Polyethylene
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Polyethylene or polyethene is an engineering thermoplastic heavily used in consumer products. Its name originates from the monomer ethene used to create the polymer.
In the polymer industry the name is sometimes shortened to PE, similar to how other polymers like polypropylene and polystyrene are shortened to PP and PS, respectively. In the United Kingdom the polymer is called 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:
Polyethylene is created through polymerization of ethene. It can be produced through radical polymerization, anionic polymerization, and cationic polymerization. This is because ethene does not have any substituent groups which influence the stability of the propagation head of the polymer. Each of these methods results in a different type of polyethylene.
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Classification of polyethylenes
Polyethylene is classified into several different categories based mostly on it's mechanical properties. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure, and the molecular weight.
- UHMWPE (ultra high molecular weight PE)
- HDPE (high density PE)
- LDPE (low density PE)
- LLDPE (linear low density PE)
UHMWPE is polyethylene with a molecular weight numbering in the millions. The high molecular weight results a very good packing of the chains into the crystal structure. This results in a very tough material. UHMWPE is made through metallocene catalysis polymerization.
HDPE has little branching and thus stronger intermolecular forces and tensile strength. The lack of branching is ensured by an appropriate choice of catalyst (e.g. Ziegler-Natta catalysts) and reaction conditions.
LDPE has many more branches than HDPE, which means that the chains do pack into the crystal structure as well. It has therefore less strong intermolecular forces as the instantaneous-dipole induced-dipole attraction is less. This results in a lower tensile strength and increased ductility. LDPE is created by free radical polymerization.
LLDPE is a substantially linear polymer, with significant numbers of short branches, commonly made by copolymerization of ethylene with longer-chain olefins.
UHMWPE is used in high modulus fibers and in bulletproof vests. The most common household use of HDPE is in containers for milk, liquid laundry detergent, etc; the most common household use of LDPE is in plastic bags. 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
Depending on the crystallinity and molecular weight, a melting point and glass transition may or may not be observable. The temperature at which these occur varies strongly with the type of PE.
References
- Macrogalleria (http://www.pslc.ws/macrog/index.htm|The)
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