Page 65 - Basic Principles of Textile Coloration

54 SYNTHETIC FIBRES

the water by distillation. The water initially hydrolyses some caprolactam to 6-
aminohexanoic acid. This then reacts with caprolactam, opening the ring and
generating a new amino end group. This amino group then undergoes the same
addition reaction with more caprolactam so that the forming polymer chain always
has amino and carboxylate end groups (Scheme 4.1). The polyamide obtained is in
equilibrium with about 10% of unreacted monomer and low molecular weight
oligomers. These are removed by water washing after extrusion.

The melting point of nylon 6 is about 215 °C. Molten nylon 6 is therefore more
stable than molten nylon 6.6. The lower melting temperature of nylon 6 allows
filament spinning directly from the polymerisation reactor without undue
decomposition or changes in properties. Nylon 6 may also be extruded as a thick
band, chipped and processed as for nylon 6.6. The chips are preferred for continuous
filament spinning because the denier is more uniform. Direct spinning from the
reactor is satisfactory for staple fibres where denier variations are less important
because the fibres will be well-blended. As for nylon 6.6, the conditioned filaments
have a sufficiently low Tg to be cold drawn to increase the molecular orientation and
crystallinity, giving improved mechanical properties.

4.2.3 Properties of nylon 6.6 and 6

The numbers of repeat units along the polymer chains of nylon 6.6 and nylon 6 are
about 65 and 130, respectively, corresponding to molecular weights of about
15 000 g mol–1. They have similar chemical structures and therefore many
common characteristics and end-uses. There are, however, some important
distinctions.

These nylons are not totally hydrophobic and do absorb water at room
temperature. The standard regains of both nylon 6.6 and 6 are around 4%. These
relatively low moisture contents do not allow effective dissipation of accumulated
static electric charge, which causes handling and soiling problems. This must be
controlled by application of appropriate anti-static chemicals to the fibres, and
grounding devices on machinery may be necessary.

Both types of nylon contain weakly basic amino and weakly acidic carboxylic acid
end groups. The number of terminal amino groups in a nylon can vary from 30 to
100 mmol kg–1. If each nylon molecule with a molecular weight of 15 000 g mol–1
had one amino end group, the number of such groups would be 67 mmol kg–1. The
actual number is usually less than this since some terminal amino groups will have
reacted with the acetic acid chain stopper (Scheme 4.2). Therefore, the number of
carboxylic acid groups in a typical nylon (90 mmol kg–1) is usually greater than the