Plastics - History, Chemistry, Polymerization, Manufacture And Processing, Thermoplastics, Crystalline And Noncrystalline Thermoplastics, Thermosets - Molecular weight
materials chain term usually
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.
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.
Additional Topics
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 cros…
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.
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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 …
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 prevent…
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 chlorid…
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…
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 pr…
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 typica…
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