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Plastics, term applied to organic polymeric materials (those consisting of giant organic molecules) that are plastic-that is, they can be formed into desired shapes through extrusion, molding, casting, or spinning. The molecules can be either natural-including cellulose, wax, and natural rubber-or synthetic-including polyethylene and nylon. The starting materials are resins in the form of pellets, powders, or solutions; from these are formed the finished plastics.
Plastics are characterized by high strength-to-density ratios, excellent thermal and electrical insulation properties, and good resistance to acids, alkalies, and solvents. The giant molecules of which they consist may be linear, branched, or cross-linked, depending on the plastic. Linear and branched molecules are thermoplastic (soften when heated), whereas cross-linked molecules are thermosetting (harden when heated).


The development of plastics began about 1860, after Phelan and Collander, a United States firm manufacturing billiard and pool balls, offered a prize of $10,000 for a satisfactory substitute for natural ivory. One of those who tried to win this prize was U.S. inventor John Wesley Hyatt. Hyatt developed a method of pressure-working pyroxylin, a cellulose nitrate of low nitration (see Cellulose) that had been plasticized with camphor and a minimum of alcohol solvent. Although Hyatt did not win the prize, his product, patented under the trademark Celluloid, was used in the manufacture of objects ranging from dental plates to men's collars. Despite its flammability and liability to deterioration when exposed to light, Celluloid achieved a notable commercial success.
Other plastics were introduced gradually over the next few decades. Among them were the first totally synthetic plastics: the family of phenol-formaldehyde resins developed by the Belgian-American chemist Leo Hendrik Baekeland about 1906 and sold under the trademark Bakelite. Other plastics introduced during this period include modified natural polymers (see Polymer) such as rayon, made from cellulose products.

Breakthrough in Plastics Chemistry

In 1920 an event occurred that set the stage for the future rapid development of plastic materials. The German chemist Hermann Staudinger hypothesized that plastics were truly giant molecules. His subsequent efforts to prove this claim initiated an outburst of scientific investigation that resulted in major breakthroughs in the chemistry of plastics. Throughout the 1920s and 1930s large numbers of new products were introduced, including cellulose acetate, used in molding resins and fibers; polyvinyl chloride, used in plastic pipe, vinyl coatings, and wire insulation; urea-formaldehyde resins, used in tableware and electrical applications; and acrylic resin, developed as a binder for laminated glass.
One of the most familiar plastics developed in this period is polymerized methyl methacrylate, which, beginning in 1937, was marketed as Lucite and Plexiglas. This material has excellent optical properties and is suitable for eyeglass and camera lenses and for producing special effects in highway and advertising illumination. Polystyrene resins, also first produced commercially about 1937, are characterized by high resistance to chemical and mechanical alteration at low temperatures and by very low absorption of water. These properties make the polystyrenes especially suitable for radio-frequency insulation and for accessories used in low-temperature situations, as in refrigeration installations and in airplanes designed for high-altitude flight. Polytetrafluoroethylene, first made in 1938, was eventually produced commercially as Teflon in 1950. Another key development during the 1930s was the synthesis of nylon, the first high-performance engineering plastic.

World War II

During World War II (1939-1945), both the Allies and the Axis powers were faced with severe shortages of natural raw materials. The plastics industry proved to be a rich source of acceptable substitutes. Germany, for example, which was cut off early from sources of natural latex, initiated a major program that led to the development of a practical synthetic rubber (see Rubber). Similarly for the United States, Japan's entry into the war eliminated most Far Eastern sources of natural rubber, silk, and many metals. The U.S. response was to accelerate the development and production of plastics. Nylon became a major souce of textile fibers, polyesters were used in fabricating armor and other war materials, and various type of synthetic rubber were produced in quantity.

