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 HIGH DENSITY POLYETHYLENE Polyethylene materials are divided into categories based on their specific gravity: (a) low density polyethylene having specific gravity of 0.910 — 0.925, (b) medium density polyethylene, specific gravity of 0.926 — 0.940, and (c) high density (linear) polyethylene, specific gravity 0.941 — 0.965. The major advantages of polyethylenes are light weight, excellent chemical resistance, low moisture absorption, good impact strength, excellent low temperature properties, superb dielectric properties and low coefficient of friction. Unmodified polyethylenes can be produced from resins which are approved for food applications. Increases in density result in: improved chemical resistance, greater hardness and tensile strength, better gas barrier properties, greater creep and temperature resistance. Polyethylenes can be shaped and formed by standard heat forming techniques. If polyethylene is heated to within 5°C of its melting point, it can be shaped easily and will retain its imposed form on cooling. High density polyethylene for instance should be heated to a range of 130°C — 135°C for forming. Polyethylenes can be readily welded with hot air. However, since oxygen at high temperatures has a tendency to degrade these materials, inert gas such as nitrogen should be used to weld this material.
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