Polyethylene (Polythene) Family
A waxy, translucent, somewhat flexible thermoplastic, prepared by polymerizing ethylene at high pressure (1,000 to 4,000 atm) and high temperature (180 to 190 deg. C.) in the presence of a trace of oxygen. It is one of the lightest of the plastics, having a specific gravity of 0.92 to 0.93. Below 60 deg. C., polyethylene is insoluble in all solvents and is resistant to the action o f most reagents, other than strong oxidizing acids. Above 115 deg. C., the polymer changes from a clear solid to a relatively low-viscosity melt. At this temperature and above, exposure to air causes relatively extensive oxidative degradation, unless antioxidants are included with the polymer.
Polyethylene is widely used as a film by itself or as a hot extrusion onto paper to provide additional strength and moisture-resistant characteristics. It is also applied to printing papers to provide finish and strength. The material is also made in sheets for use as a facing to prevent materials from sticking to a surface in operations requiring the application of pressure. The film which does not adhere permanently to waxes and many plastics in the unhardened state, is easily peeled off when the operation is completed. In sheet form, it is used in conservation work, in lieu of cellulose acetate lamination, to protect brittle paper, in which case the paper is placed between two sheets of the film, which is then sealed with double-sided adhesive tape around the edges. It may also be sealed by means of plastic welding. (Bookbinding and the Conservation of Books A Dictionary of Descriptive Terminology http://palimpsest.stanford.edu/don/dt/dt2636.html )
A family of thermoplastic resins obtained by polymerizing the gas ethylene [C2H4].
Low molecular weight polymers of ethylene are fluids used as lubricants
Medium weight polymers are waxes miscible with paraffin
High molecular weight polymers (i.e., over 6000) are the materials used in the plastics industry.
Polymers with densities ranging from about
|Low Density LDPE||.910 to .925||Polymerized at very high pressures and temperatures|
|Medium Density MDPE||.926 to .940|
High Density HDPE
|.941 to .965 and over||Polymerized at relatively low temperatures and pressures|
A relatively new type called linear low density polyethylene (LLDPE) is manufactured through a variety of processes: gas phase, solution, slurry, or high pressure conversion. A high efficiency catalyst system aids in the polymerization of ethylene and allows for lower temperatures and pressures than those required in making conventional low density polyethylene. Copolymers of ethylene with vinyl acetate, ethyl acrylate, and acrylic acid are commercially important. Major polyethylene applications can be found in packaging, housewares, toys and communications equipment.
Hexachloroethane (Hexachloroethane CAS NO. 67-72-1 First Listed in the Seventh Annual Report on Carcinogens 12 Apr 00)
It is an initiator in the formation of polyethylenes and has been used in the preparation of certain catalysts.
Hexachloroethane, when added to spectrographic samples, improves analysis. Hexachloroethane reduces corrosion when added to flue gas cleaning systems and oil when used as a coating on metal machinery. It increases specific gravity when added to drilling mud used in oil and gas wells. It is a main ingredient in production of some types of synthetic diamonds. Other uses of hexachloroethane in the U.S. have included: a plasticizer for cellulose esters; an accelerator in rubber; a retardant in fermentation processes; a moth repellent; a component of submarine paints; a constituent of various fungicidal and insecticidal formulations; and a softening agent in hair setting and styling agents. With the possible exception of use for smoke generation, only limited quantities of hexachloroethane are used in these applications.
Traditional Plastics Processing (Promising Polymers Environmental Health Perspectives v.103, n.1 Jan95)
Polymers produced on a large scale such as polystyrene, polyvinyl chloride (PVC), polyethylene co-vinylacetate, polyacrylic acid, and styrene and butadiene rubber, are created using heterogeneous dispersion polymerization. In this process, polymers form in two phases in which the initial monomer or the resulting polymer, or both, are finely dispersed in a solvent and are controlled by adding a surfactant (stabilizer) that conforms particle sizes to within a relatively narrow range. Once the polymers are formed, manufacturers remove the water or evaporate the solvents used to disperse the polymers. Companies then face the challenge of properly disposing of and remediating these harmful by-products--a daunting task as EPA regulations grow more stringent. In 1992, the U.S. plastics industry produced 567 million pounds of toxic waste, according to the EPA. In the same year, companies put about one-quarter of their total waste back into the nation's environment.
Because most organic solvents (typical ones include toluene and methylene chloride) are petroleum-based, many are flammable, volatile, and often cause narcotic effects to humans at high concentrations. Yet pharmaceutical, chemical, and polymer industries must use vast amounts of these solvents to extract, separate, and manufacture chemicals.
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