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Plastics (Spirit Markings)

Plastics (Spirit Markings)

Paranormally obtained plastics may be divided into two groups: imprints and molds. The first may be produced in any soft, yielding substance or on smoked or chemically treated surfaces; for the second, melted paraffin wax is employed.

Paranormal Imprints

Johann C. F. Zöllner, in his experiments with the medium Henry Slade, placed a dish filled to the brim with flour under the table hoping the spirit hand that took hold of him might leave an impression in the flour. Baron Lazar Hellenbach testified to having seen an impression of a hand larger than Slade's or any other individual present. None of their hands had any trace of flour. Zöllner also obtained the imprint of a foot on two sheets of paper covered with lamp black between two closed slates.

The imprint of a hand with four fingers, the imprint of a bird, two feet, and a materialized butterfly were supposedly obtained during the George Valiantine -Bradley sittings in 1925, in England. Charles Sykes, the British sculptor, was unable to give an explanation, as was Noel Jaquin, a fingerprint expert. In 1931, however, the same experts claimed to have caught Valiantine in a fraud. They smeared printing ink in secret on the modeling wax, stripped Valiantine after the séance and found a large stain on his left elbow corresponding with the lines of the imprint. Other imprints were found identical to those of his toes.

Palladino's Mediumship

Eusapia Palladino produced hand and face imprints in putty and clay. Reportedly they bore her characteristics, although she was held at a distance from the tray while the impression was made. Numerous imprints were obtained by the psychical researchers Cesare Lombroso, Enrico Morselli, Er-cole Chiaia, and Guillaume de Fontenay.

Camille Flammarion claimed to be a witness of the process at Monfort-l'Amaury in 1897. Supposedly the resemblance of the spirit head to the medium was undeniable, yet seemingly she could not have imprinted her face in the putty. Besides having been physically controlled, Ms. Z. Blech kissed Palladian on the cheeks, searching for the odor of putty on her face.

Julien Ochorowitz wrote of Palladino's mediumship at Rome:

"The imprint of this face was obtained in darkness, yet at a moment when I held two hands of Eusapia, while my arms were entirely around her. Or, rather, it was she who clung to me in such a way that I had accurate knowledge of the position of all her limbs. Her head rested against mine even with violence. At the moment of the production of the phenomena a convulsive trembling shook her whole body, and the pressure of her head on my temples was so intense that it hurt me."

Paranormal Molds

In normal wax molding, the technical process of the production of paraffin wax casts begins with the placement of buckets of hot and cold water placed side by side. The hot water will melt the paraffin. If one dips a hand in and withdraws it, a thin shell of the liquid will settle and congeal. If a hand is dipped alternately into the hot paraffin and into the cold water the shell will thicken. When the hand is freed, a wax glove is left behind. These gloves are fragile. They must be filled with plaster of Paris to preserve. Then if the paraffin wax is melted off, the texture of the skin appears in the plaster. The hand freed from the paraffin shell must be washed in soap and water before another experiment, or the second shell will stick to the fingernails. Altogether, it takes about twenty minutes to deliver a finished shell. The fingers of the hand must be held fairly straight, otherwise they will break the shell when withdrawn. For the same reason no full cast, up to the wrist, can be obtained.

Supposedly molds obtained by psychical researchers in séances with mediums have bent fingers, joined hands, and wrists. These molds are fine and delicate, whereas those obtained from living hands are thick and solid.

The first paraffin wax casts were obtained by William Denton in 1875, in Boston with the medium Mary M. Hardy. Hardy produced the paraffin wax gloves in public halls. To test Hardy's ability, the dish of paraffin was weighed before the mold appeared and after. In later years, another test was devised, locking up the liquid paraffin wax and cold water in a wire cage. After Denton, Epes Sargent investigated Hardy.

In England, William Oxley produced the first psychic molds in 1876 with Elizabeth d'Esperance and later with Mrs. A. H. Firman and the Rev. Francis W. Monck. Similar success was claimed with the Davenport Brothers, William Eglinton, and Annie Fairlamb around the same time. T. P. Barkas of Newcastle, England, mixed magenta dye in the paraffin wax during experiments with Fairlamb in 1876. The gloves had traces of the dye.

The psychical researcher Alexander Aksakof hypothesized that the plaster casts showed similar characteristics between the medium and the materialization. He noted that Oxley made similar observations and quoted his letter:

"It is a curious fact that one always recognises in the casts the distinctive token of youth or age. This shows that the materialised limbs, whilst they preserve their juvenile form, evince peculiarities which betray the age of the medium. If you examine the veins of the hand you will find in them characteristic indications which indisputably are associated with the organism of the medium."

It had been suggested the wax gloves may have been prepared from inflated rubber gloves. Gustav Geley produced some casts using rubber gloves for comparison. They were also put on display. The charge that the gloves may have been made previous to the séance could not be sustained.

One variety of plastics is the working of linen into the semblance of human features by psychic means. Reportedly Dr. Eliakim Phelps left a well-detailed description of an instance, including the appearance of 11 figures of "angelic beauty." Occasionally similar phenomena have been reported as a manifestation in haunted houses, with cushions assuming the shape of human forms.

There are also artistic efforts under the heading of direct paintings the paint appears to give three-dimensional effects. Many such pictures were produced during the late nineteenth and early twentieth centuries.

There are various methods to produce imprints. Mrs. Albert Blanchard, an American medium, produced imprints by depositing sediment under water in a dish. F. Bligh Bond discussed her work in Psychic Research (October 1930) using data collected from Horace Newhart. Blanchard put clay and water in a shallow dish, stirred the sediment with her fingers, and let it settle. When the water evaporated, supposedly the clay had assumed the outlines of a human face or head in low relief.

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Plastics

Plastics

The term plastic can be used as both an adjective and a noun. As an adjective, the term refers to any material that can be shaped or molded, with or without the application of heat. In this respect, objects such as soft waxes, asphalt, and moist clays are said to be plastic.

As a noun, the term describes a natural or synthetic polymer. A polymer is a material whose molecules consist of very long chains of one or two repeating units known as monomers. As an example, the synthetic polymer called polyethylene consists of thousands of ethylene units joined to each other in long chains. If the letter E is taken to represent an ethylene unit (monomer), then the polymer polyethylene can be represented as:

-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-E-

Although the term plastic is strictly defined as either a natural or synthetic material, it is probably understood by most people today to refer

primarily to artificial materials. Substances such as nylon, Styrofoam, Plexiglass, Teflon, and polyvinyl chloride (PVC) are examples of such materials.

Thermoplastic and thermosetting plastics

Plastics can be subdivided into two large categories: thermoplastic and thermosetting. The former term refers to a material that can be melted and shaped over and over again. Examples of thermoplastics include acetal, acrylic, cellulose acetate, polyethylene, polystyrene, vinyl, and nylon.

A thermosetting plastic, in contrast, can be melted and shaped only once. If it is then heated a second time, it tends to crack or disintegrate. Examples of thermosetting plastics (or just thermosets) include amino, epoxy, and phenolic and unsaturated polyesters.

Words to Know

Composite: A combination of a plastic and one or more additives that has special properties not possessed by the plastic alone.

Monomer: A fundamental unit of which a polymer is composed.

Polymer: A substance composed of very large molecular chains that consist of repeating structural units known as monomers.

Thermoplastic: A polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.

Thermosetting plastic (or thermoset): A polymer that solidifies when heated and that cannot be melted a second time.

Additives

Very few plastics are used in their pure state. Many different materials known as additives are added to improve their properties. Products consisting of pure plastics and additives are known as composites. For example, the strength of a plastic can be increased by adding glass, carbon, boron, or metal fibers to it. Materials known as plasticizers make the plastics more pliable and easier to work with. Some typical plasticizers include low-melting solids, organic liquids, camphor, and castor oil. Fillers are materials made of small particles that make a plastic more resistant to fire; attack by heat, light, or chemicals; and abrasion. They also can be used to add color to the plastic.

[See also Polymer ]

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vinyl

vi·nyl / ˈvīnl/ • n. 1. synthetic resin or plastic consisting of polyvinyl chloride or a related polymer, used esp. for wallpapers and other covering materials and for phonograph records: light-reflecting vinyls can be hung in the usual way. ∎ vinyl used as the standard material for phonograph records: fans had to wait almost a year before the song eventually appeared on vinyl. 2. [as adj.] Chem. of or denoting the unsaturated hydrocarbon radical −CH=CH2, derived from ethylene by removal of a hydrogen atom: a vinyl group.

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vinyl plastics

vinyl plastics, group of thermoplastics used in molded products, flexible tubing, material for raincoats, and laminated safety glass. Vinyl plastics are polymers and copolymers of vinyl derivatives (i.e., derivatives of ethylene, H2C[symbol]CH2), e.g., vinyl chloride (H2C=CHCl) and vinyl acetate (H2C=CH-OOC-CH3). Polyethylene may be considered the simplest of the vinyl polymers, and polyvinyl chloride is an important member of this group. Polytetrafluoroethylene, or Teflon, is also sometimes classed as a vinyl polymer.

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vinyl

vinyl •anthill • Edgehill • sidehill • molehill •foothill • dunghill •sigil, strigil, vigil •strongyle • Virgil • Gaitskell • orchil •roadkill • Danakil • overkill •amyl, Tamil •treadmill • windmill • gristmill •sawmill • watermill • vinyl • mini-pill •overspill • Caryl •mandrel, mandrill •Avril •beryl, Cheryl, chrysoberyl, imperil, Merrill, peril, Sheryl •tendril • April • Cyril • fibril • nombril •nostril • Bovril • tumbril • escadrille •espadrille • gracile • Cecil • utensil •codicil • windowsill •dactyl, pterodactyl •pastille • standstill •dentil, lentil, ventil •quintile • pistil • postil • tormentil •ethyl

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Plastics

Plastics


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 polymersusually animal proteins (horn and tortoise shell), tree resins, or insect secretions called shellacthat 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.

HERMAN MARK (18951992)

The influence of Herman Mark, the so-called father of polymer science, on the plastics industry still echoes today in a legacy of education and research. His work in the 1920s on the structure of cellulose opened the door for the development of synthetic fibers such as acrylic, nylon, polyester, polystyrene, and PVC.

Valerie Borek

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 ZieglerNatta 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.

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Plastics

Plastics


The term plastic refers to any material that can be shaped or molded. In this sense, ordinary clay or a soft wax is a plastic material. Perhaps more commonly, plastic has become the term used to describe a class of synthetic materials more accurately known in chemistry as polymers. Some common examples of plastics are the polyethylenes, polystyrenes, vinyl polymers, methyl methacrylates, and polyesters. These synthetic materials may or may not be "plastic" in the pliable sense.

Research on plastic-like materials began in the mid-nineteenth century. At first, this research made use of natural materials. Credit for discovery of the first synthetic plastic is often given to the American inventor, John Wesley Hyatt. In 1869, Hyatt was awarded a patent for the manufacture of a hard, tough material made out a natural cellulose. He called the product "celluloid."

