Cotton Fibre: Properties, Structure, Dyeing Behaviour, and Technical Applications Explained

1. Introduction: Why Cotton Still Matters

Cotton happens to be the most widely used natural fibre across the planet besides being a major issue of study and also of a competitive examination. Either you are studying at GATE Textile or facing a job interview, or in spinning or processing trades, it is important to know the structure and the behaviour of cotton.

It is a relatively unassuming fibre but it is probably one of the most studied and engineered fibres of the lot, due to its inner chemistry and morphology. This guide provides you with all the information you should ever know about cotton including the origin of the product, chemical and physical nature of the product, behaviour of the product undergoing processing, performance of the product during the dyeing process and application of the product.

Close-up of cotton fibres and cotton bolls showing natural structure, used to illustrate the morphology and properties of cotton fibre

2. Origin and Composition

  • Cotton is a natural seed fibre obtained from the seed coat of the Gossypium plant.
  • It is classified as a natural cellulosic fibre because its backbone is made of cellulose.
  • Cellulose content ranges from 88 to 96 percent, depending on variety and processing.
  • The molecular formula of cellulose is (C₆H₁₀O₅)ₙ — a long-chain polysaccharide.
  • In addition to cellulose, cotton contains small amounts of waxes, proteins, pectins, and mineral matter.

3. Morphology and Fibre Structure

Cotton has a multilayered structure, each with specific roles:

  • Cuticle: Thin outer waxy layer that protects the fibre.
  • Primary Wall: First cellulose layer formed during fibre development.
  • Secondary Wall: Bulk of the fibre, made up of concentric cellulose layers.
  • Lumen: Central hollow canal that collapses during drying and gives cotton its natural convoluted shape.
  • The natural twist (convolution) of cotton helps in spinning and improves inter-fibre cohesion.

4. Physical Properties

  • Cotton is a staple fiber that has a length that averages between 10mm and 65 mm.
  • It has an average 12-20 microns diameter.
  • The fibre weighs about 1.52 that is relatively dense.
  • The moisture regain of Cotton is approximately 7 to 8.5 % that makes it comfortable in a hot climate.
  • The typical range of crystallinity is between 65 and 70 %.
  • The maturity ratio of more than 0.7 shows us that the cotton is mature and this is what is required to give an even dye and a strong yarn.

5. Chemical Properties

 Solubility:

  • Dissolves in 70% sulfuric acid (H₂SO₄) — a key identification test

Alkaline Resistance:

  • Resistant to dilute alkalis

  • Undergoes mercerization in NaOH → fibre swells, improves lustre and dye uptake

Acid Sensitivity:

  • Damaged by dilute and concentrated acids

Combustion Test:

  • Burns with smell of burning paper

  • Produces fine gray ash

6. Mechanical Properties

  • Cotton exhibits moderate strength in dry condition, with a tenacity ranging from 3 to 4 grams per denier.
  • It becomes stronger when wet, increasing to about 4 to 4.5 grams per denier, due to better molecular alignment in the presence of water.
  • Elongation at break is relatively low, which provides dimensional stability in woven structures.
  • Cotton fibres are stiff and tend to wrinkle easily, as hydrogen bonds in cellulose break and reform under mechanical stress.

7. Degree of Polymerization (DP)

  • The degree of polymerisation in cotton is between 5000 and 10000.
  • This means each cellulose molecule contains 5000 to 10000 glucose units linked together.
  • Higher DP indicates stronger and more durable fibres.
  • Cotton has much higher DP than viscose rayon, which typically ranges from 300 to 600.
  • High DP contributes to chemical resistance and long service life of cotton-based products.

8. Dye Affinity

Cotton has strong affinity for dyes due to the presence of hydroxyl (–OH) groups in cellulose.

These polar groups form hydrogen bonds and covalent linkages with dye molecules.

Common dyes used for cotton include:

  • Reactive dyes – form strong chemical bonds and offer excellent wash fastness.
  • Direct dyes – easy to apply but may have moderate wash fastness.
  • Vat dyes – highly durable, used in workwear and denim.

