Pretreatment Processes of dyeing

 Dyeing Pretreatment :

Pretreatment processes of dyeing consist of cleaning operations to rid the fabric of all soil and additives that have accumulated during the weaving or knitting process. These processes are usually the first treatments a fabric undergoes after leaving the loom or knitting machine, and are required before any dyeing, printing, or finishing can be accomplished. In the strict definition of finishing, pretreatment processes do not quality as textile finishes because they are performed prior to dyeing or printing

Grey goods contain warp starches or other sizing (to add stiffness and strength to warp yarns during weaving), as well as oils, waxes, and other lubricants, plus floor dirt or other soils picked up during processing. Complete removal of all these impurities is necessary before any dyeing; printing, or finishing can be done. The processes for cleaning are varied depending on the fiber, the impurities present, and the fabric construction. In cotton, cotton-blend, silk, and manufactured-fiber fabrics, the processes are generally known as the boil-off. In woolens and worsteds, it is called a scour or scouring. Boil-off or scouring is similar to ordinary laundering. Fabrics are treated with soaps or detergents, rinsed, and then dried. If warp starches are present, the fabric is treated with an additional process known as desizing. Enzyme solutions are used to dissolve the starch.

Woolen and worsted fabrics may undergo an additional pretreatment process known as carbonizing. This process removes leaf particles and bits of grass and other cellulosic impurities that become embedded in the wool white sheep are grazing. The treatment consists of steeping the wool fabric in sulfuric acid, which destroys the cellulose but leaves the wool unharmed.

Another dyeing pretreatment process is an operation called singeing, which involves burning off projecting fibers or filament splinters from the surface of a fabric. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics. . Singeing is accomplished by passing gray goods rapidly over gas flames, usually two burners to a side, at a speed of 100 to 250 yards per minute. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics.

Dyeing pretreatment processes may also include bleaching. Fabrics to be dyed in light to medium shades, as well as most prints, are first bleached. Bleaches are required to obtain pure whites because natural fibers are rarely pure white in their natural state; they are usually slightly yellowish or grayish. Bleaches are chemical agents that react with the color compounds in the fiber and render them colorless. Our eyes see the fabric as pure white.

The effectiveness of optical brighteners after dyeing is dependent on an ultraviolet source of light. Natural sunlight is such a source. Fluorescent lighting is also, but to a small degree. Ultraviolet lamps (also called black-light lamps) are another source. Incandescent lighting (regular household bulbs) contains no ultraviolet, so optical brighteners have no effect on fabrics used under this lighting.

The Dyeing Process

The science of dyeing is highly complex, and the mechanisms of dyeing are not completely understood. This discussion about dyeing is necessarily superficial and does not provide an in-depth exploration of the subject. The medium most often used to dissolve or disperse dyes for application to textiles is water. The dye solution, called the dye liquor, is agitated or circulated to increase the migration of the dye to the fiber surface. The attraction or affinity of the dye for a particular fiber is influenced by several factors. Different dye types are attracted chemically or physically to specific fibers.

Fibers often undergo swelling in aqueous dyeing processes, increasing dye absorption. For hydrophobic fibers that do not swell in water, organic solvents can be used as swelling agents or “carriers” for the dye. Mordants can be added to increase the acceptance of dyes. Dyebath additives such as these are classed as dye auxiliaries. Surface features such as the scales on wool or waxes or finishes on fibers can inhibit attraction of the dye to the fiber. Careful preparation of fabrics for dyeing minimizes this effect by removing waxes and sizes. Further, application of aesthetic and functional finishes usually follows the dyeing process, so that the dyeing finishes do not compete for dye sites on the fiber.

Once dyes are attracted to the fiber, they should diffuse through it rather than remaining adsorbed on the surface. Dyes are absorbed into the fiber predominantly in the amorphous areas, and making these areas larger or more accessible enhances the dyeing process. Mercerization of cotton, which moves the polymer chains farther apart, allows more dye in. Higher temperatures, around the glass transition phase of thermoplastic fibers, cause some of the crystalline areas to move and separate more, increasing dye accessibility. Nylon fibers have a low glass transition temperature and can often pick up dyes from other fibers.

Exhaustion is the amount of transfer of dye from the dye bath to the fiber, either by adsorption or absorption of dyes. It is possible for all the dye to be removed from the dyeing bath and for the dye bath to be clear at the end of the dyeing process. Some dye classes exhaust better than others. It is not always prudent or even necessary, to dye to complete exhaustion of the bath.

A concern in dyeing is how evenly distributed the dye is in the fiber, a characteristic known as levelness. Low leveling will result in a streaked appearance of the fabric.

Dyes may be only adsorbed on the fiber surface, or dyes that have been absorbed by fibers may migrate back to the surface when the textile is dried, resulting in uneven dyeing. Leveling agents and retarders are dye-bath auxiliaries than can promote even dye absorption.

Once the dye has entered the amorphous regions of fiber, it must be retained within the fiber not only during the dyeing but also when fabrics are laundered or dry cleaned during use. Several factors make for dye retention. The nature of the dyeing chemical bonds and physical attraction between fiber and dye is a crucial element. Dyes may be physically bound to fibers by forces such as hydrogen bonding or ionic forces. Ionic forces are the attraction between positively and negatively charged ions, one on the dye, and the other on the fiber. The chemical structure of dye and fiber may allow the formation of covalent chemical bonds between fiber and dye. Dyes that are absorbed into swollen fibers can be physically trapped when the fibers are cooled and dried. Dyes that are soluble in water, which may also swell some fibers, can be removed when the textile is wetted during dyeing.

