Plastics

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The term "plastic" can be broadly defined as any inherently formless material that can be molded or modeled under heat and/or pressure. It is derived from the Greek word plastikos, meaning a shaped or molded substance.

The term "plastics" first included only natural polymers—usually animal proteins (horn and tortoise shell), tree resins, or insect secretions called shellac—that were subsequently mixed with fillers such as wood flour to yield substances having better molding properties. (A polymer, from the Greek word poly, meaning "many," and mer meaning "unit," is a molecule with an extremely high molecular weight.)

The use of natural polymers to make plastic products started as early as 1760, when Enoch Noyes opened a business making combs out of keratin and albuminoid organic proteins derived from animal horns and horse hoofs. However, the first commercially successful plastic material, celluloid, would not come about for another hundred years.

In the 1840s German chemist Christian Schönbein developed cellulose nitrate from a mixture of cotton, nitric acid, and sulfuric acid. Cellulose nitrate is a highly flammable doughlike substance primarily used in the manufacture of explosives. Schönbein's innovation represents the beginning of the modification of natural polymers by chemists so as to increase their processibility and functionality. Cellulose nitrate's properties as a molding substance interested other scientists of the time, and in 1855 an Englishman named Alexander Parkes developed a form of cellulose nitrate he named Parkesine. From this material, Parkes manufactured a number of buttons, pens, medallions, and combs. In 1862 he displayed this material officially at the Great International Exhibit in London. Parkes made small commercial gains with Parkesine and eventually sold the rights to Daniel Spill, who subsequently began production of the substance under the names Xylonite and Ivoride, around 1865. Spill received British patents for Xylonite and Ivoride in 1867 and 1869, respectively.

At around the same time in the United States, a billiard ball company advertised a $10,000 reward for the discovery of an alternate material to ivory. John Wesley Hyatt developed collodion, a mixture of cellulose nitrate and alcohol. Like cellulose nitrate, collodion was highly flammable and would produce a small explosion upon agitation. Hyatt reported: "[W]e had a letter from a billiard saloon proprietor in Colorado mentioning this fact … saying he did not care so much about it, but that instantly every man in the room pulled a gun." To avoid melee, camphor, a derivative of the laurel tree, was added, and in 1870 Hyatt received a U.S. patent for celluloid. In 1871 Hyatt and his brother Isaiah formed the American Celluloid Company, which is today the Plastics Division of the Celanese Corporation.

A more common perception of plastic is that it is a synthetic or man-made material, with highly engineered properties and product designs. Dr. Leo Baekeland engineered the first totally synthetic plastic in 1907. Patented in 1909 and named Bakelite after its inventor, the material was the first thermoset plastic. The term "thermoset" refers to a plastic that under initial heat and pressure can be molded into form. After cooling, the material sets and cannot be remelted or re-formed. This setting is due to the cross-linking of polymer chains, wherein strong covalent bonds form between separate oligomers, short chains of polymer units called monomers. The most common thermoset resin is vulcanized rubber, created by Charles Goodyear in the United States in 1839. Vulcanized rubber utilizes natural hevea rubber made from the gutta percha tree, and therefore is not totally synthetic (like Bakelite). Ironically, the first use of Bakelite was as a replacement for natural rubber in electrical insulations. Bakelite is formed via the reaction of phenol and formaldehyde under high heat. Initially, formaldehyde is added to the reaction mixture in small amounts (forming a resin); the mixture is then poured into a mold, into which more formaldehyde is added; and pressure is applied to create the final product.

Over the next several decades, many varieties of synthetic thermoplastic materials would be developed in Germany, England, and the United States. Thermoplastic materials such as vinyls, nylons, and acrylics are polymers that can be molded or formed under heat and pressure, and if necessary can be reheated and re-formed (and will retain most of their original mechanical properties).

Eugen Baumann created today's most common vinyl, polyvinyl chloride (PVC), in 1872. However, Friedrich Heinrich August Klatte did not patent it until 1913. At that time PVC was not well received, as illustrated by Waldo Semon's comment, "People thought of PVC as worthless back then; they would throw it in the trash." Semon was responsible for creating plasticized PVC. He had been attempting to dehydrohalogenate PVC in a high boiling solvent when he realized that the molten material was exhibiting greater flexibility and elasticity. The exposure of PVC to a boiling solvent introduced a plasticizer, or low molecular weight molecule, to the PVC matrix. Today plasticizers are commonly added to polymers (especially PVC) to enhance flexibility, prevent stress cracking, and enhance processability. This has enabled the use of PVC in diverse commercial applications, including the manufacture of rigid tubing and flexible car seats.

In 1920 German scientist Hermann Staudinger published his theories on polyaddition polymerization, the formation of long-chain molecules. (Previously, the manner in which long-chain molecules were formed was unexplained.) Nine years later, in a publication that detailed the polymerization of styrene, this method of chain formation would be laid out. During this time period Staudinger developed polystyrene into a commercial product. A division of the German chemical company IG Farben, known as Badische Anilin-und Soda-Fabrik, or BASF, produced polystyrene in 1930. The Dow Chemical Company introduced the American public to polystyrene in 1937.

In 1928 directors at E. I. du Pont de Nemours & Company (Du Pont) placed Dr. Wallace H. Carothers in charge of fundamental research into what are now classic studies on the formation of polymer chains. During his years at Du Pont, Carothers published his theory on polycondensation, and discovered both neoprene and nylon.

