Контрольные вопросы и задания для проведения текущего контроля и промежуточной аттестации по итогам освоения дисциплины
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Sci-Tech Encyclopedia: Polymer Polymers, macromolecules, high polymers, and giant molecules are high-molecular-weight materials composed of repeating subunits. These materials may be organic, inorganic, or organometallic, and synthetic or natural in origin. Polymers are essential materials for almost every industry as adhesives, building materials, paper, cloths, fibers, coatings, plastics, ceramics, concretes, liquid crystals, photoresists, and coatings. They are also major components in soils and plant and animal life. They are important in nutrition, engineering, biology, medicine, computers, space exploration, health, and the environment. Natural inorganic polymers include diamonds, graphite, sand, asbestos, agates, chert, feldspars, mica, quartz, and talc. Natural organic polymers include polysaccharides (or polycarbohydrates) such as starch and cellulose, nucleic acids, and proteins. Synthetic inorganic polymers include boron nitride, concrete, many high-temperature superconductors, and a number of glasses. Siloxanes or polysiloxanes represent synthetic organometallic polymers. See also Silicone resins. Synthetic polymers used for structural components weigh considerably less than metals, helping to reduce the consumption of fuel in vehicles and aircraft. They even outperform most metals when measured on a strength-per-weight basis. Polymers have been developed which can also be used for engineering purposes such as gears, bearings, and structural members. Nomenclature Many polymers have both a common name and a structure-based name specified by the International Union of Pure and Applied Chemistry (IUPAC). Some polymers are commonly known by their acronyms. Some companies use trade names to identify the specific polymeric products they manufacture. For example, Fortrel® polyester is a poly(ethylene terephthalate) (PET) fiber. Polymers are often generically named, such as rayon, polyester, and nylon. See also Organic nomenclature; Polyamide resins; Polyester resins. Polymers that can be molded or shaped, usually by heat and pressure. Most are lightweight, transparent, tough organic compounds that do not conduct electricity well. They fall into two classes: Thermoplastics (e.g., polyethylene, polystyrene) can be melted and formed again and again; thermosetting plastics, or thermosets (e.g., polyurethane, epoxy), once formed, are destroyed rather than melted by heating. Few plastics contain only the polymer resin; many also contain plasticizers (to change the melting point and make them softer), colorants, reinforcements, and fillers (to improve mechanical properties such as stiffness), and stabilizers and antioxidants (to protect against aging, light, or biological agents). Traditional plastics are not biodegradable (see biodegradability); recycling of plastics, especially thermoplastics, has become an important industry, and the development of low-cost biodegradable plastics and plastic substitutes is a significant pursuit of industrial research. Major commercial uses of plastics include cars, buildings, packaging, textiles, paints, adhesives, pipes, electrical and electronic components, prostheses, toys, brushes, and furniture. Common plastics include polyethylene terephthalate, or PET (beverage bottles), PVC (pipes and hoses), foamed polystyrene, or Styrofoam (insulated food containers), and Lucite (shatterproof windows). See also Leo Baekeland. 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. |
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