Dyeing methods of Direct dyes

Direct dye is populae and conventional dye. There are some popular dyeing method of direct dyeing with suitable dyeing machine those are describeb bellow:

I. Batch method: With batch dyeing, the dyeing method to be selected in each case depends on the type of dyeing equipment (loading system, winch vat, jig, paddle or jet), the nature of the material to be dyed, as well as the solubility and the affinity of the dye. Before the actual dyeing process, the material is pre-treated with a wetting agent. The dye is mixed into a paste with some warm water, and is then diluted with more water, boiled up, filtered, and then added to the dyebath.

Whilst the bath, with the fabric, heats up to the optimum temperature (usually 80–90°C), the electrolyte is added, gradually if necessary. Dyeing takes 30–60 minutes. After the dyebath has been run off, the dyed material is briefly rinsed with cold water and, in general, subjected to after-treatment. The dyeing process is divided into 3 phases: a) uptake (adsorption) through substantivity; b) penetration of the dye into the fibre (diffusion); c) bonding to the fibre (immobilisation) through van der Waals interactions. Assurance of level take-up through progessive addition of salt, temperature control, sufficiently long dyeing time, and use of levelling agents.

II. High-temperature dyeing method: With suitable dyes, one can work in a closed system at temperatures over 100°C (up to approx. 130°). Due to the rapid diffusion rate, particularly level dyeings are achieved with short dyeing times, even with fabric with difficult dye penetration. After the high-temperature phase, the dyebath is cooled down to 80–90°C, with dye pick-up continuing, and the result is the same depth of colour as in the normal dyeing method at 80–90°C.

III. Continuous and semi-continuous method: With these so-called pad methods, the dyeing material, mostly in the form of woven fabric, is first of all steeped in a concentrated dye solution, passed full-width through a trough filled with the dye solution, and subsequently the excess liquor is removed between the rubber rollers. With high fabric speed, temperatures as low as possible (30–40°C for light shades, 60–80°C for medium to dark shades), and with the minimum amount of pad liquor, the situation can be avoided where the dye already picks up substantively in the padding process. Distinction may be made between the following methods:

1. Pad-jig method: Pad, salt bath develop on jig or winch vat.

2. Pad-roll method: Pad, heat up in an IR zone, roll up and rotate in a dwell chamber for a considerable time under fixation temperature/moisture conditions.

3. Pad-salt method: Pad, pass through salt solution at boil in continuous piece-dyeing machine (light shades only; for darker shades an intermediate steam process is necessary).

4. Pad-steam method: Pad, continuous steam, optional final salt bath.

5. HT-steam method: Pad, HT-steam.

After-treatment: The wetfastness properties of direct dyeings are not adequate for the demands of everyday use, particularly in medium and deep shades.

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Direct dyes

Direct dyes are very conventional dyes. It is not used any more for industrial dyeing. Direct dye is one kind of anionic dye and having substantively for all types of cellulosic fibre like cotton, jute, hemp. They have strong affinity to the fibre which can be applied directly on the fibre surface and do not need any assistance.

Direct dye Also known as direct, substantive, benzidine or salt dyes. Group classification for dye which exhaust substantively on cellulosic fibres, cotton, viscose, cupro and high wet modulus fibres, especially in the presence of salt. Chemically they are polyazo or sulphonated disazo dye.It contains sulphonic acid group that’s why is has good solubility in water. The dye molecule contains a complex bond metal atom that’s why they have high light fastness properties. The color fastness is poor but it is easy to improve by dyeing after treatment with metal salts (Õ Aftercoppering dyes).

Benefit of Direct dye:
Direct dyes represent the main class of dyes for cellulosic fibres in terms of usage:
- Specially selected direct dyes are available for streaky dyeing viscose qualities.
- They are simple to apply.
- Economical or cost effective than any other dyes.
- It has generally good leveling properties
- A wide range of color variation and shade.
- Special types contain better wet fastness. It is possible to couple with readily soluble diazo compound by after treatment.
-Direct dyeing on cellulosic fibres exhibit considerable differences in colour fastness to light which ranges between 1 and 7–8.

Different direct dyes can therefore be used for articles which are only required to have low light fastness as well as goods with high light fastness. In general, direct dyes have poor wet fastness properties and dyeing must be after treated to improve their serviceability.

Properties of direct dye:
- Direct dye has sodium salt of sulphonic acid or carboxylic acid group. So it easily dissolve in water.
- Cheap comparative
- It has strong affinity to cellulose fibre. It is possible to dye protein fibre.
- Easily disperse in water and diffusible in to fibre.
- Wash fastness is not good. The range vary from 2 to 3
- It is possible to use in neutral and alkaline medium
- The dye is common in the practical point of view.
- The tin tropical power of this dye is very good.

Some trade and brand name of direct dyes
Benzo, Benzoform       - F.Bayer (Germany)
Coprantine                  - Ciba-geigy (Switzerland)
Solphenyl                    - Ciba-geigy (Switzerland)
Chlorazol Durazol        - Impartial Chemical Industry (ICI)
Solar                          - Sandoz A.G (Switzerland)

There are some related posts to get deep knowledge of direct dye:

# Classification of Direct dye
# How to dye with direct dyes
# Dyeing aftertreatment of direct dye
# Chemistry of dyeing related to direct dye
# Dyeing auxiliaries required for direct dye

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