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''Plastic'' covers a range of synthetic or semisynthetic Polymerization products. They are composed of Organic Condensation or addition polymers and may contain other substances to improve performance or economics. There are few natural polymers generally considered to be "plastics". Plastics can be formed into objects or Film s or Fibers . Their name is derived from the fact that many are malleable, having the property of Plasticity . Plastics are designed with immense variation in properties such as heat tolerance, hardness, resiliency and many others. Combined with this adaptability, the general uniformity of composition and light weight of plastics ensures their use in almost all industrial segments. Plastic may also refer to any material characterized by deformation or failure under Shear Stress ; see Plasticity and Ductility . Plastic can be classified in many ways but most commonly by their polymer backbone ( Polyvinyl Chloride , Polyethylene , Acrylic , Silicone , Urethane , etc.). Other classifications include Thermoplastic vs. Thermoset , Elastomer , Engineering Plastic , addition or condensation, and Glass Transition temperature or Tg. Many plastics are partially Crystalline and partially Amorphous in Molecular structure, giving them both a Melting Point (the temperature at which the Covalent Bond s dissolve) and one or more Glass Transition s (temperatures at which the degree of Cross-linking is substantially reduced). Plastics are s bonded to one another. These chains are made up of many repeating molecular units, or " Monomer s". The vast majority of plastics are composed of polymers of Carbon alone or with Oxygen , Nitrogen , Chlorine or Sulfur in the backbone. (Some of commercial interest are Silicon based.) The backbone is that part of the chain on the main "path" linking the multitude of monomer units together. To customize the properties of a plastic, different molecular groups "hang" from the backbone (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by pendant groups has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer. People experimented with plastics based on natural polymers for centuries. In the nineteenth century they discovered plastics based on chemically modified natural polymers: Charles Goodyear discovered Vulcanization of Rubber ( 1839 ) and Alexander Parkes discovered Cellulose -based plastics in 1860 s. The first plastic based on a synthetic Polymer was called Bakelite and was created by Leo Hendrik Baekeland in 1907 . The development of plastics has come from the use of natural materials (e.g., chewing gum, Shellac ) to the use of chemically modified natural materials (e.g., natural Rubber , Nitrocellulose ) and finally to completely manmade molecules (e.g., Epoxy , Polyvinyl Chloride , polyethylene). NATURAL POLYMERS People have been using natural Organic polymers for centuries in the form of Wax es and shellacs. A plant polymer named " Cellulose " provides the structural strength for Natural Fiber s and ropes, and by the early 19th Century natural Rubber , tapped from Rubber Tree s, was in widespread use. Eventually, inventors learned to improve the properties of natural polymers. Natural rubber was sensitive to temperature, becoming sticky and smelly in hot weather and brittle in cold weather. In 1834, two inventors, Friedrich Ludersdorf of Germany and Nathaniel Hayward of the U.S., independently discovered that adding sulfur to raw rubber helped prevent the material from becoming sticky. In 1839 , the American inventor Charles Goodyear was experimenting with the sulfur treatment of natural rubber when, according to legend, he dropped a piece of sulfur-treated rubber on a stove. The rubber seemed to have improved properties; Goodyear followed up with further experiments, and developed a process known as " Vulcanization " that involved cooking the rubber with sulfur. Compared to untreated natural rubber, Goodyear's "vulcanized rubber" was stronger, more resistant to abrasion, more elastic, much less sensitive to temperature, impermeable to gases, and highly resistant to chemicals and electric current. Vulcanization remains an important industrial process for the manufacture of rubber in both natural and artificial forms. Natural rubber is composed of an organic polymer named " Isoprene ". Vulcanization creates sulfur bonds that link separate isoprene polymers together, improving the material's structural integrity and its other properties. By the early 20th century, other natural polymers were extracted and used in specialty applications. Zein protein, derived from corn, was widely used to provide water and grease resistant coatings. CELLULOSE-BASED PLASTICS: CELLULOID AND RAYON All Goodyear had done with vulcanization was improve the properties of a natural polymer. The next logical step was to use a natural polymer, cellulose, as the basis for a new material. Inventors were particularly interested in developing synthetic substitutes for those natural materials that were expensive and in short supply, since that meant a profitable market to exploit. Ivory was a particularly attractive target for a synthetic replacement. An Englishman from Birmingham named Alexander Parkes developed a "synthetic ivory" named "pyroxlin", which he marketed under the trade name " Parkesine ", and which won a bronze medal at the 1862 World's Fair in London . Parkesine was made from cellulose treated with Nitric Acid and a solvent. The output of the process hardened into a hard, ivory-like material that could be molded when heated. However, Parkes was not able to scale up the process to an industrial level, and products made from Parkesine quickly warped and cracked after a short period of use. An American printer and amateur inventor named John Wesley Hyatt took up where Parkes left off. Parkes had failed for lack of a proper softener, but Hyatt discovered that Camphor would do the job very nicely. Hyatt was something of an industrial genius who understood what could be done with such a shapeable, or "plastic", material, and proceeded to design much of the basic industrial machinery needed to produce good-quality plastic materials in quantity. Since cellulose was the main constituent used in the synthesis of his new material, Hyatt named it " Celluloid ". It was introduced in 1863 . One of the first products were dental pieces, and sets of false teeth built around celluloid proved cheaper than existing rubber dentures. However, celluloid dentures tended to soften when hot, making tea drinking tricky, and the camphor taste tended to be difficult to