Polyamide-imide

Polyamide-imides are thermoplastic amorphous polymers that have exceptional mechanical, thermal and chemical resistant properties. These properties put polyamide-imides at the top of the price and performance pyramid. Other high-performance polymers in this same realm are polyetheretherketones and polyimides.

Polyamide-imides hold, as the name suggests, a positive synergy of properties from both polyamides and polyimides, such as high strength, melt processibility, exceptional high heat capability, and broad chemical resistance. Polyamide-imide polymers can be processed into a wide variety of forms – from injection or compression molded parts and ingots – to coatings, films, fibers and adhesives. Generally these articles reach their maximum properties with a subsequent thermal cure process.

Chemistry

The currently popular commercial methods to synthesize polyamide-imides are the acid chloride route and the socyanate route.

Acid Chloride Route

Reaction of the anhydride with the diamine produces an intermediate amic acid. The acid chloride functionality reacts with the aromatic amine to give the amide bond and hydrochloric acid (HCl) as a by-product. In the commercial preparation of polyamideimides,the polymerization is carried out in a dipolar, aprotic solvent such as N-methylpyrrolidone(NMP), dimethylacetamide (DMAC), dimethylformamide (DMF), or dimethylsulfoxide (DMSO) at temperatures between 20-60°C. The byproduct HCl must be neutralized in situ or removed by washing it from the precipitated polymer. Further thermal treatment of the polyamideimide polymer increases molecular weight and causes the amic acid groups to form imides with the evolution of water.

Diisocyanate Route

This is the primary route to polyamide-imides which are used as wire enamels. A diisocyanate, often 4,4’-methylenediphenyldiisocyanate (MDI), is reacted with trimellitic anhydride (TMA). The product achieved at the end of this process is a high molecular weight, fully imidized polymer solution with no condensation byproducts, since the carbon dioxide gas byproduct is easily removed. This form is convenient for the manufacture of wire enamel or coatings. The solution viscosity is controlled by stoichiometry, monofunctional reagents, and polymer solids. The typical polymer solids level is 35-45% and it may be diluted further by the supplier or user with diluents.

References

Patel, M.C. and Shah, A.D., Poly(amides-imides)based on amino end-capped polyoligomides, "Oriental J. Chem", 19(1), 2002

James M. Margolis, editor in chief, "Engineering plastics handbook ", ISBN 0071457674 , McGraw-Hill, c2006