Introduction to Textile Dyes and Their Importance
The colour is also one of the most characteristic features of textiles. Since natural dyes were used in the ancient ages to the modern dyes which are highly sophisticated, colouring has always enhanced beauty, value, and identity to fabrics. Dyes are not only used to add beauty to the textiles but also to enhance the commercial value of such textiles in fashion and industry.
Each fibre be it cotton, wool, silk, polyester or nylon has a chemical structure of its own. This implies that one type of dye cannot be applied to all fibres. As an example, cotton is a good absorbent of reactive dyes or direct dyes, whereas polyester absorbs disperse dyes. Knowing the dye that is appropriate with which fibre is crucial to students and to professionals in the textile industry.
This paper will discuss the classification of textile dyes, their properties, uses, and common methods of dyeing. It is meant to provide a clear, examination relevant, and practical knowledge of how the various dye classes work in the textile industry.
Classification of Textile Dyes
Textile Dyes can be classified into various categories based on their mechanism of action such as dyeing (reduction), oxidation, oxidizing agents (reducer), and special.
Speaking of dyes in textiles we mean coloured organic compounds which may be absorbed by fibres to give them their permanent shade. The difference between a dye and any other colouring matter lies in the fact that it is affined to fibres, and is fastened to them given favourable conditions. The matter which lacks this property is typically applied in the form of bound pigments.
Textile dyes can be classified in different ways – based on their origin, solubility, chemical structure, or application method. For practical purposes, textile professionals and students often study them in a flow that connects fibre compatibility with dye behaviour.
Fig: Classification of the Dyes
General Classification Flow of Textile Dyes
- Natural Dyes - extracted either out of plants, animals, or minerals (such as indigo, madder, cochineal). They are the first historical dyes.
- Synthetic Dyes - Artificial dyes made of coal tar or of petroleum products, and this is the biggest category in use nowadays.
Under synthetic dyes, further division is made on solubility and use:
- Water Soluble dyes - Direct dyes, reactive dyes, acid dyes, basic dyes, optical whiteners.
- Insoluble Dyes - Vat dyes, sulphur dyes, disperse dyes.
- Ingrain Dyes - Colours that are within the fibre such as azoic colours, oxidation colours, mineral colours.
- Pigments - Pigments are often called a dye, however technically they are not a dye, they are needed to colour a textile and a binder must be used to fix to the surface of the fibre.
Difference Between Dyes and Pigments
- Dyes are soluble and may be deposited in solution, and has affinity towards fibre.
- The pigments are colouring particles which are insoluble and which are not fibre affinities. They have to be fastened onto cloths by means of resins or binders.
Direct Dyes
Direct dyes, also known as substantive dyes or cotton colours, are a very important class of textile dyes. They are called “direct” because they can be applied directly to fibres from an aqueous solution without the need for a mordant.
Applications of Direct Dyes
- Mainly used for cellulosic fibres such as cotton and viscose rayon.
- Selected varieties are also applied to protein fibres like wool and silk.
- They are popular in low-cost dyeing where brilliant shades are not the main priority.
Types of Direct Dyes
- Class A – Self-levelling dyes: These dyes may start unevenly but later level out due to good migration. Salt is not required for exhaustion.
- Class B – Salt-controllable dyes: Poor migration, exhaustion depends on salt addition. Uneven dyeing is difficult to correct.
- Class C – Temperature-controllable dyes: Sensitive to both salt and temperature, require careful heating to control exhaustion.
Properties of Direct Dyes
- Water soluble and usually sodium salts of sulphonic acid.
- Good affinity for cellulose, moderate for protein fibres.
- Poor washing fastness and moderate light fastness.
- Fastness can be improved by after-treatments such as cationic fixing agents.
General Method of Application on Cotton
- Fabric must be scoured and bleached.
- Dyes are dissolved in hot water.
- Salt is added to improve exhaustion.
- Dyeing is carried out at boil or near boil depending on the dye class.
- After dyeing, fabric is washed and often after-treated for fastness improvement.
Reactive Dyes
Reactive dyes are among the most widely used for cotton and other cellulose fibres because they form a covalent bond with the fibre, making the shade highly durable.
