Polyethylene

polyethylene (abbreviation PE, becomes outdated to PL, occasionally also Polyethen mentioned) is by polymerization of Ethen <math> [CH_2=CH_2]< a /math> manufactured, part-crystalline, thermoplastic plastic with the formula <math> \ left [- CH_2-CH_2 \ right] _n< /math>. It belongs to the group of the polyolefins. Well-known trade names are: Alathon, Hostalen, Lupolen, Vestolen, Trolen.

Bild:Polyethylen.jpg
Atomic formulapolyethylene.

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historical information

polyethylene in the year 1898 of chemists Hans von Pechmann one discovered and to 27. March 1933 for the first time through Reginald Gibson and Eric Fawcett in the ICI - laboratories in England industrially under a pressure of approx. 1400 bar and a temperature of 170°C manufactured, where it formed as white, waxlike lining on the inner wall autoclaves. Only 1940 could an economically profitable manufactureProcedure to be developed. 1953 developed the German Karl Ziegler and the Italian Giulio Natta the Ziegler Natta catalyst, with whose assistance a polymerization of Ethen became possible with normal print also. But the scientists 1963 received the Nobelpreis for chemistry.

Polyethylene in large quantities becomes commercialsince 1957, particularly in piping systems for the gas and water supply for cable insulations and used in packing materials.

one

differentiates PE types between

  • PE-HD (HDPE): weakly polymer chains, therefore high density branched out between 0,94 g/cm 3 and 0.97 g/cm 3, (“hp” standsfor “high density”).
  • PE-LD (LDPE): strongly polymer chains, therefore small density branched out between 0,915 g/cm 3 and 0.935 g/cm 3, (“LD” stands for “low density”).
  • PE-LLD (LLDPE): linear polyethylene of low density, whose polymer molecule exhibits only very few and short bypasses.
  • PE-HMW: high-molecular polyethylene. The polymer chainsare longer than with PE-HD, PE-LD or PE-LLD, the middle mol mass lies with 500 to 1000 kg/mol.
  • PE-UHMW: ultrahigh-molecular polyethylene with a middle mol mass of up to 6000 kg/mol and a density of up to 0,99 g/cm 3.


Characteristic PE-LD PE-HD PE-LLD
crystallization degree in% 40 to 50 60 to 80 30 to 40
density in g/cm ³ 0.915 to 0.935 0.94 to 0.97 0,90 to 0,93
shear modulus N/mm ² with 23C ~130 ~1000 -
Kristalitschmelzbereich in °C 105 to 110 ,130 to 135 ,121 to 125
chemical resistance well better well
tensionat the yield strength in N/mm ² 8.0-10 20.0-30.0 10.0-30.0
stretch at the yield strength in % 20 12 16
elastic module (N/mm ²) 200 1000 -
linear coefficient of expansion (K -1) 1,7 * 10 -4 2 * 10 -4 2 * 10 -4
permissible constant temperature in °C 80 ,100 -
Softening temperature in °C 110 ,140 -

polyethylene

consists characteristics contrary to PVC only of hydrogen and carbon, it can thus in the waste incineration ideally to carbon dioxide and water vapour burn. It burns with dripping, bright flame and burns alsofurther, if one removes the flame. The ecological balance is good. Polyethylene possesses a high stability against the attack of acids, caustic solutions and further chemicals. Polyethylene is part crystalline, with rising crystallinity degree rises the density. By higher crystallinity also the mechanical and chemical increaseStability. Polyethylene takes up hardly water , it swims on water; Water absorption less than 0.1%, density <1 g/cm ³, and does not pour in polar solvents practically. For water vapour it is impermeable; Oxygen, carbon dioxide and flavour materials let it through however well. Its characteristics leave themselves by suitableCopolymerisation purposefully change.

A disadvantage of the polyethylene is that it is not applicable at temperatures of over 80 °C. Uncolored polyethylene is milk industrial union cloudy and matte. Polyethylene feels waxlike, it is cutable.

Due to the nonpolar surface is polyethylene without suitable pretreatment (z. B. Coronarentladung to print on or stick or Abflammen) only badly.

In principle the chemical resistance of PE with the density rises. The gas and steam permeability are higher than with most plastics. By sun exposure an embrittlement can occur, soot as UV stabilizer is usually used with PE.

General one Characteristics polyethylene:

  • low density (0,915 - 0,965 g/cm ³)
  • high tenacity and elongation of break
  • of property Gleitverhalten, small wear (v.a. PE-UHMW)
  • Temperature stability from -85°C to +90°C
  • optical, milchig knows
  • very good electrical and dielectric behavior (spec. Conductance approx. 10 18 Ohm/cm)
  • very small water absorption
  • very well machine cutting andnon-cutting to processing
  • burns well; arrears-free: CO 2 + H 2 O as Verbrennungsprodukte
  • PE is steady against nearly all polar solvents (T< 60°C), acids, caustic solutions, water, alcohols, oil, HD-PE also against gasoline.

production

polyethylene becomes through Polymerization manufactured by ethyl gas. In the high-pressure method soft polyethylene (PE-LD) develops, in the low pressure procedure develops the hard polyethylene (PE-HD). With both manufacturing processes it results first as tough liquid.

  • PE-LD becomes with pressures of 1000 to 3000 bar and temperatures of 100 °C to 300 °C under employment ofInitiators (radical starters) (oxygen or peroxides) made of the monomer Ethen.
  • PE-HD is produced industrially in the Ziegler Natta procedure. Are characteristic with this procedure the low pressure (1 to 50 bar) and the low temperature (20°C to 150°C). As catalysts titanium esters, titanium halides or aluminum alkyls are used.One keeps PE-HD alternative also with the Phillips procedure at temperatures from 85 to 180°C and pressures from 30 to 40 bar.
  • PE-UHMW is producible with modified Ziegler-Katalysatoren.

Polyethylene is with a portion of approx. 29 per cent world-wide at most produced plastic. In the year 200152 million tons were manufactured.

cross-linking

of PE macromolecules can be interlaced three-dimensional. By cross-linking the temperature stability of the material improves. In addition the impact strength and the shatter crack stability increase. Cross-linking takes place during the processing or afterwards. Interlaced polyethylene becomes asPE-X designates. There are four different cross-linking procedures:

  • Peroxide cross-linking (PE-Xa)
  • silane cross-linking (PE-Xb)
  • radiation cross-linking (PE-Xc)
  • Azo cross-linking (PE-Xd)

areas of application

  • PE-LD and PE-LLD: the material is used particularly in foil production. Typical products are garbage bags, shrink wrappings and agriculture foils. To small extent PE-LD and PE-LLD becomealso to the production of sheaths, pipes and hollow bodies uses.
  • PE-HD: most important area of application are in the Blasformverfahren manufactured hollow body, for example bottles for cleaning agents in the household, in addition, large volume containers with a capacity of up to 1000 l (IBC so mentioned). In addition PE-HD becomes spritzgussteilen (predominantly) Processes packing, besides foils and pipes become from polyethylene in the extruding procedure manufactured.
  • PE-UHMW: for example for pump parts, gear wheels, floating bushings, implants and prostheses one uses. Fibers from PE-UHMW belong, to their weight referred, to the stärkesten well-known artificial fibers (Dyneema©, DSM). They are verwendent as surgical seam material. Itthe only well-known fibers are which at least theoretically a space elevator to probably make possible could.
  • PE-X: among other things for warm water pipes and as isolation of central and high-voltage cables one uses.

see also

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