Details
Material hierarchy: Polymer
Plastics consist of two categories:
Plastics are man-made materials that can be shaped into almost any form. They are one of the most useful materials ever created. Our homes, schools, and businesses are filled with plastic products. Engineers have developed plastics that are as rigid as steel or as soft as cotton. They can make plastics that are any color of the rainbow--or as clear and colorless as crystal. Plastics can be rubbery or rigid, and they can be shaped into an endless variety of objects, ranging from automobile fenders to squeezable bottles to soft fabrics. Plastic products, especially those used by industries, often have a useful life of many years.
Plastics consist of long chains of molecules called polymers. These chains are made of repeating patterns of smaller molecules. Each of the smaller molecules forms a "link" in the polymer's chain. In some plastics, the chains are rigid and are lined up like logs flowing down a river. In others, they are flexible and tangled like spaghetti on a plate. These different structures give plastics their most notable characteristic, the ability to be shaped. In fact, the word plastics comes from the Greek word plastikos, which means able to be shaped.
As useful as they are, plastics do have drawbacks. The biggest problem is that most plastics take a very long time to decompose (break down into simple compounds). Figuring out how to dispose of plastic wastes has become a major environmental concern.
How Plastics Are Used
Engineers have created hundreds of different plastics, each with its own properties. They have developed plastics that can replace metals, natural fibers, paper, wood and stone, and glass and ceramics. Manufacturers use these plastics to make products stronger, lighter, longer lasting, easier to maintain, or less expensive to make. In addition, inventors have used plastics to create items that could be made with no other materials.
To Replace Metals. Plastics are used to replace metals in a variety of products. Automakers commonly use plastic bumpers, fenders, and wheel covers in their products. In some cars, the entire body is made of plastics. Plastic auto parts do not rust, nor do they dent as easily as metal ones. Plastic auto parts are easier and often less expensive to repair. Replacing metal parts with plastic ones also reduces the weight of a vehicle, resulting in more efficient fuel use. Airplane manufacturers use plastic wing and body assemblies for many of these same reasons. These large, seamless parts also create less wind resistance than do riveted metal sections.
Plastics have also replaced metals in many building construction materials, such as pipe and home siding. Plastic siding does not dent as easily as that made from aluminum. Pipe made from plastics is lightweight and easy to cut and join. Moreover, it does not corrode like metal pipe.
Surgeons mend broken bones with plastic parts rather than metal ones, because the plastics are less likely to trigger a harmful reaction. Dentists often use plastic fillings because--unlike metal fillings--the plastic ones can match the patient's tooth color.
To Replace Natural Fibers and Hides. The textile industry uses plastics to replace such natural fibers as cotton, ramie, silk, and wool. Plastic fibers may have such qualities as strength, durability, and resistance to stains and wrinkling. Some plastic fibers are tough enough to be used for automobile safety belts or bulletproof vests. Others are delicate enough to be made into sheer hosiery. Durability and resistance to stains make plastic fibers an excellent material for clothing, carpeting, and furniture coverings. Manufacturers can also treat plastic fibers to make them more difficult to burn. Plastic fibers are often mixed with natural fibers to produce fabrics with qualities similar to an all-natural fabric but with added durability.
Plastics are also used to create synthetic leathers, suedes, and furs. Spun plastic fibers replace down or feathers in insulated jackets and pillows.
To Replace Paper. Plastics have replaced paper in many packaging applications. Plastic-foam packing materials provide more protection for boxed products than crushed paper does. Many products, particularly delicate electronic equipment, are packed in foamed plastic inserts that fit the shape of the item exactly.
Plastic wraps have many uses. They preserve foods longer than paper wraps can. Plastic wrap can stretch to form a seal around the opening of a container. Many items that are sold in cardboard packages, such as jigsaw puzzles and record albums, are sealed in clear plastic wrap. Such hardware items as nuts and bolts are packaged in clear plastic boxes that allow a buyer to see the product.
To Replace Wood and Stone. Plastics have replaced wood and stone in many applications. Laminated plastic countertops come in a variety of patterns. Some look like marble. Laminated countertops are lighter, less expensive, and easier to install than marble ones. They also resist marring and stains.
Furniture makers use plastics to produce cabinet doors and tabletops that look like wood but are easier to clean and do not warp. Plastics have also replaced wood in boat hulls. Plastic boats are stronger than wooden ones and require less maintenance. Unlike wooden hulls, plastic hulls can be constructed easily in one piece. These smooth, one-piece hulls can be shaped to cut through water with less resistance.