suppress. Celluloid's real breakthrough products were waterproof shirt collars, cuffs, and the false shirtfronts known as " Dickie s", whose unmanageable nature later became a stock joke in silent-movie comedies. They did not wilt and did not stain easily, and Hyatt sold them by trainloads. Corsets made with celluloid stays also proved popular, since perspiration did not rust the stays, as it would if they had been made of metal. Celluloid proved extremely versatile in its field of application, providing a cheap and attractive replacement for ivory, Tortoiseshell , and bone, and traditional products that had used these materials were much easier to fabricate with plastics. Some of the items made with cellulose in the nineteenth century were beautifully designed and implemented. For example, celluloid combs made to tie up the long tresses of hair fashionable at the time are now jewellike museum pieces. Such pretty trinkets were no longer only for the rich. Celluloid could also be used in entirely new applications. Hyatt figured out how to fabricate the material in a strip format for movie film. By the year 1900, movie film was a major market for celluloid. However, celluloid still tended to yellow and crack over time, and it had another more dangerous defect: it burned very easily and spectacularly, unsurprising given that mixtures of nitric acid and cellulose are also used to synthesize Smokeless Powder . Ping-pong balls, one of the few products still made with celluloid, sizzle and burn if set on fire, and Hyatt liked to tell stories about celluloid billiard balls exploding when struck very hard. These stories might have had a basis in fact, since the billiard balls were often celluloid covered with paints based on another, even more flammable, nitrocellulose product known as " Collodion ". If the balls had been imperfectly manufactured, the paints might have acted as primer to set the rest of the ball off with a bang. Cellulose was also used to produce cloth. While the men who developed celluloid were interested in replacing ivory, those who developed the new fibers were interested in replacing another expensive material, Silk . In 1884, a French chemist, the Comte de Chardonnay, introduced a cellulose-based fabric that became known as "Chardonnay silk". It was an attractive cloth, but like celluloid it was very flammable, a property completely unacceptable in clothing. After some ghastly accidents, Chardonnay silk was taken off the market. In 1894 , three British inventors, Charles Cross, Edward Bevan, and Clayton Beadle, patented a new "artificial silk" or "art silk" that was much safer. The three men sold the rights for the new fabric to the French Courtauld company, a major manufacturer of silk, which put it into production in 1905, using cellulose from wood pulp as the "feedstock" material. Art silk became well known under the trade name " Rayon ", and was produced in great quantities through the 1930s , when it was supplanted by better artificial fabrics. It still remains in production today, often in blends with other natural and artificial fibers. It is cheap and feels smooth on the skin, though it is weak when wet and creases easily. It could also be produced in a transparent sheet form known as " Cellophane ". BAKELITE (PHENOLIC) The limitations of celluloid led to the next major advance, known as " Phenol ic" or "phenol-formaldehyde" plastics. A chemist named Leo Hendrik Baekeland , a Belgian-born American living in New York state, was searching for an insulating shellac to coat wires in electric motors and generators. Baekeland found that mixtures of Phenol (C6H5OH) and Formaldehyde (HCOH) formed a sticky mass when mixed together and heated, and the mass became extremely hard if allowed to cool and dry. He continued his investigations and found that the material could be mixed with wood flour, asbestos, or slate dust to create "composite" materials with different properties. Most of these compositions were strong and fire resistant. The only problem was that the material tended to foam during synthesis, and the resulting product was of unacceptable quality. Baekeland built pressure vessels to force out the bubbles and provide a smooth, uniform product. He publicly announced his discovery in 1909 , naming it " Bakelite ". It was originally used for electrical and mechanical parts, finally coming into widespread use in consumer goods in the 1920s. When the Bakelite patent expired in 1927, the Catalin Corporation acquired the patent and began manufacturing Catalin plastic using a different process that allowed a wider range of coloring. Bakelite was the first true plastic. It was a purely synthetic material, not based on any material or even molecule found in nature. It was also the first "thermoset" plastic. Conventional " Thermoplastic s" can be molded and then melted again, but Thermoset plastics form bonds between polymers strands when "cured", creating a tangled matrix that cannot be undone without destroying the plastic. Thermoset plastics are tough and temperature resistant. Bakelite was cheap, strong, and durable. It was molded into thousands of forms, such as radios, telephones, clocks, and, of course, billiard balls. The U.S. government even considered making one-cent coins out of it when World War II caused a copper shortage. Phenolic plastics have been largely replaced by cheaper and less brittle plastics, but they are still used in applications requiring its insulating and heat-resistant properties. For example, some electronic Circuit Board s are made of sheets of paper or cloth impregnated with phenolic resin. Phenolic sheets, rods and tubes are produced in a wide variety of grades under various brand names. The most common grades of industrial phenolic are Canvas, Linen and Paper. POLYSTYRENE AND PVC After the First World War , improvements in chemical technology led to an explosion in new forms of plastics. Among the earliest examples in the wave of new plastics were " Polystyrene " (PS) and " Polyvinyl Chloride " (PVC), developed by IG Farben of Germany. Polystyrene is a rigid, brittle plastic that is now used to make plastic model kits, disposable eating utensils, and similar knickknacks. It would also be the basis for one of the most popular "foamed" plastics, under the name "styrene foam" or " Styrofoam ". Foam plastics can be synthesized in an "open cell" form, in which the foam bubbles are interconnected, as in an absorbent sponge, and "closed cell", in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and floatation devices. H H H H H H |
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