High Temperature High Performance Polymers:
High Performance Polymers films are thermoplastic and are generally transparent. The are needed for use in applications where temperatures vary from very low (- 269°C) to very high (+ 400°C) values. Such applications include space shuttle solar arrays, high speed locomotive and other motor armature slot liners, capacitors, transformers, miniaturized electronic components, microprocessor chip carriers, cryogenic cables and other applications at cryogenic temperatures. Successful application of these polymers is due to the unique combination of their electrical, physical and mechanical properties and their ability to retain these properties over a wide temperature range, where other insulating materials may fail.
(a) Perfluoro Carbon Films:
High Temperature High Performance Polymers materials used in practice include Fluorocarbon films such as Teflon (P FFE) and Perfluoroalkoxyl (PFA), Polyimide films such as Kapton and its grades (made by DuPont), Polycarbonate (PC), Polyethylene (PE) and composites such as Polybenzomidole (PEI), Nomex and Kavalar. Important properties of these materials and their applications in electrical and electronic equipments/devices are described below.
Perfluoro carbon film is a transparent thermoplastic that can be heat sealed, metallized, laminated and used as an excellent adhesive. Teflon (PTFE), perfluoroalkoxy (PFA) and polyvinylidene fluoride (PVDF) are the pefluoro carbon films used for electrical insulation purposes. These films are chemically inert and will not dissolve in virtually any chemical except in molten alkyl-metal fluorine gas and chlorine trifluoride at higher temperatures and pressures. Teflon is the most inert of all the plastics. Perfluoro carbon films have high dielectric strength 26.5 kV/mm for 25 micrometers thick film and are non-wettable and non-charring. They have very low dielectric constant of 2.0 and low dielectric loss of 0.0002 at a frequency of 100 Hz which increases to 0.0007 at 100 MHz.
(b) lmidepolymers: Polyimide Film:
(i) General properties Polyimide film has the best thermal properties of all organic films. The film made by DuPont under the trade name Kapton in USA and by ICI with the trade name Upilex. Polyimide films possess unique properties of combinations that permit them to be used at extremely high and low temperatures.
Kapton film has no melting point and can be successfully used over the temperature range of-269°C to +350°C. It can function continuously at 240°C without losing any of its properties. It retains its dielectric strength even at High Temperature High Performance Polymers. Physical properties include high tensile strength and high resistance to creep and abrasion. The dielectric properties other than the dielectric strength show reduction with increase in temperature. Resistance to radiation and ultraviolet light are very good. However, the film is vulnerable to attacks by alkalies and strong inorganic acids. The moisture absorption rate is the highest for all dielectric films.
Corona resistance of polyinride films:
Like all organic materials Kapton film is subject to damage by a corona discharge. At moderate rate of corona, equipments insulated with this material have not shown any damage due to corolla up to 3000 hours. This means that brief exposure to corona discharges will not significantly affect the working life of the film. The experimental investigations on 25µm thick I-IN film showed that corona inception level does not depend on duration of applied voltage and pre-stressing of the insulatiing HN Therefore this film can be used to make insulated magnet wire, wrappers and slot insulation without fear of damage due to corona.
Types of Kapton film:
There are three main types of Kapton films which are commercially available. They are:
- HN, an all-purpose that can be laminated, metallized or adhesive coated. It has been successfully used in a variety of applications at temperature as low as – 269°C and as high as + 400°C.
- Type VN similar to type HN but with better dimensional stability.
- Type FN, a type HN film coated on both the sides with Teflon PTFE fluoro- carbon resin to give it heat sealing, provide resistance to moisture entry and to enhance resistance to chemicals.
(v) Applications in electrical/electronic equipments/devices:;
Kapton polyimide film can be used in a variety,of electronic and electrical insulation applications, such as motor slot insulation, transformer and capacitor insulation, formed coil insulation, magnet wire insulation and for flexible printed circuits. Because of its relatively high cost, this film is mainly used where its unique properties make it the only suitable and also where its use permits economy in the design. It is used in motors where it replaces thicker dielectrics, thus making the motor more powerful without increasing its size. It is also used in aerospace electrical equipment where it is subjected to temporary overvoltages and in the construction of wire and cable where the film provides considerable savings in space with weight.
Further, the Kapton film is compatible with many High Temperature High Performance Polymers impregnating varnishes including polyimides, epoxies, silicones and esterimides which are used in the manufacture of electrical equipment.
(c) Polycarbonate (PC) and Imide Polymers:
One of the most recently developed resins is polycarbonate which is used for a variety of electrical and electronic applications. This polymer is manufactured by General Electric of USA. Polycarbonate film has good heat resistance, flexibility and dielectric characteristics. It is resistant to attack by oils, fats and dilute acids but is affected by alkalies and esters and aromatic hydrocarbon. Water vapour and gas transmission rates are relatively high. Because of its greater cost efficiency and heat resistance it is widely used in electrical industry for capacitors, conductor insulation, coil insulation and slot insulation. This is called lexan polymer.
Noryl, is a modified Polypropylene Oxide (PPC), is an amorphous material which has excellent electrical, mechanical and thermal properties and is used widely in electrical industry. On account of its flexibility it is primarily used for wire and coil insulation. It can be operated continuously up to temperatures of 190°C.
Valox is a thermoplastic polyester and can be used up to 200°C with no loss in electrical properties. It can also be filled with mineral and glass and these grades give higher temperature resistance and increased flexibility. These properties make it an ideal choice for use in circuit breakers, switches and plugs.