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Polyimide (sometimes abbreviated PI) is a polymer of imide monomers. The structure of imide is as shown. Polyimides have been in mass production since 1955. Typical monomers include pyromellitic dianhydride and 4,4'-oxydianiline.
According to the composition of their main chain, polyimides can be:
- Aliphatic (linear polyimides),
- Aromatic: R' and R" are two carbon atoms of an aromatic ring. These are the most used polyimides because of their thermostability.
According to the type of interactions between the main chains, polyimides can be:
- Thermoplastic: very often called pseudothermoplastic.
- Thermosetting: commercially available as uncured resins, polyimide solutions, stock shapes, thin sheets, laminates and machined parts.
Several methods are possible to prepare polyimides, among them:
- The reaction between a dianhydride and a diamine (the most used method).
- The reaction between a dianhydride and a diisocyanate.
Thermosetting polyimides are known for thermal stability, good chemical resistance, excellent mechanical properties, and characteristic orange/yellow color. Polyimides compounded with graphite or glass fiber reinforcements have flexural strengths of up to 50,000 psi (340 MPa) and flexural moduli of 3,000,000 psi (21,000 MPa). Thermoset polyimides exhibit very low creep and high tensile strength. These properties are maintained during continuous use to temperatures of up to 452 °C (846 °F) and for short excursions, as high as 704 °C (1,299 °F). Molded polyimide parts and laminates have very good heat resistance. Normal operating temperatures for such parts and laminates range from cryogenic to those exceeding 500 °F (260 °C). Polyimides are also inherently resistant to flame combustion and do not usually need to be mixed with flame retardants. Most carry a UL rating of VTM-0. Polyimide laminates have a flexural strength half life at 249 °C (480 °F) of 400 hours.
Typical polyimide parts are not affected by commonly used solvents and oils — including hydrocarbons, esters, ethers, alcohols and freons. They also resist weak acids but are not recommended for use in environments that contain alkalis or inorganic acids. Some polyimides, such as CP1 and CORIN XLS, are solvent-soluble and exhibit high optical clarity. The solubility properties lend them towards spray and low temperature cure applications.
The polyimide materials are lightweight, flexible, resistant to heat and chemicals. Therefore, they are used in the electronics industry for flexible cables, as an insulating film on magnet wire and for medical tubing. For example, in a laptop computer, the cable that connects the main logic board to the display (which must flex every time the laptop is opened or closed) is often a polyimide base with copper conductors. Examples of polyimide films include Apical, Kapton, UPILEX, VTEC PI, Norton TH and Kaptrex.
The semiconductor industry uses polyimide as a high-temperature adhesive; it is also used as a mechanical stress buffer. Some polyimide can be used like a photoresist; both "positive" and "negative" types of photoresist-like polyimide exist in the market.
An additional use of polyimide resin is as an insulating and passivation layer in the manufacture of digital semiconductor and MEMS chips. The polyimide layers have good mechanical elongation and tensile strength, which also helps the adhesion between the polyimide layers or between polyimide layer and deposited metal layer. The minimum interaction between the gold film and the polyimide film, coupled with high temperature stability of the polyimide film, results in a system that provides reliable insulation when subjected to various types of environmental stresses.
Polyimide powder can be used to produce parts and shapes by sintering technologies (hot compression molding, direct forming, and isostatic pressing). Because of their high mechanical stability even at elevated temperatures they are used as bushings, bearings, sockets or constructive parts in demanding applications. To improve tribological properties compounds with solid lubricants like graphite, PTFE or molybdenum sulfide are common. Polyimide parts and shapes include P84 NT, VTEC PI, Meldin, Vespel and Plavis.
In coal-fired power plants, waste incinerators or cement plants, polyimide fibres are used in hot gas filtration. A polyimide needle felt separates dust and particulate matter from the exhaust gas.
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