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Topic last updated: 09 Nov 2023

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The Earth's resources

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High density and low density polymers

Poly(ethene) can either be a high-density or a low-density polymer, and each has a different structure. These differences depend on what monomers they are made from and the conditions in which they are made. These conditions could include the temperature at which the polymer is made, for example.

Low density poly(ethene) (LDPE)

  • Made from ethene monomers
  • Soft and flexible
  • Polymer chains are branched
  • Makes carrier bags (as an example)

High density poly(ethene) (HDPE)

  • Made from ethene monomers
  • Strong and sometimes flexible
  • Polymer chains are not branched
  • Makes plastic bottles (as an example)

Plastics can also be thermosetting or thermosoftening, and this is again due to the conditions in which they are made. The thermosetting or thermosoftening status of the plastic determines whether it can be recycled or not.

Thermosoftening polymers:

  • Melt when heated and can be reshaped back into a solid when cooled. This means that they can be recycled.
  • This is because the polymer chains are not cross linked with covalent bonds, which means that the strands can separate as the polymer melts.
  • When cooled, the plastic resets and can take a new shape.
  • Most plastics are thermosoftening, including all types of poly(ethene).

Thermosetting polymers:

  • Do not melt when heated and so cannot be reshaped.
  • They cannot be recycled.
  • This is because the polymer chains are cross linked with covalent bonds, which means that the strands cannot separate when heated.
  • They are used when the plastic must withstand a really high temperature, such as plastics used in electronics.
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