Page 425 - Handbook of Properties of Textile and Technical Fibres

398 Handbook of Properties of Textile and Technical Fibres

12.4.5 Processing of PA 66 fibers

The history of PA 66 (nylon) invention and development is described in a book by
(Seymour and Kirshenbaum, 1986), Chapter 3 on History and Development of Nylon
66, by Melvin I. Kohan. Here, the basic technologies of ADA and hexamethylenedi-
amine (HMD) preparation and polycondensation into PA 66 and technologies neces-
sary for fiber formation are described. The reasons why fibers from PA 6 were not
developed at DuPont are also discussed. It is interesting that the melt spin process
for this polymer had already been licensed in 1939 to IG Farbenindustrie for the pro-
duction of PA 6 filaments.
PA 6 and PA 66 form strong, tough, and durable fibers useful in a wide variety of
textile and technical applications. The major differences in the fibers are that PA 66 has
a more compact structure with more hydrogen bonds and a higher melting point. The
PA 66 fiber has good dye wash fastness and UV light fastness, and excellent perfor-
mance in high-speed spinning processes but handling is stiffer with a slightly harsher
feel as compared to PA 6 (Richards, 2005; Mather and Wardman, 2015).
PA 66 exhibits only about half the shrinkage of PA 6 in steam, for instance. The
most important advantages of PA 66 over PA 6 are its high tensile strength, excellent
abrasion resistance, creep resistance, excellent gas barrier properties, and resistance
against chemicals. PA 66 provides high tensile strength for tough fibers at fine deniers
and excellent performance for tire applications. Excellent abrasion resistance makes it
ideal for use in carpets, upholstery, and conveyor belts. The rubber industry takes
advantage of the higher melting point of PA 66 in high-temperature tire curing. A
high melting point also results in a fiber with high stretch and recovery in false twist
textured yarns (e.g., hosiery and socks) and thermal stability in high-temperature
coating operations.
Nylon (PA 66) is a condensation product of ADA and HMD. Preparation of ADA
starts with phenol, via cyclohexanol and cyclohexanone with oxidation in air using
manganese nitrate as a catalyst. The simplest method for the production of ADA is
direct oxidation of cyclohexane. Manufacture of HMD is exclusively achieved by
the hydrogenation of the dinitriles.
The first step before polycondensation is creation of AH salt (“nylon salt”)by
precipitation of 80% HMD and 20% hot ADA in methanol. This salt, containing equi-
molar quantities of both monomers crystallizes well, which is important for its purifi-
cation. For step growth polymerization the AH salt concentration in water is raised to
>80% and a small quantity of a monofunctional agent, such as ethanoic acid, is added.
Polycondensation is carried out in an atmosphere of nitrogen. Oxygen should be
excluded, because the polymer is prone to thermooxidation at the reaction tempera-
tures. As water in the form of steam is a by-product of the reaction, the molar mass
of PA 66 is controlled by the pressure of the steam during the reaction. Initially, PA
66 of low molar mass (about 4000) is formed at a temperature of 260e280 C and a

pressure of 1.8 MPa. The pressure is then released by allowing the steam to escape
and the molar mass of the PA 66 increases to approximately 12,000. A higher molar
mass can be achieved by conducting the postpolymerization of lower molecular weight
polymer in the molten state in the presence of a phosphonic acid catalyst or in the solid