While SAMPE is celebrating its 75th anniversary in 2019, the anniversary of polyimide mass production is only 10 years behind. Although the raw materials and monomers were discovered much earlier, polyimides (PI) were made practical for use by Edwards and Robertson’s patent in 1955. The excellent electrical, thermal, and high-temperature mechanical properties make these organic polymers attractive. Originally, PI resins were developed for the microelectronics industry in the 1960s (wire insulation and flexible electronics substrates) but very quickly saw aerospace applications during the 1970s and 1980s (e.g., radomes, warm exhaust wash structure, and armament components) and also medical and biomedical applications in the late 1990s (e.g., implants, sensors, and instruments). All of these applications are focused on PI as a resin matrix or film. While rarer, applications can also include PIs as synthetic fibers (i.e. Evonik’s P84).
PIs are aromatic-heterocyclic polymeric resins that consist of many imides and cure by two types of mechanisms: 1.) condensation reactions involving polyamic acids and exhibiting thermoplastic characteristics and 2.) addition reactions between unsaturated groups in preformed imide monomers or oligomers exhibiting cross-linked thermosetting behavior.
Condensation-type PI chemistry is a two-step process involving the condensation of anhydrides and diamines to form polyamic acid followed by cyclodehydration to form the PI. Another family of condensation PIs with very high thermal stability is Avimid® products from DuPont, now DowDuPont, and Skybond® from Monsanto, now Industrial Summit Technology Corp. The limitation of processing in the polyamic acid stage during the evolution of water by-products often makes it difficult to fabricate thick parts.
Addition-type PIs were developed originally by NASA Lewis Research Center in the 1970s. Known as Polymerization of Monomer Reactants (PMR), the chemistry involves ester solutions dissolved in methanol or ethanol to give a varnish. Imidization via condensation occurs, and finally, in the last stage, the oligomers undergo a cross-linking addition reaction without evolving volatiles. The ability to use low boiling point solvents is appealing in contrast to the high molecular weight polyamic acids found in condensation PIs (i.e. these are soluble only in high boiling point, difficult to remove solvents, like n-methyl-2-pyrrolidone (NMP)). Most notably NASA Lewis Research Center developed PMR-15 in 1971.
Significant work from the U.S. Government agencies (NASA Langley and Lewis, Navy, Army, and the Air Force Research Laboratory) led to more addition reaction PI development. In particular, the Air Force and Hughes Aircraft Company developed acetylene terminated PIs which allowed for high-temperature curing via the triple bond of an addition reaction, without volatile evolution. Thermid 600® by Hughes Aircraft is one notable example. Similarly, NASA Langley developed a series of Phenyl Ethynly Terminated Imide (PETI) systems in the mid-1990’s for Supersonic Transport that offered a much larger processing window and even higher thermal oxidative stability. In contrast to the PMR solution, Version Five (PETI-5) with an average molecular weight of 5,000 g/mol is prepared as a polyamic acid/NMP solution followed by imidization and then addition crosslinking.
Benefits of PIs include high glass transition temperatures (250-300+°C), high thermal and thermal oxidative stability, good chemical resistance, and high mechanical strength and modulus. Nevertheless, when compared to epoxies, PIs are prone to more part springback and warpage (likely due to the high cure temperatures and pressures required), higher levels of porosity (especially for condensation-type PIs), higher moisture uptake (near 3% and possibly related to large free volume), substantial reductions in wet glass transition temperature (due to hydrolytic instability of nonbornene crosslinks), resin microcracking, and blistering. While this is a brief synopsis of the historical progress of PIs, goals remain to overcome the weaknesses mentioned here, and the motivation to continue to push the thermal stability of organic polymers to extreme limits will enable polyimide research for the next 75 years.