Polymers natural fibers

Textile fibers are normally broken down into two main classes, natural and man-made fibers. All fibers which come from natural sources (animals, plants, etc.) and do not require fiber formation or reformation are classed as natural fibers. Natural fibers include the protein fibers such as wool and silk, the cellulose fibers such as cotton and linen, and the mineral fiber asbestos. Man-made fibers are fibers in which either the basic chemical units have been formed by chemical synthesis followed by fiber formation or the polymers from natural sources have been dissolved and regenerated after passage through a spinneret to form fibers. Those fibers made by chemical synthesis are often called synthetic fibers, while fibers regenerated from natural polymer sources are called regenerated fibers or natural polymer fibers. In other words, all synthetic fibers and regener-... 

There are many different types of fibers. Most fibers have diameters greater than 1 micrometer, and they can be divided into polymer fibers and non-polymer fibers. Polymer fibers include synthetic polymer fibers and natural polymer fibers. Synthetic polymer fibers are made from polymers synthesized from raw... 

The chemical and physical structures of natural polymer fibers are more complex than those of synthetic polymer fibers. Two most important building imits for natural polymer fibers are cellulose and protein. Natural cellulose fibers come from the stringy portions of plants ranging from the fine seed fibers of the cotton plant to the coarse pineapple leaf fibers. Natural protein fibers are hairs of animals, like the sheep and the dehcate filaments spun by silkworms and insects. In addition to these natural fibers, manufactured cellulose and protein fibers also are based on natural biopolymers. Although these fibers are processed like synthetic polymer fibers, they are discussed in this book together with natural cellulose and protein fibers since they have similar chemical structures. The chemical and physical structures of natural polymer fibers are discussed in Chapters 4 and 5, respectively. 

Synthetic polymer fibers and manufactured natural polymer fibers typically are produced by melt, solution, dispersion, or gel spinning. In these processes, polymer fluids are extmded through dies to form one-dimensional fiber stmctures (Figure 8.11). The dies typically have small diameters so thin fibers can be obtained. As a result, in order to form fibers with desired structure and properties, it is critically important to understand the capillary flow of polymers, i.e., the flow behavior in a small-diameter pipe. 

Due to the long lengths and small diameters, fibers typically have large aspect ratios, i.e., length-to-diameter ratios. All natural polymer fibers except silk are staple fibers, and the aspect ratios range from 1000 to 5000. Synthetic polymer fibers are produced as filaments, bnt they can be cut into staple fibers with desired aspect ratios. Inorganic fibers and nanofibers often are produced in continuous filament form. But they also can be made into staple fibers for certain applications. 

Spectra Natural Polymer Fibers (including regenerated fibers) 25 3... 

The physical shape of fibers affects their appearance, luster, hand and feel, cohesiveness, and many other properties. The shape of fibers can be examined both in longitudinal and cross-sectional directions. The simplest longitudinal shape of fibers is straight. But many natural polymer fibers, such as cotton and wool, have crimp longitudinal shape. Synthetic nanofiber fibers can be made to have straight, crimp, coiled, or spiral shape. 

The cross-sectional shape of fibers is more complex. Many natural polymer fibers have unique cross-sectional shapes. For example, the cross-section of dry cotton is kidney-shaped, while that of degummed silk is nearly a triangle. These cross-sectional shapes are controlled by genetic codes, and human has limited influence. However, the cross-sectional shapes of synthetic polymer flbers, inorganic flbers, and nanofibers can be manipulated by controlling the fiber formation processes. The cross-sectional shapes of these fibers range from circular to oval, triangular, dog bone, trilobal or multilobal, hollow, etc. 

Although un-oriented amorphous and semicrystalline polymer fibers are important and they often are the intermediate products of moleculariy oriented fibers, most commercial polymer fibers ate moleculariy oriented, except for a few elastomeric polymer fibers. The molecular orientation in natural polymer fibers occurs by itself under the control of genetic codes. For synthetic polymer fibers, molecular orientation is introduced purposely by high-speed spinning or drawing to enhance the mechanical properties of these fibers so they can be converted into useful final products. 

The stress-strain curves shown in Figure 15.17 just present the general property trends. In reality, the curve shapes may vary from fiber to fiber. For example, the stress-strain curves of natural polymer fibers are affected by the fanning conditions of the plants or animals. Depending on the applications, the processing conditions of synthetic polymer fibers can be selectively adjnsted to produce different physical stractures and mechanical properties. Table 15.2 shows typical tensile strength, modulus and strain-at-break valnes of some commercial fibers. 

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