Polyimide (sometimes abbreviated PI) is a

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

containing

imide In organic chemistry, an imide is a functional group consisting of two acyl groups bound to nitrogen. The compounds are structurally related to acid anhydrides, although imides are more resistant to hydrolysis. In terms of commercial applications, ...

groups belonging to the class of

high-performance plastics High-performance plastics are plastics that meet higher requirements than ''standard'' or ''engineering'' plastics. They are more expensive and used in smaller amounts. Definition High performance plastics differ from standard plastics and eng ...

. With their high heat-resistance, polyimides enjoy diverse applications in roles demanding rugged organic materials, e.g. high temperature

fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most batteries in requ ...

s, displays, and various military roles. A classic polyimide is

Kapton Structure of poly-oxydiphenylene-pyromellitimide Kapton insulating pads for mounting electronic parts on a heat sink Kapton is a polyimide film used in flexible printed circuits (flexible electronics) and space blankets, which are used on spac ...

, which is produced by condensation of

pyromellitic dianhydride Pyromellitic dianhydride (PMDA) is an organic compound with the formula C6H2(C2O3)2. It is the double carboxylic acid anhydride that is used in the preparation of polyimide polymers such as Kapton. It is a white, hygroscopic solid. It forms a ...

and

4,4'-oxydianiline 4,4′-Oxydianiline (ODA) is an organic compound with the formula O( C6 H4 NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and cross-linking agent for polymers, especially the polyimides, such as Kapton. Us ...

.Wright, Walter W. and Hallden-Abberton, Michael (2002) "Polyimides" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim.

History

The first polyimide was discovered in 1908 by Bogart and Renshaw. They found that 4-amino phthalic anhydride does not melt when heated but does release water upon the formation of a high molecular weight polyimide. The first semialiphatic polyimide was prepared by Edward and Robinson by melt fusion of diamines and tetra acids or diamines and diacids / diester. However, the first polyimide of significant commercial importance - Kapton - was pioneered in the 1950s by workers at Dupont who developed a successful route for synthesis of high molecular weight polyimide involving a soluble polymer precursor. Up to today this route continues being the primary route for the production of most polyimides. Polyimides have been in

mass production Mass production, also known as flow production or continuous production, is the production of substantial amounts of standardized products in a constant flow, including and especially on assembly lines. Together with job production and batch ...

since 1955. The field of polyimides is covered by various extensive books and review articles.

Classification

According to the composition of their main chain, polyimides can be: *

Aliphatic In organic chemistry, hydrocarbons ( compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. ''aleiphar'', fat, oil). Aliphatic compounds can be saturated, like hexane, or ...

, * Semi-aromatic (also referred to as ''alipharomatic''), *

Aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

: these are the most used polyimides because of their

thermostability In materials science and molecular biology, thermostability is the ability of a substance to resist irreversible change in its chemical or physical structure, often by resisting decomposition or polymerization, at a high relative temperature. ...

. According to the type of interactions between the main chains, polyimides can be: *

Thermoplastic A thermoplastic, or thermosoft plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling. Most thermoplastics have a high molecular weight. The polymer chains associate ...

: very often called ''pseudothermoplastic''. * Thermosetting: commercially available as uncured resins, polyimide solutions, stock shapes, thin sheets, laminates and machined parts.

Synthesis

Several methods are possible to prepare polyimides, among them: * The reaction between a di anhydride and a

diamine A diamine is an amine with exactly two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to primary diamines, as those are the most reactive. In terms of quantities p ...

(the most used method). * The reaction between a dianhydride and a di isocyanate. The polymerization of a diamine and a dianhydride can be carried out by a two-step method in which a

poly(amidocarboxylic acid) Poly, from the Greek πολύς meaning "many" or "much", may refer to: Businesses * China Poly Group Corporation, a Chinese business group, and its subsidiaries: ** Poly Property, a Hong Kong incorporated Chinese property developer ** Poly Real ...

is prepared first, or directly by a one-step method. The two-step method is the most widely used procedure for polyimide synthesis. First a soluble poly(amidocarboxylic acid) (2) is prepared which is cyclized after further processing in a second step to the polyimide (3). A two-step process is necessary because the final polyimides are in most cases infusible and insoluble due to their aromatic structure. Polyimide Formation (schematic) V1

Dianhydrides used as precursors to these materials include

, benzoquinonetetracarboxylic dianhydride and

naphthalene tetracarboxylic dianhydride Naphthalenetetracarboxylic dianhydride (NTDA) is an organic compound related to naphthalene. The compound is a beige solid. NTDA is most commonly used as a precursor to naphthalenediimides (NDIs) (such as napthalenetetracarboxylic diimide), a fam ...

. Common diamine building blocks include

4,4'-diaminodiphenyl ether 4,4′-Oxydianiline (ODA) is an organic compound with the formula O( C6 H4 NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and cross-linking agent for polymers, especially the polyimides, such as Kapton. Us ...

