Considerable points of grading of cotton

The assessment of cotton is carried out traditionally by the cotton ‘classer’ who depends upon personal skill and long experience in judging cotton quality by inspection and feel.

 In arriving at his assessment, the classer takes note of (1)The staple length,(2)The colour and (3)The amount of impurity in the cotton and the quality of its preparation.

Grading of cotton of different countries:

         (From high grade to low grade)

American cotton	Egyptian cotton	Indian cotton
Middling fair	Extra fine	Super choice
Strict good middling	Fine	Choice
Good middling	Good	Super fine
Strict middling	Fully good fair	Fine
Middling	Good fair	Fully good
Strict low middling	Fair	Good
Low middling		Fully good fair
Strict good middling
Strict Good Ordinary
Good ordinary

Cross sectional structure of cotton fibre: 

1.                   Cuticle: the outermost layer of cotton is known as the cuticle and is a thin film of fats, pectines and waxes.

2.                   Primary cell wall: Inside the cuticle the primary wall is composed mainly of cellulose in which the fibers are arranged in a criss-cross pattern.

3.                   Secondary cell wall:

• Under primary wall, secondary wall is composed of cellulose which constitutes the balk of the fiber.
• The outer most S₁ is comparatively thin layer of fibrils oriented vertically with an angle of 20-35⁰. In this region the fibrils periodically reverse the direction of the spiral.
• The S₂ zone which is situated inside S₁, accounts for the bulk of the cellulose content and in this case, the fibrils are again in the form of helices with a somewhat lower pitch of 20⁰-30⁰ and which like those S₁, are organized so that there are periodical reversal in the direction of the spiral.
• In very mature fiber the S₃ layer can be detected adjacent to the linning of the lumen and it consists of the mineral salt and the proteins derived from the dyeing up of the cell.

4.                   Lumen: The hollow central core of cotton fibers is known as the lumen.

Fibril:

Fibrils are bundles of linear polymer which form the small fibrous structure. A fibre is made by the combination of many fibrils. In primary cell wall of the cotton fibre, the fibrils are about 20 mm thick and in secondary cell were are 10 mm thick, but their length is yet unknown.

Convolution:

The mature fiber can be recognized by the  Under the microscope, the cotton fiber looks a twisted ribbon or a collapsed and twisted tube. These twists are called convolution. Convolution of Egyptian cotton: 230 /cm and of Indian cotton: 60/cm.

Chemical structure of cotton fiber:  

                                   Link                   Link

 - Glycolcydic link between C₁-C₄

Features (structural) of cellulose:    

• Cellulose is a polymer of  – D glucose.
• Branchless linear large polymer.
• -D glucose are linked by -glycocydic bond.

·         Bond is made between C₁ and C₄ atom of -D glucose.

·         Molecular weight is above 5,00,000.

·         One cellulose polymer contains 300-1500 glucose polymer.

Chemical Characteristics:

• Water insoluble but soluble in NaOH and CS₂ mixture, Ammoniacal Cu (OH) ₂ etc.
• Do not react with I_2.   

Properties of cotton fibre:

1.         Molecular structure: Cellulosic.

2.         Physical properties:

i)          Length: .3 cm to 5.5 cm.

ii)        Diameter: 16 to 20 microns.

iii)               Cross section: Kidney shape

iv)               Colour: White, grey, cream.

3. Tensile properties:

i) Tenacity(gm/den): 3.0-5.0  (In dry state)

                                      3.6-6.0   (In wet state)

ii) Elasticity: Elongation at break 3.0-7.0%

iii) Resiliency: Low.

iv) Moisture regain: 8.5%and raised to around

                                    25-30% water at 100% Relative humidity.

v) Specific gravity: 1.54         

4.. Chemical properties of cotton fiber:

i) Effects of bleaches: – Resistance to bleach, but H₂O₂, NaOCl, NaOCl₂, Ca (OCl)Cl are used as bleaching agents for cotton processing.

ii) Effects of acids and alkalis: Dissolved in high concentrated mineral acids such as HCl, H₂SO₄ but high resistant to alkalis that’s why NaOH, Na₂CO₃, and used for scouring of cotton. Concentrated NaOH is also used for cotton mercerizing.

iii) Effect of organic solvents: Have resistant ability to most organic solvents such as dry cleaning agents.

v)                  Effect of heat: Cotton has an excellent resistance to degradation by heat. It begins to turn yellow after several hours at 1200c and decomposes markedly at 1500c as a result of oxidation.

vi)               Effect of sunlight: Ther is a gradual loll of strength when cotton is expressed to sunlight and the fiber turns yellow.

vii)             Effectof insects: Cotton is not attacked by moth grubs or beetles.

viii)          Micro-organism: Cotton is attacked by fungi and bacteria.

ix)                Dyeability: Direct, Reactive, Sulphur and Vat dye.

Chemical composition of raw cotton:

Component	Main location	Relative amount (%)
Cellulose	Secondary wall	86.8
Oil and waxes	Cuticle	0.7
Pectins	Primary wall	1.0
Carbohydrate	Primary wall	0.5
Proteins	Lumen	1.2
Salts	Lumen	1.0
Water		6.8
Others		2.0