20.1 Synthetic Fibers

Although many of the polymers used for synthetic fibers are identical to those in plastics, the two industries grew up separately with completely different terminologies, testing procedures, etc. Many of the requirements for fabrics are stated in nonquantitative terms such as “hand” and “drape” that are difficult to relate to normal physical property measurements, but which can be critical from the standpoint of consumer acceptance, and therefore the commercial success, of a fiber.

A fiber is often defined as an object with a length-to-diameter ratio of at least 100. Synthetic fibers are spun (Chapter 17) in the form of continuous filaments, but may be chopped to much shorter staple, which is then twisted into thread before weaving. Natural fibers, with the exception of silk, are initially in staple form. The thickness of a fiber is most commonly expressed in terms of denier, which is the weight in grams of a 9000-m length of the fiber. Stresses and tensile strengths are reported in terms of tenacity, with units of grams/denier.

20.2 Fiber Processing

The polymer molecules in synthetic fibers are only slightly oriented by flow as they emerge from the spinnerette. To develop the tensile strengths and moduli necessary for textile fibers, the fibers must be drawn (stretched) to orient the molecules along the fiber axis and develop high degrees of crystallinity. All successful fiber-forming polymers are crystallizable and so, from a molecular standpoint, the polymer must have polar groups, between which strong hydrogen bonding holds the chains in a crystal lattice (e.g., polyacrylonitrile, nylons), or be sufficiently regular to pack closely in a lattice held together by dispersion forces (e.g., isotactic polypropylene).

In Chapter 17, it was pointed out that the cross section of fibers is determined by the cross section of the spinnerette holes and the nature of the spinning process. This plays an important role in establishing the properties of the fiber. For certain applications, the spun fibers are textured after spinning. Carpet fibers, for example, are often given a heat twist and/or are crimped by passing them through a pair of gear-like rollers.

20.3 Fiber Dyeing

The dyeing of fibers is a complex art in itself. A successful dye must either form strong secondary bonds to polar groups on the fiber or react to form covalent bonds with functional groups on the polymer. Furthermore, since the fibers are dyed after spinning, the dye must penetrate the fiber diffusing into it from the dye bath. The dye molecules cannot penetrate the crystalline areas of the polymer, so it is mainly the amorphous regions that are dyed. This often conflicts with the requirement of high crystallinity. The chains of polyacrylonitrile, for example, while possessing the necessary polar sites for dye attachment in abundance, are so strongly bound to each other that it is difficult for the dye to penetrate. For this reason, acrylic fibers usually contain minor amounts of plasticizing comonomers to enhance dye penetration. Nonpolar, nonreactive fibers such as polypropylene, on the other hand, have no sites to which the dye can bond even if it could penetrate. This was a problem long with polypropylene fibers and was overcome by incorporating a finely divided solid pigment in the polymer before melt spinning. Many of these dyes are also subject to leaching out, as you may have observed when washing a new bright red sweater with white clothes.

20.4 Other Fiber Additives and Treatments

Static electricity can be a big problem with carpets. Many carpet fibers therefore incorporate an antistatic agent (such as quaternary ammonium salts or alkyl esters of poly(ethylene glycol) to bleed off static charge. These additives are designed to bloom to the surface of the host polymer reducing the build-up of static charge.

The same polar bonding sites used for dyeing fibers also make them stainable. In the past, the finished item, a carpet, for example, would be treated with an anti-staining agent such as a fluorocarbon telomer (such as 3M's Scotchguard) or the anti-staining agents could be applied to the fibers before weaving. However, many of these have gone out of use due to toxicity and bioaccumulation problems of the fluorocarbons. Another approach is to use bicomponent fibers that mix the feel and texture of the original fiber (e.g., using nylon 6/6 as a core fiber) with a highly stain-resistant sheath (e.g., polypropylene or Teflon). These additives are also commonly found in many stain-resistant clothing lines.

20.5 Effects of Heat and Moisture on Polymer Fibers

The polarity of the polymer also directly influences its degree of water absorption. Other things being equal, the more polar the polymer, the higher its equilibrium moisture content under any given conditions and humidity. As with dyes, however, moisture content is reduced by strong interchain bonding. The moisture content exerts a strong influence on the feel and comfort of fibers. Hydrophobic fibers tend to have a “clammy” feel in clothing and can build up static electricity charges. Recent advances in athletic apparel has led to clothing that actively wicks away moisture, using fibers that are “breathable.”

Perhaps the most important effect of moisture on polar polymers is as a plasticizer. Since fiber-forming polymers are linear, heat acts basically as a plasticizer. This explains why suits wrinkle on hot, humid days, and why the wrinkles can be removed by steam pressing. “Wash-and-wear” and “permanent-press” fabrics are produced by operations that crosslink the fibers by reacting with functional groups on the chains, such as the hydroxyls on cellulose. The more hydrophobic polymers are inherently more wrinkle resistant because they are not plasticized by water. Wash-and-wear shirts, therefore, usually are made of blends of poly(ethylene terephthalate), a polyester, and cotton, about 65%/35%.