For the Purposes of This Chapter, Polyamides Are Defined As Polymers Which Contain Recurring
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 fibre-forming 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 fibre-forming 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 hexamethylene-diamine : 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 :
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 capro-lactam 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.
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’-diamino-diphenyl 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’-diaminodiphenyl-methane 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.