("DAPE"), meta-phenylenediamine ("MDA"), and 3,3-diaminodiphenylmethane. Hundreds of diamines and dianhydrides have been examined to tune the physical and especially the processing properties of these materials. These materials tend to be insoluble and have high softening temperatures, arising from charge-transfer interactions between the planar subunits.

Analysis

The imidization reaction can be followed via IR spectroscopy. The IR spectrum is characterized during the reaction by the disappearance of absorption bands of the poly(amic acid) at 3400 to 2700 cm⁻¹ (OH stretch), ~1720 and 1660 (amide C=O) and ~1535 cm⁻¹ (C-N stretch). At the same time, the appearance of the characteristic imide bands can be observed, at ~1780 (C=O asymm), ~1720 (C=O symm), ~1360 (C-N stretch) and ~1160 and 745 cm⁻¹ (imide ring deformation).⁠　Detailed analyses of polyimide and carbonized polyimide and graphitized polyimide have been reported.

Properties

Thermosetting polyimides are known for thermal stability, good chemical resistance, excellent mechanical properties, and characteristic orange/yellow color. Polyimides compounded with

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...

or glass fiber reinforcements have flexural strengths of up to and flexural moduli of .

Thermoset polymer matrix A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as gla ...

polyimides exhibit very low

creep Creep, Creeps or CREEP may refer to: People * Creep, a creepy person Politics * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign Art ...

and high

tensile strength Ultimate tensile strength (UTS), often shortened to tensile strength (TS), ultimate strength, or F_\text within equations, is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials t ...

. These properties are maintained during continuous use to temperatures of up to and for short excursions, as high as . Molded polyimide parts and laminates have very good heat resistance. Normal

operating temperature An operating temperature is the allowable temperature range of the local ambient environment at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the de ...

s for such parts and laminates range from cryogenic to those exceeding . 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 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.

Applications

Insulation and passivation films

Polyimide materials are lightweight, flexible, resistant to heat and chemicals. Therefore, they are used in the electronics industry for flexible cables and as an insulating film on

magnet wire Magnet wire or enameled wire is a copper (Cu) or aluminium (Al) wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, generators, speakers, hard disk head actuators, electroma ...

. 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,

UPILEX Upilex is a heat-resistant polyimide film that is the product of the polycondensation reaction between biphenyl tetracarboxylic dianhydride ( BPDA) monomers and a diamine. Its properties include dimensional stability, low water absorption, high ch ...

, VTEC PI, Norton TH and Kaptrex. Polyimide is used to coat

optical fibers An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass (silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a means ...

for medical or high temperature applications. An additional use of polyimide resin is as an insulating and passivation layer in the manufacture of

Integrated circuit An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...

s 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 is also used as a substrate for cellphone antennas. Multi-layer insulation used on

spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...

is usually made of polyimide coated with thin layers of

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...

, silver, gold, or germanium. The gold-colored material often seen on the outside of spacecraft is typically actually single aluminized polyimide, with the single layer of aluminum facing in. The yellowish-brown polyimide gives the surface its gold-like color.

Mechanical parts

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

PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. It is one of the best-known and widely applied PFAS. The commonly known brand name of PTFE-based composition is Teflon by Chemour ...

, or

molybdenum sulfide Molybdenum disulfide (or moly) is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is . The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenit ...

are common. Polyimide parts and shapes include P84 NT, VTEC PI, Meldin,

Vespel Vespel is the trademark of a range of durable high-performance polyimide-based plastics made by DuPont. The one shown in the structure on the right was the first to be commercialized. Characteristics and applications Vespel is mostly used in ...

, and Plavis.

Filters

In coal-fired power plants, waste incinerators, or cement plants, polyimide fibres are used to filter hot gases. In this application, a polyimide needle felt separates dust and particulate matter from the exhaust gas. Polyimide is also the most common material used for the reverse osmotic film in purification of water, or the concentration of dilute materials from water, such as maple syrup production.

Other

Polyimide is used for medical tubing, e.g. vascular catheters, for its burst pressure resistance combined with flexibility and chemical resistance. The semiconductor industry uses polyimide as a high-temperature

adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...

; 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. The

IKAROS IKAROS (Interplanetary Kite-craft Accelerated by Radiation Of the Sun) is a Japan Aerospace Exploration Agency (JAXA) experimental spacecraft. The spacecraft was launched on 20 May 2010, aboard an H-IIA rocket, together with the ''Akatsuki'' (V ...

solar sailing Solar sails (also known as light sails and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large mirrors. A number of spaceflight missions to test solar propulsion and navigation have been ...

spacecraft uses polyimide resin sails to operate without rocket engines.

References

External links

MIT {{Plastics Biomaterials Dielectrics Organic polymers Thermoplastics Thermosetting plastics