The quality of the fibres varies greatly according to the variety, the seasonal conditions during cultivation and the way the crop is handled after harvest.
100 kg seed cotton gives 35 to 42 kg of cotton lint. Cotton fibres are characterised by a longitudinal spiral twist (see picture magnified 630 times), which contributes to an elasticity of about 10%. Some of the most important quality criteria are:
Staple length: The longer cotton fibres are, the better they are suited to producing high quality yarns and apparels. Fibre lengths are divided into short-staple (less than 24.6 mm), medium-staple (24.6 – 30.9 mm), long-staple (more than 30,9 mm) and extra long-staple fibres (more than 33.4 mm).
Tensile strength: The tensile strength is measured in grams per tex (US) or PSI for Pressley (outside US) and indicates the maximum load that a bundle of fibres will hold before they break.
Micronaire: This parameter measures fibres’ fineness and maturity. The thinner the fibre, the lower the number of micronaire. However, low micronaire values can be an indication of either low intrinsic fineness (diameter of fibre) or low maturity.
Colour: Variation in the colour of seed cotton causes variation in the colour of dyed cotton products. A distinction is made between white, creamy, light spotted and spotted colour. Some varieties and specific strains, however, are of greenish, beige or brown colour.
Contamination: The degree of contamination with non-fibre material or foreign fibres is an important quality criterion. Contaminated fibres cause problems during processing, reducing the quality of the resulting yarn and garments. Frequent contaminants are dust, cotton foliage and packing material.
Damage: Pest and disease infestation can damage cotton fibres, reducing the quality of the harvest. A common problem is honeydew (secretion) from lice, a sugary liquid that causes cotton fibres to stick together.
Source: Chaudhry, M. Rafiq; Guitchounts, Andrei (2003): Cotton Facts.
Size matters and it's in your jeans, denim hunters, January 2015