Polyimide (sometimes abbreviated PI) is a polymer 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 cells, displays, and various military roles. A classic polyimide is Kapton, which is produced by condensation of pyromellitic dianhydride and 4,4'-oxydianiline. [1]
The first polyimide was discovered in 1908 by Bogart and Renshaw. [2] 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. [3]
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 since 1955. The field of polyimides is covered by various extensive books [4] [5] [6] and review articles. [7] [8]
According to the composition of their main chain, polyimides can be:
According to the type of interactions between the main chains, polyimides can be:
Several methods are possible to prepare polyimides, among them:
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. Common diamine building blocks include 4,4'-diaminodiphenyl ether (DAPE), meta-phenylenediamine (MDA) and 3,3'-diaminodiphenylmethane. [1] 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. [9]
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). [10] Detailed analyses of polyimide [11] and carbonized polyimide [12] and graphitized polyimide [13] have been reported.
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 340 MPa (49,000 psi) and flexural moduli of 21,000 MPa (3,000,000 psi). Thermoset polymer matrix polyimides exhibit very low creep and high tensile strength. These properties are maintained during continuous use to temperatures of up to 232 °C (450 °F) and for short excursions, as high as 704 °C (1,299 °F). [14] Molded polyimide parts and laminates have very good heat resistance. Normal operating temperatures for such parts and laminates range from cryogenic to those exceeding 260 °C (500 °F). 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.
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. 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.
Polyimide is used to coat optical fibers for medical or high temperature applications. [15]
An additional use of polyimide resin is as an insulating and passivation [16] layer in the manufacture of Integrated circuits 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. [17] [18] Polyimide is also used as a substrate for cellphone antennas. [19]
Multi-layer insulation used on spacecraft is usually made of polyimide coated with thin layers of aluminum, 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. [20] The yellowish-brown polyimide gives the surface its gold-like color.
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 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. [21] [22]
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. [23]
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; 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 solar sailing spacecraft uses polyimide resin sails to operate without rocket engines. [24]
Carbon fibers or carbon fibres are fibers about 5 to 10 micrometers (0.00020–0.00039 in) in diameter and composed mostly of carbon atoms. Carbon fibers have several advantages: high stiffness, high tensile strength, high strength to weight ratio, high chemical resistance, high-temperature tolerance, and low thermal expansion. These properties have made carbon fiber very popular in aerospace, civil engineering, military, motorsports, and other competition sports. However, they are relatively expensive compared to similar fibers, such as glass fiber, basalt fibers, or plastic fibers.
Flexible electronics, also known as flex circuits, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates, such as polyimide, PEEK or transparent conductive polyester film. Additionally, flex circuits can be screen printed silver circuits on polyester. Flexible electronic assemblies may be manufactured using identical components used for rigid printed circuit boards, allowing the board to conform to a desired shape, or to flex during its use.
A polyamide is a polymer with repeating units linked by amide bonds.
Kapton is a polyimide film used in flexible printed circuits and space blankets, which are used on spacecraft, satellites, and various space instruments. Invented by the DuPont Corporation in the 1960s, Kapton remains stable across a wide range of temperatures, from 4 to 673 K. Kapton is used in electronics manufacturing, space applications, with x-ray equipment, and in 3D printing applications. Its favorable thermal properties and outgassing characteristics result in its regular use in cryogenic applications and in situations where high vacuum environments are experienced.
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, imides are best known as components of high-strength polymers, called polyimides. Inorganic imides are also known as solid state or gaseous compounds, and the imido group (=NH) can also act as a ligand.
Polyamide-imides are either thermosetting or thermoplastic, amorphous polymers that have exceptional mechanical, thermal and chemical resistant properties. Polyamide-imides are used extensively as wire coatings in making magnet wire. They are prepared from isocyanates and TMA in N-methyl-2-pyrrolidone (NMP). A prominent distributor of polyamide-imides is Solvay Specialty Polymers, which uses the trademark Torlon.