It was not until 1907, however, that an entirely synthetic plastic was invented. In that year, the Belgian-American chemist Leo Baekeland discovered a new compound that was hard, water- and solvent-resistant and electrically non-conductive. He named the product Bakelite. Almost immediately, the new material was put to use in the manufacture of buttons, radio cases, telephone equipment, knife handles, counter tops, cameras, and dozens of other products.

Today, thousands of different plastics are known. Their importance is illustrated by the fact that of the 50 chemicals produced in the greatest volume in the United States, 24 are used in the production of polymers. Products that were unknown until the 1920s are now manufactured by the millions of tons each year in the United States.

Despite the bewildering variety of plastics now available, most can be classified in one of a small number of ways. First, all plastics can be categorized as thermoplastic or thermosetting. A thermoplastic polymer is one that, after being formed, can be re-heated and re-shaped. If you warm the handle of a toothbrush in a flame, for example, you can bend it into another shape. A thermosetting polymer is different, however, since, once formed, it can be re-heated, but not re-shaped.

Polymers can also be classified according to the chemical reaction by which they are formed. Addition polymers are formed when one kind of molecule reacts with a second molecule of the same kind. This type of reaction can occur only when the molecules involved contain a special grouping of atoms containing double or triple bonds.

As an example one molecule of ethylene can react with a second molecule of ethylene. This reaction can continue, with a third ethylene molecule adding to the product. In fact, this reaction can be repeated many times until a very large molecule, polyethylene, results.

The prefix poly -means "many" indicating that many molecules of ethylene were used in its production. In the formation of a polymer like polyethylene, "many" can be equal to a few hundred or few thousand ethylene molecules. The basic unit of which the polymer is made (ethylene, in this case) is called the monomer. The process by which monomers combine with each other many times is known as polymerization.

A second type of polymer, the condensation polymer, is formed when two different molecules combine with each other through the loss of some small molecule, most commonly, water. For example, Baekeland's original Bakelite is made by the reaction between a molecule of phenol and a molecule of formaldehyde. Phenol and formaldehyde fragments condense to make a new molecule (C6H5CHO) when water is removed. As with the formation of polyethylene, this reaction can repeat hundreds or thousands of times to make a very large molecule.

The names of polymers often reveal how they are made. For example, polyethylene is made by the polymerization of ethylene, polypropylene by the polymerization of propylene, and polyvinyl chloride by the polymerization of vinyl chloride . The names of other polymers give no hint as to the way they are formed. One would not guess, for example, that nylon is a condensation polymer of adipic acid and hexamethylene diamine or that Teflon is an addition polymer of tetrafluorethylene.

Listing all the uses to which plastics have been put is probably impossible. Such a list would include squeeze bottles, electrical insulation, film, indoor-outdoor carpeting, floor tile, garden hoses, pipes for plumbing, trash bags, fabrics, latex paints, adhesives, contact lenses, boat hulls, gaskets, non-stick pan coatings, insulation, dinnerware, and table tops.

Plastics technology has become highly sophisticated in the last few decades. Rarely is a simple polymer used by itself in a product. Instead, all types of additives are available for giving the polymer special properties. Ultraviolet stabilizers, as one example, are added to absorb ultraviolet light that would otherwise attack the polymer itself. Many polymers become stiff and brittle when exposed to ultraviolet light.

Plasticizers are compounds that make a polymer more flexible. Foaming agents are used to convert a polymer to the kind of foam used in insulation or cooler chests. Fillers are materials like clay, alumina, or carbon black that add properties such as color, flame resistance , hardness, or chemical resistance to a plastic.

Some of the most widely used additives are reinforcing agents. These are fibers made of carbon, boron, glass, or some other material that add strength to a plastic. Reinforced plastics find application in car bodies and boat hulls and in many kinds of sporting equipment such as football helmets, tennis rackets, and bicycle frames.

One of the most interesting new variations in polymer properties is the development of conducting polymers. Until 1970, the word plastic was nearly synonymous with electrical non-conductance. In fact, one important use of plastics has been as electrical insulation. In 1970, however, a Korean university student accidentally produced a form of polyacetylene that conducts electricity.

For a number of reasons, that discovery did not lead to a commercial product until 1975. Then, two researchers at the University of Pennsylvania discovered that adding a small amount of iodine to polyacetylene increases its conductivity a trillion times.

A number of technical problems remain, but the day of plastic batteries is no longer a part of the distant future. Indeed, scientists are now studying dozens of ways in which conduction plastics can be substituted for metals in a variety of applications.

For all their many advantages, plastics have long posed some difficult problems for the environment . Perhaps the most serious of those problems, is their stability . Since plastics have not occurred in nature for very long, microorganisms that can degrade them have not yet had an opportunity to evolve. Thus, plastic objects that are discarded may tend to remain in the environment for hundreds or thousands of years.

Sometimes, this problem translates into one of sheer volume. In 1988, for example 19.9 percent by volume of all municipal solid wastes consisted of plastic products. That translated into 14.4 million tons of waste products, third only to paper products and yard and food wastes. At other times, the stability of plastics is actually life-threatening to various organisms. Stories of aquatic birds who are strangled by plastic beer can holders are no longer news because they occur so often.

The presence of plastics can interfere with some of the methods suggested for dealing with solid wastes. For example, incineration has been recommended as a way of getting rid of wastes and producing energy at the same time. But the combustion of some types of plastics results in the release of toxic hydrochloric acid, hydrogen cyanide, and other hazardous gases.

Scientists are now moving forward in the search for ways of dealing with waste plastic materials. Some success has been achieved, for example in the development of photodegradable plastics, polymers that degrade when exposed to light. One problem with such materials is that they are often buried in landfills and are never exposed to sunlight.

Research also continues to progress on the recycling of plastics. A major problem here is that some kinds of polymers are more easily recycled than others, and separating one type from the other is often difficult to accomplish in everyday practice.

The use of plastics is also interconnected with the world's energy problems. In the first place, the manufacture of most plastics is energy intensive. It takes only 24 million Btu to make one ton of steel, but 49 million Btu to make one ton of polyvinyl chloride and 106 million Btu to make one ton of low-density polyethylene.

Perhaps even more important is the fact that petroleum provides the raw materials from which the great majority of plastics are made. Thus, as our supplies of petroleum dwindle, as they inevitable will, scientists will have to find new ways to produce plastics. While they will probably be able to meet that challenge, the change-over from an industry based on petroleum to one based on other raw materials is likely to be long, expensive, and disruptive.

[David E. Newton ]


RESOURCES

BOOKS


Denison, R. A., and J. Wirka. Degradable Plastics: The Wrong Answer to the Right Question. New York: Environmental Defense Fund, 1989.

Joesten, M. D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Selinger, B. Chemistry in the Marketplace. 4th ed. Sydney: Harcourt Brace Jovanovich, 1989.

Williams, A. L., H. D. Embree, and H. J. DeBey. Introduction to Chemistry. 3rd ed. Reading, MA: Addison-Wesley, 1981.

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Plastics

PLASTICS

PLASTICS. Perhaps the most prevalent manufactured material in society today is plastics. About 200 billion pounds of plastics are produced annually in the world, 90 billion pounds in the United States alone. In the 1967 movie The Graduate, the title character, played by Dustin Hoffman, was offered one word of advice for future success: "plastics." It is difficult to imagine society without plastics. Plastics come in innumerable forms, types, and items. They can take the form of adhesives, casting resins, coating compounds, laminates, or molded plastics. They are formed through extrusion, injection, compression, blowing, transfer (fusing), or by a vacuum. There are thermoplastics of nylon, polyester, polyethylene, polypropylene, polyvinyl chloride, polystyrene, and many other substances. There are also thermoset plastics, made of phenols, urea-formaldehydes, melamines, or epoxies. A single object may involve many different types of plastics. For example, the plastics in a car include phenolic and glass (fiberglass), acetal, nylon, polypropylene, fluorocarbon, polyethylene, acrylic, butyrate, and melamine. Plastic can be a natural substance or a synthetic one. In other words, "plastics" can mean any number of different substances and products.

History

Resin is the key to plastics. Until the mid-nineteenth century, societies used natural plastic materials such as amber, sealing wax, shellac, or animal horns. These materials could be softened and molded. When cooled, they retained the new shape. Sealing wax was used to close documents with a personal mark. Items made from animal horns included buttons, cups, hornbooks, and lantern windows. Shellac (a gutta-percha molded plastic) was often

used for lamination and for phonograph records (until vinyl was introduced).

In the mid-nineteenth century, the organic chemical industry began, which led to a study of the chemical makeup of materials and many man-made products. Early plastics were created from cellulose wood fibers treated with nitrate. A German, Christian Friedrich Schönbein, was one of the first to develop cellulose nitrate plastics in 1846. Later, in England, Alexander Parkes developed Parkesine, a pressure-molded collodoin (cellulose nitrate in ethanol). He displayed many Parkesine objects at the 1862 London International Exhibition. However, as happened with so many inventions from Europe, it was the Americans who developed them as commercial successes. John Wesley Hyatt and his brother created the Celluloid Manufacturing Company in Newark, New Jersey, in 1872; this company became the renowned Celanese Corporation of America, renamed CelaneseAG in 1999. Hyatt used camphor as a plasticizer with cellulose, which proved safer and, therefore, more commercially viable. Camphor is still used as a natural plasticizer. Hyatt also introduced injection molding, extrusion molding (forcing molten plastics through an opening), and blow molding (like glass blowing). His work in celluloid made possible motion picture film for Thomas Edison, photographic film for George Eastman, and other products such as collars, eyeglass frames, and side curtains for automobiles. The great disadvantage of celluloid nitrate was its flammability. However, by World War I (1914–1918) the Tennessee Eastman Corporation had developed cellulose fibers mixed with acetate, which proved much less flammable and was used widely on airplane wings.

Leo Baekeland, a Belgian who came to the United States in 1889, developed the first commercial synthetic resin and the first thermoset resin in the early 1900s. He created a substance from phenolics (found in coal tar) and formaldehyde to impregnate fibrous sheets. His new synthetic was called Bakelite, which became the foremost name in plastics.

The work of Hermann Staudinger in Zurich in the 1920s was critical in explaining how the plastic molecules, polymers, were created. Once his work was accepted in the 1930s, the plastics industry developed rapidly with diversified products for commercial uses. In the 1930s the new plastics materials included urea resins, acrylics, and polyethylene in 1931; vinyl resins in 1933; melamine, fiberglass, and styrene in 1937; Teflon and epoxy in 1938; and nylon in 1939. After World War II (1939–1945), society entered the "Plastics Age."

What Are Plastics?

Plastics are inexpensive substances that are soft and malleable during manufacturing and are fabricated into lightweight, tough, rigid or flexible, clear or opaque, corrosive-resistant objects. There are some inorganic substances that conform to this definition—concrete, mortar, and plaster of Paris for example. However, as we think of them, plastics are organic substances made up of huge molecules called polymers. The organic material generally used is coal, oil, natural gas, or wood. Plastics have a high molecular weight; for instance, the molecular weight of oxygen is 32, and that of a polymer is between 10,000 and 500,000. Chemicals are used to distill and modify the organic substance. Chemicals found in plastics include carbon, hydrogen, oxygen, and nitrogen. Chlorine, fluorine, sulfur, or silicon may also be present. To make the polymers more flexible or tougher, a plasticizer is added. There are many different plasticizers, and it is important to use the right one in the right amount for the particular substance or object desired. If the wrong plasticizer is used, the polymer loses its plasticity in a short time. In the early days of the plastics industry, this happened often with raincoats, handbags, curtains, and other objects, which soon became brittle and cracked.

