Polyimide
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Polyimide (sometimes abbreviated PI) is a
monomer A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization. Classification Chemis ...
containing imide groups belonging to the class of high-performance plastics. With their high heat-resistance, polyimides enjoy diverse applications in roles demanding rugged organic materials, such as high temperature
fuel cell A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen fuel, hydrogen) and an oxidizing agent (often oxygen) into electricity through a pair of redox reactions. Fuel cells are different from most bat ...
s, displays, and various military roles. A classic polyimide is
Kapton file:Kaptonpads.jpg, 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 spacecraft, satellites, and variou ...
, which is produced by condensation of pyromellitic dianhydride and
4,4'-oxydianiline 4,4′-Oxydianiline (ODA) is an organic compound with the chemical formula, formula Oxygen, O(Carbon, C6Hydrogen, H4Nitrogen, NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and Cross-link, cross-linking agent ...
.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 mass production, series production, series manufacture, or continuous production, is the production of substantial amounts of standardized products in a constant flow, including and especially on assembly lines ...
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 (in which all ...
, * Semi-aromatic (also referred to as ''alipharomatic''), *
Aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...
: 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. T ...
. According to the type of interactions between the main chains, polyimides can be: *
Thermoplastic A thermoplastic, or thermosoftening 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 as ...
: very often called ''pseudothermoplastic''. *
Thermoset In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
ting: 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 An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid. In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...
and a
diamine A diamine is an amine with two amino groups. Diamines are used as monomers to prepare polyamides, polyimides, and polyureas. The term ''diamine'' refers mostly to Primary (chemistry), primary diamines, as those are the most reactive. In terms of ...
(the most used method). * The reaction between a dianhydride and a di
isocyanate In organic chemistry, isocyanate is the functional group with the formula . Organic compounds that contain an isocyanate group are referred to as isocyanates. An organic compound with two isocyanate groups is known as a diisocyanate. Diisocyan ...
. The polymerization of a diamine and a dianhydride can be carried out by a two-step method in which a poly(amidocarboxylic acid) 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. Dianhydrides used as precursors to these materials include pyromellitic dianhydride, 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 fami ...
. Common diamine building blocks include
4,4'-diaminodiphenyl ether 4,4′-Oxydianiline (ODA) is an organic compound with the chemical formula, formula Oxygen, O(Carbon, C6Hydrogen, H4Nitrogen, NH2)2. It is an ether derivative of aniline. This colourless solid is a useful monomer and Cross-link, cross-linking agent ...
(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 Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functio ...
. 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 Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
or glass fiber reinforcements have
flexural strength Flexural strength, also known as modulus of rupture, or bend strength, or transverse rupture strength is a material property, defined as the Stress (mechanics), stress in a material just before it Yield (engineering), yields in a flexure test. T ...
s of up to and flexural moduli of . Thermoset polymer matrix polyimides exhibit very low creep and high
tensile strength Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
. 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 retardant Flame retardants are a diverse group of chemicals that are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an combustion, ignition source and pr ...
s. 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
freon Freon ( ) is a registered trademark of the Chemours Company and generic descriptor for a number of halocarbon products. They are stable, nonflammable, low toxicity gases or liquids which have generally been used as refrigerants and as aerosol p ...
s. 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 The electronics industry is the industry (economics), industry that produces electronic devices. It emerged in the 20th century and is today one of the largest global industries. Contemporary society uses a vast array of electronic devices that ar ...
for flexible cables and as an insulating film on magnet wire. 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 file:Kaptonpads.jpg, 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 spacecraft, satellites, and variou ...
, UPILEX, VTEC PI, Norton TH and Kaptrex. Polyimide is used to coat optical fibers 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 (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
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 that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...
is usually made of polyimide coated with thin layers of
aluminum Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium 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 Tribology is the science and engineering of understanding friction, lubrication and wear phenomena for interacting surfaces in relative motion. It is highly interdisciplinary, drawing on many academic fields, including physics, chemistry, material ...
properties, compounds with solid lubricants like
graphite Graphite () is a Crystallinity, crystalline allotrope (form) of the element carbon. It consists of many stacked Layered materials, layers of graphene, typically in excess of hundreds of layers. Graphite occurs naturally and is the most stable ...
,
PTFE Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...
, or molybdenum sulfide are common. Polyimide parts and shapes include P84 NT, VTEC PI, Meldin, Vespel, 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 Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline (petrol), diesel fuel, fuel oil, biodiesel blends, or coal. According to the type of engine, it is discharged into the atmosphere through ...
. 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.


Flexible circuits

Polyimide is used as the core of flexible circuit boards and flat-flex cables. Flexible circuit boards are thin and can be placed in odd-shaped electronics.


Other

Polyimide is used for medical tubing, e.g. vascular
catheters In medicine, a catheter ( ) is a thin tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgical procedure. Catheters are man ...
, 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 advantage ...
; it is also used as a mechanical stress buffer. Some polyimide can be used like a
photoresist A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry. T ...
; both "positive" and "negative" types of photoresist-like polyimide exist in the market. The IKAROS
solar sailing Solar sails (also known as lightsails, light sails, and photon sails) are a method of spacecraft propulsion using radiation pressure exerted by sunlight on large surfaces. A number of spaceflight missions to test solar propulsion and navigatio ...
spacecraft uses polyimide resin sails to operate without rocket engines.


See also

* * * *


References


Further reading

*Modern Plastic Mid-October Encyclopedia Issue, Polyimide, thermoset, p. 146.
Varun Ratta: POLYIMIDES: Chemistry & structure-property relationships – literature review
(Chapter 1).


External links



MIT The Massachusetts Institute of Technology (MIT) is a private research university in Cambridge, Massachusetts, United States. Established in 1861, MIT has played a significant role in the development of many areas of modern technology and sc ...
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