polyimide

Polyimide
Density	1430 kg/m3
Young's modulus	3200 MPa
Tensile strength	75-90 MPa
Elongation @ break	4-8%
notch test	4-8 kJ/m
Glass temperature	>400 °C
melting point	none
Vicat softening point	220(?) °C[1]
Thermal conductivity	0.52 W/(m·K)
Coefficient of thermal expansion	5.5×10-5 /K
Specific heat capacity	1.15 kJ/(kg·K)
Water absorption (ASTM)	0.32
Dielectric constant at 1 MHz	3.5

Polyimide (sometimes abbreviated PI) is a polymer of imide monomers. The structure of imide is as shown.

Thermosetting polyimides are commercially available as uncured resins, stock shapes, thin sheets, laminates and machines parts. Thermoplastic polyimides are very often called pseudothermoplastic. There are two general types of polyimides. One type, so-called linear polyimides, are made by combining imides into long chains. Aromatic heterocyclic polyimides are the other usual kind, where R' and R" are two carbon atoms of an aromatic ring. Examples of polyimide films include Apical, Kapton, UPILEX, VTEC PI, Norton TH and Kaptrex. Polyimide parts and shapes include VTEC PI, Meldin, Vespel and Plavis. Polyimides have been in mass production since 1955. Typical monomers include pyromellitic dianhydride and 4,4'-oxydianiline.

Properties

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 p.s.i. and flexural moduli of 3 million p.s.i. Thermoset polyimides exhibit very low creep and high tensile strength. These properties are maintained during continuous use to temperatures of 450 °F (232 °C) and for short excursions, as high as 900 °F (482 °C). 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 480 °F (249 °C) 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.

Application

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. 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 an an insulating layer in the manufacture of digital isolator semiconductor 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.^[2][3]

References

See also

• Polyamide
• Polyamide-imide
• Polyimide Film

Further reading

• Modern Plastic Mid-October Encyclopedia Issue, Polyimide, thermoset, page 146.

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