Thousands of logs are

stored to meet the vora­cious appetite of a pulp mill. Wood contains cellulose (see diagram, opposite), a versatile organic substance. Cellulose will probably gain even greater importance with the decline in reserves of petroleum and the petro­chemicals from which most polymers are currently man­ufactured.

important synthetic rubbers, including nitrile rubber and chloroprene rubber.

Polyamides

This group of high-molecular weight polymers is characterized by the amide linkages that bind the monomers together. Amides are a type of compound containing nitrogen. Poly­amides are formed by reactions in which an organic acid and an amine (a nitrogen-contain­ing compound) add together. They may be nat­ural (as in proteins) or synthetic. By far the best known synthetic polyamide is nylon. It was dis­covered shortly before World War II when sci­entists at Du Pont in the U.S. were searching for silklike polymers. The original nylon was made from two compounds containing hydro­gen and carbon. Still used today, these two compounds are chemically combined to form nylon salt. This salt is put into solution at high heat under pressure. After all the water has been driven off by the heat, the polymer is spun into fibers that may be woven to make textiles. Different types of nylon can be pro­duced by altering the numbers of carbon atoms in the monomers. One modern form uses only a single monomer. Besides its use in fibers and fabrics, nylon is used in the plastics industry to make gears, tubing, electrical equipment, sporting equipment, and even body panels on some automobiles.

Other synthetic polyamides are the ar-amids, renowned for their flame-retardant properties. One of them, Kevlar, was specially developed for use in the belting for radial tires.

Polyesters

Polyesters are formed by reacting organic acids with polyfunctional alcohols, discussed in the article "Alcohols." Polyethylene (PET) is the best-known of all polyesters. It is used as a textile fiber under various trade names such as Terylene and Dacron. PET can also be formed into film. Polyester films are used in magnetic tapes for computers and tape recorders. They also serve as insulation for electrical wires or as containers. In recent years, PET has become important as the raw material for plastic con­tainers for soft drinks.

Polyurethanes

Like polyesters, polyurethanes are manufac­tured from polyfunctional alcohols. The best-known of all polyurethanes is polyurethane foam. It is prepared by adding water to the re­action mixture for polyurethane. This causes cross-links in the polymer to form. This liber­ates carbon dioxide gas, which causes foam­ing. Water is thus said to act as a blowing agent. Flexible or rigid foams can be made, de­pending on the addition of other organic com­pounds. The foam has become so familiar since it has been widely used in furniture, es­pecially as mattresses and as seat cushions. Polyurethanes are also excellent thermal insu­lators, used to shield storage tanks and ships' boJds and to insulate pipes.

Date: 2015-12-11; view: 1114