In ancient times the cotton plant was a perennial big shrub which bore fruits at all seasons; in time and after careful selections, almost in every country it has become a more or less branched out plant which is sown every year, grows and crops in 5 to 6 months.
The blooming takes place eight weeks after sowing, after few days the white and yellow flowers fall, leaving the boll which contains the seeds, around which the fibre fluff develops. In the subsequent weeks the capsule boll grows up to the size of a hen egg and bursts, releasing the white and bright cotton fibre which will continue to ripen until the harvesting time.
The cotton fluffs are picked either mechanically or manually and brought to the ginning room, where special machines called ″gins″ complete the separation of the fibres from the seed. The fibre mass is tightly compressed into bales, hence the term ″raw cotton″ used for this fibre. At this point the seeds are separated from the shorter cotton fibrils named ″linters″, which are used for the production of valuable artificial fibres, as we shall see later on.
From the botanical point of view, there are four basic species of cotton: Gossypium arboreum, herbaceum, hirsutum and barbadense. The first two species yield short staple, the third medium staple and the fourth long and extralong staple cottons (the term ″staple″ identifies the fibre length). In this connection we have to remind the prestigious varieties called ELS (extra-long staple), which are grown in relatively small quantities in Egypt, Sudan, Israel, USA and Peru. The staple length is one of the most decisive characteristics of the cotton fibre because the longer the staple length, the finer the spinnable yarn count. According to the official American standards, the staple lengths are divided into four ranges:
- below 0.99" : short staple
- 0.99" to 1.10": medium staple - 1.11 " to 1.26": long staple
- over 1.26": extra-long staple.
Fibre characteristics
The cotton fibre is characterised by the presence of a cavity in its interior, named ″lumen″. The quality assessment of the cotton fibres is based on following characteristics:
· staple length
· cross-section size, usually called fineness
· linear mass
The spinnability of a fibre, i.e. the finest yarn count attainable in spinning, depends on the length and on the fineness, while the yarn tenacity is related to the strength of the individual fibres as well as to the number of fibres in the yarn cross-section, which has to range between 90 and 120.
The fibre fineness is defined by the American standard ASTM D 123-85 and D 1448-84 as the weighted average linear mass expressed in micrograms per inch, but also in millitex µg/cm. For many years these values were assessed with a gravimetric test method, by weighing measured fibre lengths. For commercial purposes, the linear mass is assessed today more rapidly through special micronaire testing (or similar) instruments.
Besides fineness, a very important fibre property is the maturity degree, which is the ratio between the lumen length and the thickness of the fibre wall.
In fact there is a correlation between fibre maturity, linear mass and micronaire test readings, which is expressed by following values:
- micronaire lower than 3.0 = very fine; maturity degree 0.70-0.80 = immature
- micronaire from 3.0 to 3.9 = fine; maturity degree 0.80-0.85 = maturity below average - micronaire from 4.0 to 4.9 = medium fineness; maturity degree 0.85-0.95 = ripe
- micronaire from 5.0 to 5.9 = coarse fineness; maturity degree 0.95-1.00 = maturity
above average
- micronaire 6.0 and higher = very coarse fineness; maturity degree 1.00 and higher = very ripe
The fibre strength is, like fineness, the property which most affects the yarn characteristics: in fact the two properties are closely connected one another. Owing to the enormous difference in the values of these characteristics among the various fibres, cotton strength is measured with a so-called Pressley tester a flat fibre bundle composed by 500 to 1500 fibres.
For the assessment of fibre quality, also following factors are significant:
- colour: white, slightly spotted, spotted, slightly coloured, yellow stained, slightly grey, grey. The spinner requires anyway colour evenness.
- purity: contents of foreign matters. In fact another requirement of the spinner is the presence of a limited quantity of trash (coarse dusts).
- fibre preparatory process: this corresponds to the ginning process, which is a decisive factor for fibre softness, for uniform and open condition of the tufts and for the persistence of fibre entanglements, called ″neps.
Maturity degree of the cotton fibre— Relationship between lumen width and fibre wall thickness
The typical cotton characteristics all together as colour, foreign matter contents, kind of preparation as well as length and strength contribute to determine the ″grade″ and consequently the commercial value of the fibre, which in the trade is usually referred to as ″good middling″, ″strict middling″, ″middling″, ″low middling″, ″strict good ordinary″ or ″good ordinary″.