There are two types of plastics—thermoplastics and thermoset plastics. Thermoplastics are formed from long linear chains of molecules (polymers). These polymers can be softened and when cooled regain a solid state. These plastics can be first formed as sheets, pellets, films, tubes, rods, or fibers. These forms can then be reheated and molded into other shapes. For example, nylon thread can be made into fabric. The various chemical and molecular properties of thermoplastics determine whether they are called nylon, polyester, polypropylene, polystyrene, polyethylene, polyvinyl chloride (PVC), or other names.

Thermoset plastics are different. These polymers are formed from two directions and produce three-dimensional networks of molecules, not linear chains. Such substances cannot be remelted. They are formed through compression molding or casting. Thermoset plastics include phenolic laminates (the original Bakelite), urethane, melamine, epoxy, acrylic, silicone, fluorocarbons, and others.

Uses of Plastics

Plastics are prolific and have many advantages over other heavier, easily corroded, breakable, or more expensive materials. A home provides a good example of the ubiquity and versatility of plastics. The house may use vinyl concrete, vinyl siding, vinyl window frames, vinyl wallpaper, and vinyl venetian blinds. These are long lasting and require little upkeep. The wiring in the house could be polyethylene with epoxy coating. The insulation may be silicone or polystyrene. The house will also have polyvinyl chloride pipes. The outdoor furniture is likely to be molded PVC. Windows may be acrylic and so, too, the sofa. Seat cushions and pillows will likely be made with urea-formaldehyde foam; the carpets, nylon. The tables and cabinets may be polyurethane. Dishes may be melamine, which is easily dyed, durable, and very scratch resistant. The family car is also likely to be melamine coated. Pots and pans often use Teflon, a fluorocarbon invented in 1943. Serving dishes may be the acrylic Lucite,

and small windows may be of another acrylic, Plexiglas. Clothing may also be of plastics, including nylon stockings and nylon underwear. In the late 1960s, clothing often was all polyester; today, polyester fibers are often mixed with natural fibers such as wool or cotton for a more natural look. The home's air ducts are also likely to be polyester, and if there is a boat, it is most likely fiberglass, made from polyester and glass fiber mix. The glass fibers reinforce the plastics and allow for repairs. Foods in a home, especially meats, are packaged in Styrofoam, made from polystyrene, as are some carry-out containers. Polystyrenes are thermoplastics that are easily molded, rigid, and good insulators.


The five most prevalent plastics are all thermoplastics and account for 90 percent of the plastics of the early twenty-first century. These include polyethylene, used in all types of bags, diaper liners, agricultural covers, and milk and juice jugs; polyethylene terephthalate (PET), used principally for soda bottles and videotapes; polystyrene, used as clear packaging, as a foam (Styrofoam), or for furniture, toys, utensils, and dishes; polypropylene, used for battery cases, crates, film, molded car parts, appliances, fish nets, and wire coating; and polyvinyl chloride, used as a flexible substance in film, hoses, rainwear, and wall coverings, or as a rigid substance in pipes, buildings, and credit cards. The most prevalent thermoset plastics are phenolics, used with formaldehyde and fillers in plywood, fiberglass, and circuit boards; and urea resins, used in polyurethane foam fillers.

The uses of plastics are always expanding and new polymers are being created. One example of thermoset plastics whose uses are expanding is silicone. It is an oxygen-based, and not the usual carbon-based, substance. Because it is highly resistant to ozone, chemicals, sunlight, and aging, it has a wide variety of uses, such as polishes, insulation, waterproofing, adhesives, and implants. Two very versatile thermoplastics are polyethylene and polycarbonate. Polyethylene is used for toys, electronic devices, wires, and milk carton coatings. Polyethylene is also now used widely in medical procedures, for example, to replace aortas or as prosthetic devices. Polycarbonates are fairly new polymers that are formed from bonding oxygen and silicon. Polycarbonates are easy to use yet highly rigid and very corrosive resistant. They have replaced phenol laminates in spacecraft, automobiles, and ships.

Disadvantages of Plastics

Though plastics are ubiquitous and versatile, they also have several disadvantages. The original plastic, cellulose nitrate, was highly flammable; celluloid acetate lessened that danger. Later plastics have included flame retardants, which delay the outbreak of flames but not the decomposition before reaching flammable temperatures. Because of the flame retardants, plastics produce thick, dense smoke that is acrid from the chemicals, especially carbon monoxide. In some of the most disastrous fires, more people suffocated from the plastics smoke and soot than died from the flames. Also, once plastic does flame, it burns faster and hotter than natural substances.

Decomposition is another issue. Because plastics are made from long chains of molecules that receive high heat to set or mold them, decomposition can emerge as weaknesses in the chain. When thermoplastics are remolded, weaknesses can increase. Some plastics also decompose more rapidly than others, especially the less expensive plastics such as PVC and urethane foam. Some critics claim that the phthalate plasticizers used in PVC create low-level toxicity. The urethane foam cushions begin to break down fairly quickly, leaving bits of foam and dust. Leaving plastics exposed to sunlight and heat also causes decomposition and cracking. As plastics decompose, they release chemicals such as carbon monoxide, chlorine, and benzene into the air. For example, the "office worker's illness" is caused by decomposing polymers of the air ducts, furniture, and equipment, and too little fresh air.

Future

Another problem with plastics is waste disposal. In the United States alone, some 60 billion pounds of plastics

are discarded annually and over 90 percent of the waste is not yet recycled. Thermoset plastics cannot be reused; neither can some thermoplastics because of impurities (including disposable diapers, food packaging, and trash bags). Nevertheless, in the United States and Europe plastics recycling has become a major industry, tripling in the United States since 1990. Recycled bottles alone have grown from 411 million pounds in 1990 to 1,511 million pounds in 2000. There are over 1,400 products made from recycled plastics, most of the same items as new synthetic plastics—furniture, packaging, household items—but also new items such as lumber and posts.

Composting is a principal method of recycling plastics. Synthetic plastics may decompose through photo-degradation, oxidation, or hydrolysis—naturally or chemically. Success in composting depends on the environment and the chemicals used in the plastics. Some, such as the polyolefins, are hydrophobic (water-resistant) and thus highly resistant to biodegradation.

The newest research and development in plastics is in bioplastics, biodegradable plastics whose components are principally derived from renewable raw materials. This often means a return to many of the natural polymers used in the nineteenth century, with late-twentieth or twenty-first-century technology added. In 1941 Henry Ford produced a prototype Ford made of soybean plastics. Due to war needs and the rise of synthetic plastics, the work was abandoned, but such innovation is typical of today's research and development. Bioplastics are already used in a wide variety of products including all types of bags, packaging, fishnet and lines, pet toys, wall coverings, razors, and golf tees.

Starch is a prolific raw material that makes a good plastic. It is now used in many fast-food containers and for the "peanuts" used in shipping. The water solubility of starch is both an advantage for decomposition and a limitation, which technology may overcome. For example, some eating utensils are now made of 55 percent cornstarch and 45 percent poly(lactic acid), which is insoluble in water but biodegradable in seawater. Poly(lactic acid) is a polyester synthesized from lactic acid. It shows solid commercial production growth and is used, for example, in compost bags, agricultural films, fibers, and bone repair. Cellulose is another bioplastic from the past. It is contained in 40 percent of organic matter and thus is renewable. Its limitation is that it is not thermoplastic, though it can be made into films.

Though bioplastics have limitations such as tensile strength, solubility, and cost, they produce less toxicity to humans and the environment and are based on renewable resources. Improved technology may overcome the limitations.

BIBLIOGRAPHY

Hooper, Rodney. Plastics for the Home Craftsman. London: Evans Brothers, 1953.

Simonds, Herbert R., and James M. Church. A Concise Guide to Plastics. New York: Reinhold Publishing Company, 1963.

Stevens, E. S. Green Plastics: An Introduction to the New Science of Biodegradable Plastics. Princeton, N.J.: Princeton University Press, 2002.

Wallace, Deborah. In the Mouth of the Dragon. Garden City Park, N.Y.: Avery Publishing Group, 1990.

Diane NagelPalmer

See alsoBuilding Materials ; Carpet Manufacture ; Industrial Research ; Manufacturing ; Textiles .

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Plastics

Plastics

History

Chemistry

Molecular Weight

Polymerization

Manufacture And Processing

Thermoplastics

Crystalline And Noncrystalline Thermoplastics

Thermosets

Manufacturing Methods

Fillers And Other Modifications

Applications

Resources

In the twentieth century, the term plastic has come to refer to a class of materials that, under suitable conditions, can be deformed by some kind of shaping or molding process to produce an end product that retains its shape. When used as an adjective, the term plastic (from Greek plastikos meaning to mold or form) describes a material that can be shaped or molded with or without the application of heat. With few exceptions, plastics do not flow freely like liquids, but retain their shapes like solids even when flowing.

When used in a chemical sense, the term plastic usually refers to a synthetic high molecular weight chain molecule, or polymer, that may have been combined with other ingredients to modify its physical properties. Most plastics are based on carbon, being derived from materials that have some relationship to living, or organic, materials, although, although some plastics, like acetal resins and silicones, contain oxygen or silicon atoms in their chains.

As plastics are heated to moderate temperatures, the polymer chains are able to flow past each other. Because of the organic nature of most plastics, they usually cannot withstand high temperatures and begin to decompose at temperatures around 392°F (200°C).

The oldest known examples of plastic materials are soft waxes, asphalts, and moist clays. These materials are capable of flowing like synthetic plastics, but because they are not polymeric, they are usually not referred to as plastics.

History

The history of synthetic plastics goes back over 100 years to the use of cellulose nitrate (celluloid) for billiard balls, mens collars, and shirt cuffs. Before plastics were commercialized, most household goods and industrial products were made of metals, wood, glass, paper, leather, and vulcanized (sulfurized) natural rubber.

The first truly synthetic polymer was Bakelite, a densely cross-linked material based on the reaction of phenol and formaldehyde. It has been used for many applications, including electrical appliances and phonograph records. Among the first plastics developed that could be reformed under heat (thermoplastics) were polyvinyl chloride, polystyrene, and nylon 66.

The first polymers used by man were actually natural products such as cotton, starch, proteins, or wool. Certain proteins that are in fact natural polymers once had commercial importance as industrial plastics, but they have played a diminishing role in the field of plastics production in recent years.

Chemistry

There are more than 100 different chemical atoms, known as elements. They are represented by the chemist by the use of simple symbols such as H for hydrogen, O for oxygen, C for carbon, N for nitrogen, Cl for chlorine, and so on; these atoms have atomic weights of 1, 16, 12, 14, and 17 atomic units, respectively.

