Linear polyimides

Developed in the 1960s, linear polyimides (IP) are a fast-growing family of specialty polymers, particularly sought-after for applications in harsh environments. Indeed, their properties make them the materials of choice when good dielectric properties or high thermal stability are required, as well as for the design of mechanical parts with low coefficients of friction and high wear resistance.

Their highly aromatic chemical structure, which is responsible for many of their properties, notably through charge-transfer-type interchain interactions, is such that some of these linear polyimides show neither glass transition nor observable melting prior to oxidative decomposition at around 500°C. These particular characteristics make this category of polyimides (type A) a special family of polymers, distinguished from thermoplastics despite their linear nature, and often requiring specific processing techniques. They are classified into two types.

The oldest types, such as Kapton or Vespel, are obtained by sintering a powder under pressure at high temperature. Parts obtained in this way can only be modified by machining, as softening and degradation temperatures are very close. They can be shaped by isostatic sintering or by machining semi-finished parts. They can also be obtained in film form from a solution.

A second, more recent family of polyimides (type B) includes thermoplastics that can be processed using conventional thermoplastic transformation methods such as injection, extrusion or thermoforming (Ultem, Aurum).

The properties sought in polyimides are heat resistance, solvent resistance, good mechanical properties, dimensional stability, low coefficient of friction, excellent radiation resistance, etc. Common applications for polyimides include automotive and aerospace components, surgical instruments and microwave ovens. Their dielectric constant makes them the materials of choice for electrical and electronic applications (circuits, etc.). All polyimides have remarkable properties, but these vary considerably from one PI to another. Type A polyimides, for example, retain their mechanical properties at high temperatures, but their non-processability limits their applications. Type B thermoplastic polyimides have slightly lower temperature resistance, are less resistant to solvents and can suffer thermal degradation.