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260 Engineering Plastics

O C N CH 2 N C O

Figure 12.3 MDI.

With a high concentration of amic acid groups along the polymer back-
bone, the polymer at this stage can be formulated into aqueous solutions
with the aid of a tertiary amine, which reacts to create the ammonium
salt of the amic acid [11].
Further thermal treatment (k ) of the polyamide-imide polymer
3
increases the molecular weight and imidization level. In addition to
thermal treatment, polyamide-imides can subsequently cyclize through
chemical imidization. The chemical treatment is accomplished by using
a tertiary amine and an anhydride; typically triethylamine and acetic
anhydride are common, while other variants have also been used with
success [12].
All polyamide-imides which are fabricated in some fashion, whether
it is an injection-molded part, an extruded rod, or a thin coating, must
be thermally cured in order for the material to reach its maximum prop-
erties. The cure schedule depends on the geometry and thickness of the
part. Injection- and compression-molded parts require a longer postcure
due to their inherent thickness and geometry, compared to polyamide-
imide coatings, which are likely in the thickness range of 1 mil (25 µm)
or less. The curing process further increases the molecular weight of the
polymer, drives imidization of the amic acid, and induces cross-linking
reactions. Once fully cured, polyamide-imide articles are no longer melt-
processable or soluble. A general guide to cure schedules for polyamide-
imide coatings and injection molded and compression-molded parts is
indicated in the Processing section (see Table 12.8).

Isocyanate route
The other most popular method of synthesizing commercial PAI uses the
so-called isocyanate route. This is the primary route to polyamide-imides
which are then utilized as wire enamels or fibers. This PAI chemistry
involves the reaction of a di-isocyanate, often 4,4′-methylenediphenyldi-
isocyanate (MDI) (Fig. 12.3) with trimellitic anhydride (TMA) (Fig. 12.4).
To overcome the lower reactivity of the carboxylic acid functionality in

O
O
HO
O O
Figure 12.4 TMA.