A chemical reaction between two or more atoms forms a molecule. Each molecule is characterized by its elemental constitution and its molecular weight. For example, when carbon is burned in oxygen, one atom of carbon (C) reacts with two atoms of oxygen (O2; equivalent to one molecule of molecular oxygen) to form carbon dioxide (CO2). The chemist represents this reaction by a chemical equation, i.e.,

C + O2 = CO2

Similarly, when four atoms of hydrogen (2H2; equivalent to two molecules of molecular hydrogen) and two atoms of oxygen (O2; equivalent to one molecule of oxygen) react to form two molecules of water (2H2O), the chemist writes

2H2 +O2 = 2H2O

Note that one molecule of oxygen combines with two molecules of hydrogen, and one atom of carbon combines with one molecule of hydrogen. This is because different elements have different combining capacities. Thus hydrogen forms one bond, oxygen two bonds, and carbon four bonds. These bonding capacities, or valences, are taken for granted when writing a chemical formula like H2O.

In the case of methane, or CH4, the carbon is bonded to four hydrogen atoms. But carbon can also form double bonds, as in ethylene (C2H4) where two CH2molecules share a double bond. The chemist could also describe the ethylene molecule by the formula CH2 = CH2, where the double bond is represented by an equal sign.

Plastic materials consist of many repeating groups of atoms or molecules (called monomers) in long chains, and hence are also known as polymers or macromolecules. Elements present in a polymer chain typically include oxygen, hydrogen, nitrogen, carbon, silicon, fluorine, chlorine, or sulfur. The way the polymer chains are linked together and the lengths of the chains determine the mechanical and physical properties of the plastic.

Molecular Weight

Polymers exist on a continuum that extends from simple gases to molecules of very high molecular weights. A relatively simple polymer has the structure

H - (CH2)n-H

where the number (n) of monomers (CH2 groups, in this case) in the chain may extend up to several thousand. Table 1 shows how the physical properties and uses of the polymer change with the number of repeating monomer units in the chain.

Polymerization

Most commercial plastics are synthesized from simpler molecules, or monomers. The simple chemicals from which monomers, and ultimately polymers, are derived are usually obtained from crude oil or natural gas, but may also come from coal, sand, salt, or air.

For example, the molecules used to form polystyrene, a widely used plastic, are benzene and ethylene. These two molecules are reacted to form ethyl benzene, which is further reacted to give a styrene monomer. With the aid of a catalyst, styrene monomers may form a chain of linked, bonded styrene units. This method of constructing a polymer molecule is known as addition polymerization, and characterizes the way most plastics-including polystyrenes, acrylics, vinyls, fluoroplastics-are formed.

When two different molecules are combined to form a chain in such a way that a small molecule such as water is produced as a by-product, the method of building the molecule is known as condensation polymerization. This type of polymerization characterizes a second class of plastics. Nylons are examples of condensation polymers.

Manufacture And Processing

When polymers are produced, they are shipped in pelletized, granulated, powdered, or liquid form to plastics processors. When the polymer is still in its raw material form, it is referred to as a resin. This term antedates the understanding of the chemistry of polymer molecules and originally referred to the resemblance of polymer liquids to the pitch on trees.

Plastics can be formed or molded under pressure and heat, and many can be machined to high degrees of tolerance in their hardened states. Thermoplastics are plastics that can be heated and reshaped; thermo-sets are plastics that cannot.

Table 1. Change in Molecular Properties with Molecular Chain(Thomson Gale)
Change in molecular properties with molecular chain length
Number of CH2 units in chainAppearance at room temperatureUses
1 to 4simple gascooking gas
5 to 11simple liquidgasoline
9 to 16medium viscosity liquidkerosene
16 to 25high viscosity liquidoil and grease
25 to 50simple solidparaffin wax candles
1,000 to 3,000tough plastic solidpolyethylene bottle and containers

Thermoplastics

Thermoplastics are plastics that become soft and malleable when heated, and then become hard and solid again when cooled. Examples of thermoplastics include acetal, acrylic, cellulose acetate, nylon, polyethylene, polystyrene, vinyl, and nylon. When thermoplastic materials are heated, the molecular chains are able to move past one another, allowing the mass to flow into new shapes. Cooling prevents further flow. Thermo-plastic elastomers are flexible plastics that can be stretched up to twice their length at room temperature and then return to their original length when released.

The state of a thermoplastic depends on the temperature and the time allowed to measure its physical properties. At low enough temperatures, amorphous, or noncrystalline, thermoplastics are stiff and glassy. This is the glassy state, sometimes referred to as the vitreous state. On warming up, thermoplastics soften in a characteristic temperature range known as the glass transition temperature region. In the case of amorphous thermoplastics, the glass transition temperature is the single-most important factor determining the physical properties of the plastic.

Crystalline And Noncrystalline Thermoplastics

Thermoplastics may be classified by the structure of the polymer chains that comprise them.

In the liquid state, polymer molecules undergo entanglements that prevent them from forming regularly arranged domains. This state of disorder is preserved in the amorphous state. Thus, amorphous plastics, which include polycarbonate, polystyrene,

Thermoplastics
TypeChemical basisUses
ABS plasticsDerived from acrylonitrile, butadiene, and styreneElecroplated plastic parts; automotive components; business and telecommunication applications such as personal computers, terminals, keyboards, and floppy disks; medical disposables; toys; recreational applications; cosmetics packaging; luggage; housewares
AcetalsConsist of repeating CH2 O units in a polymer backboneRollers, bearings and other industrial products; also used in automotive, appliance, plumbing and electronics applications
AcrylicsBased on polymethyl methacrylateAutomobile lenses, fluorescent street lights, outdoor signs, and boat windshields; applications requiring high resistance to discoloration and good light transmission properties
CellulosicsDerived from purified cotton or special gradesInsulation, packaging, toothbrushes
FluoroplasticsConsist of carbon, fluorine, and or hydrogen atoms in a repeating polymer backboneApplications requiring optimal electrical and thermal properties, almost complete moisture resistance, chemical inertness; non-stick applications
NylonsDerived from the reaction of diamines and dibasic acids; characterized by the number of carbon atoms in the repeating polymeric unitElectrical and electronic components; industrial applications requiring excellent resistance to repeated impact; consumer products such as ski boots and bicycle wheels; appliances and power tool housings; food packaging; wire and cable jacketing; sheets,rods, and tubes; and filaments for brush bristles, fishing line, and sewing thread
PolyarylatesAromatic polyestersAutomotive appliance, and electrical applications requiring low shrinkage, resistance to hydrolysis, and precision void-free molding
PolyarylsulfonesConsist of phenyl and biphenyl groups linked by thermally stable ether and sulfone groupsElectrical and electronic applications requiring thermal stability including circuit boards, connectors, lamp housings, and motor parts
PolybutylensesPolymers based on poly(1butene)Cold- and hot-water pipes; hotmetal adhesives and sealants
Polybutylene terephthalate (PBT)Produced by reaction of dimethyl terephthalate with butanediolAutomotive applications such as exterior auto parts; electronic switches; and household applications such as parts for vacuum cleaners and coffee makers
PolycarbonatesDerived from the reaction of bisphenol A and phosgeneApplications requiring toughness, rigidity, and dimensional stability; high heat resistance; good electrical properties; transparency; exceptional impact strength. Used for molded products, solutioncast or extruded films, tubes and pipes, prosthetic devices, nonbreakable windows, street lights, household applicances; compact discs; optical memory disks; and for various applications in fields related to transportation, electronics sporting goods, medical equipment, and food processing
PolyestersProduced by reacting dicarboxylic acids with dihydroxy alcoholsReinforced plastics, automotive parts, foams, electrical encapsulation, structural applications, lowpressure laminates, magnetic tapes, pipes, bottles. Liquid crystal polyesters are used as replacements for metals in such applications chemical pumps, electronic components, medical components, and automotive components
PolyetherimidesConsist of repeating aromatic imide and ether unitsTemperture sensors; electrical/electronic, medical (surgical instrument parts), industrial; applicance, packaging, and specialty applications
PolyetherketonesPolymerized aromatic ketonesFine monofilaments, films, engine parts, aerospace composites, and wire and cables, and other applications requiring chmical resistance; exceptional toughness, strength, and rigidity; good radiation resistance; and good fire-safety characteristics
PolyethersulfonesConsist of diaryl sulfone groups with ether linkagesElectrical applications including multipin connectors, integrated circuit sockets, edge and round multipin connectors, terminal blocks, printed circuit boards
Polyethylenes, polypropylenes, and polyallomersPolyethylenes consist of chains of repeated ethylene units; polypropylenes consist of chains of repeated propylene units; polyallomers are copolymers of propylene and ethyleneLow density polyethylene is used for packaging films, liners for shipping containers, wire and cable coatings, toys plastic bags, electrical insulation. High density polyethylene is used for blow-molded items, films and sheets, containers for petroleum products. Low molecular weight polyethylenes are used as mold release agents, coatings, polishes, and textile finishing agents. Polypropylenes are used as packaging films, molded parts, bottles, artificial turf, surgical casts, nonwoven disposable filters. Polyallomers are used as vacuum formed, injection molded, and extruded products, films, sheets, and wire cables
Polyethylene terephthalatePrepared from ethylene glycol and either terephthalic acid or an ester of terephthalic acidFood packaging including bottles, microwave/conventional oven-proof trays; x-ray and other photographic films; magnetic tape
Polyimides and polyamideimidesPolyimides contain imide (CONHCO) groups in the polymer chain; polyamideimides also contain amide (CONH) groupsPolyimides are used as high temperature coatings,laminates, and composites for the aerospace industry; ablative materials; oil sealants; adhesive; semi- conductors; bearings; cable insulation; printed circuits; magnetic tapes; flame-resistant fibers. Polyamideimides have been used as replacements for metal parts in the aerospace industry, and as mechanical parts for business machines
PolymethylpentenePolymerized 4methylpentene-1Laboratory ware (beakers, graduates, etc.); electronic and hospital equipment; food packaging; light reflectors
Polyphenylene ethers, modifiedConsist of oxidatively coupled phenols and polystyreneAutomobile instrument panels, computer keyboard bases
Polyphenylene sulfidesPara-substituted benzene rings with sulful linksMicrowave oven components, precision molded assemblies for disk drives
PolystyrenesPolymerized ethylene and styrenePackaging, refrigerator doors, household wares, electrical equipment; toys, cabinets; also used as foams for thermal insulations, light construction, fillers in shipping containers, furniture construction.
PolysulfonesConsist of complicated chains of phenylene units linked with isopropylidene, ether, and suflfone unitsPower tool housings, electrical equipment, extruded pipes and sheets, automobile components, electronic parts, appliances, computer components; medical instrumentation and trays to hold instruments during sterilization; food processing equipment; chemical processing equipment, water purification devices
VinylsPolymerized vinyl monomers such as polyvinyl chloride and polyvinylidene chlorideCrystal-clear food packaging, water pipes, monolayer films

Table 2. Thermoplastics (cont_d). (Thomson Gale.)

acrylonitrile-butadiene-styrene (ABS), and polyvinyl chloride, are made up of polymer chains that form randomly organized structures.

