MICROSCOPIC CHARACTERISTICS
A polarized light microscope is the primary tool for the identification and analysis of manufactured ibers. Many characteristics of manufactured ibers can be viewed in nonpolarized light, however, and these characteristics provide a fast, direct, and accurate method for the discrimination of similar fibers. A comparison light microscope is required to confirm whether the known and the questioned fibers truly present the same microscopic characteristics. The cross-section is the shape of an individual fiber when cut at a right angle to its long axis. Shapes for manufactured fibers vary with the desired end result, such as the fiber’s soil-hiding ability or a silky or coarse feel to the final fabric. Figure 1 shows some variations in fiber cross-sections. The particular cross-section also may be indicative of aiber’s intended end use: Many carpet ibers have a lobed shape to help hide dirt and create a specific visual texture to the carpet. The way a fiber’s diameter is measured depends on its cross-sectional shape; there is more than one way to measure the diameter of a nonround iber. Manufactured fibers can be made in diameters from about 6 µm (so-called microfibers) up to a size limited only by the width of the spinneret holes. By comparison, natural fibers vary in diameter from cultivated silk (10–13 µm) to US sheep’s wool (up to 40 µm or more) and human head hairs range from 50 to 100 µm. Delustrants are finely ground particles of materials, such as titanium dioxide, that are introduced into the spinning dope. These particles help to diffract light passing through the fibers and reduce their luster, as illustrated in Figure 2. The size, shape, distribution, and concentration of delustrants should be noted.
FIGURE 1 The cross-section of a fiber may relate to its end use. Carpet fibers, for example, often have cross-sections that are meant to keep the fiber upright (making the carpeting feel plush) and to hide dirt. About 500 cross-sectional shapes are used in the manufacture of fibers.
FIGURE 2 Tiny grains of material, usually titanium dioxide, are incorporated into a fiber as it is spun; these are called delustrants. Delustrants break up the light entering the fiber and make it appear dull or give a matte finish. A fiber with no delustrant is described as being “bright.