9. Microscopic Identification

  • Cotton shows a characteristic twisted ribbon-like appearance when viewed under a microscope.
  • The fibre cross-section appears kidney-shaped.
  • The collapsed lumen and convoluted structure help distinguish it from other cellulosic fibres like flax or viscose.
  • Microscopic identification is a common technique in forensic and quality control labs.

10. Quick Revision Notes

  • Cotton is a natural cellulosic seed fibre with high cellulose content and degree of polymerisation.
  • It dissolves in 70% H₂SO₄ and is damaged by acids but resists dilute alkalis.
  • Stronger when wet and has good dye affinity due to hydroxyl groups.
  • Requires desizing, scouring, and bleaching before dyeing.
  • Used in both apparel and technical textiles.
  • Exam questions often focus on its microscopic appearance, chemical solubility, DP, and dyeing behaviour.

How Does Cotton Change Before It Is Dyed?

Several processes have to be carried out on cotton fabric before the dyeing starts. With these actions, impurities are removed, the colour takes evenly and the fabric receives an overall improved dye uptake. Let’s go over what generally occurs during the process:

1. Desizing

Desizing takes away the size materials that were included during weaving. Size matters as it can hinder the dye from getting into your cotton. Sizing agents can be removed using enzymes or acids in a process known as desizing.

2. Scouring

Scouring helps remove substances such as waxes, oils, pectins and extra size from the material. Boiling cotton in a solution of sodium hydroxide makes it able to attract water molecules. As a result of this step, dyes are absorbed and spread out more consistently across the fabric.

3. Bleaching

Achieving evenness on the fabric, especially when dyeing pastel colors, involves bleaching the cotton first. Hydrogen peroxide helps to lighten the fabric without causing harm to the cellulose in it. It provides an even and clean backdrop for stronger and brighter dye colors.

4. Mercerization

The purpose of the optional step for luster, strength and dye attraction is to increase shine. Being exposed to a harsh caustic soda, under tension, causes the cotton fibers to grow bigger. The repositioning of cellulose improves the fabric’s look as well as the way it takes in dye.

Cotton in Dyeing, Printing, and Finishing

Cotton is highly suitable for multiple dyeing methods like exhaust dyeing, pad-dry-cure, and continuous dyeing.

In printing, it responds well to pigment, reactive, and resist techniques.

Various functional finishes can be applied, such as:

  • Anti-crease finishes to reduce wrinkling.
  • Water-repellent coatings for rainwear or uniforms.
  • Antimicrobial treatments for hygiene textiles.
  • Flame retardant finishes for industrial fabrics.
  • UV-blocking finishes for outdoor use.

Using Cotton in Other Industries

Although most people associate cotton with clothing and home decor, it is also used in areas many people fail to recognize.
  • For bandages and gauze, as well as surgical products, cotton is used because it is soft, breathable and safe for use on the body.
  • Filtration – This material is convenient for use in industry and filter devices because it has a porous structure and soaks up substances.
  • Reinforced composites – Resin and cotton fiber are mixed to produce lightweight and environmentally friendly materials.

Innovation and New Progress in Cotton

Researchers are always dedicated to boosting cotton’s efficiency and ability to be sustainable. These developments are important:
  • By modifying cotton using enzymes, it becomes easier to dye and less chemicals are needed.
  • Some cotton farmers grow organic cotton without applying chemical fertilizers and pigmented varieties that take less dye to become colorful.
  • Clothing and e-textiles that collect body temperature or similar data are made by combining cotton with conductive yarns.

🎓 Final Thoughts

Cotton is one of the most important fibres in the textile science not only in terms of its extensive spread in applications but also in terms of the level of technical information that it presents. The cotton has cellulose based structure, large number of polymerisation, excellent wet strength and dyeing ability factors that makes the fibre a perfect role model in the industry as well as in an academic perspective.

Be it if you are going to take competitive exams like GATE, or doing spinning or dyeing industries or doing research in the labs you would always be ahead of the game with a good knowledge of properties of cotton.

All important aspects were addressed in this post of structure, chemical behaviour, mechanical properties, pre-treatment, and applications in an exam-relevant way and even with practical explanation.

Continue revisiting these notions. Cotton is a natural fibre but I need to learn it in order to become a textile professional.

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