Polyimide resins are soluble thermoplastic or thermoset polyimides used for high-temperature adhesives and composites. They are a class of thermally stable resins that have proven to be technically important due to their strength at high temperatures and their high resistance to oxidative degradation.
Polyimide resins are amber to transparent in color and compete with polyketones, polyarylates, and polysulfones in engineering applications. The limitations of polyimide resins include their expense. their high processing temperatures, and their inability to be used above their glass transition temperature unless post annealed. Most polyimides are processed in precursor form, otherwise they decompose before their melting or softening points are reached. Some can be processed by injection molding and extrusion. They are available as films and common stock shapes.
Polyimides belong to a class of high-performance plastics with high-heat resistance, which enjoy diverse application sin roles demanding rugged organic materials, including temperature fuel cells, displays, and various military roles.
Polyimide resins are used to make a permanent and leak-free union between two pieces of silica tubing, stainless steel, or glass and press fit unions. They are also used in the electronics industry for flexible cables as an insulating film on magnet wire and for medical tubing. These resins have been proven to be good electrical insulators over a wide range of temperatures, from below zero to 400 degrees Celsius. And it can be used as a mechanical stress buffer. Some polyimide resins are manufactured for use in high-temperature composite parts for the aerospace industry and industries where parts are replaced by metal and exposed to high temperatures.
Polyimide resins are prepared by condensation polymerization of aromatic primary diamines and aromatic tetracarboxylic dianhydrides. The aromatic ring structure along the polymer backbone results in the polyimide resin's thermal, mechanical, and chemical properties. Most polymer composites consist of some sort of fiber, such as glass, used to reinforce the matrix; in turn, these fibers are embedded in an epoxy or similar material resin to result in a stronger material. The glass used in the material creates the glass transition temperature limitation.
Polyimide resin classification
The first polyimide was discovered in 1908 by Bogart and Renshaw. There was early interest in the use of polymers and polyimide resins for the aerospace industry after the Second World War. These attempts were focused on the synthesis of aliphatic polyimides.
In the 1970s, the United States Air Force and NASA-funded programs formulated the first polyimides. These early polymerized monomeric reactant (PMR) resins, based in aromatic dianhydride ester acid and aromatic diamine chemistry polymerized through solvent addition, have since become an aerospace industry standard, with the PMR-15 and PMR-II-50 formulations. These PMR composites (also known as carbon fiber) outperform metals with greater strength-to-weight properties in aircraft engine nozzles and nacelles, helicopter gear cases, and missile fins operating in the range of 242 to 342 degrees Celsius.
NASA began the development of another series of polyimides that differ from PMRs after the PMR resin's methylene dianiline (MDA) was suspected of causing cancer in the liver. As well, they were looking for improved polyimide resins. This resulted in phenylethynyl-terminated imide (PETI) oligomers, which received consideration for "hot section" military engine components. PETI resins deliver improved processing properties, including prolonged melt stability, minimized post-cure moisture uptake, and better control of the outgassing of volatiles.
In 2019, the Air Force Research Laboratory, in cooperation with researchers from the University of Louisville and NASA's Glen Research Center, developed a polyimide resin capable of being used in additive manufacturing and printed, reinforced polymer composite parts. This resin presented the highest temperature-capable material of its kind to be on record. The success of the material could be an element in a more cost-efficient generation of manufacturing for the United States Air Force and other aviation entities. These materials could have practical applications for 3D printed parts in turbine engines and engine exhaust hot areas.