To Replace Glass and Ceramics. Because they are lighter and far less likely to break, plastics have replaced glass or ceramics in a variety of products. Plastic wall tiles, bathtubs, and sinks are cheaper and easier to install than ceramic ones. Airplane windows made of acrylic plastics are lighter and less brittle than glass. Safety and comfort have made lightweight, shatterproof plastic eyeglass lenses a popular substitute for glass lenses. Plastic bottles are also shatterproof, and they have replaced glass ones in packaging such products as milk, ketchup, household cleaners, and many other foods and household goods. Plastic bottles also weigh much less than glass ones and so help cut down shipping costs.
To Provide New Characteristics. Plastics are used in many ways that would not be possible for other materials. They have many medical applications because they are not harmful to the body and can be formed into any shape. Parts made from plastics can replace damaged hip, knee, and finger joints. Plastic pieces are used to rebuild facial structures damaged by accidents. Sometimes, plastic parts are used to replace faulty heart valves.
Plastics are also used to make insulating foam that blocks the flow of heat and sound. The foam can be blown into the walls of a home through a small hole. Integrated circuits, which hold thousands of transistors that control the flow of electricity, are sealed in plastics. The plastics protect the delicate transistors without interrupting the flow of electricity.
Types of Plastics
Although there are hundreds of different plastics, all of them belong to one of two basic types, based on how they behave when heated. These types are (1) thermosetting plastics and (2) thermoplastics.
Thermosetting plastics--or thermosets, for short--can be heated and set only once. They cannot be remelted or reshaped. When a thermoset is heated, it undergoes a chemical reaction called cross-linking, which binds its polymer chains together. This reaction is similar to the hardening of an egg when it is boiled. Once it has hardened, it cannot become a liquid again. Because thermosets cannot be remelted, engineers use them in applications that require high resistance to heat. Products made from thermosetting plastics include pot handles and trays for sterilizing medical instruments.
Thermoplastics can be melted and re-formed again and again. Their polymer chains do not form crosslinks. Thus, the chains can move freely each time the plastics material is heated.
Thermoplastics are used much more widely than are thermosets. Manufacturers prefer thermoplastics because they are easier to handle. They also require less time to set--as little as 10 seconds, compared to as long as 5 minutes for thermosets. And unlike thermosets, most thermoplastics can be dispersed in liquids to produce durable, high-gloss paints and lacquers. Because their molecules can slide slowly past one another, some thermoplastics tend to lose their shape when exposed to constant pressure over a long period of time. For this reason, manufacturers prefer to use thermosets for such products as plastic seats on buses.
How Plastics Are Made
The substances used to make plastic products are called synthetic resins. These resins are made primarily from petroleum, but some come from such other natural sources as coal, natural gas, cotton, and wood. Chemical manufacturers produce the resins and sell them to companies that make plastic products.
The Chemistry of Plastics. To understand how plastics are made, it is helpful to know something about the chemistry of polymers. The polymers in plastics are made up of small molecules called monomers. Most of these molecules are composed of carbon, hydrogen, nitrogen, and oxygen atoms. Some include chlorine, fluorine, silicon, or sulfur atoms. A polymer chain consists of hundreds, thousands, or even millions of monomer links. In some polymers, these links are made up of the same kind of monomer, repeated over and over. Others are composed of two or more kinds of monomers, which may be linked randomly or in alternating sequences. In some polymers, blocks of one kind of monomer are joined to blocks of another kind.
Polymer chains may or may not have branches. A chain may have branches on only one side or alternating from one side to the other. The chains may pack together in straight rows to make a stiff, crystalline solid. Or they may remain tangled and spread out, to make a soft, rubbery material. The properties of plastics depend on the types of monomers in their polymer chains, the lengths of the chains, and the arrangement of the chains.
Different kinds of polymer molecules can be mixed together to form polymeric alloys, or blends. Alloys are often easier to create than new synthetic polymers. They may have properties that lie between those of their component polymers, or they may have properties that are better than either.
Making Synthetic Resins. Resin manufacturers make polymers by combining chemical compounds. These range from familiar chemicals like ammonia and benzene to compounds with tongue-twisting names such as hexamethylenediamine. When a manufacturer combines appropriate compounds, chemical reactions cause atoms to cluster together to form monomers. Further reactions cause the monomers to polymerize--that is, to form long chains of molecules. Polymerization produces the synthetic resin.