These polymer chains may themselves have attached side chains, and the side chains may also be quite long. When the side chains are particularly bulky, molecular branching prevents the molecules from forming ordered regions, and an amorphous plastic will almost certainly result.

Under suitable conditions, however, the entangled polymer chains can disentangle themselves and pack into orderly crystals in the solid state where the chains are symmetrically packed together; these materials are known as crystalline polymers.

Crystalline thermoplastics consist of molecular chains packed together in regular, organized domains that are joined by regions of disordered, amorphous chains. Examples of crystalline thermoplastics include

Table 3. Thermosetting Plastics. (Thomson Gale.)
Thermosetting plastics
TypeChemical basisUses
Alkyd polyestersPolyesters derived from the reaction of acids with two acid groups, and alcohols with three alcoholic groups per moleculeMoldings, finishes; applications requiring high durability, excellent pigment dispersion, toughness, good adhesion, and good flowing properties
AllylsPolyesters derived from the reaction of esters of allyl alcohol with dibasic acidElectrical insulation, applications requiring high resistance to heat, humidity, and corrosive chemicals
BismaleimidesGenerally prepared by the reaction of a diamine with maleic anhydridePrinted wire boards; high performance structural composites
EpoxiesDerived from the reaction of epichlorohydrin with hydroxyl-containing compoundsEncapsulation, electrical insulations, laminates, glass-reinforced plastics, floorings, coatings, adhesives
MelaminesDerived from the reaction of formaldehyde and amino compounds containing NH2Molded plates, dishes, and other food containers
PhenolicsDerived from the reaction of phenols and formaldehydesCements, adhesives
PolybutadienesConsist of polyethylene with a cross-link at every other carbon in the main chainMoldings, laminating resins, coatings, cast-liquid and formed-sheet products; applications requiring outstanding electrical properties and thermal stability
Polyesters (thermosetting)Derived from reactions of dicarboxylic acides with dihydroxy alcoholsMoldings, laminated or reinforced structures, surface gel coatings, liquid castings, furniture products, structures
PolyurethanesDerived from reactions of polysiocyanates and polyolsRigid, semi-flexible, and flexible foams; elastomers
SiliconesConsist of alternating silicon and oxygen atoms in a polymer backbone, usually with organic side groups attached to the chainApplications requiring uniform properties over a wide temperature range; low surface tension; high degree of lubricity; excellent release properties; extreme water repellency; excellent electrical properties over a wide range of temperatures and frequency; inertness and compatibility; chemical inertness; or weather resistance
UreasDerived from the reaction of formaldehyde and amino compounds containing NH2 groupsDinnerware, interior plywood, foams, insulation

acetals, nylons, polyethylenes, polypropylenes, and polyesters.

Liquid crystalline plastics are polymers that form highly ordered, rodlike structures. They have good mechanical properties and are chemically unreactive, and they have melting temperatures comparable to those of crystalline plastics. But unlike crystalline and amorphous plastics, liquid crystalline plastics retain molecular ordering even as liquids. Consequently, they exhibit the lowest shrinkage and warpage of any of the thermoplastics.

Thermosets

Thermosetting plastics, or thermosets, include amino, epoxy, phenolic, and unsaturated polyesters. These materials undergo a chemical change during processing and become hard solids. Unlike the linear molecules in a thermoplastic, adjacent molecules in a thermosetting plastic become cross-linked during processing, resulting in the production of complex networks that restrain the movement of chains past each other at any temperature.

Typical thermosets are phenolics, urea-formaldehyde resins, epoxies, cross-linked polyesters, and most polyurethanes. Elastomers may also be thermosetting. Examples include both natural and synthetic rubbers.

Manufacturing Methods

At some stage in their processing, both thermoplastics and thermosetting plastics are sufficiently fluid to be molded and formed. The manufacture of most plastics is determined by their final shape.

Many cylindrical plastic objects are made by a process called extrusion. The extrusion of thermoplastics consists of melting and compressing plastic granules by rotating them in a screw conveyor in a long barrel, to which heat may be applied if necessary. The screw forces the plastic to the end of the barrel where it

is pushed through a screen on its way to the nozzle. The nozzle determines the final shape of the extruded form. Thermosets may also be extruded if the screw in the conventional extruder is replaced with a plunger-type hydraulic pump.

Plastic powders are directly converted into finished articles by molding. Two types of molding processes are compression molding and injection molding. In compression molding, which is used with thermosetting materials, steam is first circulated through the mold to raise it to the desired temperature; then a plastic powder or tablets are introduced into the mold; and the mold is closed under high pressure and the plastic is liquefied so that it flows throughout the mold. When the mold is re-opened, the solid molded unit is ejected. Injection molding differs from compression molding in that plastic material is rendered fluid outside the mold, and is transferred by pressure into the cooled mold. Injection molding can be used with practically every plastic material, including rubbers.

Sheets, blocks, and rods may be made in a casting process that in effect involves in situ, or in-place, polymerization. In the case of acrylics, sheets are cast in glass cells by filling cells with a polymer solution. The polymer solution solidifies and the sheet is released by separating the glass plates after chilling the assembly in cold water. Blocks can be made in the same way using a demountable container; and rods can be made by polymerizing a polymer syrup under pressure in a cylindrical metal tube.

Plastic foams are produced by compounding a polymer resin with a foaming agent or by injecting air or a volatile fluid into the liquid polymer while it is being processed into a finished product. This results in a finished product with a network of gas spaces or cells that makes it less dense than the solid polymer. Such foams are light and strong, and the rigid type can be machined.

Fillers And Other Modifications

Very few plastics are used in their commercially pure state. Additives currently used include the following: Finely divided rubbers added to more brittle plastics to add toughness; glass, carbon, boron, or metal fibers added to make composite materials with good stress-strain properties and high strength; carbon black or silica added to improve resistance to tearing and to improve stress-strain properties; plasticizers added to soften a plastic by lowering its glass transition temperature or reducing its degree of crystallinity; silanes or other bonding agents added to improve bonding between the plastic and other solid phases; and fillers such as fire retardants, heat or light stabilizers, lubricants, or colorants.

Filled or reinforced plastics are usually referred to as composites. However, some composites includes neither fillers nor reinforcement. Examples are laminates such as plastic sheets or films adhered to non-plastic products such as aluminum foil, cloth, paper or plywood for use in packaging and manufacturing. Plastics may also be metal plated.

Plastics, both glassy and rubbery, may be cross-linked to improve their elastic behavior and to control swelling. Polymers may also be combined to form blends or alloys.

Applications

Plastics have been important in many applications to be listed here. Table 2, Thermoplastics, and Table 3, Thermosetting Plastics, list hundreds of commercial applications that have been found for specific plastics.

Engineering plastics are tough plastics that can withstand high loads or stresses. They can be machined and remain dimensionally stable. They are typically used in the construction of machine parts and automobile components. Important examples of this class of plastics include nylons, acetals, polycarbonates, ABS resins, and polybutylene terephthalate. The structure of their giant chains makes these plastics highly resistant to shock, and gives them a characteristic toughness.

KEY TERMS

Amorphous Noncrystalline; lacking a definite crystal structure and a well-defined melting point. CastingFormation of a product either by filling an open mold with liquid monomer and allowing it to polymerize in place, or by pouring the liquid onto a flat, moving surface.

Composite A mixture or mechanical combination (on a macroscopic level) of materials that are solid in their finished state, that are mutually insoluble, and that have different chemistries.

Crystalline Having a regular arrangement of atoms or molecules; the normal state of solid matter.

Extrusion An operation in which material is forced through a metal forming die, followed by cooling or chemical hardening.

Glass An amorphous, highly viscous liquid having all of the appearances of a solid.

Inorganic Not containing compounds of carbon.

Molding Forming a plastic or rubber article in a desired shape by applying heat and pressure.

Monomer A substance composed of molecules that are capable of reacting together to form a polymer. Also known as a mer.

Organic Containing carbon atoms, when used in the conventional chemical sense. Originally, the term was used to describe materials of living origin.

Plastic Materials, usually organic, that under suitable application of heat and pressure, can be caused to flow and to assume a desired shape that is retained when the pressure and temperature conditions are withdrawn.

Polymer A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.

Synthetic Referring to a substance that either reproduces a natural product or that is a unique material not found in nature, and which is produced by means of chemical reactions.

Thermoplastic A high molecular weight polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.

Thermoset A high molecular weight polymer that solidifies irreversibly when heated.

Plastics are almost always electrically insulating, and for this reason they have found use as essential components of electrical and electronic equipment (including implants in the human body).

Major applications have been found for plastics in the aerospace, adhesives, coatings, construction, electrical, electronic, medical, packaging, textile, and automotive industries.

Resources

BOOKS

Brandrup, J. and E.H. Immergut. Polymer Handbook, 4th ed. New York: John Wiley & Sons, 2003.

Braungart, Michael, and William McDonough.Cradle to Cradle: Remaking the Way We Make Things. North Point Press, 2002.

Harper, Charles A. and Edward M. Petrie. Plastics Materials and Processes: A Concise Encyclopedia New York: John Wiley & Sons, 2003.

Sperling, L.H. Introduction to Physical Polymer Science. New York: John Wiley & Sons, 1992.

Randall Frost

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Plastics

Plastics

In the twentieth century, the term plastic has come to refer to a class of materials that, under suitable conditions, can be deformed by some kind of shaping or molding process to produce an end product that retains its shape. When used as an adjective, the term plastic (from Greek plastikos meaning to mold or form) describes a material that can be shaped or molded with or without the application of heat . With few exceptions, plastics do not flow freely like liquids, but retain their shapes like solids even when flowing.

When used in a chemical sense, the term plastic usually refers to a synthetic high molecular weight chain molecule , or polymer , that may have been combined with other ingredients to modify its physical properties. Most plastics are based on carbon , being derived from materials that have some relationship to living, or organic, materials, although, although some plastics, like acetal resins and silicones, contain oxygen or silicon atoms in their chains.

As plastics are heated to moderate temperatures, the polymer chains are able to flow past each other. Because of the organic nature of most plastics, they usually cannot withstand high temperatures and begin to decompose at temperatures around 392°F (200°C).

The oldest known examples of plastic materials are soft waxes, asphalts, and moist clays. These materials are capable of flowing like synthetic plastics, but because they are not polymeric, they are usually not referred to as plastics.


History

The history of synthetic plastics goes back over 100 years to the use of cellulose nitrate (celluloid) for billiard balls, men's collars, and shirt cuffs. Before plastics were commercialized, most household goods and industrial products were made of metals, wood , glass , paper , leather, and vulcanized (sulfurized) natural rubber.

The first truly synthetic polymer was Bakelite, a densely cross-linked material based on the reaction of phenol and formaldehyde. It has been used for many applications, including electrical appliances and phonograph records. Among the first plastics developed that could be reformed under heat (thermoplastics) were polyvinyl chloride, polystyrene, and nylon 66.

The first polymers used by man were actually natural products such as cotton , starch, proteins , or wool. Certain proteins that are in fact natural polymers once had commercial importance as industrial plastics, but they have played a diminishing role in the field of plastics production in recent years.


