Textile Dyeing

Dyeing:

Dyeing is the process of imparting colours to a textile material in loose fibre, yarn, cloth or garment form by treatment with a dye.

Textile dyeing is concerned with organic (that is, carbon-based) compounds that can be dissolved in appropriate solvents, usually water. The dyes in solution are absorbed on the surface of the textile fibre then pass into the interior of the material by a process called diffusion.

The process of transferring the dye from solution to the fibre is called exhaustion, with 100% exhaustion meaning that there is no dye left in the dyebath solution. An important property of a dyeing is its levelness, in other words when the same depth of colour can be seen all over the material.

Another factor is good penetration, when the dye has penetrated deeply into the structure of the fibre, colouring it from the outer surface of the fibre to its interior.

Dye molecules are attracted by physical forces at the molecular level to the textile. The amount of this attraction is known as 'substantivity': the higher the substantivity the greater the attraction of the dye for the fibre.

Dye types:

For most of the thousands of years in which dyeing has been used by humans to decorate clothing, or fabrics for other uses, the primary source of dye has been nature, with the dyes being extracted from animals or plants. In the last 150 years, man has 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 dye 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, nylon and protein fibers such as wool and silk are dyed with acid dyes, polyester yarn is dyed with disperse dyes. Cotton is dyed with a range of dye types including vat dyes which are similar to the ancient natural dyes and modern synthetic reactive and direct dyes.

Methods:

Dyeing in Fes, Morocco.

Dyes are applied to textile goods by dyeing from dye solutions and by printing from dye pastes.

Direct application:

The term "direct dye application" stems from some dyestuff having to be either fermented as in the case of some natural dye or chemically reduced as in the case of synthetic vat and sulfur dyes before being applied. This renders the dye soluble so that it can be absorbed by the fiber since the insoluble dye has very little substantivity to the fiber. Direct dyes, a class of dyes largely for dyeing cotton, are water soluble and can be applied directly to the fiber from an aqueous solution. Most other classes of synthetic dye, other than vat and sulfur dyes, are also applied in this way.

The term may also be applied to dyeing without the use of mordants to fix the dye once it is applied. Mordants were often required to alter the hue and intensity of natural dyes and improve their color fastness. Chromium salts were until recently extensively used in dying wool with synthetic mordant dyes. These were used for economical high color fastness dark shades such as black and navy. Environmental concern has now restricted their use and they have been replaced with reactive and metal complex dyes which need no mordant.

Yarn dyeing:

There are many forms of yarn dyeing. Common forms are: at package form and at hanks form. Cotton yarns are mostly dyed at package form, and acrylic or wool yarn are dyed at hank form. In continuous filament industry, polyester or polyamide yarns are always dyed at package form, viscose rayon yarns are partly dyed at hank form because of technology

The common dyeing process of cotton yarn with reactive dyes at package form is given below in short: firstly the raw yarn is wound on a spring tube to achieve package suitable for dye penetration. Then, these softened packages are loaded on a dyeing carrier's spindle one on another. The packages are next pressed up to a desired height to achieve suitable density of packing. The carrier is then loaded on the dyeing machine and the yarn is dyed. After dyeing, the packages are unloaded from the carrier into a trolly. Next, all the packages are hydro extracted to remove the maximum amount of water. All the packages are then dried to achieve the final dyed package. At last the dyed yarn packages are packed and delivered.

Removal of dyes:

In order to remove natural or unwanted colour from material, the opposite process of bleaching is carried out.

If things go wrong in the dyeing process the dyer may be forced to remove the dye already applied by a process that normally known as stripping. This normally means destroying the dye with powerful reducing agents (sodium hydrosulphite) or oxidizing agents (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, black is often the easiest or last option.

Dye-fibre association:

In all dyeing processes the first step is to impregnate the textile material with dye solution. An important requirement is the movement of the dye into the structure of the fibre. In addition, the ability of certain types of dye to interact with the polymer material that makes up the fibre determines whether or not that type of dye is suitable for a particular fibre.

Organic dyes are classified into groups according to their ability to dye various fibres, and some of the main dye classes and their suitability for different fibres are shown opposite.

Cellulosic: Direct, reactive, vat, sulphur

Wool Acid, premetallised, reactive (some)

Polyamide Acid, premetallised, reactive (some)

Acrylic Basic, disperse (some)

Polyester Disperse

Cellulose acetate Disperse

There are broadly four types of interaction between colorant and fibre molecules: hydrogen bonding; van der Waals forces; ionic or electrostatic bonds and covalent bonds.

The first three usually act in conjunction to varying degrees in most dye classes, although depending on the colorant and the fibre and the dyeing conditions used, one of these physical forces usually predominates. Covalent forces are only present between fibres and reactive dyes.

Some of the important dye classes include:

Direct dyes:

The first direct dye was called Congo red and was introduced in 1884. It was called a direct dye because it was the first dye to become available for colouring cellulose 'directly', without the use of a mordant. Direct dyes are applied in a boiling dyebath in the presence of salt.

Advantages:

• Cheap

• Simple to apply

• Wide range of colours

• Very good fastness to light

• Disadvantages:

• Poor wet fastness

• Moderate brightness

Reactive dyes:

Developed in the 1950s, these were the first dyes produced capable of reacting chemically with the fibre (usually cellulose) under alkaline conditions. The dye thereby becomes part of the fibres, rather than merely remaining as an independent chemical entity within the fibre. Reactive dyes are applied at relatively low temperatures, with controlled amounts of salt and alkali.

Advantages:

• Simple to apply

• Wide range of bright colours

• Good fastness to light and washing

Disadvantage:

• Relatively expensive

Disperse dyes:

Disperse dyes were introduced in the 1920s to dye acetate fibres, which were otherwise undyeable, although nowadays they are mainly used for polyester fibres. They are applied at relatively high temperatures

(over 100 °C).

Acid dyes:

These are most commonly used for dyeing protein fibres such as wool and silk, the term 'acid' referring to the fact that acid or an acid-producing compound is used in the dyebath. There are different types including:

Levelling acid dyes: these are available in a range of bright colours and have good light fastness, but their wash fastness is only moderate

Milling acid dyes: these are also available in a range of bright colours and have good light and wash fastness, but are more difficult to apply correctly than levelling dyes.

The first synthetic dyes for wool were acid dyes, a class of dye that has since grown into a large, diverse, versatile and widely used group. Acid dyes are sufficiently soluble to allow direct application from an aqueous solution. Some acid dyes may also be used for coloration of other animal fibres, including silk, and also of nylon, which like protein fibres has both amide linkages and amino groups in its structure.