The steps in polymer building can be illustrated by the production of polystyrene resin. To make polystyrene, a chemical manufacturer begins with the liquid benzene and the gas ethylene, two chemicals derived from petroleum. The manufacturer bubbles the ethylene through the benzene. During this process, the compounds react to form the liquid ethylbenzene. Ethylbenzene is used to make liquid styrene by heating ethylbenzene gas to a high temperature and bringing it into contact with certain metal oxides. This process removes some hydrogen atoms from the ethylbenzene. The remaining atoms form molecules of styrene.
Finally, the manufacturer polymerizes the styrene to make solid polystyrene. One way this can be done is by suspending the styrene in water, adding chemicals, and heating it. A chemical reaction causes the styrene molecules to link together and form chainlike molecules of polystyrene. The manufacturer then forms the solid polystyrene into grainlike particles. These particles are the raw material used to mold polystyrene products.
Manufacturers frequently use additives to change the properties of a plastics resin. Common additives include (1) reinforcements, (2) fillers, (3) plasticizers, and (4) pigments.
Resin makers use such reinforcements as glass fibers or carbon fibers to give plastics extra strength or rigidity. The resulting mix, called a composite or a reinforced plastic, may contain from 10 to as much as 80 per cent reinforcement. Composites are lightweight and can replace metals in missiles, aircraft, and automobiles.
Resin manufacturers may use fillers to improve the quality of plastics or to extend an expensive resin. Common fillers include powdered wood, talc, and clay.
Manufacturers add plasticizers to certain synthetic resins to make them softer, more flexible, and easier to shape. Plasticizers overcome the attractive forces between the polymer chains and separate them to prevent intermeshing.
Pigments change the color of plastics. Resin makers use pigments to produce unlimited varieties of color.
Additives enable resin manufacturers to make plastics even more useful. For example, vinyl plastics are naturally clear and rigid. But thanks to additives, vinyl plastics can be made into products ranging from rigid, gray pipe to slightly flexible, black phonograph records to soft, transparent windows for convertible automobile tops.
Making Plastic Products. Manufacturers use seven main processes to shape plastics into products. These processes are (1) molding, (2) casting, (3) extrusion, (4) calendering, (5) laminating, (6) foaming, and (7) thermoforming.
Molding. There are a variety of molding processes, including compression, injection, blow, and rotational molding. In all these processes, force is applied to the plastic material during and after it enters the mold. Once the product has hardened, it is released from the mold.
Compression molding is the most common method of molding thermosetting plastics. Compression-molded products include automobile hoods and fenders, electrical switches, and handles for cooking utensils and irons. In compression molding, resin powder is put into a mold. The manufacturer then heats the mold and, at the same time, applies pressure. After the plastics have set, the mold is opened and the product is released. The mold can then be refilled.
Injection molding is the most widely used method of molding thermoplastics. Injection-molded products include telephones, computer housings, automobile steering wheels, and a variety of other items. In injection molding, resin pellets fall from a hopper into a heated, horizontal barrel, where they melt. A plunger or revolving screw inside the barrel pushes the liquid resin under pressure into a mold. Most injection-molded products take only 10 to 30 seconds to harden. The mold is opened, and ejector pins push the formed product out of the mold. The mold is then closed and refilled.
Blow molding is used to make bottles and other hollow objects. In this process, a tube of molten resin, called a parison, is inserted into a mold. Compressed air or steam then is forced into the parison, which expands much like a balloon being inflated. This action forces the resin against the walls of the mold, where it is held until it hardens.
Rotational molding also forms hollow objects, such as soccer balls, dolls, and automobile fuel tanks. In this process, a mold is partly filled with powdered resin. The mold is then heated while a motor spins it rapidly, creating a centrifugal force. This force pushes the melting resin against the mold walls and holds it there as the mold is cooled and the object solidifies.
Casting, unlike molding, does not depend on any external pressure to shape the plastics. Manufacturers use this method to shape both thermoplastics and thermosetting plastics. To cast thermosets, they pour a liquid resin containing chemicals into a mold and harden it by applying heat. For thermoplastics, the molten resin is poured into a mold and cooled until it sets. Processors employ casting to make thick plastic panels and to produce gears, paperweights, and other solid objects.
Extrusion is used to produce pipe, rods, fibers, wire coatings, and other products that have the same shape along their entire length. Solid thermoplastics particles from a hopper enter a stationary, heated barrel. One or more rotating screws force the particles through the barrel, where they melt as they are pushed along. The molten material is forced out through a shaping die.