An organic acid anhydride is an acid anhydride that is also an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the parent acid is a carboxylic acid, the formula of the anhydride being (RC(O))2O. Symmetrical acid anhydrides of this type are named by replacing the word acid in the name of the parent carboxylic acid by the word anhydride. Thus, (CH3CO)2O is called acetic anhydride.Mixed (or unsymmetrical) acid anhydrides, such as acetic formic anhydride (see below), are known, whereby reaction occurs between two different carboxylic acids. Nomenclature of unsymmetrical acid anhydrides list the names of both of the reacted carboxylic acids before the word "anhydride" (for example, the dehydration reaction between benzoic acid and propanoic acid would yield "benzoic propanoic anhydride").
In polymer chemistry, step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Many naturally-occurring and some synthetic polymers are produced by step-growth polymerization, e.g. polyesters, polyamides, polyurethanes, etc. Due to the nature of the polymerization mechanism, a high extent of reaction is required to achieve high molecular weight. The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain—each person has two hands. There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers production take place.
Polyphthalamide is a subset of thermoplastic synthetic resins in the polyamide (nylon) family defined as when 55% or more moles of the carboxylic acid portion of the repeating unit in the polymer chain is composed of a combination of terephthalic (TPA) and isophthalic (IPA) acids. The substitution of aliphatic diacids by aromatic diacids in the polymer backbone increases the melting point, glass transition temperature, chemical resistance and stiffness.
Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.
Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include naturally occurring chemicals, such as in plants and insects, as well as synthetics such as polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not. Synthetic polyesters are used extensively in clothing.
4,4′-Oxydianiline (ODA) is an organic compound with the formula O(C6H4NH2)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.
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 glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.
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 chemical resistance and high mechanical properties, high heat and chemical resistance. It was developed by UBE Industries. Upilex-S is the standard grade but other grades include Upilex-RN, VT, CA and SGA. Upilex-S is used when excellent mechanical properties are required. Upilex-RN possesses excellent molding processability, while Upilex-VT has superior heat bonding characteristics. General applications of Upilex include their use in flexible printed circuits, electric motor and generator insulation, high temperature wire and cable wrapping, and specialty pressure sensitive tapes. Polyimides have also been extensively studied in gas and humidity sensors, where the concentration is determined by monitoring the capacitance of modified Upilex films. With advantages of flexibility and easy functionalization, Upilex films are often used as substrate materials in biosensor platforms. For instance, it is possible to electropolymerize onto these films or attach enzymes to it for the detection of glucose.
BPDA or biphenyl-tetracarboxylic acid dianhydride is a monomer used in the production of some polyimides.
Graphitic carbon nitride (g-C3N4) is a family of carbon nitride compounds with a general formula near to C3N4 (albeit typically with non-zero amounts of hydrogen) and two major substructures based on heptazine and poly(triazine imide) units which, depending on reaction conditions, exhibit different degrees of condensation, properties and reactivities.
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 hydrate.
Cardo polymers are a sub group of polymers where ring structures are pendent to the polymer backbone. The backbone carbons bonded to the pendent ring structures are quaternary centers. As such, the cyclic side group lies perpendicular to the plane of the extended polymer chain. These side rings are bulky structures which sterically hinder rotation of the backbone bonds; they also disrupt chain packing and thus create greater free volume than found in conventional polymer structures. The rotational restrictions placed on the polymer backbone increase the value of the characteristic ratio, leading to what is referred to as a "rigid" or "stiff" backbone. The sterically driven large rotational potential produces a high glass transition temperature and the disrupted packing yields comparatively high values for gas and other small molecule solubilities in these polymers. Because of these physical effects, recent advances in membranes used for gas separation have used cardo polymers.
4,4′-(Hexafluoroisopropylidene)diphthalic anhydride (6FDA) is an aromatic organofluorine compound and the dianhydride of 4,4′-(hexafluoroisopropylidene)bisphthalic acid.
3,3’,4,4’-Benzophenone tetracarboxylic dianhydride (BTDA) is chemically, an aromatic organic acid dianhydride. It may be used to cure epoxy-based powder coatings. It has the CAS Registry Number of 2421-28-5 and a European Community number 219-348-1. It is REACH and TSCA registered. The formula is C17H6O7 with a molecular weight of 322.3.
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