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Polyethylene

Polyethylene or polyethene is one of the simplest and most inexpensive polymers. It is a waxy, chemically inert plastic.

It is named thus because it is obtained by the polymerization of ethylene (ethene).

It is sometimes shortened to PE or, chiefly in the United Kingdom, polythene (e.g. in the Beatles song Polythene Pam).

The ethene molecule (known almost universally by its non-IUPAC name ethylene), C2H4 is CH2 = CH2, Two CH2 connected by a double bond, thus:

image:Ethene.png

In the polymerization process of ethene, the double bonds of the monomers are broken to allow additional single bonds to form amongst the carbon atoms, thus a macromolecule is created.

image:polyethene_monomer.png

Classification of polyethylenes

Polyethylenes are classified according to their density, which in turn depends on the extent and type of branching present in the material.

  • HDPE (high density PE)
  • MDPE (medium density PE)
  • LDPE (low density PE)
  • LLDPE (linear low density PE)

LDPE has many more branches than HDPE, which means that the chains do not "fit well" together. It has therefore less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower density and tensile strength, increased malleability and faster biodegradation. LDPE is created by free radical polymerization.

HDPE has virtually no branching and thus stronger intermolecular forces and tensile strength. The lack of branching is ensured by an appropriate choice of catalyst (e.g. Ziegler catalysts) and reaction conditions.

LLDPE is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins.

The most common household use of LDPE is in plastic bags; the most common household use of HDPE is in containers for milk, liquid laundry detergent , etc. Significant uses of MDPE include plumbing fittings and enclosures for inexpensive consumer devices. LLDPE is used primarily in flexible tubing.

Recently, much research activity has focused on Long Chain Branched polyethylene. This is essentially HDPE, but has a small amount (perhaps 1 in 100 or 1000 branches per backbone carbon) of very long branches. These materials combine the strength of HDPE with the processability of LDPE.

History

Polyethylene was first synthesized by the German chemist Hans von Pechmann , who prepared it by accident in 1898 while heating diazomethane. When his colleagues Eugen Bamberger and Friedrich Tschirner characterized the white, waxy subsance he had created, they recognized that it contained long -CH2- chains and termed it polymethylene.

The first industrially practical polyethylene synthesis was discovered (again by accident) by Eric Fawcett and Reginald Gibson at ICI Chemicals in 1933. Upon applying extremely high pressure (several hundred atmospheres) to a mixture of ethylene and benzaldehyde, they again produced a white waxy material. Since the reaction had been initiated by trace oxygen contamination in their apparatus, the experiment was at first 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.

Subsequent landmarks in polyethylene synthesis have centered around the development of several types of catalyst that promote ethylene polymerization at more mild temperatures and pressures. The first of these was a chromium trioxide based catalyst discovered in 1951 by Robert Banks and John Hogan at Phillips Petroleum. In 1953, the German chemist Karl Ziegler developed a catalytic system based on titanium halides and organoaluminum compounds that worked at even milder conditions than the Phillips catalyst. The Phillips catalyst is less expensive and easier to work with, leading to both methods being used in industrial practice.

By the end of the 1950s both the Phillips and Ziegler type catalysts were being used for HDPE production. Phillips' initially had difficulties producing a HDPE product of uniform quality, and filled warehouses with off-specification plastic. However, financial ruin was unexpectedly averted in 1957, when the hula hoop, a toy consisting of a circular polyethylene tube, became a fad among teenagers throughout the United States.

A third type of catalytic system, one based on metallocenes, was discovered in 1976 in Germany by Walter Kaminsky and Hansjörg Sinn . The Ziegler and metallocene catalyst families have since proven to be very flexible at copolymerizing ethylene with other olefins and have become the basis for the wide range of polyethylene resins available today, including VLDPE, LLDPE, and MDPE. Such resins, in the form of fibers like Dyneema, have (as of 2005) begun to replace aramids in many high-strength applications.

Until recently, the metallocenes were the most active single-site catalysts for ethylene polymerisation known - new catalysts are typically compared to zirconocene dichloride. Much effort is currently being exerted on developing new single-site (so-called post-metallocene) catalysts, that may allow greater tuning of the polymer structure than is possible with metallocenes. Recently, work by Fujita at the Mitsui corporation has demonstrated that certain iminophenolate complexes of Group IV metals show substantially higher activity than the metallocenes.

Physical Properties

Polyethylene variants do not have specific melting points, but gradually soften as the temperature is increased. LDPE may melt at approximately 110 C, while HDPE melts at approximately 130 C.

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