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Polyethene    (2003)

CAS no 9002-88-4

Synonyms
Polyethylene
PE
HDPE
LDPE
LLDPE


Physical data
Melting point: 85-145░C depending on the polymer
Boiling point: decomposes
Vapour pressure: very low
Water solubility: insoluble

Polyethene is a plastic. It is a white, waxy polymer constituted of only carbon and hydrogen. It burns well, does not conduct electricity and is insoluble in water and most organic solvents. Polyethene is a thermoplastic. Thermoplastics melt when heated and can thus be remodelled.

Polyethene is manufactured by cracking/decomposing petroleum distillates into small, reactive ethane molecules that can be rearranged. Ethene molecules polymerize to long chains of polyethene, chains that are kept close to each other by weak bonds to form a material, comparable with the fibers of a felt.

There are two main groups of polyethene, HDPE och LDPE. HDPE stands for High Density Poly Ethene and is produced at such a low pressure that there is time for formation of linear polymer chains. This makes it possible for the molecules to be packed densely together and the material becomes rigid. LDPE, Low Density Poly Ethene, on the other hand is manufactured at high pressure, the ethene molecules have high energy and move vigorously. This results in polymer chains that are rich in branching and become too bulky to be closely packed. Hence, LDPE is soft and thus suitable for manufacture of packaging films and soft tubes and hoses, whereas stiff HDPE is used for e.g. piping. The density of LDPE and HDPE is between 0.91 to 0.96 g /cm3. There is also a modification of LDPE that has a growing field of application, Linear Low Density Poly Ethene, LLDPE. For this, some α-olefin like 1-butene or 1-octene is used in addition to ethene for polymerization, resulting in an even distribution of similar side chains. This gives a soft polymer that can be produced at a lower pressure than LDPE.

Global consumption of polyethene was in 2000 about 30 million tons, LDPE constituting a little more than half of it. The use of polyethene is expected to grow. PE is also manufactured in Sweden and the capacity is planned to increase.

A polyethene molecule can be 1000 to 10 000 ethene units long, that means 2000 to 20 000 carbon atoms in a line. A PE-plastic consists of polymer chains with different lengths; the longer molecule the better mechanical properties for the plastic material. At the same time it will be harder to work, e.g. a melt of PE with 50 000 ethene units will be to viscous for injection-moulding.

Polyethene is a material used for articles in most areas but particularly for folios/films, household articles, pipes and hoses. Paper is coated with polyethene to render it a waterproof surface. In paints and adhesives it acts as the binder and constitutes the remaining film/coating. Polyethene can be used as wax in polish. Packaging made of LDPE-film is recycled and used for garbage and carrier bags.
 

 

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