Plastic

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". more...

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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.

Overview

Plastic can be classified in many ways, but most commonly by their polymer backbone (poly(vinyl chloride), polyethylene, poly(methyl methacrylate)and other acrylics, silicones, polyurethanes, etc.). Other classifications include thermoplastic thermoset, elastomer, engineering plastic, addition or condensation or polyaddition (depending polymerization method used), and glass transition temperature or T_g.

Some 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 above which the extent of localized molecular is substantially increased). So-called semi-crystalline plastics include polyethylene, polypropylene, poly(vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly(methyl methacrylate), all thermosets.

Plastics are polymers: long chains of atoms bonded to one another. Common hermoplastics range from 20,000 to 500,000 in moleclar weight, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen 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 a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone, (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat unit's molecular structure 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 made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented by Leo Hendrik Baekeland in 1907, the product being known as Bakelite. Subsequently poly(vinyl chloride), polystyrene, polyethylene (polyethene), polypropylene (polypropene), polyamides (nylons), polyesters, acrylics, silicones, polyurethanes were amongst the many varieties of plastics developed and have great commercial success.

Read more at Wikipedia.org