Polyethylene
Polyethylene is a long chain polymer produced by the polymerization reaction of liquid ethylene at high temperatures and pressures. It is a solid plastic which has a milky transparent appearance.
The discovery of polyethylene in the 1930s greatly aided the Allied war effort in World War II. Polyethylene was used as insulation for cables vital to the Allied information network. Polyethylene was accidently discovered by J. Swallow and M. Perrin at Imperial Chemical Industries in Britain. In 1933, while researching the effects of high pressure on chemical reactants, a fellow scientist, R. Gibson, managed to produce a waxy solid from ethylene and benzaldehyde. He repeated the experiment with ethylene alone with no success. The experiment had taken place at 280°F (138 °C) with ethylene at a pressure of 1,400 atmospheres. Theorizing that a higher pressure was needed, the three colleagues set to work designing and building improved laboratory equipment.
Nearly three years later, Swallow and his companions tried the experiment at a higher temperature. In the course of the polymerization process the pressure dropped and they added more ethylene to compensate. The procedure yielded approximately eight grams (.28 g) of polyethylene. After inspecting the equipment they discovered that there had been a leak. The polyethylene they had added contained oxygen. Further investigation revealed that the oxygen was vital to the polymerization process. The polyethylene, like polyesters, benefitted from the cold drawing technique developed by Wallace Carothers at DuPont.
Polyethylene, or polythene as it was marketed in Britain, began as a polymer with little practical use. It would have remained so if an employee had not noticed that the mechanical properties of polyethylene were comparable to those of gutta percha, a natural product used to insulate telegraph cables. Polyethylene was used to insulate the cables laid between France and Britain, providing a crucial line of communication towards the end of World War II. After the war, polyethylene film was used for packaging, liners, tank and pool covers, and drop cloths.
The early polyethylene was a low density branched polymer. This means that there was a good deal of empty space in each molecule, and the molecules were formed in a branching pattern. These characteristics accounted for the fact that this polyethylene was not a very strong material. However, in 1953, German scientist Karl Ziegler and his staff discovered a method of producing high density, linear polyethylene (whose molecules had less empty space and were arranged in rope-like strands, both of which give a substance greater tensile strength). Ziegler was researching organometallic compounds (carbon compounds which contain metals) and their effects on polymerization reaction. He attempted to polymerize ethylene using catalysts at a much lower pressure than that used by the British process.
At first, Ziegler was puzzled when an experiment yielded a dimer (a compound of two radicals, rather than the several radicals which compose a polymer) of polyethylene rather than the expected low molecular weight polymer. He discovered trace amounts of nickel on the laboratory equipment had inhibited the reaction. As a result of his investigation of this occurrence, Ziegler eventually succeeded in producing a very high molecular weight polyethylene with a very high melting point using metal chloride and organoaluminum compounds as catalysts. Polyethylene could now be carried out at low temperatures and normal pressure, which greatly simplified the industrial production process.
High density polyethylene is currently used in dishes, squeezable bottles, and other soft plastics products. It is recyclable. Linear low density polyethylene (LLPE) now substitutes for the older branched low density polyethylene (LDPE). LLPE is formed using the Ziegler process using hydrogen to regulate molecular weight. All of the various polyethylenes may be processed by injection molding or extrusion.
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