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Plastics are materials that can be molded, spun, or shaped to make products for human consumption. Plastics may be natural or synthetic. Natural plastics are materials such as asphalt, bitumen, cellulose, horn, rubber, shellac, tortoise shell, and waxes. Humans have crafted products for millennia using natural plastics. Ancient peoples in all parts of the globe used natural plastics for decorative or functional purposes. The ancient Egyptians soaked the winding cloths for mummies in natural resins to protect the body of the deceased from decay. Musical instruments, parts of weapons, or other devices have been made in part or in whole from the horns of sheep, cattle, or other animals.
In the 1850s, shellac, a resinous secretion of the lac insect, was imported from southern Asia into the United States, where it was patented as a natural plastic. Another natural plastic from southern Asia was gutta percha, which is sap from trees of the same name in Malaysia. It was used to insulate electric wires that were just coming into use. In 1897, milk was used to develop casein plastics. Other natural plastics materials derived from tropical forests, such as rubber, began to be employed by manufacturers.
By 1900, scientists were finding more ways to manufacture modified natural plastics, such as artificial horn, celluloid film, and cellophane. While natural plastics were useful, there were drawbacks to using them. The supply was insufficient and unreliable. They were not always amenable to being molded, and they were prone to degrading or, in the case of celluloid, were highly flammable.
In the early 1900s, developments in the coal and oil industry made available vast quantities of material that could be employed in the manufacture of synthetic plastics. In 1909, Leo Hendrik Baekeland, an American chemist, created Bakelite. Baekeland developed a thermosetting synthetic resin (phenolic resin). His invention was commercially successful and was employed in telephones, pot handles, and many other products. As research into the chemistry of long chain organic polymers expanded enormously, many new ways to combine or modify them to make synthetic plastics were developed. In Germany, the I.G. Farben Company discovered how to make polystyrene and polyvinyl chloride by 1932. In the mid-1930s, it patented polyepoxide (epoxy). In 1937 Friedrich Bayer & Co. invented polyurethane. Du Pont’s research team, led by Wallace Carothers, discovered how to synthesize vinylacetylene with chlorine to make synthetic rubber (neoprene) in 1930. In 1935, Carother’s team found that a polyamid formed by combining hexamethylene diamine and adipic acid produced nylon. His discovery of nylon made parachutes available in vast quantities.
After World War II, more new plastics were invented. The textile industry began to use many plastic fibers, which were blended with natural fibers and marketed as polyester blends. Among the new products were Tupperware, plastic bottles, film, and safety helmets. Thermoplastics (polysulfone) were used in facemasks or in hospital equipment. Everything from spacecraft parts to artificial body parts are now made with plastics.
Plastics are made from synthetic resins. The resins are made from chemicals manufactured from natural sources such as coal, petroleum, salt, water, and limestone. Chemically, synthetic resins are long chain polymers. Monomers are used to build the polymers by combining them together with chemicals such as ammonia and benzene. The process of polymerization joins the monomers into long chains of atoms and molecules.
Plastic products are manufactured by molding, casting, laminating, extruding, or calendaring. Other methods include vacuum forming and press forming. Molded plastic products are formed by squeezing a molten resin into a mold that will be hardened by pressure and by chemical reactions. Thermoplastic resins harden when they are allowed to cool, while thermosetting resins harden under heat and pressure. Once set, the plastic object is permanently molded.
Casting is similar to molding, but the plastic resin is poured instead of squeezed into the mold. This method is useful when using thermosetting and thermoplastic resins. Other casting resins contain acrylic, polyimide, polyurea, and silicone adhesives. Often, the casting resins are mixed with a hardener. They are used to make semiconductors and insulated parts for generators, transformers, switches, and circuit breakers, and even in artwork. Some photonic and optical equipment made with casting resins use polymers and elastomers. Formica table or counter tops are made with the laminating method of applying plastics. Vast quantities of laminating plastics are sold to businesses and home users.
Extruding forces solid resin material into a heating device that then pushes (extrudes) it out through an opening to form products such as fibers, garden hoses, piping, gaskets, or tubing. The extruded material may be forced through a rectangular opening that will form molding. The textile industry uses vast quantities of synthetic fibers for products such as carpet. Calendaring pushes plastic resins between rollers to coat different products, such as plastic cards used in business or other services. Plastic film is made by extrusion and by calendaring.
The amount of plastic consumed every year has grown greatly over the last 50 years. Unfortunately, so has the negative impact of plastics upon the environment. Apart from the petroleum and other materials used to make plastics, discarded plastic products remain in the environment for a long time. Large amounts of waste are dumped into landfills. Also, large amounts scatter as unsightly trash. Efforts to reduce the amount of plastics have focused on recycling. Great quantities of plastics are reused after they have been melted and recast into other products. Plastics are also incinerated for energy.
Some progress has been made toward making plastics much more biodegradable. Nylon naturally degrades in direct sunlight because the light breaks some of the bonds in the polymers; researchers are seeking ways to ensure that more plastics will degrade in sunlight. Another way to ensure that plastics degrade is to add starch-based resins to the formula. However, the “rotting” is only partial. Researchers are also experimenting with plastic-eating bacteria to create biodegradable plastics, and two nylon-eating bacteria have been identified. These methods are in the experimental stage because of costs and the danger that bacteria could evolve and end the age of plastics.
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- Daniel Imhoff and Roberto Carra, Paper or Plastic?: Searching for Solutions to an Overpackaged World (Sierra Club Books, 2005);
- L. Meikle, American Plastic: A Cultural History (Rutgers University Press, 1995);
- Nabil Mustafa, Plastics Waste Management: Disposal, Recycling and Reuse (Marcel Dekker, 1993);
- National Research Council, Clean Ships, Clean Ports, Clean Oceans: Controlling Garbage and Plastic Wastes (National Academies Press, 1995);
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