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POLYESTER HEAVEN - Fibers, Part II

© Elizabeth Becka Lansky

Cotton is still the most widely worn fiber, but synthetics are becoming more popular as new forms are developed. Take "microfiber", today's current craze, which is nothing but polyester. Polyester got a bum rap when it first came out because it was formed in large, thick fibers with heavy weaves that didn't breathe, and people rebelled. Now it is produced in light, airy fabrics that float on the skin, but they still didn't dare call it polyester. Hence, "microfiber". (Not to be confused with Tencel, which really is a new type of synthetic fiber.)

As you recall from Fibers, Part I, natural fibers can only be identified as to type and approximate color. Synthetic fibers have more characteristics to compare, and therefore can make more compelling evidence. Cotton fibers always look like cotton, the same approximate size and shape, varying only in color, but synthetic fibers can come in different shapes, thicknesses, and conformations. Every similarity between a fiber from the suspect's shirt and the fibers of the victim's clothing helps the case.

To begin with, there are a limited number of synthetic fibers. The most common are polyester and nylon, commonly used in everything from shirts to carpeting. Acrylic fibers are almost always used for thick, fluffy things like socks and sweaters. Acetate is used for shiny, smooth fabrics, a sort of fake satin. Olefin, because of its excellent stain-resistant qualities, is used most in carpeting and sometimes in activewear. Spandex is basically rubber; this is why it tends to pull out of its stitching. Then there are specialty fibers made for flame- and heat- resistant qualities, such as aramid, saran and vinyon.

The analysis of these fibers usually begins with a microscope. Because the liquid fiber is extruded through a sort of sieve (which sets a constant shape and thickness) the long-chain polymer molecules tend to line up. This gives synthetic fibers a consistent shape, as well as optical properties not found in natural fibers. The cross section can be narrow, for fine, silk-like fibers, or thick and strong for jackets and ropes. They can be round, bone-shaped, hollow, etc. Acrylic fibers can be crimped in a zig-zag shape to give more fluff and volume to sweaters and knits. A three-lobed shape (trilobal) is popular for carpeting because it tends to hide dirt. Manufacturers have a reason for everything they do to a fiber, so their shape can hold a clue as to their purpose.

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