Chemistry

There are more than 100 different chemical atoms, known as elements. They are represented by the chemist by the use of simple symbols such as "H" for hydrogen , "O" for oxygen, "C" for carbon, "N" for nitrogen , "Cl" for chlorine , and so on; these atoms have atomic weights of 1, 16, 12, 14, and 17 atomic units, respectively.

A chemical reaction between two or more atoms forms a molecule. Each molecule is characterized by its elemental constitution and its molecular weight. For example, when carbon is burned in oxygen, one atom of carbon (C) reacts with two atoms of oxygen (O2; equivalent to one molecule of molecular oxygen) to form carbon dioxide (CO2). The chemist represents this reaction by a chemical equation, i.e.,

Similarly, when four atoms of hydrogen (2H2; equivalent to two molecules of molecular hydrogen) and two atoms of oxygen (O2; equivalent to one molecule of oxygen) react to form two molecules of water (2H2O), the chemist writes

Note that one molecule of oxygen combines with two molecules of hydrogen, and one atom of carbon combines with one molecule of hydrogen. This is because different elements have different combining capacities. Thus hydrogen forms one bond, oxygen two bonds, and carbon four bonds. These bonding capacities, or valences, are taken for granted when writing a chemical formula like H2O.

In the case of methane, or CH4, the carbon is bonded to four hydrogen atoms. But carbon can also form double bonds, as in ethylene (C2H4) where two CH2molecules share a double bond. The chemist could also describe the ethylene molecule by the formula CH2=CH2, where the double bond is represented by an equal sign.

Plastic materials consist of many repeating groups of atoms or molecules (called monomers) in long chains, and hence are also known as polymers or macromolecules. Elements present in a polymer chain typically include oxygen, hydrogen, nitrogen, carbon, silicon, fluorine,

TABLE 1. CHANGE IN MOLECULAR PROPERTIES WITH MOLECULAR CHAIN LENGTH
Number of CH 2units in chain Appearance at room temperature Uses
1 to 4 simple gas cooking gas
5 to 11 simple liquid gasoline
9 to 16 medium viscosity liquid kerosene
16 to 25 high viscosity liquid oil and grease
25 to 50 simple solid paraffin wax candles
1000 to 3000 tough plastic solid polyethylene bottle and containers


chlorine, or sulfur . The way the polymer chains are linked together and the lengths of the chains determine the mechanical and physical properties of the plastic.


Molecular weight

Polymers exist on a continuum that extends from simple gases to molecules of very high molecular weights. A relatively simple polymer has the structure


where the number (n) of monomers (CH2 groups, in this case) in the chain may extend up to several thousand. Table 1 shows how the physical properties and uses of the polymer change with the number of repeating monomer units in the chain.

Polymerization

Most commercial plastics are synthesized from simpler molecules, or monomers. The simple chemicals from which monomers, and ultimately polymers, are derived are usually obtained from crude oil or natural gas , but may also come from coal , sand , salt , or air.

For example, the molecules used to form polystyrene, a widely used plastic, are benzene and ethylene. These two molecules are reacted to form ethyl benzene, which is further reacted to give a styrene monomer. With the aid of a catalyst, styrene monomers may form a chain of linked, bonded styrene units. This method of constructing a polymer molecule is known as addition polymerization, and characterizes the way most plastics-including polystyrenes, acrylics, vinyls, fluoroplastics-are formed.

When two different molecules are combined to form a chain in such a way that a small molecule such as water is produced as a by-product, the method of building the molecule is known as condensation polymerization. This type of polymerization characterizes a second class of plastics. Nylons are examples of condensation polymers.


Manufacture and processing

When polymers are produced, they are shipped in pelletized, granulated, powdered, or liquid form to plastics processors. When the polymer is still in its raw material form, it is referred to as a resin. This term antedates the understanding of the chemistry of polymer molecules and originally referred to the resemblance of polymer liquids to the pitch on trees.

Plastics can be formed or molded under pressure and heat, and many can be machined to high degrees of tolerance in their hardened states. Thermoplastics are plastics that can be heated and reshaped; thermosets are plastics that cannot.



Thermoplastics

Thermoplastics are plastics that become soft and malleable when heated, and then become hard and solid again when cooled. Examples of thermoplastics include acetal, acrylic, cellulose acetate, nylon, polyethylene, polystyrene, vinyl, and nylon. When thermoplastic materials are heated, the molecular chains are able to move past one another, allowing the mass to flow into new shapes. Cooling prevents further flow. Thermoplastic elastomers are flexible plastics that can be stretched up to twice their length at room temperature and then return to their original length when released.

The state of a thermoplastic depends on the temperature and the time allowed to measure its physical properties. At low enough temperatures, amorphous, or noncrystalline, thermoplastics are stiff and glassy. This is the glassy state, sometimes referred to as the vitreous state. On warming up, thermoplastics soften in a characteristic temperature range known as the glass transition temperature region. In the case of amorphous thermoplastics, the glass transition temperature is the single-most important factor determining the physical properties of the plastic.


Crystalline and noncrystalline thermoplastics

Thermoplastics may be classified by the structure of the polymer chains that comprise them.

In the liquid state, polymer molecules undergo entanglements that prevent them from forming regularly arranged domains. This state of disorder is preserved in the amorphous state. Thus, amorphous plastics, which include polycarbonate, polystyrene, acrylonitrile-butadiene-styrene (ABS), and polyvinyl chloride, are made up of polymer chains that form randomly organized structures.

These polymer chains may themselves have attached side chains, and the side chains may also be quite long. When the side chains are particularly bulky, molecular branching prevents the molecules from forming ordered regions, and an amorphous plastic will almost certainly result.

Under suitable conditions, however, the entangled polymer chains can disentangle themselves and pack into orderly crystals in the solid state where the chains are symmetrically packed together; these materials are known as crystalline polymers.

TABLE 2. THERMOPLASTICS
Type Chemical basis Uses
ABS plastics Derived from acrylonitrile, butadiene, and styrene Electroplated plastic parts; automotive components; business and telecommunication applications such as personal computers, terminals, keyboards, and floppy disks; medical disposables; toys; recreational applications; cosmetics packaging; luggage; housewares
Acetals Consist of repeating -CH2-O-units in a polymer backbone Rollers, bearings and other industrial products; also used in automotive, appliance, plumbing and electronics applications
Acrylics Based on polymethyl methacrylate Automobile lenses, fluorescent street lights, outdoor signs, and boat windshields; applications requiring high resistance to discoloration and good light transmission properties
Cellulosics Derived from purified cotton or special grades of wood cellulose Insulation, packaging, toothbrushes
Fluoroplastics Consist of carbon, fluorine, and or hydrogen atoms in a repeating polymer backbone Applications requiring optimal electrical and thermal properties, almost complete moisture resistance, chemical inertness; non-stick applications
Nylons Derived from the reaction of diamines and dibasic acids; characterized by the number of carbon atoms in the repeating polymeric unit Electrical and electronic components; industrial applications requiring excellent resistance to repeated impact; consumer products such as ski boots and bicycle wheels; appliances and power tool housings; food packaging; wire and cable jacketing; sheets, rods, and tubes; and filaments for brush bristles, fishing line, and sewing thread
Polyarylates Aromatic polyesters Automotive appliance, and electrical applications requiring low shrinkage, resistance to hydrolysis, and precision void-free molding



Crystalline thermoplastics consist of molecular chains packed together in regular, organized domains that are joined by regions of disordered, amorphous chains. Examples of crystalline thermoplastics include acetals, nylons, polyethylenes, polypropylenes, and polyesters.

TABLE 2. THERMOPLASTICS (cont'd)
Type Chemical basis Uses
Polyarylsulfones Consist of phenyl and biphenyl groups linked by thermally stable ether and sulfone groups Electrical and electronic applications requiring thermal stability including circuit boards, connectors, lamp housings, and motor parts
Polybutylenes Polymers based on poly(1-butene) Cold- and hot-water pipes; hot-metal adhesives and sealants
Polybutylene terephthalate (PBT) Produced by reaction of dimethyl terephthalate with butanediol Automotive applications such as exterior auto parts; electronic switches; and household applications such as parts for vacuum cleaners and coffee makers
Polycarbonates Derived from the reaction of bisphenol A and phosgene Applications requiring toughness, rigidity, and dimensional stability; high heat resistance; good electrical properties; transparency; exceptional impact strength. Used for molded products, solution-cast or extruded films, tubes and pipes, prosthetic devices, nonbreakable windows, street lights, household appliances; compact discs; optical memory disks; and for various applications in fields related to transportation, electronics sporting goods, medical equipment, and food processing
Polyesters Produced by reacting dicarboxylic acids with dihydroxy alcohols Reinforced plastics, automotive parts, foams, electrical encapsulation, structural applications, low-pressure laminates, magnetic tapes, pipes, bottles. Liquid crystal polyesters are used as replacements for metals in such applications chemical pumps, electronic components, medical components, and automotive components
Polyetherimides Consist of repeating aromatic imide and ether units Temperature sensors; electrical/electronic, medical (surgical instrument parts), industrial; appliance, packaging, and specialty applications
TABLE 2. THERMOPLASTICS (cont'd)
Type Chemical basis Uses
Polyetherketones Polymerized aromatic ketones Fine monofilaments, films, engine parts, aerospace composites, and wire and cables, and other applications requiring chemical resistance; exceptional toughness, strength, and rigidity; good radiation resistance; and good fire-safety characteristics
Polyethersulfones Consist of diaryl sulfone groups with ether linkages Electrical applications including multipin connectors, integrated circuit sockets, edge and round multipin connectors, terminal blocks, printed circuit boards
Polyethylenes, polypropylenes, and polyallomers Polyethylenes consist of chains of repeated ethylene units; polypropylenes consist of chains of repeated propylene units; polyallomers are copolymers of propylene and ethylene Low density polyethylene is used for packaging films, liners for shipping containers, wire and cable coatings, toys, plastic bags, electrical insulation. High density polyethylene is used for blow-molded items, films and sheets, containers for petroleum products. Low molecular weight Polyethylenes are used as mold release agents, coatings, polishes, and textile finishing agents. Polypropylenes are used as packaging films, molded parts, bottles, artificial turf, surgical casts, nonwoven disposable filters. Polyallomers are used as vacuum-formed, injection molded, and extruded products, films, sheets, and wire cables
Polyethylene terephthalate Prepared from ethylene glycol and either terephthalic acid or an ester of terephthalic acid Food packaging including bottles, microwave/conventional oven-proof trays; x-ray and other photographic films; magnetic tape
Polyimides and polyamide-imides Polyimides contain imide (-CONHCO-) groups in the polymer chain; polyamide-imides also contain amide (-CONH-) groups Polyimides are used as high temperature coatings, laminates, and composites for the aerospace industry; ablative materials; oil sealants; adhesive; semiconductors; bearings; cable insulation; printed circuits; magnetic tapes; flame-resistant fibers. Polyamide-imides have been used as replacements for metal parts in the aerospace industry, and as mechanical parts for business machines
TABLE 2. THERMOPLASTICS (cont'd)
Type Chemical basis Uses
Polymethylpentene Polymerized 4-methylpentene-1 Laboratory ware (beakers, graduates, etc.); electronic and hospital equipment; food packaging; light reflectors
Polyphenylene ethers, modified Consist of oxidatively coupled phenols and polystyrene Automobile instrument panels, computer keyboard bases
Polyphenylene sulfides Para-substituted benzene rings with sulfur links Microwave oven components, precision molded assemblies for disk drives
Polystyrenes Polymerized ethylene and styrene Packaging, refrigerator doors, household wares, electrical equipment; toys, cabinets; also used as foams for thermal insulations, light construction, fillers in shipping containers, furniture construction
Polysulfones Consist of complicated chains of phenylene units linked with isopropylidene, ether, and sulfone units Power tool housings, electrical equipment, extruded pipes and sheets, automobile components, electronic parts, appliances, computer components; medical instrumentation and trays to hold instruments during sterilization; food processing equipment; chemical processing equipment; water purification devices
Vinyls Polymerized vinyl monomers such as polyvinyl chloride and polyvinylidene chloride Crystal-clear food packaging, water pipes, monolayer films



Liquid crystalline plastics are polymers that form highly ordered, rodlike structures. They have good mechanical properties and are chemically unreactive, and they have melting temperatures comparable to those of crystalline plastics. But unlike crystalline and amorphous plastics, liquid crystalline plastics retain molecular ordering even as liquids. Consequently, they exhibit the lowest shrinkage and warpage of any of the thermoplastics.


