Chemistry Reference
In-Depth Information
is a result of interaction of a carboxyl group with a molecule of caprolactam or with an amide
group [
58
]:
O
N
COOH
5
H
N
4
O
O
O
N
+
CO
2
O
+
OH
N
H
O
O
N
Polymerizations of caprolactam should be conducted in inert atmospheres to prevent oxidative
decompositions. These can result in formations of carbon monoxide, carbon dioxide, acetaldehyde,
formaldehyde, and methanol. Caprolactam can even oxidize in air at temperatures between 70 and
100
C[
64
], according to the following scheme:
O
O
H
[O]
O
O
-H
2
O
N
N
N
H
H
H
O
O
COOH
COOH
Much of nylon 6 is used in producing fibers. Polycaprolactam prepared by water catalyzed
polymerizations is best suited for this purpose. It can also be used in molding, though anionically
polymerized caprolactam can be used as well [
65
]. The polymerizations are carried out both in batch
and in continuous processes. Often tubular flow reactors are employed.
A typical polymerization reaction is carried out as follows. Caprolactam, water (5-10% by weight
of monomer), and acetic acid (about 0.1%) are fed into the reactor under nitrogen atmosphere. The
reaction mixture is heated to about 250
C for 12 h. Internal pressure is maintained at 15 atmospheres
by venting off steam. The product of polymerization is extruded as a ribbon, quenched, and chopped
into chips. It consists of about 90% polymer and about 10% low molecular weight compounds and
monomer. The polymer is purified by either water leaching at 85
C or by vacuum extraction of the
undesirable by-products at 180
C.
Castings of nylon 6 are commonly formed in situ in molds. Here the preparation of the polymer
by anionic mechanism is preferred. The catalyst systems consist of 0.1-1.0 mole percent of acetyl
caprolactam and 0.15-0.5 mole percent sodium caprolactam. The reaction temperature is kept
between 140 and 180
C. An exotherm can raise it as much as 50
C as the polymerization
proceeds.
Nylon 7
and
are part of a process developed in Russia to form polyamides for use in fibers.
The process starts with telomerization of ethylene [
66
]. A free-radical polymerization of ethylene is
conducted in the presence of chlorine compounds that act as chain-transferring agents. The reaction is
nylon 9