Calendering produces a continuous plastic sheet or film by pressing molten plastics between pairs of polished, heated rollers. Manufacturers feed fabric, paper, or metal foil through the rollers to produce such items as plastic-coated playing cards and tablecloths.
Laminating uses plastics to bind together stacks of glass-fiber, wood, paper, cloth, or metal-foil sheets. The sheets are coated or soaked with a resin. They are then placed one on top of the other. A machine squeezes the sheets together and heats them until the resin has joined them firmly. Laminating produces strong materials with a wide range of thicknesses for such products as plywood, electronic circuit boards, and tabletops.
Foaming refers to any of several methods that produce plastic foams. All these methods involve introducing a gas into heated plastic resins. The gas expands and creates bubbles in the cooling resin. The resulting material is lightweight plastic foam, which is sometimes called cellular plastic. Depending on the resins and the method, plastic foams can be stiff and strong, such as those used in packaging and home insulation. Others can be soft and rubbery, such as the foams in furniture cushions and pillows.
Thermoforming is an inexpensive process used to mold items from sheets of plastics. In this process, workers clamp a plastic sheet over a mold. They then heat the sheet until it becomes soft. Next, a pump sucks air out through tiny holes in the mold. This creates a vacuum that pulls the soft plastic sheet down until it covers the surface of the mold. There it cools and hardens in the shape of the mold. Manufacturers use thermoforming to produce such objects as bathtubs, shower bases, and yogurt containers.
Development of Plastics
For thousands of years, people used natural gums and resins with properties similar to plastics. For example, the ancient Greeks and Romans created decorative objects from amber, a fossil resin. During the Middle Ages, Europeans used the natural resin lac, and its purified form, shellac, to coat objects.
By the mid-1800's, the commercial molding of plasticslike natural substances had developed. Manufacturers molded items from lac, gutta-percha (a tree resin), and other substances obtained from animal, vegetable, and mineral sources. Products made from these natural "plastics" included brush handles, knobs, electrical insulation, phonograph records, and novelty items. Museums and collectors treasure the beautiful molded objects created during the late 1800's and early 1900's.
Despite their beauty, these natural molding materials had several disadvantages. Manufacturers often had difficulty obtaining the raw materials. Some materials proved difficult to mold, and many of the finished products turned brittle and broke easily.
The Invention of Celluloid. In the late 1860's, John W. Hyatt, a printer from Albany, N.Y., developed a material to replace the scarce ivory used to make billiard balls. In 1870, he and his brother Isaiah received a patent for the material, which they later named Celluloid. Celluloid was the first synthetic plastic material to receive wide commercial use.
Hyatt made Celluloid by first treating cellulose, a substance found in cotton, with nitric acid. He then combined the resulting substance, pyroxylin, with the solvent camphor. The end product, Celluloid, was a hard, stiff material that could be shaped under heat and pressure to form useful items.
Celluloid was used for years to make such products as combs, dentures, and photographic films. But it was highly flammable. During the early 1900's, researchers produced a similar, but less flammable, material called cellulose acetate. Today, manufacturers use cellulose acetate to make films, fibers, and molded objects. Celluloid itself is still used to make ping-pong balls.
The Invention of Bakelite. During the early 1900's, Leo Baekeland, a chemist from New York City, attempted to make a synthetic shellac by combining the chemicals carbolic acid (also known as phenol) and formaldehyde. Chemists had experimented with combining these chemicals for several years, but the reaction had been too violent to contain. Baekeland succeeded in controlling the reaction, which created phenolic resin.
The resin did not turn out to be the synthetic shellac that Baekeland had sought. But his research was hardly a failure, for he had created the first completely synthetic resin and the first of the thermosetting plastics. He patented it in 1909 and named it Bakelite, after his own name. Bakelite soon became widely used to make such items as telephones and handles for pots and irons. It continues to be used today in the electrical and automotive industries.
Growth of the Plastics Industry. The introduction of Bakelite in 1909 gave scientists a better understanding of polymer chemistry. The plastics industry expanded steadily throughout the next three decades. Scientists in the United States, Great Britain, and Germany conducted a great deal of research in plastics, shedding new light on their structures.
The most dramatic developments occurred in the 1930's. Four important thermoplastics--acrylics, nylon, polystyrene, and polyvinyl chloride (PVC or vinyl)--came into commercial use. Acrylics are strong and clear. They became widely used for airplane windows. Nylon was used to make women's hosiery and, later, such molded products as bearings and gears. Manufacturers used polystyrene in many products, including clock and radio housings, toys, wall tile, and food containers. PVC, too, had numerous applications, finding its way into such diverse products as garden hoses, raincoats, wire insulation, and electric plugs. The introduction of specialized machinery to form and mold plastics into useful items also helped the growth of the industry.
