Although it a common practice to use the terms ‘dead’ and ‘immature’ interchangeably, it is useful to use these terms to indicate two different levels of maturity in cotton fibers. The normal mature cotton fiber is bean-shaped in cross-section and has a thick cell-wall. The other extreme, dead cotton, has virtually no cell-wall thickness. The intermediate range between mature and dead is classified as immature. The immature (sometimes called thin-walled) fiber does have some secondary wall thickening. The thinner wall of the immature fiber lacks the rigidity of mature cotton. This increased flexibility of immature or dead fibers makes them prone to be mechanically knotted into a clump during ginning, lint cleaning and carding. These neps or clusters of fibers may resist dye and appear as white specks in the dyed material.
The distinction between dead and immature fibers is very important. Both dye lighter than fully mature fibers but only immature fibers respond to mercerization or any other swelling treatment. In contrast, dead fibers lack the ability to accept some dye even if pre-treated with a swelling agent.
The white or light-colored specks caused by immature/dead fibers may be of one of the following three types. The first type of the defect occurs when a surface knot of entangled immature fibers is flattened during processing and takes on a glazed, shiny appearance. The knot then becomes a small, reflective mirror on the surface of the dyed material. Its greater reflectance makes the knot appear lighter at some viewing angles than the surrounding area although it has actually been dyed to the same depth. The second type occurs when the fabric is poorly penetrated during dyeing. Since the clumps of immature fibers are often loosely attached to the material, they can be moved or knocked loose during subsequent processes. If the clump, or the yarn behind it, is not properly penetrated during dyeing, a light spot will be seen when the clump changes its position. The third type is the classic case of the clump of immature or dead fibers not dyeing to the same depth as the surrounding material.
The coverage of immature cotton depends upon the following factors:
Fiber preparation: There are several stages in the fiber preparation where an attempt can be made to decrease the amount of neps of the immature and/or dead fibers that are usually clumped together. It is important to try to remove these clumps prior to the carding process. Once past the main cylinder of the card, the clumped fibers go into the subsequently formed yarn and the fabric.
Preparation sequence: The preparation sequence has little, if any, impact on the coverage of immature cotton. Only pre-treatments that swell the cell wall, giving it greater thickness, are effective in improving the dyeability of immature cotton.
Swelling pre-treatment: Treatment with swelling agents at optimum concentration (e.g. caustic soda with a 14% or greater concentration) is effective in swelling the secondary wall of immature cotton, and improving its dyeing affinity. On the other hand, dead cotton lacks the necessary cell-wall thickness to be effectively treated by any type of swelling pre-treatment system.
Dye selection: Dyes vary widely in their ability to effectively eliminate the white or off-shade specks. It is recommended that dye suppliers be consulted for data on the immature cotton coverage capabilities of specific dyes. Since caustic pre-treatment is ineffective in eliminating white or off-shade specks caused by dead cotton, dye selection is the best alternative in this case. Although the exact mechanisms are unknown, one theory is that dyes that cover dead cotton are those which do not penetrate into the cellulose of the fiber (the core) but are deposited mainly in the outside layer. This gives the dead fiber a ‘colored’ skin.
After-treatments: Swelling treatments such as mercerization or ammonia treatment may be effective after dyeing, as well as before, if the problem is the presence of reflective surfaces and not a genuine difference in dye uptake by the immature cotton. However, such a procedure is justified only in extreme cases, as there is an inevitable change of shade even when the fabric is dyed with dyes that are resistant to strong alkalis.
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