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  • Nature of neps and short fibres

    Chapter 2 - Cotton value addition - Neps and short fibres

     
     

    The general definition of neps is ‘hopelessly entangled masses of fibres’ (see figure 2.15). The appearance of common neps on yarns and fabrics is shown in figure 2.16. Neps may be further categorized into ‘seedcoat neps’ – which have a piece of the seedcoat attached to the fibres (see figure 2.17) – and ‘shiny neps’ – which consist of dead fibres, with insufficient cellulose to even absorb dye (see figure 2.18). If neps are incorporated into the yarn, it is quite likely they will survive into the fabric. Generally, if neps exceed a fairly low threshold, the resulting fabric is not suitable for high-value textile products.


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    A very small portion of observed neps may exist in unprocessed cotton, but the vast majority of neps are caused by handling and processing. Almost any mechanical process can cause the formation of neps, but the most likely ones include harvesting, ginning, and opening/cleaning in the textile mill. Neps are generally removed from the cotton fibres at only two places in the textile mill: the carding machine and the combing machine. If the cotton is not combed, only the carding machine is left to do the job. A state-of-the-art, well-adjusted carding machine can remove about 90% of the neps that are fed into the machine. Thus, if the cotton feeding into the carding machine has 200 neps/gram, then the count for cotton coming out in the card sliver may, in the best of circumstances, be reduced to about 20 neps/gram. And 20 neps/gram is a threshold above which the fibres’ usefulness for making high-quality textile products rapidly deteriorates.

    While mechanical processes are the chief cause of neps, some cotton fibres are more susceptible to nep formation than others. In other words, there are significant interaction effects between some fibre properties and mechanical processes with regard to nep formation. Susceptibility to neps tends to increase as the perimeter of the fibres decreases, the maturity of the fibres decreases, the length of the fibres increases, and with either very high or very low moisture content. Also, the more trash the cotton contains, the more the fibres must be cleaned, which will result in the formation of more neps.

    Short fibres

    The traditional definition of ‘short fibres’ is those fibres less than 1/2" long. It has long been apparent, however, that this traditional definition is inadequate. Most spinning systems can be adjusted to accommodate the ‘dominant long fibre content’ – which is practically synonymous with the ‘staple length’ – of cotton. If the staple length is quite long, then the critical designation for short fibre may be longer than 1/2". If the staple length is short, then the critical designation for short fibre may be shorter than 1/2".

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    Figure 2.19: Length distribution (by weight) of two cottons with similar staple length

    Figure 2.19 shows frequency distributions for the length of two different cottons, as obtained from AFIS®. While the mean lengths of the two cottons are approximately equivalent, sample A exhibits a larger portion of shorter fibres than does sample B (as shown by the shaded area in figure 2.19). These shorter fibres are negatively correlated with good yarn properties (e.g. strength and elongation) and positively correlated with bad yarn properties (e.g. CV%, thin and thick places, and hairiness).

    The critical need, after all, is information on the entire length distribution of the cotton being spun. There can be no doubt that, regardless of the staple length of cotton, the more uniform the length distribution is, the better the cotton will perform in spinning. Furthermore, even a slight elevation of very short fibres (say, less than 1/4") is likely to disproportionately damage spinning performance and yarn quality. These statements apply regardless of whether the poor length distribution is caused by genetic factors or by fibre breakage in harvesting, ginning, or textile manufacturing.

    It may have already occurred to the reader that there is likely to be a strong interaction effect between neps and short fibres, since in the previous section short fibres were given as one of the causes for an increased tendency toward nep formation. This is in fact true; indeed, most fibre properties that tend to elevate the number of neps also tend to elevate the number of short fibres.

    • Not only do immature fibres readily tangle up to form neps; they also readily break under any kind of mechanical stress. This, in turn, fosters the creation of even more neps.
    • Fibres that are long and fine are more subject to breakage in high-speed manufacturing than fibres that are short and coarse. (It is common practice to slow machinery down when processing long, fine fibres.) Elevated levels of breakagemay be a primary reason that neps aremanifested in long, fine fibres.
    • Fibres with very low moisture levels are much more likely to break, and then are susceptible to forming neps.
    • Fibres that have elevated levels of trash must be cleaned more aggressively, resulting in more broken fibres and, therefore, more neps.
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