Page 452 - Handbook of Properties of Textile and Technical Fibres

Tensile failure of polyester fibers 425

ethylene glycol. Direct esterification (TPA process) is becoming more widely used
than the process using DMT. Industrially PET is prepared with a high molecular
weight, generally in two successive stages:

1. In the first stage, a mixture of ethylene glycol esters of TPA as oligoesters and bishydroxye-
thylterephthalate (molecular weight M n 100e2000 g/mol, depending on the molar ratio of the
starting compounds) is produced at 180e240 C either by ester interchange (transesterifica-

tion) or by direct esterification with an excess of ethylene glycol under pressure. During the
reaction water should be eliminated. Mn, Cd, Zn, or Co acetate are used as catalysts of ester
interchange. During direct esterification the acid-catalyzed side reaction can be prevented by
adding small amounts of sodium hydroxide or a quarternary organic hydroxide.

2. The ethylene glycol esters of the TPA mixture is subjected to polycondensation at 285 C and
reduced pressure of 1 mbar that produces fiber-forming PET (M n > 10 000 g/mol). The
condensation reaction is virtually reversible and glycol as a by-product should be removed.
Effective methods for expelling glycol have been reported in Stevenson (1969). Antimony
trioxide (Sb 2 O 3 ) is intended to serve as a catalyst. The trioxide reacts with the glycol to
form various glycol oxides, which are probably the true catalysts. Antimony trioxide func-
tions very well but due to the forming of a very fine colloidal metallic antimony suspension
in the PET a grey discoloration occurs. This is especially true if the stabilizer is a trivalent
phosphorous compound (Duh, 2002). There are 200e300 ppm of antimony and
20e100 ppm of phosphorus in most commercial PET (Duh, 2002). Either TiO 2 or a mixture
with SiO 2 or TiO 2 /ZrO 2 compositions can be used as polymerization catalysts (AKZO pat-
ent) as well (McIntyre, 2005). The discoloration, occurring when titanium alkoxides are
employed, is usually attributed to organic contaminants formed during the polymerization
process (Finelli et al., 2004). It was found that the C-94 catalyst (TiO 2 and SiO 2 with a ratio
of 9:1 w/w) (Finelli et al., 2004)is6e8 times as active as the antimony oxide catalyst
(Thier-Grebe and Rabe, 2000). Germanium oxide is also very effective. Other common cata-
lyst systems are listed in the review (Pang et al., 2006). Antimony catalyst typically leads to
the production of 1e2 mol% of diethylene glycol (DEG) but for other catalysts the amount of
DEG can be higher (Chen and Chen, 1998a).
Since melt spinning can take place at temperatures above the melting point degra-
dation processes occur. It is then necessary to realize technologies under controlled
degradation or side reactions. One of the most important side reactions is the forming
of a DEG unit HO(CH 2 ) 2 O(CH 2 ) 2 OH in the chain. The polymer chain length is not
changed but modifying DEG components are included in the backbone. The presence
of DEG reduces crystallinity and lowers both thermal and hydrolytic stability. PET
produced by the direct esterification of TPA generally contains more DEG than
PET produced by the transesterification of DMT (Hovenkamp and Munting, 1970).
During the heat treatment of PET fibers, crystallization occurs by squeezing the
DEG-rich portion out to the amorphous region (Akahane et al., 1980). The kinetics
and mechanism of DEG formation is described in the work by Chen and Chen
(1998b). It is impossible to completely eliminate DEG formation and around
1.5 mol% is always present (McIntyre, 2005).
In PET chips stable cyclic trimer (see Fig. 13.3) occurs in a concentration of about
2% w/w.
Cyclic trimer can be extracted from PET chips with hot xylene but if the extracted
polymer is remelted, the same level of trimer reforms.