Polyimide: This polymer is presently the most advanced of all T.S. matrices. It has characteristics of high temperature (H/T) physical and mechanical properties. It is available as uncured resin, prepreg, stock shapes, thin sheets, laminates, and machined parts. Along with the H/T properties, this polymer must be processed at very high temperatures and relative pressure to produce these characteristics.
With prepreg materials, 600 °F (316 °C) to 650 °F (343 °C) temperatures and 200 psi (1,379 kPa) pressures are required. The entire cure profiles are inherently long as there are a number of intermediate temperatures dwells, duration of these are dependent on part size and thickness. The cut of P.I. is 450 °F (232 °C), highest of all T.S., with short term exposure capabilities of 900 °F (482 °C). Normal operating temperatures range from cryogenic to 500 °F (260 °C).
General Description:
Most important group of highly heat resistant plastics, often referred to as "exotics".
- the polyamide imides,
- the polyesters and
- the polyetherimides as well as
- imides with heterocyclic groups
- Manufacture by polycondensation or polyaddition.
- Polycondensates are usable as thermosets or thermoplastics. However this does not mean that they are moldable as a melt.
- Polyaddition products are useful only as thermosets.
- The Du Pont polyimides, such as Vespel (polycondensation polymer), Pyroline laminates and Kapton films have melting points which are far above the decomposition temperature.
- Torlon (Amoco) and Polyimid 2080 (Upjohn) are thermoplastic polyimides.
- Some polyimides demand special processing techniques, so they are available only as semi-finished products.
- The semi-finished products are manufactured by the powder sinter technique.
- There are also polyimide grades which can be processed by compression molding but at very high temperatures and further annealing is required for best results.
Some polyimide grades can be injection molded.
- The melt temperature is 350 °C and
the injection pressures are very high.
- The cycle times are long because of the large temperature difference during cooling.
- Injection moldable polyimides can also be extruded.
- Multiple laminates are manufactured in heated presses, using the vacuum bag method, or in low-pressure autoclaves.
- Filament winding is also common.
- This method involves a heat-, cure- and cool-program. For condensation resins, pauses must be interspersed for escape of the volatile by-products so as to achieve a product essentially free of voids.
- [The volatile side-products of polycondensation reactions are aromatic acids, aromatic nitrogen compounds and solvent vapor.]
- The polyimides can also be applied by spraying.
- Polyimide on aluminum gives glossy, heat-resistant coatings which are resistant to solvents and prevent stain formation.
- There are many adhesives for bonding polyimide molded articles to each other or to other materials.
General Description
- Often the advantages of polyimides cannot be exploited as only a limited selection of moldings & semi-finisheds can be manufactured.
- Up to now it has not been usual to process molding compounds or prepregs in plastics processing plant.
- The raw material manufacturers prefer to carry out the complicated processing themselves and produce moldings to clients' specifications. Some polymer manufacturers thus also offer finished parts commercially
Structure and General Properties
1. As the aromatic or heterocyclic ring compounds in the chains of the macromolecule are closely pressed together, the properties are
- high strength over a wide temperature range (- 240 to + 370°C),
- high stiffness and hardness,
- high thermal stability,
favorable friction and abrasion performance,
- good electrical properties,
- high resistance to high energy radiation,
- highly flame resistant,
- low outgassing under high vacuum,
- good resistance to chemicals and water.
Additives
- Functional additives and reinforcements are of importance:
- Molybdenum disulfide (MoS2), graphite and PTFE for bearing material.
- Glass, Carbon- and aramide-fibers are used as reinforcements.
Availability
- A wide variety of imide-based thermosets and thermoplastics with functional additives and reinforcements are commercially available:
- As resins and compounds in the form of dry powders or dissolved in solvents
- Compressed and machined moldings
- Prepregs in the B-state, honeycombs, films, laminates, foams, coating materials, wire enamel and adhesives.
- The polyimides - Vespel moldings and Kapton films are products of the firm Du Pont de Nemours (US) and can only be obtained as finished precision moldings and films from Du Pont.
- There is a limited availability of semi-finisheds in the form of rods, tubes, profiles, sheets, rings and disks.
Resistance to Chemicals
- Polyimide is resistant to solvents, fuels, grease, oils, detergents, dilute acids and alkalis
- But not resistant to strong acids and alkalis or oxidants.
- It is not resistant to long-term exposure to boiling water and steam although the decrease in strength is reversible.
- It is resistant to stress crack formation except in alkaline media.
Weathering Resistance
- Molded articles lose strength and elongation at break after long exposure outdoors and should therefore not be used. However they resist mold.
Resistance to High Energy Radiation
- Molded items of PI are resistant up to an absorbed dose of 107 J kg-l, higher than most other plastics.
Water Absorption
- Moisture content of polyimide at equilibrium under normal environmental conditions (23°C of, 50% RH) is 1.25% - the dimensional changes caused by alteration of the moisture content of the surroundings is small
- After 1000 hrs - 0.11 % in MD & 0.06% in TD
- After 10000 hours - 0.28 in MD & 0.16% in TD
Only very narrow tolerances and extreme conditions preclude the use of PI.
Machining
- Machining is performed with standard carbide-tipped steel metalworking tools.
- The materials must remain cool enough to be touched with the bare hand.
Joining
- The preferred method of joining is by adhesion.
- Polyimide molded items can be bonded to each other and to other materials.
- Choice of adhesive is made according to the operating temperature.
- For service temperatures below 150°C Epoxide resins are suitable. Phenolic resin adhesives are somewhat more heat resistant.
Applications
- In the form of moldings and machined items, PI are used for piston rings, valve seats, bearings, seals, welding-torch handle grips, jet engine accessories, electronic connectors, coil cores, covers for meters and automobile accessories.
- PIs are also used as valve stems in valves. High thermal stability and heat resistance are the reasons for application in connection with soldering and welding.
- The largest and most important area of application for PI items up to now-is in the engines, for example in the Boeing 747 for seals in the different compressor stages.
- More recently because of the favorable friction and wear characteristics and lubricant-free operation, polyimide molded articles have been used in office machinery and computers as bearings, rollers, slide and guide rails.
- Kapton films are successfully used in a wide range of applications from -269°C to +400°C. These films are flame resistant, do not melt, are resistant to solvents and to high-energy radiation.
- PI films are used for insulating washers, cable and wire sheathing as well as printed circuit substrates.
- Films coated on one or both sides with FEP can be heat sealed and act as a moisture barrier. Films coated by vaporizing aluminium protect space ships against the rays of the sun and the effect of heat on the power plant. Space suits are also insulated with such films.
- Polyimide films serve as sound insulation at high operating temperatures.
Properties of Polyimide:
1. Good mechanical properties at H/T
2. Good electrical properties
3. High wear resistance
4. Low creep at high temperatures
5. Good compression with glass or graphite fiber reinforcement
6. Good chemical resistance
7. Inherently flame resistant
8. Unaffected by most solvents and oils.
Polyimide film: Polyimide film possesses a unique combination of properties that make it ideal for a variety of applications in many different industries.
High-performance resin: The high-performance resin is used in electrical, wear resistant and as structural materials when combined with reinforcement for aircraft-aerospace applications, which are replacing heavier more expensive metals. High temperature processing causes some technical problems as well as higher costs compared to other polymers. Hysols PMR series is an example of this polymer.
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