Organic+Chemistry+and+the+Plastic+Industries

Plastic Industries:

The use of plastics is constrained chiefly by their organic chemistry, which seriously limits their properties, such as hardness, density, heat resistance, organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt or decompose when heated to a few hundred degrees Celsius. While plastics can be made electrically conductive, with the conductivity of up to 80 kS/cm in stretch-oriented polyacetylene, they are still no match for most metals like copper which have conductivities of several hundreds kS/cm. Plastics are still too expensive to replace wood, concrete and ceramic in bulky items like ordinary buildings, bridges , dams , pavement , and railroad ties. 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 flexibility is substantially increased). The 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 ), and all thermosets. Common thermoplastics range from 20,000 to 500,000 amu, 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 thousand repeating 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 interests are silicon based.)