Nylon, not publicly announced until 1938, was first used for bristles on combs, but made headlines in 1939 when nylon stockings debuted at the World's Fair in New York City. Nylon is known by its chemical name, poly(hexamethylene) adipamide, but more often simply as nylon. The first nylon manufacturing plant went into production at Seaford, Delaware, in 1940. Commercial production of nylon 6 by IG Farben in Germany began in 1941. These two plants would go on to produce millions of pounds of nylon annually. This mass production was essential to the World War II effort, as nylon was used for everything from belts, ropes, and straps to tents and parachutes.

Another polymer that came into use during World War II was polytetrafluoroethylene (PTFE), which received the trademark Teflon. Dr. Roy J. Plunkett and his assistant Jack Rebock at Du Pont discovered PTFE accidentally on April 6, 1938. They had been conducting research on alternate refrigeration methods when they discovered the polymerization of tetrafluoroethylene. Plunkett received a patent for PTFE in 1941. It was found that the material was resistant to corrosion by all the solvents, acids, and bases that were available for testing at that time. This led to the U.S. military's interest in PTFE, and its subsequent use as a cover for proximity fuses on the nose cones of artillery shells. It was not until the material was declassified in 1946 that the public learned of the material Du Pont had named Teflon two years earlier. Teflon has since become a household name; its best-known use being its contribution to nonstick surfaces on pots and pans.

Today's most widely produced and perhaps most versatile plastic, polyethylene, was discovered at the Imperial Chemical Industries (ICI) in England in 1933. E. W. Fawcett and R. O. Gibson set off a reaction between ethylene and benzaldehyde under 2,000 atmospheres of pressure, resulting in the polymerization of ethylene and the birth of polyethylene. By 1936, ICI had developed a larger volume compressor that made the production of useful quantities of polyethylene possible. Among polyethylene's first applications were its uses as underwater cable coatings and as insulation for radar during World War II.

In 1943, Karl Ziegler began work that would drastically alter the production of polyethylene. Ziegler used organometallic compounds, which have both metallic and organic components, as catalysts. At very modest pressures, these catalysts generated a linear, more rigid, high molecular weight polyethylene, and the innovation increased the number of the polymer's applications. Today polyethylene is used in the production of detergent bottles and children's toys, and is even replacing Kevlar as a bulletproof material.

In 1957, at the Montecatini Laboratories in Italy, Giulio Natta continued the work of Ziegler and used what is now termed Ziegler–Natta polymerization to create polypropylene. When Natta reported the polymerization of ethylene with a titanocene catalyst, it became clear that polymer chains with specific tacticities, or specific ordered structures, were possible. Polypropylene rose to become a substitute for polyethylene in products in which slightly higher temperature stability was necessary, for example, dishwasher-safe cups and plates.

Polycarbonate, a popular plastic used originally to make eyeglass lenses, was first discovered by A. Einhorn in 1898. But it would be more than fifty years before further research was performed on the material. In the 1950s Dr. Herman Schnell, working at Bayer, a division of IG Farben, along with Daniel Fox of General Electric's Corporate Laboratory in Schenectady, New York, conducted concurrent research on the synthesis of polycarbonate. Schnell and Fox each achieved a polymerization that produced polycarbonate via different methods, and received patents in 1954 and 1955, respectively. Upon his achievement of polymerization, Fox described his attempts to remove the newly formed polymer from the reaction vessel: "The remnants of the glass were broken away to yield a hemispherical, glass fragment embedded, glob of plastic on the end of a steel stirrer shaft. The glob was pounded on the cement floor and struck with a hammer in abortive attempts to remove the remaining glass, and/or, shatter the plastic. The pseudo plastic mallet was even used to drive nails into wood." That glob would eventually be developed into bulletproof glass and provide General Electric and Bayer with billions of dollars in revenue.

Means to improve the material properties of plastics have been sought for decades. Improvement has sometimes come in the form of compounds such as mineral fillers, antioxidants, and flame-retardants. One of the first searches for an improved material was centered on cellulose nitrate. Cellulose nitrate is colorless and transparent, which enabled it to be used as photographic film. However, it is extremely flammable, and its early use in motion picture film and concomitant exposure to hot lights led to numerous fires. In 1900, Henri Dreyfus substituted acetic acid for nitric acid in the synthesis of cellulose nitrate, and created instead a less flammable material, cellulose acetate. Today, polymers are often halogenated in order to achieve flame-retardation.

Plastics have been designed to be chemically resistant, stable compounds, and have been extremely successful in these regards. In fact, they have been so successful that an environmental problem has been created. Plastic products discarded in landfills decay slowly. They sometimes contain heavy metal additives. In addition, the millions of pounds of plastic discarded annually have engendered a crisis over landfill space. In the early 1980s plastic recycling programs began to spring up across the United States in response to the large number of polyethylene terephthalate (PET or PETE) bottles being discarded. In 1989, 235 million pounds of PET bottles were recycled. The number rose to 1.5 billion pounds in 1999.

Most plastics can be recycled. Even mixed plastic waste can be recycled into artificial lumber or particleboard. Plastic "wood" is easy to saw, and it has better resistance to adverse weather and insects than real wood.

SEE ALSO Baekeland, Leo ; Carothers, Wallace ; Goodyear, Charles ; Staudinger, Hermann ; Nylon ; Polymers, Synthetic .

Paul E. Koch

Bibliography

DiNoto, Andrea (1984). Art Plastic Designed for Living. New York: Abbeville Press.

Morawetz, Herbert (1985). Polymers: The Origins and Growth of a Science. New York: John Wiley.

Seymour, Raymond B. (1986). High Performance Polymers: Their Origin and Development. New York: Elsevier Science.