Production of Polyamide 6,6

Polymerisation
The nylon salt is polymerised by heating to 350°C under pressure, with the addition of a small amount of ethanoic acid to regulate polymer chain length. Polymerisation always results in a chain with an amine at one end and an acid at the other, so the reaction can continue.
chemical equation: polymerisation

Monomer production

PA 6,6 requires two monomers, hexanedioic acid and 1,6 diaminohexane, both produced from the Ketone-Alcohol mixture ("KA"), made usually from benzene.

Hexanedioic acid (commonly called adipic acid) is first made from KA using nitric acid, and a proportion of this is then converted to 1,6 diaminohexane.

chemical equation: hexanedioic acid from KA

Nylon Salt

Hexanedioic acid and 1,6 diaminohexane are made into a salt, commonly referred to as nylon salt, that can either remain in solution or be crystalised as a solid. This allows it to be transported or stored for use at a later date.

chemical equation: polyamide salt

chemical equation: 1,6 diaminohexane from hexanedioic acid

By-Products

The production of hexanedioic acid by this route suffers from one serious disadvantage - one of the other by-products is dinitrogen monoxide (N2O, nitrous oxide). This gas is produced naturally from soil and rivers, is commonly used as an anaesthetic ("laughing gas") and as a propellant for aerosols, but there are concerns over increases in atmospheric concentration, for two main reasons:

  • N2O has a "greenhouse" effect in the atmosphere, which is over 200 times as strong as carbon dioxide
  • N2O is involved in several reactions responsible for the thinning of the ozone layer

Production of hexanedioic acid for PA 6,6 is estimated to account for 10% of nitrous oxide emission, and so alternative production methods are being developed that either recycle the nitrous oxide, or use reactions that do not produce it.

Find out more about these developments by clicking here.


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