Polyamides are polymers which contain repeating amide, -CO-NH-, linkages. Proteins are examples of naturally occurring polyamides.
The best known manufactured polyamides are often called nylons (the trade name given by the manufacturer, DuPont) and these are aliphatic polyamides.
Uses of polyamides
The properties of the polyamides (nylons), which include high strength, abrasion resistance, and resilience, make them very important in the manufacture of clothing and carpets. Although these polyamides account for 95% of the material used in women's hosiery, this still only accounts for about 5% of the total fibres used to make clothing. Nevertheless this is more than either the polypropenoates (acrylics) or wool but it is substantially less than either cotton or polyesters.
The polyamides (nylons) are also used in engineering plastics, for example, in cars, and for making films for food packaging. They are used in films for their good balance between mechanical strength and barrier properties against oxygen, smells and oils.
Polyamide 6,6 was first produced in the laboratory in 1935 by W H Carothers whilst working for DuPont in the US. Commercial production started in 1938, the same year as I G Farbenindustrie developed polyamide 6 in Germany.
For use as an engineering plastic, polyamides are often compounded with fillers, pigments, glass fibre and toughening agents to give specific properties to the polymer. However, for either continuous filament or staple fibres, which are melt spun at very high speeds (ca 6 km every minute), there is great emphasis on controlling the polymer chemistry and the way the yarn is produced in order to ensure the production of the high quality material needed for particular purposes. For example, the thread for use in stockings needs to be strong, as well as very fine, so the molecular mass and hence tensile properties of the polymer must be carefully controlled.
Annual production of polyamides
Caprolactam (monomer of polyamide 6)
Manufacture of polyamide 6 and 6,6
Both polyamides are manufactured from benzene via cyclohexane. Hydrogen is passed through liquid benzene in the presence of a nickel catalyst under pressure:
Cyclohexane is oxidized by passing air through the liquid under pressure in the presence of a catalyst (often a cobalt salt) to yield two products:
The mixture of cyclohexanol and cyclohexanone is known as "mixed oil" or KA (ketone/alcohol).
A more recent route to cyclohexanol is the Asahi process from benzene via its hydrogenation to cyclohexene and subsequent hydration to alcohol. This is more energy efficient than the other processes.
To make polyamide 6, pure cyclohexanone is required. When the mixed oil is heated under pressure with copper(ll) and chromium(lll) oxides, the cyclohexanol, which is a secondary alcohol, is dehydrogenated to the corresponding ketone, cyclohexanone:
Cyclohexanone is then converted into caprolactam via the oxime (produced by the reaction of the ketone with hydroxylamine - in the form of the salt, hydroxylamine hydrogensulfate):
The isomerisation of the oxime to caprolactam by sulfuric acid is an example of the Beckmann rearrangement in which an oxime is transformed into an amide in the presence of acid.
This is an example of a batch process .
Polyamide 6,6 is produced by reacting 1,6-diaminohexane (hexamethylenediamine) with hexanedioic acid (adipic acid) by condensation polymerization.
One of the monomers, hexanedioic acid is also produced from KA mixed oil (cyclohexanol and cyclohexanone). The mixed oil is oxidized in the liquid phase using moderately concentrated (60%) nitric acid and a copper(II) nitrate and ammonium vanadate(V) catalyst, at 330 K to form hexanedioic acid:
This process has a considerable disadvantage. A side-product is nitrogen(I) oxide (nitrous oxide), N2O, a powerful greenhouse gas but it is carefully removed by thermal or catalytic treatment units.
The chain length is regulated by controlling process conditions, such as reaction time, temperature and pressure. An aqueous solution of the salt is heated, in the absence of air, to ca 500 K. A pressure develops in the vessel. The temperature is then raised to 540 K, and the steam is bled off to keep the pressure constant. Eventually, the pressure is reduced and the polymer is extruded under nitrogen to yield a lace which is then granulated (Figure 4).
Date last amended: 7th May 2013