Important thermosetting plastics called polyesters were introduced commercially in the 1940's. Important thermoplastics developed during the 1940's included polyethylene, silicones, and epoxy resins. All of these plastics found new uses during the early 1950's. Polyethylene proved an excellent material for dishes, squeezable bottles, plastic bags, artificial flowers, and other products. Manufacturers used silicones in lubricants and electrical insulation, and physicians used them in body implants. Epoxy resins gained wide use as strong adhesives. Manufacturers used polyesters to make boat hulls. In 1953, the General Motors Corporation introduced the Chevrolet Corvette--a sports car with a body made of polyester reinforced by glass fibers. The Corvette rolled into history as the first mass-produced automobile with a plastic body.
The uses of plastics continued to grow during the late 1950's and the 1960's. This growth corresponded directly to the growth of the petrochemical industry, the major producer of the raw materials for plastics. Engineers found new uses for plastics in medicine, nuclear and space research, industry, and architecture. Polymer chemists developed several new plastics that are especially resistant to chemicals and extreme heat.
Throughout the 1970's and 1980's, plastics continued to find new applications, appearing in such products as microwave cookware, personal computer housings, and compact discs. Aerospace engineers used heat-resistant polyurethane foam to cover the external fuel tanks of the United States space shuttles. This plastic foam acts as heat insulation to prevent loss of fuel by evaporation. During the late 1980's, scientists developed the first practical conductive plastics, which--unlike other plastics--can carry an electric current. Conductive plastics have possible uses in batteries, wiring, and static-resistant fabrics.
The Plastics Industry
The United States, Japan, and other industrialized nations lead the world in plastics production. The plastics industry continues to grow rapidly in these countries. The growth of the industry in any country depends on a plentiful supply of petroleum.
Plastics companies may be divided into three general groups: resin manufacturers (mostly chemical companies) who make and supply resins; processors who shape the resins into products; and finishers and assemblers who make products by cutting, drilling, decorating, and assembling plastic parts. Most resin manufacturers are located in regions that allow easy access to great supplies of petroleum. Most processors, finishers, and assemblers operate in areas where they can serve many industries.
The plastics industry offers a variety of job opportunities, including careers in research, design, machine operation, quality control, and sales. For information about careers in plastics, write to the Society of Plastics Engineers, 14 Fairfield Drive, Brookfield, CT 06804.
Plastics and the Environment
As more and more plastic packaging materials are used by consumers, more plastic waste is generated. Because most plastics do not readily break down, this waste contributes significantly to environmental pollution.
Recycling has emerged as one method of combating the problem of plastics waste. Industries that produce or use large amounts of plastics have recycled their wastes for years. Usually they clean and separate the plastics by type. They recycle the thermoplastics by remelting and re-forming them into new products. Thermosets are either ground into fine powders or shredded. The powders are used as fillers. The shreds can be used as insulation in such products as quilted jackets and sleeping bags.
In the 1980's, many cities and towns turned to recycling to help dispose of consumer plastics waste. These communities require citizens to sort certain plastic items--such as polyester soft drink bottles and polyethylene milk bottles--from other waste materials. These plastics can be reused in the same manner as industrial plastics waste.
Some communities do not separate the plastics but instead burn the mixed municipal waste. This process yields energy that can be used for electricity or heating. It requires, however, sophisticated incinerators that remove the acid gases produced by the burning of PVC and other plastics.
Another approach to the disposal problem is to design plastics that can be broken down by nature and time. In the 1970's, chemists introduced biodegradable plastics that break down through the actions of microorganisms. In products made from these plastics, molecules of starches or cellulose separate the polymer chains of the plastics. Microorganisms attack and consume the starches and then cause the products to deteriorate. Scientists also created photodegradable plastics that break down through long exposure to sunlight. The polymers in these plastics are decomposed by a chemical additive that breaks down when exposed to sunlight.
In the mid-1980's, manufacturers began using degradable plastics to make trash bags, foam cups, and other disposable products. But such plastics have come under fire from environmental groups and even members of the plastics industry. These critics argue that even under the best conditions, degradable plastics products leave plastics residue behind, and that the products will not break down at all when buried in landfills. They are also concerned that the additives used to enable plastics to degrade also make the plastics unfit for recycling.