Types of Reactive Dyes
- Cold brand dyes (M brand) – Fixed at room temperature with soda ash.
- Hot brand dyes (H brand, X brand) – Require higher temperature fixation.
- High Exhaust (HE) dyes – High exhaustion and fixation efficiency.
- Medium Exhaust (ME) dyes – Balanced properties.
- Vinyl Sulphone and Low Salt dyes (LS dyes) – Used for dyeing and printing with better ecological profile.
Properties of Reactive Dyes
- Water soluble, contain sulphonic acid groups.
- Form covalent bonds with hydroxyl groups of cellulose.
- Good washing fastness and moderate to good light fastness.
- Available in wide shade range with excellent brightness.
Cold Brand Reactive Dyeing on Cotton (General Procedure)
- Material preparation: Fabric must be neutralised to remove alkali.
- Dye solution: Dye paste made with cold water, dissolved at ~50°C.
- Dye bath: Fabric worked in solution at room temperature.
- Salt addition: Common salt added after 10–15 minutes to promote exhaustion.
- Alkali addition: Soda ash added gradually to fix the dye.
- After-treatment: Thorough washing, soaping, and sometimes softener treatment for maximum fastness.
Acid Dyes
Acid dyes are highly important for protein fibres such as wool and silk, and also for polyamides like nylon. They are applied from acidic dye baths, which is why they are called acid dyes.
Types of Acid Dyes
- Levelling acid dyes – Good migration, even shades, but poor fastness.
- Milling acid dyes – Better fastness, suitable for wool fabrics.
- Super milling acid dyes – High fastness, used for demanding applications.
Properties of Acid Dyes
- Soluble in water, usually sodium salts of sulphonic acids.
- Wide range of brilliant shades, especially useful for fashion textiles.
- Do not work on cellulose fibres.
- Light fastness is generally moderate (3–4 on blue wool scale).
Methods of Application
- Silk with acid dyes: Fabric entered in bath at 40°C, dye added, then heated gradually to boil. Finally, acetic acid is added for fixation.
- Wool with acid dyes: Dyebath set with glauber salt, acetic acid, and sometimes sulphuric acid. Dyeing is carried out gradually from 40°C to 80°C for about 1 hour.
- Nylon with acid dyes: Similar to wool dyeing, but fixation is faster because nylon has high affinity for acid dyes.
Basic Dyes
Basic dyes, also known as cationic dyes, are among the earliest synthetic dyes discovered. They are particularly valued for their brilliance and intensity of colour.
Applications of Basic Dyes
- Best suited for protein fibres such as wool and silk.
- Extensively used for acrylic fibres, which have high affinity for cationic dyes.
- Also used for paper, leather, and some modified cellulosic fibres.
Properties of Basic Dyes
- Possess high tinctorial value (small amount gives intense colour).
- Outstanding brilliance and bright shades, especially reds, oranges, and violets.
- Generally poor light fastness and moderate washing fastness.
- No affinity for natural cellulose like cotton unless a mordant is used.
Methods of Dyeing with Basic Dyes
- Wool: Dyed in an acidic bath containing sodium sulphate and acetic acid. Dyeing is carried out at boil for 1–2 hours.
- Silk: Dyed in a similar acidic medium, often with glauber salt and acetic acid. The process ensures bright, deep shades but requires after-treatment for fastness.
- Acrylic fibres: Excellent affinity for basic dyes, dyed easily under mild conditions with good exhaustion.
Disperse Dyes
Disperse dyes were originally developed for acetate fibres, and later became the most important dyes for polyester and other synthetic hydrophobic fibres.
Applications of Disperse Dyes
- Used mainly for polyester, acetate, nylon, and acrylic blends.
- Essential for synthetic textile industry, especially sportswear and fashion fabrics.
- Can be applied by high-temperature dyeing or thermosol process.
Properties of Disperse Dyes
- Insoluble in water – applied in the form of a fine dispersion with dispersing agents.
- Moderate light fastness (2–3 for some shades, higher for selected dyes).
- Moderate washing fastness, but suitable after-treatment improves durability.