The Postwar Boom

The scientific and technological momentum in the plastics industry carried over into the postwar years. Of particular interest were the advances in such engineering plastics as polycarbonates, acetals, and polyamides; other synthetics were used in place of metal in machinery, safety helmets, high-temperature devices and many other products used in environmentally demanding settings. In 1953 the German chemist Karl Ziegler developed polyethylene, and in 1954 the Italian chemist Giulio Natta developed polypropylene-two of today's most important plastics. A decade later, these two men shared the 1963 Nobel Prize in Chemistry for their studies of polymers.

Kinds of Plastics

Three of the ways in which plastics can be categorized are by the polymerization process that forms them, by their processibility, and by their chemical nature.
The two basic polymerization processes for producing resins are condensation and addition reactions. Condensation produces a variety of polymer chain lengths, whereas addition reactions produce only specific lengths. Furthermore, condensation polymerizations produce small by-product molecules such as water, ammonia, and glycol, whereas no by-products are generated in addition reactions. Typical condensation polymers are nylons, polyurethanes, and polyesters. Addition polymers include polyethylene, polypropylene, polyvinyl chloride, and polystyrene. The average molecular weights for the addition polymers are generally orders of magnitude larger than those of condensation polymers.


The processibility of a plastic depends on whether it is thermoplastic or thermosetting. Thermoplastics, which are made up of linear or branched polymers, are fusible: They soften when heated and harden when cooled. This is also true of thermosets that are lightly cross-linked. Most thermosets, however, harden when heated. This final cross-linking, which fixes the true thermosets, takes place after the plastic has already been formed.
Chemical Nature
The chemical nature of a plastic is defined by the monomer (repeating unit) that makes up the chain of the polymer. For example, polyolefins are made up of monomer units of olefins, which are open-chain hydrocarbons with at least one double bond. Polyethylene is a polyolefin; its monomer unit is ethylene. Other categories are acrylics (such as polymethylmethacrylate), styrenes (such as polystyrene), vinyl halides (such as polyvinyl chloride), polyesters, polyurethanes, polyamides (such as nylons), polyethers, acetals, phenolics, cellulosics, and amino resins.


The manufacture of plastic and plastic products involves procuring the raw materials, synthesizing the basic polymer, compounding the polymer into a material useful for fabrication, and molding or shaping the plastic into its final form.
Raw Materials
Originally, most plastics were made from resins derived from vegetable matter, such as cellulose (from cotton), furfural (from oat hulls), oils (from seeds), starch derivatives, or coal. Casein (from milk) was among the nonvegetable materials used. Although the production of nylon was originally based on coal, air, and water, and nylon 11 is still based on oil from castor beans, most plastics today are derived from petrochemicals. These oil-based raw materials are more widely available and less expensive than other raw materials. However, because the world supply of oil is limited, other sources of raw materials, such as coal gasification, are being explored.

Synthesizing the Polymer

The first stage in manufacturing plastic is polymerization. As noted, the two basic polymerization methods are condensation and addition reactions. These methods may be carried out in various ways. In bulk polymerization, the pure monomer alone is polymerized, generally either in gaseous or liquid phase, although a few solid-state polymerizations are also used. In solution polymerization, an emulsion is formed and then coagulated. In interfacial polymerization, the monomers are dissolved in two immiscible liquids, and the polymerization occurs at the interface of the two liquids.


Chemical additives are often used in plastics to produce some desired characteristic. For instance, antioxidants protect a polymer from chemical degradation by oxygen or ozone; similarly, ultraviolet stabilizers protect against weathering. Plasticizers make a polymer more flexible, lubricants reduce problems with friction, and pigments add color. Among other additives are flame retardants and antistatics.
Many plastics are manufactured as composites. This involves a system where reinforcements (usually fibers made of glass or carbon) are added to a plastic resin matrix. Composites have strength and stability comparable to that of metals but generally with less weight. Plastic foams, which are composites of plastic and gas, offer bulk with low weight.

