Structure of Cotton Fiber

By Eman Abdallah Kamel

Eman is a writer and an engineer. She holds a Bachelor’s degree in Textile Science from the Faculty of Applied Arts, Egypt.

In this article, you will learn about cellulose, cotton fiber structure, including 4 main parts: Cuticle, the primary wall, the secondary wall, and lumen.

Image source: sciencedirect.com

Introduction

Cotton is a plant fiber in the genus Gossypium, which includes 32 species. The native Asian species are G. arboreum and G. herbaceum, while the American species are G. barbadense and G. hirsutum. G. barbadense includes long-staple varieties of Pima, Egyptian, and Sea Island cotton, while G. hirsutum is grown in highland regions.

Organic cotton is grown without the use of chemicals or fertilizers to preserve soil fertility and the environment. Cotton is also biodegradable, making it a more sustainable option. Being a cellulose fiber, it naturally decomposes and returns to the soil.

Cotton is a natural fiber extracted from the seeds of the cotton plant. Chemically, cotton is a polysaccharide, represented by the chemical formula (C6H10O5)n.

Cotton fibers contain 80-90% cellulose, 6-8% water, 0.5-1% waxes and fats, 0-1.5% proteins, 4-6% hemicelluloses and pectins, and 1-1.8% ash.

Whole cotton fibers contain between 88% and 96.5% cellulose, while non-cellulosic polysaccharides account for up to 10% of the total fiber weight. The primary wall in mature fibers is only 0.5–1 micrometer thick and contains about 50% cellulose. The non-cellulosic components consist of pectins, fats, waxes, proteins, and natural colorants. The secondary wall, which includes about 92% to 95% cellulose, consists of concentric layers with alternating twists. These layers consist of densely packed primary fibers arranged in microfibrils and macrofibrils. Strong hydrogen bonds hold these fibers together. The center of the fiber is the lumen.

Did you know?

Cotton fibers have the inherent capacity to hold water up to 24–27 times their weight. The length of cotton fibers generally ranges from 20 to 32 mm, with a micronaire value of between 3.5 and 5.5.

Cellulose chemical structure. Image source: researchgate.net

Chemical Structure of Cellulose

Cellulose consists of anhydroglucose units connected by beta-1,4-glucosidic bonds to form linear polymer chains. A cotton cellulose molecule’s degree of polymerization (DP), or its chain length, is the number of anhydroglucose units joined to form the chain molecule.

Remember,

The degree of polymerization of a cellulose molecule may reach 14,000, but it can be easily reduced to 1,000-2,000 by various purification treatments using alkali.
Cotton typically has a molecular weight of 50,000 to 1,500,000, depending on the source of the cellulose.
Chains are held together along their lengths by van der Waals forces and hydrogen bonds.

Cellulose and other non-cellulosic materials are distributed throughout the fiber layers in cotton. Image source: researchgate.net

Cotton Fiber Morphology

Cotton fibers consist of four main parts, including:

The Cuticle,
The Primary Wall,
The Secondary Wall,
The Lumen.

1. The Cuticle

The cuticle is the outer waxy layer, containing waxes, oils, and mineral matter. It acts as a water repellent.

2. The Primary Wall

The primary wall is a tough, protective layer that forms during the first days of growth within the boll. Chemical analysis of the primary wall material reveals that it contains wax, protein, pectinaceous material, and some cellulose.

Did you know?

The first layer of secondary thickening is the winding layer, and its structure differs from that of the primary wall and the rest of the secondary wall. It consists of fibrils aligned at an angle of 40–70° to the fiber axis in an open lattice pattern.

3. The Secondary Wall

The secondary wall is almost pure cellulose, accounting for 90% of the fiber’s total weight. This cellulose is laid down during the second stage of fiber growth, after the fiber has reached its full length. The secondary wall fibrils are arranged in a near-parallel arrangement. The fibril layers form a spiral along the fiber axis, and the direction of the spirals is often reversed within the layer itself. This reversal is the cause of the twists seen in the longitudinal view of the fiber. This configuration results in tremendous longitudinal strength.

Did you know?

A single cotton fiber contains between 150 and 400 twists per inch. The twist direction may change along the fiber axis. These twists are critical to the cohesion of the yarn fibers, which is essential for its durability.

4. The Lumen

Lumen is the term for the hollow canal that runs the length of the fiber. It is filled with living protoplasts during development. After the fiber matures and the boll opens, the protoplasts dry out, and the lumen naturally shrinks, leaving pore space in each fiber.

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