- Available in powder and liquid forms for easy application.
General Method of Dyeing Polyester with Disperse Dyes
- Dye is dispersed in water using dispersing agents.
- Polyester fabric is introduced and dyeing carried out at high temperature (130°C) under pressure.
- Alternatively, the thermosol process is used: fabric is padded with dye dispersion, dried, and then heated at 180–200°C to fix the dye.
- After-treatment includes reduction clearing with alkaline hydrosulphite to remove unfixed dye.
Other Important Dye Classes
Apart from the commonly used direct, reactive, acid, basic, and disperse dyes, there are other significant dye classes that hold an important place in both textile practice and examinations. These include vat dyes, sulphur dyes, optical whiteners, and pigments.
Vat Dyes
Vat dyes are one of the oldest and most durable classes of dyes. The best-known example is indigo, traditionally used for denim.
Properties of Vat Dyes
- Insoluble in water in their natural form.
- Applied in a soluble “leuco” form by using reducing agents like sodium hydrosulphite.
- After dyeing, the fibre is exposed to air, and the dye is oxidised back to its insoluble form inside the fibre.
- Excellent fastness properties to washing, light, and rubbing.
Applications
- Widely used on cotton and other cellulosic fibres.
- Ideal for workwear, uniforms, and denim due to high durability.
Sulphur Dyes
Sulphur dyes are an economical group used mainly for dark shades like navy, brown, and black.
Properties of Sulphur Dyes
- Insoluble dyes, applied in soluble reduced form similar to vat dyes.
- Generally give dull shades compared to reactive or vat dyes.
- Good washing fastness, but light fastness is moderate.
- May cause fibre tendering if not properly washed after dyeing.
Applications
- Mostly applied on cotton for inexpensive, dark-coloured fabrics.
- Common in casual wear, workwear, and low-cost apparel.
Optical Whiteners
Optical whiteners, also called fluorescent brightening agents (FBAs), are not true dyes but play a vital role in textile finishing.
Properties
- Colourless compounds that absorb ultraviolet (UV) light and re-emit it as blue visible light.
- This masks yellowish shades in fibres and makes fabrics appear brighter and whiter.
- Applied during finishing, washing, or bleaching processes.
Applications
- Widely used on cotton, polyester, nylon, and blends.
- Important in household textiles, shirts, and bed linen to achieve a brilliant white effect.
Pigments
Pigments are different from dyes because they have no natural affinity for fibres. They are coloured particles that must be bound to fabrics using a polymeric binder or resin.
Properties
- Completely insoluble in water and other solvents.
- Available in a very wide colour range, including shades that dyes cannot easily produce.
- Fastness depends on the quality of the binder used.
Applications
- Used in textile printing, particularly on cotton and blends.
- Popular in fashion fabrics due to bright shades and cost-effectiveness.
- Also used for special finishes such as denim pigment washes.
Conclusion
Textile dyes are the heart of the fabric colouring, and it's shaping both beauty and utility. From natural dyes of ancient times to the wide range of synthetic dyes today, each class direct, reactive, acid, basic, disperse, vat, sulphur, and pigments serves a unique role in various applications. Choosing the right dye depends mainly on the fibre type, desired fastness, cost, and end-use. For students, mastering dye classification ensures strong exam preparation, while for professionals, it builds expertise in practical dyeing applications. So, make sure you know about these topics which are important.
Colour in textiles is not just about appearance it is about performance, durability, and commercial value.
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FAQs on Textile Dyes
Q1. What is the main difference between a dye and a pigment?
Dyes are soluble colourants that chemically or physically bond with fibres, while pigments are insoluble and require a binder to attach to the fabric.
Q2. Which dye is most commonly used for cotton?
Reactive dyes are most commonly used for cotton because they form covalent bonds, giving bright shades and good wash fastness.
Q3. Why are disperse dyes used for polyester?
Polyester is hydrophobic and does not absorb water-soluble dyes. Disperse dyes are non-ionic and finely dispersed, making them suitable for polyester.
Q4. Which dyes are most durable in terms of fastness?
Vat dyes, such as indigo, are known for their excellent washing and light fastness.