Thermosets

Thermosetting plastics, or thermosets, include amino, epoxy, phenolic, and unsaturated polyesters. These materials undergo a chemical change during processing and become hard solids. Unlike the linear molecules in a thermoplastic, adjacent molecules in a thermosetting plastic become cross-linked during processing, resulting in the production of complex networks that restrain the movement of chains past each other at any temperature.

Typical thermosets are phenolics, urea-formaldehyde resins, epoxies, cross-linked polyesters, and most polyurethanes. Elastomers may also be thermosetting. Examples include both natural and synthetic rubbers.


Manufacturing methods

At some stage in their processing, both thermoplastics and thermosetting plastics are sufficiently fluid to be molded and formed. The manufacture of most plastics is determined by their final shape.

Many cylindrical plastic objects are made by a process called extrusion. The extrusion of thermoplastics

TABLE 3. THERMOSETTING PLASTICS
Type Chemical basis Uses
Alkyd polyesters Polyesters derived from the reaction of acids with two acid groups, and alcohols with three alcoholic groups per molecule Moldings, finishes; applications requiring high durability, excellent pigment dispersion, toughness, good adhesion, and good flowing properties
Allyls Polyesters derived form the reaction of esters of allyl alcohol with dibasic acids Electrical insulation, applications requiring high resistance to heat, humidity, and corrosive chemicals
Bismaleimides Generally prepared by the reaction of a diamine with maleic anhydride Printed wire boards; high performance structural composites
Epoxies Derived from the reaction of epichlorohydrin with hydroxylcontaining compounds Encapsulation, electrical insulations, laminates, glass-reinforced plastics, floorings, coatings adhesives
Melamines Derived from the reaction of formaldehyde and amino compounds containing NH2 groups Molded plates, dishes, and other food containers
Phenolics Derived from the reaction of phenols and formaldehydes Cements, adhesives
Polybutadienes Consist of polyethylene with a cross-link at every other carbon in the main chain Moldings, laminating resins, coatings, cast-liquid and formed-sheet products; applications requiring outstanding electrical properties and thermal stability
Polyesters (thermosetting) Derived from reactions of dicarboxylic acids with dihydroxy alcohols Moldings, laminated or reinforced structures, surface gel coatings, liquid castings, furniture products, structures
Polyurethanes Derived from reactions of polyisocyanates and polyols Rigid, semi-flexible, and flexible foams; elastomers


consists of melting and compressing plastic granules by rotating them in a screw conveyor in a long barrel, to which heat may be applied if necessary. The screw forces the plastic to the end of the barrel where it is pushed through a screen on its way to the nozzle. The nozzle determines the final shape of the extruded form. Thermosets may also be extruded if the screw in the conventional extruder is replaced with a plunger-type hydraulic pump.

Plastic powders are directly converted into finished articles by molding. Two types of molding processes are compression molding and injection molding. In compression molding, which is used with thermosetting materials, steam is first circulated through the mold to raise it to the desired temperature; then a plastic powder or tablets are introduced into the mold; and the mold is closed under high pressure and the plastic is liquefied so that it flows throughout the mold. When the mold is reopened,

TABLE 3. THERMOSETTING PLASTICS (cont'd)
Type Chemical basis Uses
Silicones Consist of alternating silicon and oxygen atoms in a polymer backbone, usually with organic side groups attached to the chain Applications requiring uniform properties over a wide temperature range; low surface tension; high degree of lubricity; excellent release properties; extreme water repellency; excellent electrical properties over a wide range of temperature and frequency; inertness and compatibility; chemical inertness; or weather resistance
Ureas Derived from the reaction of formaldehyde and amino compounds containing NH2 groups Dinnerware, interior plywood, foams, insulation


the solid molded unit is ejected. Injection molding differs from compression molding in that plastic material is rendered fluid outside the mold, and is transferred by pressure into the cooled mold. Injection molding can be used with practically every plastic material, including rubbers.

Sheets, blocks, and rods may be made in a casting process that in effect involves in situ, or in-place, polymerization. In the case of acrylics, sheets are cast in glass cells by filling cells with a polymer solution . The polymer solution solidifies and the sheet is released by separating the glass plates after chilling the assembly in cold water. Blocks can be made in the same way using a demountable container; and rods can be made by polymerizing a polymer syrup under pressure in a cylindrical metal tube.

Plastic foams are produced by compounding a polymer resin with a foaming agent or by injecting air or a volatile fluid into the liquid polymer while it is being processed into a finished product. This results in a finished product with a network of gas spaces or cells that makes it less dense than the solid polymer. Such foams are light and strong, and the rigid type can be machined.


Fillers and other modifications

Very few plastics are used in their commercially pure state. Additives currently used include the following: Finely divided rubbers added to more brittle plastics to add toughness; glass, carbon, boron, or metal fibers added to make composite materials with good stress-strain properties and high strength; carbon black or silica added to improve resistance to tearing and to improve stress-strain properties; plasticizers added to soften a plastic by lowering its glass transition temperature or reducing its degree of crystallinity; silanes or other bonding agents added to improve bonding between the plastic and other solid phases; and fillers such as fire retardants, heat or light stabilizers, lubricants, or colorants.

Filled or reinforced plastics are usually referred to as composites. However, some composites includes neither fillers nor reinforcement. Examples are laminates such as plastic sheets or films adhered to nonplastic products such as aluminum foil, cloth, paper or plywood for use in packaging and manufacturing. Plastics may also be metal plated.

Plastics, both glassy and rubbery, may be crosslinked to improve their elastic behavior and to control swelling. Polymers may also be combined to form blends or alloys.


Applications

Plastics have been important in many applications to be listed here. Table 2, "Thermoplastics," and Table 3, "Thermosetting Plastics," list hundreds of commercial applications that have been found for specific plastics.

Engineering plastics are tough plastics that can withstand high loads or stresses. They can be machined and remain dimensionally stable. They are typically used in the construction of machine parts and automobile components. Important examples of this class of plastics include nylons, acetals, polycarbonates, ABS resins, and polybutylene terephthalate. The structure of their giant chains makes these plastics highly resistant to shock, and gives them a characteristic toughness.

Plastics are almost always electrically insulating, and for this reason they have found use as essential components of electrical and electronic equipment (including implants in the human body).

Major applications have been found for plastics in the aerospace, adhesives, coatings, construction, electrical, electronic, medical, packaging, textile, and automotive industries.


Resources

books

Brandrup, J., and E.H. Immergut, eds. Polymer Handbook. 3rd ed. New York, NY: Wiley-Interscience, 1990.

Braungart, Michael,and William McDonough. Cradle to Cradle: Remaking the Way We Make Things. North Point Press, 2002.

Juran, Rosalind, ed. Modern Plastics Encyclopedia. Hightstown, NJ: McGraw-Hill, 1988.

Sperling, L.H. Introduction to Physical Polymer Science. New York, NY: John Wiley & Sons, 1992.


Randall Frost

KEY TERMS


. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Amorphous

—Noncrystalline; lacking a definite crystal structure and a well-defined melting point.

Casting

—Formation of a product either by filling an open mold with liquid monomer and allowing it to polymerize in place, or by pouring the liquid onto a flat, moving surface.

Composite

—A mixture or mechanical combination (on a macroscopic level) of materials that are solid in their finished state, that are mutually insoluble, and that have different chemistries.

Crystalline

—Having a regular arrangement of atoms or molecules; the normal state of solid matter.

Extrusion

—An operation in which material is forced through a metal forming die, followed by cooling or chemical hardening.

Glass

—An amorphous, highly viscous liquid having all of the appearances of a solid.

Inorganic

—Not containing compounds of carbon.

Molding

—Forming a plastic or rubber article in a desired shape by applying heat and pressure.

Monomer

—A substance composed of molecules that are capable of reacting together to form a polymer. Also known as a mer.

Organic

—Containing carbon atoms, when used in the conventional chemical sense. Originally, the term was used to describe materials of living origin.

Plastic

—Materials, usually organic, that under suitable application of heat and pressure, can be caused to flow and to assume a desired shape that is retained when the pressure and temperature conditions are withdrawn.

Polymer

—A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.

Synthetic

—Referring to a substance that either reproduces a natural product or that is a unique material not found in nature, and which is produced by means of chemical reactions.

Thermoplastic

—A high molecular weight polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.

Thermoset

—A high molecular weight polymer that solidifies irreversibly when heated.

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Plastics

PLASTICS

Technologies have world-shaping powers. They have fundamentally changed ways of thinking as much as they influenced social practices. Plastics form a striking case.

Human beings are surrounded by plastics, in their computers, clothes, cars, kitchens, and beds, on their noses, and often in their bodies, in the form of hearing aids, hip replacements, and heart valves. In the early-twentieth century they were an odd curiosity; a century later a world without plastics is unthinkable and unlivable. They have permeated every conceivable practice and in most of these made themselves indispensable. It would, for example, be impossible to have twenty-first century supermarkets without plastic packaging, because the supermarket system is dependent on lightweight, airproof, and pre-packaged goods. In fact the transition from the traditional grocery store to the supermarket system was strongly encouraged by the emerging availability of plastic packaging materials in the 1950s and 1960s.


Plastics Science and Technology

The noun plastics is derived from the Latin plasticus, itself rooted in the Greek plasein meaning to mold; by connotation plastics are thus pliable, malleable, and adaptable. In scientific language, plastics are called polymers, a large and divergent group of materials with a wide range of properties. Their shared characteristic is that they consist of synthetically produced macromolecules, molecules about 1,000 to 100,000 times larger than, for example, the molecules of water or sugar. In a broad sense, synthetic rubbers and resins may also be called plastics. Some macromolecular materials (such as rubbers and resins) are found in nature, but the revolutionary thing about plastics is that they can be synthesized in the laboratory.

