Tuesday, January 15, 2013

Fibre Blends; Properties of fiber Blended Yarns

Fibre Blends 
Fibre Blending is the process of mixing fibers together. As noted earlier, it can take place at any of several points during the preparation of a yarn. The purposes of blending are (1) the thorough intermixing of fibers and/or (2) combining fibers with different properties to produce yarns with characteristics that cannot be obtained by using one type of fiber alone. Self blending of bales of the same fiber is done routinely in processing natural fibers because the fibers may vary from bale to bale. In this type of blending, the mixing of as many bales as possible is done early in the processes preparatory to spinning so that the subsequent steps can help to mix the fiber still more completely. For the same reasons, even when two or more different fiber types are combined, blending is done as early as possible. Carding helps to break up fiber clusters and intermix fibers more thoroughly. However, if the fibers being blended require different techniques for opening, cleaning, and carding, as with polyester and cotton, then slivers can be blended. For blended yarns of different fibers, the blend level is the percentage by weight of each fiber. Blending is not limited to staple-length fibers. Filament fibers of different generic types can be combined into a single yarn. This can be done either by extruding these fibers side by side, during drawing, or during texturing. As described earlier, a blended yarn can be core spun with one fiber at the center and a different fiber as the covering or be wrapped with one fiber making up the central section and another the wrapping yarn. As yarn spinning and texturing technologies grow more sophisticated, we expand the possibilities of combining several different fibers into one yarn. Multiple-input texturing machines can produce specialty yarn blends. 

It should be noted that fabrics woven from two or more yarns each made of different fibers are not considered blends. These fabrics are, instead called combination fabrics. They do not behave in the same way as those fabrics in which, the fibers are more intimately blended and may require special care procedures. Regrettably, the Textile Fibers Products Identification Act (TFPIA) labeling requirements do not distinguish between blended fabrics and combination fabrics when fiber percentage contents of fabrics are given. 

Properties of Blended Yarns 
Fibers with different characteristics, blended into a yarn, can each contribute desirable properties to the final textile material. The ultimate performance is an average of the properties of the component fibers. For example, a fabric of 50 percent cotton and 50 percent polyester would have an absorbency intermediate between that of cotton or polyester. In some cases, however, the observed fabric property is not determined simply by the relative amounts of each fiber in the blend. In blends of nylon with cotton, the tenacity of the blended yarn initially decreases with increasing amounts of nylon because of differences in the breaking elongation of the two fibers. At the breaking elongation of the cotton fibers, the nylon fibers are not assuming their share of the stress, leaving the cotton to bear the load. 

The stage at which blending occurs also affects the properties of the fabrics. In general, the more intimate the mixing of fibers in the blends, the better the resulting properties. Yarns blended at the fiber stage exhibit a more effective averaging of properties than ply-blended yarns. Even though considerable study and evaluation have been made of optimum fiber proportions required to achieve desired results in blends, no certain conclusions have been reached. It is clear that extremely small proportions of fibers have no appreciable influence on performance, although they may have some effect on appearance.

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