Polyimide
Polyimides (abbreviated PI) are plastics whose main structural feature is the imide group. These include Polybismaleinimid ( PBMI ), polybenzimidazole ( PBI) and Polyoxadiazobenzimidazol (PBO), polyimide sulphone ( PISO ) and polymethacrylimide (PMI).
Polyimides containing more structural elements such as ester groups, amide groups, etc., form their own groups of substances such as polyetherimide (PEI ), polyamide-imides ( PAI).
Production
Polyimides prepared by polycondensation of tetracarboxylic acid dianhydrides and diamines. For thermally maximum load only polyimides are suitable for containing aromatic building blocks in the polymer chain. These products can not be processed in liquid form because of the insolubility and extremely high melting points or missing. Therefore, the preparation takes place in a two - step process wherein the first stage can be processed smoothly and only in the second step to the polyimide (C) is condensed. The starting components are aromatic tetracarboxylic acid anhydrides such as pyromellitic dianhydride ( 1,2,4,5- benzenetetracarboxylic dianhydride ) or biphenyltetracarboxylic acid dianhydride and aromatic diamines such as 4,4 ' -diaminodiphenyl ether. The following diagram illustrates the procedure:
The anhydride (A) is reacted with the diamine in an anhydrous polar solvent such as N- methyl-2- pyrrolidone (NMP) or dimethylformamide (DMF ) for the polyamic acid (B). The solution thus obtained can be poured or applied as a coating. After evaporation of the solvent and the use of high temperatures ( enamel ), the reaction to produce polyimide ( C) elimination of water takes place. A typical application is the magnet wire insulation. Since the polyamidocarboxylic are quite corrosive and the process for the user is often complex, attempts, in the liquid products preferable to the step of imide formation as much as possible. Photosensitive polyimides for printed circuits, etc. can be obtained with methacrylic acid over glycols by esterification of the acid groups in ( B). This can be fixed by exposure, dissolved the unexposed parts again and then the fixed material can be thermally converted to polyimide.
Properties and Applications
Purely aromatic polyimides are often infusible and chemically resistant (including to many solvents ). For use as a coating agent is in the solvents: DMF, DMAc or NMP dissolved polyimide. Polyimides are brought to application in electrical / electronics because of their heat resistance, low outgassing, radiation resistance and insulating properties in the form of light brownish, semitransparent films. A well-known product is Kapton from DuPont. High continuous operating temperatures up to 230 ° C and for short periods up to 400 ° C are possible. Property profiles can be found in the dealer and the manufacturer's instructions.
Polyimides are often used for very thin, yet quite stable varnish insulation of electrical wiring, including for weight savings in aircraft technology. Due to their small compared to the conductor thickness such insulations are independent of the material sensitive to mechanical loads (see arc tracking).
For the generation of X-ray radiation, polyimides are widely used as a versatile, robust and inexpensive materials for X-ray windows. Prerequisite for this is their thermal stability and high transmittance for X-rays. In addition, when a high absorption and reflection is necessary for light in the visible range, beryllium is most likely used.
Polyimide also be used in reverse osmosis as a selectively permeable membrane - such as dialysis or in the seawater desalination.
Structural parts or semi-finished products made of polyimide prepared by sintering techniques such as hot compression molding, isostatic pressing or forming Direct from polyimide. Due to the high mechanical strength even at high temperatures, bushings, bearings, guides or gaskets of PI are used in thermally demanding applications. The tribological properties can be specifically adapted by compounds with solid lubricants such as graphite, molybdenum disulfide or PTFE. Well-known trade name for polyimide parts are Vespel, Meldin, Plavis or P84 NT.
Polyimide fibers are used as needle felts in hot gas filtration. Here are removed from the flue gas of coal-fired power plants, waste incineration plants or cement factories fine dusts on filter fabrics from PI.