Chemistry Reference
In-Depth Information
Nylon 6,6
is a condensation product of hexamethylene diamine and adipic acid. This polyamide
was originally synthesized in 1935 and first produced commercially in 1938. It is still one of the major
commercial nylons produced today. Because high molecular weight is required for such polymers to
possess good physical properties, it is necessary to follow exact stoichiometry of the reactants in the
condensation. To achieve that, the practice is to initially form a “nylon salt,” prior to the polymeriza-
tion. To do this, equimolar quantities of adipic acid and hexamethylene diamine are combined in
aqueous environment to form solutions of the salt. The end point is controlled electrochemically. An
alternate procedure is to combine the diacid with the diamine in boiling methanol. A 1:1 adduct
precipitates out, is filtered off, and dissolved in water.
+
n
H
2
N
NH
2
n
6
HOOC
COOH
4
OOC
COO
n
6
H
3
N
NH
3
4
A 60-75% solution of the salt in water is then fed into a reaction kettle. In a typical batch
process, some acetic acid may also be added if it is desired to limit molecular weight
(10,000-15,000). The temperature in the reaction kettle is raised to 220
C, and due to water and
steam in the reactor, internal pressure of about 20 atmospheres develops. After 1-2 h, the temperature
is raised to 270-280
C. Some steam is bled off to maintain internal pressure at 20 atmospheres. The
temperature is maintained and the bleeding out of the steam is continued for 2 h. During that period,
the internal pressure is gradually reduced to atmospheric. In some processes, vacuum is applied at this
point to the reaction kettle if high molecular weight products are desired. When the reaction is
complete, the molten polymer is ejected from the kettle by applying pressure with nitrogen or carbon
dioxide.
In one continuous process, the desired conditions are maintained while the reaction mixture moves
through various zones of the reactor. Tubular reactors are also often employed in continuous
polymerizations.
Nylon 6.10
is prepared by the same procedure as nylon 6,6 from a salt of hexamethylene diamine
and sebacic acid, while
is prepared from a salt of hexamethylene diamine and azelaic acid.
The melting points of various nylons that are formed from diamines and dicarboxylic acids are
presented in Table
7.5
.
One commercial polyamide is prepared by condensation of a cycloaliphatic diamine with a twelve
carbon dicarboxylic acid. The diamine, bis(
nylon 6,9
p
-aminocyclohexyl)methane, is prepared from aniline:
NH
2
+
2
O
H
2
N
NH
2
[H]
H
2
N
NH
2
Ru catalyst
