Natural fibers such as cotton and wool are some of the oldest materials. These fibers were used by early man when strength and light weight were critical. However, only in the past 50 years, with the development of analytical techniques such as X-ray diffraction, has the reason for the unusual properties of materials in fiber form been understood. Scientists now know that the molecules within fibers tend to align along the fiber axis. This preferred alignment makes the strength and modulus (stiffness) of both natural fibers and synthetic fibers superior to the same material in a randomly oriented bulk form. As an example, Table 1.2 shows the strength and modulus of a typical polymer in various forms. While the strength of an injection-molded polyamide plate in only 0.08 GPa, the tensile strength of the same polymer is over five times greater when it is extruded into a textile-grade fiber. If this same textile-grade fiber is stretched in an extensive drawing process, an industrial-grade tire cord fiber can be produced that is 10 times stronger and nearly twice as stiff as the injection-molded polymer. Chemically, all of these materials are identical, differing only in the orientation and structure of the solid polymer. When both natural and synthetic polymers are extruded and/or drawn into fiber form, the processes of extrusion and extension orient the structure along the fiber axis. This results in high strength and increased stiffness for much the same reason that an oriented mass of strings (a rope) is stronger and stiffer than the same mass of strings with no orientation.

TABLE 1.2. Properties of Polyamid in Various Forms

Form	Tensile Strength (GPa)	Tensile Modulus (GPa)	Orientation
Injection molded	0.08	2.5
Textile-grade fiber	0.43	2.5
Industrial-grade fiber	0.92	4.5
Kevlar®†	3.50	186.0
† Trademark of E.I. DuPont de Nemours and Co

Rigid, liquid-crystal-forming polymers (e.g., aramid fibers) can develop nearly perfect orientation and alignment during fiber formation. This allows