Structure of Cotton Fiber

Structure of Cotton Fiber:
Cotton, the seed hair of plants of the genus Gossypium, is the purest form of cellulose readily available in nature. It has many desirable fibre properties making it an important fibre for textile applications. Cotton is the most important of the raw materials for the textile industry. The cotton fibre is a single biological cell with a multilayer structure The layers in the cell structure are, from the outside of the fiber to the inside, cuticle, primary wall, secondary wall, and lumen. These layers are different structurally and chemically. The primary and secondary walls have different degrees of crystallinity, as well as different molecular chain orientations. The cuticle, composed of wax, proteins, and pectins, is 2.5% of the fiber weight and is amorphous. The primary wall is 2.5% of the fiber weight, has a crystallinity index of 30%, and is composed of cellulose. The secondary wall is 91.5% of the fiber weight, has a crystallinity index of 70%, and is composed of cellulose. The lumen is composed of protoplasmic residues. 

Cotton fibres have a fibrillar structure. The whole cotton fibre contains 88 to 96.5% of cellulose, the rest are non-cellulosic polysaccharides constituting up to 10% of the total fibre weight. The primary wall in mature fibres is only 0.5-1 µm thick and contains about 50% of cellulose. Non-cellulosic constituents consist of pectins, fats and waxes, proteins and natural colorants. The secondary wall, containing about 92- 95% cellulose, is built of concentric layers with alternatic shaped twists. The layers consist of densely packed elementary fibrils, organized into micro fibrils and macro fibrils. They are held together by strong hydrogen bonds. The lumen forms the centre of the fibres. Cotton is composed almost entirely of the polysaccharide cellulose. Cotton cellulose consists of crystalline fibrils varying in complexity and length and connected by less organized amorphous regions with an average ratio of about two-thirds crystalline and one-third non-crystalline material, depending on the method of determination. 

Figure: Chemical structure of Cellulose.

The chemical composition of cellulose is simple, consisting of anhydroglucose units joined by β-1,4-glucosidic bonds to form linear polymeric chains. The chain length, or degree of polymerisation (DP), of a cotton cellulose molecule represents the number of anhydroglucose units connected together to form the chain molecule. DP of cotton may be as high as 14 000, but it can be easily reduced to 1000–2000 by different purification treatments with alkali. The crystalline regions probably have a DP of 200 to 300. Correspondingly, the molecular weight (MW) of cotton usually lies in the range of 50,000–1,500,000 depending on the source of the cellulose. The individual chains adhere to each other along their lengths by hydrogen bonding and Van der Waals forces. The physical properties of the cotton fibre as a textile material, as well as its chemical behaviour and reactivity, are determined by arrangements of the cellulose molecules with respect to each other and to the fibre axis.

Non Cellulosic Constituents of Cotton: 
The primary wall is about 1 µm thick and comprises only about 1 % of the total thickness of cotton fibre. The major portion of the non-cellulosic constituents of cotton fibre is present in or near the primary wall. Non cellulosic impurities, such as fats, waxes, proteins, pectins, natural colorants, minerals and water-soluble compounds found to a large extent in the cellulose matrix of the primary wall and to a lesser extent in the secondary wall strongly limit the water absorbency and whiteness of the cotton fiber. Pectin is located mostly in the primary wall of the fibre.

Figure: A schematic representation of cotton fibre showing its various layers.

It is composed of a high proportion of D-galacturonic acid residues, joined together by α(1→4)-linkages. The carboxylic acid groups of some of the galacturonic acid residues are partly esterified with methanol. Pectic molecule can be called a block-copolymer with alternating the esterified and the non-esterified blocks. In the primary cell wall pectin is covalently linked to cellulose or in other plants to hemicellulose, or that is strongly hydrogen- bonded to other components. Pectin is like powerful biological glue. The mostly water-insoluble pectin salts serve to bind the waxes and proteins together to form the fiber`s protective barrier.

The general state of knowledge of the chemical composition of a mature cotton fiber is presented in Table

Composition of a Fiber				Composition of the Cuticle%
Constituent	Typical%	Low%	High%
Cellulose	94.0	88.0	96.0
Protein (N-6.25)	1.3	1.1	1.9	30.4
Pectic substances	0.9	0.7	1.2	19.6
Wax	0.6	0.4 1	1.0	17.4
Mineral matters	1.2	0.7	1.6	6.5
Maleic, citric, and other organic acids	0.8	0.5	1.0
Total sugars	0.3
Cutin				8.7

Table shows that non-cellulosic materials account for only a very small amount of the fiber weight. These materials are amorphous and are located in the cuticle and the lumen. The cuticle forms a protective layer to shield the cotton from environmental attacks and water penetration. Waxy materials are mainly responsible for the non-absorbent characteristics of raw cotton. Pectins may also have an influence, since 85% of the carboxyl groups in the pectins are methylated.

Row cotton fibres have to go through several chemical processes to obtain properties suitable for use. With scouring, non-cellulose substances (wax, pectin, proteins, hemicelluloses…) that surround the fibre cellulose core are removed, and as a result, fibres become hydrophilic and suitable for bleaching, dyeing and other processing.

By removing pectin, it is easier to remove all other non-cellulosic substances. The processes of bio-scouring that are in use today are based on the decomposition of pectin by the enzymes called pectinases.