reactive dye will be applied to the cotton fibers in the union fabric, while a disperse dye will colour the polyester fibres. Fibre reactive dyes exhibit excellent wash fastness because they are covalently bonded to the cotton fibre.

Dye fixation with heat or chemicals and finally: Then fixing the dye in the polyester by dry heating to a high temperatures about 190° – 205°C for 60-90 seconds to promote complete fixation, penetration, and heat-setting. During this process the fibre molecular chains open up at these elevated temperatures and the dispersed dyes vaporize and diffuse into the polymer. On cooling, the dyes are trapped within the fibre yielding coloured fibres that have good fastness properties.

If reactive dyes are to be fixed by dry heat, as in the Thermosol process, 50 - 200 g/l urea should be added to the pad-bath.

Washing. After dyeing dyed fabric should be washed properly to wash down unfixed dye.