Owing to their different characteristics, the fibers take up different positions in the body of the yarn. Grouping arises mostly during drawing.
Thus, long fibers are often located in the core, since they exhibit more cohesive friction, and therefore higher resistance to the draft, and remain in the interior. Short fibers are often found on the yarn exterior. This tendency is reinforced by fiber migration (wandering of the fibers), since the fibers do not always stay in the positions they first take up.
For example, if any traction of power (even minimal) acts on the yarn, highly tensioned fibers of the outer layers press inward wholly or partly (the fiber ends, for example). In doing so, they press out the lower-tensioned fibers from the interior.
Migration takes place from the sheath to the core and vice versa. Such migration is, of course, most prevalent during yarn formation but still occurs after yarn formation is completed.
When the smallest forces are exerted on the yarn, e.g. during bending, tensile loading, etc., the persisting tensions in the fibers constituting the yarn lead to continuation of the process of fiber migration even after the completion of yarn formation.
For example, the short fibers work their way to the surface and are then partly rubbed off. Moreover, some fibers in the body of the yarn lose their helical dispositions during fiber migration; this effect is more prominent the shorter the fibers and the more random their arrangement.
In addition to its dependence on length, fiber migration is dependent upon degree of elasticity, stiffness, fineness, crimp, etc. Short, coarse, stiff fibers move out towards the sheath while long, fine, flexible fibers move towards the core.
Strongly crimped fibers are also found predominantly in the sheath, since they can exert greater resistance to binding-in. Fiber migration should be adequately taken into account in determining the composition of blends.
Possibilities for imparting strength
In order to obtain strength in the yarn, which consists of individual fibers of relatively short length, the inherent strength of one fiber must be made wholly or partly transferable to another. In principle, there are two alternatives: adhesives and twist.
Total exploitation of the inherent strength of the fibers can be achieved only by using adhesives, as was done, for example, in the Twilo process. The adhesive effect can be produced by means of adhesive substances or adhesive fibers (polyvinyl-alcohol fibers).
Since this process can be used only for a small market segment, twisting of the fiber strand remains the sole possibility for imparting strength, even for the future.
The extension of the fibers that arises during twisting leads, via the associated fiber tension, to increased pressure directed towards the yarn interior, i.e. to an increase in the frictional forces between the fibers and thus finally to the desired, immensely strong coherence of the body of the yarn (Figure 4). Fiber strands that are not held together by adhesives cannot completely exploit the inherent strength of the individual fibers.