Shaping and Finishing

The techniques used for shaping and finishing plastics depend on three factors: time, temperature, and flow (also known as deformation). Many of the processes are cyclic in nature, although some fall into the categories of continuous or semicontinuous operation.
One of the most widely used operations is that of extrusion. An extruder is a device that pumps a plastic through a desired die or shape. Extrusion products, such as pipes, have a regularly shaped cross section. The extruder itself also serves as the means to carry out other operations, such as blow molding and injection molding. In extrusion blow molding, the extruder fills the mold with a tube, which is then cut off and clamped to form a hollow shape called a parison. This hot, molten parison is then blown like a balloon and forced against the walls of the mold to form the desired shape. In injection molding, one or more extruders are used with reciprocating screws that move forward to inject the melt and then retract to take on new molten material to continue the process. In injection blow molding, which is used in making bottles for carbonated beverages, the parison is first injection molded and then reheated and blown.
Compression molding uses pressure to force the plastic into a given shape. Another process, transfer molding, is a hybrid of injection and compression molding: The molten plastic is forced by a ram into a mold. Other finishing processes include calendering, in which plastic sheets are formed, and sheet forming, in which the plastic sheets are formed into a desired shape. Some plastics, particularly those with very high temperature resistance, require special fabrication procedures. For example, polytetrafluoroethylene has such a high melt viscosity that it is first pressed into shape and then sintered-exposed to extremely high temperatures that bond it into a cohesive mass without melting it. Some polyamides are produced by a similar process.

Uses Plastics have an ever-widening range of uses in both the industrial and consumer sectors.


The packaging industry is the leading user of plastics, accounting for about one-third of total United States production. In the early 1990s, U.S. sales of low-density polyethylene (LDPE) exceeded 10 billion pounds per year. Most of the LDPE produced is marketed in rolls of clear-plastic wrap. High-density polyethylene (HPDE) is used for some thicker plastic films, such as those used for plastic trash bags and containers. Other packaging plastics include polypropylene, polystyrene, polyvinyl chloride, and polyvinylidene chloride. Polyvinylidene chloride is used primarily for its barrier properties, which can keep gases such as oxygen from passing into or out of a package. Similarly, polypropylene is an effective barrier against water vapor. Polypropylene also is often used in housewares and as a fiber for carpeting and rope.

The United States building industry is the second largest consumer of plastics, including many of the packaging plastics mentioned above. HDPE is used for pipes, as is polyvinyl chloride (PVC); PVC is also used in sheets for siding and similar components. Many plastics are used to insulate cables and wires, and polystyrene in the form of foam serves as insulation for walls, roofs, and other areas. Other plastic products are roofing, door and window frames, moldings, and hardware.

Other Uses

Many other industries, especially automobile and truck manufacturing, also depend on plastics. Tough engineering plastics are found in components like air-intake manifolds, fuel lines, emission canisters, fuel pumps, and electronic devices. Plastics are also used for interior paneling, seats, and trim. Many automobile bodies are made of fiberglass-reinforced plastic.
Among the other uses of plastic are housings for business machines, electronic devices, small appliances, and tools. Consumer goods range from sports equipment to luggage and toys.

Health and Environmental Hazards

Because plastics are relatively inert, they do not normally present health hazards to the maker or user. However, some monomers used in the manufacture of plastics have been shown to cause cancer. Similarly, benzene, which is an important raw material for the synthesis of nylon, is a carcinogen. The problems involved in the manufacture of plastics parallel those of the chemical industry in general.
Most synthetic plastics are not environmentally degradable; unlike wood, paper, natural fibers, or even metal and glass, they do not rot or otherwise break down over time. (Some degradable plastics have been developed, but none has proven compatible with the conditions required for most sanitary landfills.) Thus, there is an environmental problem associated with the disposal of plastics. Recycling has emerged as the most practical method to deal with this problem, especially with products such as the polyethylene terephlalate bottles used for carbonated beverages, where the process of recycling is fairly straightforward. More complex solutions are being developed for handling the commingled plastic scrap that constitutes a highly visible, albeit relatively small, part of the problem of solid waste disposal.

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