For the purposes of this chapter, polyamides are
defined as polymers which contain recurring amide
groups
(-CO-NH-) in the main chain
The most important commercial polymers of
this type are synthetic linear aliphatic polyamides
which are capable of fibre formation : these
polymers are very commonly termed nylons.
There are also available more complex
synthetic aliphatic polyamides which are not fibreforming but which have found use in such
applications as adhesives and coating.
Various synthetic aromatic polyamides and
derivatives have been prepared principally for
evaluation in high temperature applications. Some
of these materials have found limited commercial
use in industry.
Finally a very important class of polyamides
consists of the naturally occurring proteins. The
importance of proteins extends beyond their
various fundamental roles in living matter to
several technological applications such as
adhesives and fibres.
The commercial development of nylons was an
outcome of the fundamental researches into
polymerization which were begun in 1929 by
Carothers of E.I. du pond de Nemours and Co.
Several polyamides and polyesters were
investigated : nylon 6, 6 was first synthesized in
1935 and was selected as most promising fibreforming material. Commercial production was
started 1938 : nylon stacking were put on trial sale
in 1939 and become generally available in the
U.S.A. in 1940. The first nylon mouldings were
produced in 1941 but the polymer did not become
well known in this form until about 1950.
Nylon 6 ,6 continues to be the major commercial
nylon but several other nylons have now achieved
commercial status. The principal nylons which are
currently commercially available are shown below.
For the preparation of these nylons, three general
methods have been developed, namely reaction of
* a diamine and a dicarboxylic acid :
* self condensation of an -aminoacid :
* and ring opening polymerization of a lactam.
The methods usually applied for each nylon is also
shown below. The various nylons are
distinguished from one another by a numbering
system based on the number of carbon atoms in
the starting material.
For the production of polyamide fibres, it is
essential that the polymer is of high molecular
weight i.e. 10 000 or more. In order to achieve
such molecular weight when the polyamide is
prepared by reaction of a diamine and a dibasic
acid, it is necessary to have exact equivalence of
the reactants. It is common practice to ensure
such equivalence by the preparation of a nylon
salt prior to polymerization.
Thus in the production of nylon 6 ,6 the first
step consist of the neutralization of an aqueous
of adipic acid by addition of hexamethylenediamine : the exact end point is determined
electrometrically. The aqueous salt solution is
then concentrated to 60 – 75 % solid content
before transfer to the polymerization reactor. In
an alternative procedure, approximately
equivalent quantities of adipic acid and
hexamethylenediamine are mixed in boiling
methanol, from which the 1 : 1 salt is
precipitated. The salt is isolated by filtration
and then dissolved in water and the solution fed
to the polymerization reactor. The nylon salt,
hexamethylenediammonium adipate may be
represented as follows :
  OOC  (CH 2 )6  COO  



 H 3 N  (CH 2 )6  NH 3 
Both batch and continuos processes are used for
the production of nylon 6. In a typical batch
process, a mixture of caprolactam, water (5-10
% by weight) and acetic acid (about 0.1 %) is
fed into a reactor which has been purged with
nitrogen ( to prevent discoloration by oxygen).
The mixture is heated at about 250 C for 12
hours: a pressure of about 15 atm. is
maintained by venting off steam. The product is
then extruded as ribbon, quenched, quenched in
water and chopped into chips. At this point the
material consists of high molecular weight
polymer (about 90 %) and low molecular
weight compounds ( about 10 %) the latter are
caprolactam (mainly) higher lactams and
aminoacids. In order to obtain the best physical
properties in the final product, the low
molecular weight compounds are removed
either by leaching with water at about 85 C or
by heating at about 180 C at 0.05 atmosphere.
The water-initiated polymerization of caprolactam is believed to proceed according to the
following scheme :
Initiation
Propagation
Besides the amine interchange reaction shown
above, propagation may also involve reaction of
amine and carboxyl end groups :
A newer technique for the preparation of nylon
6 in bulk form consists of the polymerization
casting of caprolactam in situ in the mould. In
this process, rapid polymerization is achieved
by the use of anionic initiators. Anionic
polymerization of caprolactam is effected by
strong bases (B-M+) such as metal amides, metal
hydrides and alkali metals according to the
following scheme:
Initiation
Propagation
The various types of nylons described above
have generally similar physical properties,
being characterized by
* High impact strength,
* Toughness,
* Flexibility and
* Abrasion resistance
See table given below for some comparative
properties.
It bay be noted that the mechanical properties
of nylons are considerably affected by the
amount of crystallization in the piece,
temperature and humidity and it is necessary to
control carefully these factors in the
determination of comparative properties.
The principal structure differences between the
various types of nylons is in the length of the aliphatic
chain segments separating adjacent amide groups.
The amine groups lead to intermolecular hydrogen
bonding so that crystallization ready occurs to give
materials to high melting point and tensile strength.
Polyimides were commercially introduced in
1961 and are made by a two-stages reaction
between a dianhdyride and diamine. Although,
in principal, any dianhdyride and diprimary
amine may be used to synthesize polyimides,
most attention has been given to those cases
where both precursors are aromatic since the
resulting polymers generally have outstanding
thermal stability.
The polyimide which has been preferred for
commercial development is that based on
promellitic dianhyride and 4, 4’-diaminodiphenyl ether and the reaction between these
compounds will illustrate the general method of
preparation of polyimides.
There is also commercially available a class of
polymers related to both polyimides and
polyamides. These polymers are called
poly(amide-imide)s. One such material is
prepared from trimellitic anhydride (such as
acid chloride) and
4,4’-diaminodiphenylmethane as follows :
Poly(amide-imide)s. Are most easily prepared
that polyimides and have better head stability
than polyamides : they have found use as
magnet wire coatings.