The first industrially practical polyethylene

The first industrially practical polyethylene

The first industrially practical polyethylene synthesis was discovered in 1933 by Eric Fawcett and Reginald Gibson, again by accident, at the Imperial Chemical Industries (ICI) works in Northwich, England. Upon applying extremely high pressure to a mixture of ethylene and benzaldehyde they again produced a white, waxy material. Because the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was, difficult to reproduce. It was not until 1935 that another ICI chemist, Michael Perrin, developed this accident into a reproducible high-pressure synthesis for polyethylene that became the basis for industrial LDPE production beginning in 1939. Because polyethylene was found to have very low-loss properties at very high frequency radio waves, commercial distribution in Britain was suspended on the outbreak of World War II, secrecy imposed, and the new process was used to produce insulation for UHF and SHF coaxial cables of radar sets. During World War II, further research was done on the ICI process and in 1944 Bakelite Corporation at Sabine, and Du Pont at Charleston began large-scale commercial production under license from ICI.

The breakthrough landmark in the commercial production of polyethylene began with the development of catalysts that promoted the polymerization at mild temperature and pressures. The first of these was a chromium-trioxide based catalyst discovered in 1951 by Robert Banks and Paul Hogan at Phillips Petroleum. In 1953 the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organo aluminium compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, however, and both methods are heavily used industrially. By the end of the 1950 decade both the Phillips- and Ziegler-type catalysts were being used for HDPE production. In the 1970decade, the Ziegler system was improved by the incorporation of magnesium chloride. Catalytic systems based on soluble catalysts, the metallocenes, were reported in 1976 by Walter Kaminsky and Hansjörg Sinn. The Ziegler- and metallocene-based catalysts families have proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available nowdays, including linear low-density polyethylene and very low density polyethylene . Such resins, in the form of UHMWPE fibers, have (as of 2005) started to replace aramids in many high-strength products.

 

Properties of polyethylene

The properties of polyethylene can be divided into mechanical, chemical, electrical, optical, and thermal properties.

Polyethylene consists of nonpolar, saturated, high molecular weight hydrocarbons. Therefore, its chemical behavior is similar to paraffin. The individual macromolecules are not covalently linked. They tend to crystallize because of their symmetric molecular structure; overall polyethylene is partially crystalline. Higher crystallinity increases density and mechanical and chemical stability.

Thermal properties
The usefulness of polyethylene is limited by its melting point of 80 °C. For common commercial grades of medium- and high-density polyethylene the melting point is typically in the range 120 to 180 °C. The melting point for average, commercial, low-density polyethylene is typically 105 to 115 °C . These temperatures vary strongly with the type of polyethylene.

Mechanical properties
Polyethylene is of low strength and rigidity, but has a high ductility and impact strength as well as low friction. It shows strong creep under persistent force and It feels waxy when touched.

 

Polyethylene consists of nonpolar, saturated, high molecular weight hydrocarbons. Therefore, its chemical behavior is similar to paraffin. The individual macromolecules are not covalently linked. They tend to crystallize because of their symmetric molecular structure; overall polyethylene is partially crystalline. Higher crystallinity increases density and mechanical and chemical stability.
Most LDPE and MDPE, and HDPE grades have excellent chemical resistance, meaning they are not attacked by strong bases or strong acids, and are resistant to gentle oxidants and reducing agents. Crystalline samples do not dissolve at room temperature. Polyethylene (other than cross-linked polyethylene) usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as chlorinated solvents such as trichloroethane or trichlorobenzenetoluene or xylene.
Polyethylene absorbs almost no water. The gas and water vapor permeability (only polar gases) is lower than for most plastics; oxygen, carbon dioxide and flavorings on the other hand can pass it easily.
PE can become brittle when exposed to sunlight; carbon black is usually used as a UV stabilizer.
Polyethylene burns slowly with a blue flame having a yellow tip and gives off an odour of paraffin (similar to candle flame). The material continues burning on removal of the flame source and produces a drip.
Polyethylene cannot be imprinted or stuck together without pretreatment.
Optical properties
Depending on thermal history and film thickness PE can vary between almost clear (transparent), milky-opaque or opaque. LDPE thereby owns the greatest, LLDPE slightly less and HDPE the least transparency. Transparency is reduced by crystallites if they are larger than the wavelength of visible light.

Electrical properties
Polyethylene is a good electrical insulator. It offers good tracking resistance; however, it becomes easily electro statically charged (which can be reduced by additions of graphite, carbon black or antistatic agents).