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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 films or fibers. Their name is derived from the fact that many are malleable, having the property of plasticity. 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 attractive intermolecular forces are overcome) and one or more glass transitions (temperatures at which the degree of cross-linking is substantially reduced).

Plastics are polymers: long chains of atoms bonded to one another. These chains are made up of many repeating molecular units, or "monomers". 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 the 1860s. 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)...

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