Page 303 - Engineering Plastics Handbook

Polyamide-imide (PAI) 263

The monoacid functionality of TMA reacts directly with the isocyanate
functionality to give the mixed anhydride, again reducing to the amide
bond with release of a molecule of CO (Fig. 12.10).
2
In isocyanate chemistry many unintended side reactions can occur.
The amine formed from the unstable carbamic acid can react with iso-
cyanate, forming a urea. This secondary nitrogen urea or any other
amide nitrogen may react further with another isocyanate, forming a
cross-link at this new biuret functional group. The amine may also react
with the anhydride on TMA, forming the amic acid. And, depending on
temperature, the isocyanate may react with the solvent itself. All these
side reactions and their influences on stoichiometry must be fully under-
stood and controlled.
Commercially the isocyanate process is also done in a dipolar aprotic
solvent, typically NMP. The isocyanate route is performed at higher tem-
peratures to evolve CO gases and promote imidization reactions. The
2
reaction temperature is ramped over time to promote all the various reac-
tions shown above. One typical profile starts the reaction at 80°C with the
temperature increased at a rate of 30°C/h to 200°C over 4 h. Here the max-
imum CO evolution occurs in the 120 to 130°C range, with no more evi-
2
dence of isocyanate functional groups present [13].
The polyamide-imide achieved at the end of this process is a high-
molecular-weight, fully imidized polymer in solvent with no condensation
by-products, since the carbon dioxide gas is easily removed. This con-
venient form makes it especially beneficial for the manufacturer where
the primary end use is wire enamel or another coating application. The
solution viscosity is thus controlled by stoichiometry, monofunctional
reagents, and polymer solids. The typical polymer solids level is 35 to 45%
and it may be diluted further by the supplier or customer with diluents.
The polymer solution may also be spun into polyamide-imide fibers
®
by using a wet or dry process. Fibers under the trade name Kermel are
manufactured in this manner, formerly by Rhodia.
As inferred from the reaction schemes shown above, there is the
potential for many side reactions. These complex reactions lead to a
polyamide-imide with potential branch points and a slightly irregular
polymer backbone compared to the acid chloride route polyamide-
imide. All the possible reactions are dependent on many parameters
including the solvent, monomer concentration, purity of reagents,
water content, and temperature reaction profile. Because of the slightly
irregular backbone and branching which reduce thermal stability,
isocyanate-based PAIs are not seen in melt-processable PAI compounds.
Another reason is that MDI is often the di-isocyanate of choice. The meth-
ylene moiety is not stable at melt processing temperatures, leading to
cross-linked material which does not flow well. In the acid chloride case,