Page 426 - Handbook of Properties of Textile and Technical Fibres

The chemistry, manufacture, and tensile behavior of polyamide fibers 399

state (Yang, 2006). The level of cyclic oligomers formed in the process is usually of an
acceptable level, less than 2%. Results are PA 66 chips. For extrusion spinning of fine
PA 66 filaments, the chips need to be dried thoroughly to 0.05 0.01% moisture level.
It is important to avoid excessive exposure of the chip to oxygen, which may lead to
their degradation and yellowing.
Melt spinning is carried out at the polymer temperature 280e290 C. The thermal

instability of the melt requires very quick processing into fibers (Murthy et al.,

1990). The cooling air, having a temperature of less then 14 C should cool the fila-
ments as closely as possible under the spinneret (Fourné, 1999).

12.4.6 Processing of PA 6 fibers
The history of the invention and development of PA 6 is described in a book by
Seymour and Kirshenbaum (1986), Chapter 4 on History and Development of Nylon
6, by Paul Matthies and Wolfgang F. Seydl. Here the basic technologies of capro-
lactam preparation and its polymerization are described in an historical context. In
recent years, production of polyamide fibers has undergone structural changes caused
by the preparation of high-performance technical fibers and competition between PA 6
and polyester fiber manufacturers. Fiber spinning efficiency is continuously increasing
and new spinning or processing technologies are appearing (Najafi et al., 2017b). The
other important factor in increasing the competitiveness of polyamides is the reduction
of the cost of raw materials (Mishra and Deopura, 1982).
The original raw material for manufacturing caprolactam (monomer for PA 6) was
phenol gained from the carbonization of pit coal. By a series of chemical reactions,
cyclohexanol, cyclohexanone, and cyclohexa nonoxim are successively prepared
and finally caprolactam is obtained by Beckman’s rearrangement in concentrated
H 2 SO 4 . An alternative route is starting from benzene. Cyclohexanol is obtained by
oxidation of cyclohexane obtained by hydration of benzene (Fourné, 1999).

The caprolactam has a melting point 69.2 C, boiling point 270.8 C, and it is easily

dissolved in water (solubility in water at 20 C is 4560 g/L). It does not polymerize
upon heating to elevated temperatures. The ring-opening reaction occurs readily in
the presence of amine and carboxyl groups. Thus, ε-aminocaproic acid, PA 66 salt,
or simply water, is employed to hydrolyze lactam to form [eCOOH] and [eNH 2 ]
end groups (Mishra and Deopura, 1982).
PA 6 is commonly prepared by hydrolytic polymerization of caprolactam in the
presence of water used as a reaction initiator by discontinuous (autoclave) or contin-
uous (VK Tube) processes at 250e270 C under atmospheric pressure. In both cases,

the polymer preparation and spinning is carried out in an inert nitrogen atmosphere due
to sensitivity of the monomer and polymer to thermooxidation. The PA 6 chips contain
approx. 10% low cyclic oligomers, mainly caprolactam (8.4%) and dimers/trimers
(1.3%), which should be removed before spinning by extraction. Today wet extraction
is generally used, and the remaining extract content in the chips is usually between
0.2% and 0.6%.
Under anhydrous conditions, lactams may be protonated and these lactam cations
may initiate and propagate cationic polymerization. It is characterized by the absence