Page 451 - Handbook of Properties of Textile and Technical Fibres

424 Handbook of Properties of Textile and Technical Fibres

methods were used to prepare shrinkable fibers but to obtain selective shrinking in
boiled water or at higher temperature in air, physical and chemical modification
methods had to be combined.
In 1959 Eastman Kodak described a fiber in which ethylene glycol was replaced by
1,4-bis(hydroxymethyl) cyclohexane (Martin and Bush, 1962). By substituting
butylene glycol for ethylene glycol PBT was obtained, and by substituting propylene
glycol for ethylene glycol PTT fibers were produced. Fibers made of PBT and PTT
were rapidly accepted, especially in the carpet industry (Militký et al., 1991).
From other types of polyester used in the production of fibers, mention should be
made of polyethylene oxybenzoate (Tomita, 1977), produced by polymerizing
4-hydroxybenzoic acid, and polylactic acid (Perepelkin, 2002), produced from lactic
acid. Some other fiber-forming polyester developments are mentioned, for instance,
in the books (Scheirs and Long, 2003; Militký et al., 1991; Fakirov, 2002).
The traditional way of synthesizing polyesters is polycondensation using diols and
a diacid (or an acid derivative), or from a hydroxy acid. Very high conversion is desir-
able to get polymer chains of sufficiently high molecular mass to provide useful
mechanical properties in the fibers. In spite of all precautions, a high degree of poly-
merization is very difficult to achieve by this method because of side-reactions and the
volatilization of monomers, which lead to a stoichiometric imbalance of reactants.
Ring-opening polymerization of lactones, cyclic diesters (lactides and glycolides),
and cyclic ketene acetals is an alternative method which has been successfully
employed to yield high-molecular-mass polymers under relatively mild conditions.
This polyaddition reaction can be carried out with no or very limited side-reactions
making it possible to control properties like molecular weight and molecular weight
distribution (Brunelle, 2008).

13.2.1 Poly(ethylene terephthalate) fibers
PET is still produced commercially by the dimethyl terephthalate (DMT) process and
the TA process (Davis and Talbot, 1988). The first commercially successful route to
prepare TPA was the oxidation of p-xylene under pressure using dilute nitric acid.
The final TPA contained colored and color-forming impurities that could not have
been removed, so it was necessary to react it with methanol to form DMT. By recrys-
tallization and distillation of DMT it was possible to remove these impurities. The stan-
dard route to DMT preparation (Katzschmann) is based on two air oxidations (starting
p-xylene to p-toluic acid and methyl p-toluate to monomethyl terephthalate) and two
esterifications (p-toluic acid esterified by methanol to form methyl p-toluate and mono-
methyl terephthalate esterified by methanol to make DMT) (Scheirs and Long, 2003;
Fakirov, 2002).
Direct oxygen oxidation of p-xylene to TPA uses acetic acid as the solvent at

temperature of about 200 C and a combination of cobalt, manganese acetates with bro-
mide ions as catalysts. A process adopted by Amoco also incorporated a purification of
TPA and simultaneous catalytic hydrogenation of impurities from superheated water

under pressure at about 250 C(Scheirs and Long, 2003; Landau and Saffer, 1968).
This process allowed the preparation of PET by direct esterification of TPA by the