Launched in 1868 with the synthesis of celluloid by the American inventor John Wesley Hyatt (1937–1920), polymer synthesis was followed around the turn of the nineteenth century into casei formaldehyde (synthetic horn) and fenolformaldehyde, better known as Bakelite. These more or less accidental findings preceded the scientific understanding of macromolecular structures, which were first elucidated by the German chemist and Nobel Prize winner Hermann Staudinger (1881–1965) and his students in the 1920s. Chemistry thus opened the door to a riot in plastics design. New types that turned out to be especially successful included polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), nylon, polystyrene (PS), and the synthetic rubber styrene butadiene rubber (SBR).

Cultural History

On top of this scientific and technological history is another even more exciting one concerning the public image and social embedding of plastics. As an exemplary case of the cultural response to controversial technology the history offers rich material for philosophical and ethical reflection. In all European and North American countries, the public appreciation of plastics exhibits a whimsical pattern, filled with opposite emotions and paradoxes, soaked with utopian and dystopian fantasies.

One peculiarity in the cultural history of plastics is that appreciation was out of step with development. Parallel to dramatic advances in quality and numbers of applications, the image of plastics deteriorated rapidly. The same qualities that were initially praised—such as their cheapness, lightness, unnaturalness, durability, moldability, imitative properties, ability to be mass produced, and resistance to wear and tear—subsequently became the basis of criticisms.

Jeffrey Meikle's American Plastic (1995) offers an excellent overview of this cultural transformation. The book is a gold mine of facts, stories, and opinions on plastics during the twentieth century, focusing on the United States but with some foreign perspectives as well. As an historian, Meikle does not articulate nor theorize the patterns of extreme and opposite public reactions, which call for philosophical interpretation. The public reactions from fascination to abomination cannot be explained by any simple irrationality or gut feelings on the part of the public, as is often claimed. Rather the ambiguous position of plastics in the cultural scheme is part of a deep-seated nature-culture dichotomy.

In the beginning, plastics were warmly embraced by scientists and the nonscientific public alike. Until World War II plastics existed mostly in chemical labs. Insofar as they were, like Bakelite, commercially produced, their quality and functions were rather poor, yet dreams of their potential were sky-high. Inventors and promoters portrayed plastics as unnatural or even supernatural substances. Plastics thus began with a positive reputation.

For the first time in history, human beings had been able to produce a raw material artificially. This was the general sentiment. Previously raw materials were products of nature that required human processing. Plastics were looked upon as unique exceptions to this rule, miraculous substances just waiting for human use. Their alleged unnaturalness gave rise to a widespread euphoria, their development considered a triumph of humans over nature.

At the end of World War I, Edwin Slosson, a journalist and director of the Science News Service, portrayed plastics chemists as agents of applied democracy. Rare and expensive materials, such as ebony and precious metals, which formerly had been "confined to the selfish enjoyment of the rich," were now "within the reach of every one" thanks to the imitative qualities of plastics. For Slosson "a state of democratic luxury" based on synthetic chemistry was at hand (Slosson 1919, p. 132–135). Fulfilling the ancient alchemists dream of transforming dirt into gold, chemists would gradually "substitute for the natural world an artificial world, molded nearer to its heart's desire" (Meikle 1995, p. 69).

Near the beginning of World War II, the applied chemists Victor Emmanuel Yarsley and Edward Gordon Couzens announced The Expanding Age of Plastics that would created a world brighter and clear than any previously known, "a world free from moth and rust and full of color" (Yarsley and Couzens 1941, p. 57). In such a world, Plastic Man would live in an abundance of safe, hygenic, strong, soft, and light objects, "a world in which man, like a magician makes what he wants for almost every need, out of what is beneath him and around him: coal, water, and air" (Yarsley and Couzens 1941, p. 68). Indeed, because of scarcities in traditional raw materials during World War II, war production of plastic or synthetic substitutes laid the base for postwar mass utilization.

But during the war the best plastics were reserved for the military and consumer plastics were often of inferior quality. As historian Meikle notes, U.S. civilians were faced with "shower heads of cellulose acetate that softened in hot water, with laminated products that separated when wet or stressed, with small moldings so devoid of resin that they shattered when dropped" (Meikle 1995, p. 166). Initial enthusiasm turned into ambivalence, as plastics came to connote inferior substitutes for real materials. When the war ended, the people felt free to demand genuine not artificial materials.

Yet postwar plastics were a booming business. Already in 1946, the average American used 3.5 kilos of plastics per year. Between 1950 and 1974, world production grew by an average 16 percent annually. Compared to other materials, plastics were the most expansive sector in many economies. At the same time, a growing call for "real," natural materials emerged. The quality of artificialness and unnaturalness now had become the essence of plastics supposed flaw. Plastics started to symbolize a fake, cheap, materialist world that would lead to human alienation, cultural decay, and loss of control over technology.

An early sign of this kind of discomfort was expressed by the young biologist and journalist Rachel Carson (the future author of Silent Spring [1962]) in a women's magazine in 1947: "The witchery the chemist performs, turns them first into something unearthly, that gives you the creeps. You feel, when you go into a chemical plant where plastics are made, that maybe man has something quite unruly by the tail" (Carson 1947, p. 127). Roland Barthes, the French literary critic, voiced a similar distrust after he saw a large exposition on plastics in Paris. After his visit, Barthes feared that the whole world would become plasticized, even life. "Even one has already begun to produce plastic aortas," he wrote with disgust (Barthes 1957). But meanwhile Barthes supposed that living materials would not be imitated adequately. Plastics would remain inferior to natural materials, he declared, ignorant of the high-quality biomedical materials that would follow.

Although science, technology, and industry worked to overcome the inferior qualities of consumer plastics—and were remarkably successful in doing so—the nadir in public image was yet to come. This occurred in the 1960s and 1970s as environmental concerns turned plastics, along with nuclear radiation, into central emblems of self-destructiveness in high-tech society. According to novelist Norman Mailer, for instance, plastics were spreading through the country "like the metastases of cancer cells" (Meikle 1995, p. 177). In this climate most viewers of the film The Graduate (1967) immediately recognized its praise of plastics as a cynical joke, as a metaphor for the phony, banal and materialist world the protagonist has entered. The unsolicited career advice given to the new college graduate Benjamin Braddock (played by Dustin Hoffman) is simple: "Plastics. There is a great future in plastics." The words came to reflect dense cultural irony, because, of course, the future of plastics was the problem of waste.

An early spokesman of the plastics waste problem was the American biologist and environmentalist Barry Commoner. According to Commoner, the strength of plastics was also their essential flaw, an inability to degrade when discarded as waste: Only "human beings are uniquely capable of producing materials not found in nature [such as] is synthetic plastic, which unlike natural materials is not degraded by biological decay. It therefore persists as rubbish or is burned—in both cases causing pollution" (Commoner 1971, p. 127). Not being biodegradable had lost its meaning of triumph over nature; on the contrary, it made that plastics were perceived as a permanent threat to nature, and the durability of plastic became a permanent threat to nature.

Then in the 1980s and 1990s, the public response to plastics shifted again. The issues of acid rain and greenhouse gases replaced the emblematic status of plastics as a source of environmental problems (Hajer 1995). Instead of condemning all plastics wholesale, even strict environmentalists began to distinguish different types associated with different degrees of environmental burden. Several organizational and technical strategies emerged to cope with plastics waste—from recycling to decomposing polymer materials into oil-like products and the development of biopolymers that degraded in sunlight. The plastics waste problem was not solved, but with technological and organizational fixes it became manageable.

Toward an Anthropological Ethics

How can one account for the fierce and contradictory emotions and changes in perception about plastics during the last century? They cannot be explained by the improving qualities of the material. Neither can they be explained by the dimension of plastics waste risks in comparison with other environmental risks. Explaining the whimsical pattern of public fascination and disgust about plastics by appealing to the emotional approach of the public—as chemists and spokespersons of the plastics industry were apt to do in reaction to environmental criticism—is unsatisfactory as well.

A richer understanding calls for taking into account fundamental, cultural assumptions toward new technologies. Technologies must be appropriated in order to make them fit into people's lives and practices. During the appropriation process both technologies and existing social orders often have to shift and adjust to one another. Plastics are ambiguous substances that did not always fit into existing cultural, symbolic categories. Under such circumstances erratic reactions are common.

In her pioneering work on impurity ideas in traditional societies, the British anthropologist Mary Douglas (1966) has described how border-crossing phenomena that do not fit into the cultural orders cause extreme reactions both of fascination and fear. Such a dual reaction is especially strong when something fits into two categories that were previously considered to be mutually exclusive such as the human and animal, organism and machine, or nature and culture. The Nuer Tribe in Africa, for example experienced malformed babies as ambivalent beings, crossing the border between man and animal. Therefore they were treated as hippopotamus babies and put across the river. In the case of plastics, it is the nature–culture dichotomy that is decisive for its experienced ambivalence.

From the beginning, plastics were unlike natural raw materials, because they were artificially synthesized and therefore products of culture. This led to the interpretation of plastics as a miracle. Then in the climate of increasing environmental concern the nondegradability of plastics turned the miracle into monster. The coping strategies can be understood as attempts to put plastics in an acceptable cultural category. Product recycling brings the waste back into culture, while biodegradation makes nature out of it again. Although the waste problem is not solved, plastics have been culturally domesticated. They have become ethically accepted.


MARTIJNTJE W. SMITS

SEE ALSO Artificiality.

BIBLIOGRAPHY

Barthes, Roland. (1972). Mythologies, trans. Annette Lavers. New York: Hill and Wang. Original from Paris: Éditions du Seuil (1957).

Carson, Rachel. (1947). "Plastic Age." Colliers 120: 22, 49–50.

Commoner, Barry. (1971). The Closing Circle; Nature, Man and Technology. New York: Knopf.

Douglas, Mary. (1966). Purity and Danger: An Analysis of the Concepts of Pollution and Taboo. London: Routledge.

Hajer, Maarten A. (1995). The Politics of Environmental Discourse: Ecological Modernization and the Policy Process. Oxford: Clarendon Press; New York: Oxford University Press.

Meikle, Jeffrey L. (1995). American Plastic: A Cultural History. New Brunswick, NJ: Rutgers University Press.

Mossman, Susan, ed. (1997). Early Plastics, Perspectives 1850–1950. London: Leicester University Press; Washington, DC: Science Museum. Paperback edition from New York: Continuum (2000).

Slosson, Edwin E. (1919). Creative Chemistry. New York: Century.

Smits, Martijntje. (2002). Monsterbezwering. De Culturele Domesticatie Van Nieuwe Technologie [Taming monsters: the cultural domestication of new technology]. Amsterdam: Boom.

Sparke, Penny, ed. (1990). The Plastics Age: From Modernity to Post-modernity. London: Victoria and Albert Museum. Published in the United States as The Plastics Age: From Bakelite to Beanbags and Beyond. Woodstock, NY: Overlook Press (1993).

Yarsley, Victor Emmanuel, and Edward G. Couzens. (1941). "The Expanding Age of Plastics." Science Digest 10(December): 57–59.

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