Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Synthetic Fibers

Synthetic fibers derived from natural cellulose were first developed at the end of the 19th century and became known as rayons. In a typical rayon-making process, natural cellulose made from wood pulp is treated with chemicals to form a thick liquid. This liquid is then extruded as filaments into a weak acid bath that converts the filaments back into pure cellulose. Rayons are not, therefore, completely synthetic but are actually regenerated fibers. Acetates and triacetates, which are true synthetic fibers, were developed shortly after rayon. They are derived from cellulose acetate (see Esters) in a process similar to that used for making rayon.

Most synthetic fibers are now derived from organic polymers, materials consisting of large organic molecules. Most of them are thermoplastic—that is, they are softened by heat. The first commercially successful organic synthetic fiber, nylon (polyamide), dates from 1938. Since then many other fibers, including acrylic (polyacrylonitrile), aramid (aromatic polyamide), olefins (polyethylene and polypropylene), polyester, and spandex (polyurethane), have been developed. In a typical fiber-spinning process, a molten polymer or polymer solution is extruded through tiny holes in a spinneret into an environment that causes the filaments to solidify. The fiber's properties depend on the base polymer, the spinning process, and the post-spinning treatment of the fiber, which can include drawing, annealing, applying a finish, and coating. Fiber properties such as weight, abrasion resistance, heat resistance, chemical resistance, moisture resistance, strength, stiffness, elasticity, and ease of dyeing and coloring can be optimized by such treatments.

Carbon and graphite fibers are high-strength materials that are used as reinforcing agents in composites. Carbon fibers are produced by using heat to chemically change rayon or acrylic fibers. Carbonization occurs at temperatures of 1000° C to 2500° C (1832° to 4532° F) in an inert atmosphere. Carbon fibers are converted to graphite at temperatures above 2500° C. Carbon and graphite fibers can also be made from pitch, a residual petroleum product. Products that use carbon fibers include heat-shielding materials, aircraft fuselages and wings, spacecraft structures, and sports equipment. High-strength fibers such as aramid and polyethylene are also used in armor applications such as bullet-resistant garments, car doors, and crew seats for military helicopters and aircraft.