Polyimides
Normally infusible, coloured (often amber) high performance polymers with predominantly aromatic molecules of high thermal stability. Semi-fabricated shapes are usually supplied by the polymer manufacturer and made by powder sintering or working with more tractable pre-cursors and completing polymerization in final form, though some melt-process able grades of resin are manufactured.
They have excellent high temperature properties and radiation resistance, inherently low flammability and smoke emission, low creep and high wear resistance and are very expensive. They have moderately high water absorption and are prone to hydrolysis and attack by alkalis and concentrated acids.
| General Properties : | ||
|---|---|---|
| Chemical Resistance | ||
| Acids – concentrated | Poor | |
| Acids – dilute | Fair | |
| Alcohols | Poor | |
| Alkalis | Poor | |
| Aromatic hydrocarbons | Good | |
| Greases and Oils | Good | |
| Halogenated Hydrocarbons | Good | |
| Halogens | Fair | |
| Ketones | Good | |
| Electrical Properties | ||
| Dielectric constant @1MHz | 3.4 | |
| Dielectric strength ( kV mm-1 ) | 22 | |
| Dissipation factor @ 1kHz | 0.0018 | |
| Surface resistivity ( Ohm/sq ) | 10 16 | |
| Volume resistivity ( Ohmcm ) | 10 18 | |
| Mechanical Properties | ||
| Coefficient of friction | 0.42 | |
| Elongation at break ( % ) | 08 to 70 | |
| Hardness – Rockwell | E52 to 99 | |
| Izod impact strength ( J m -1 ) | 80 | |
| Tensile modulus ( GPa ) | 2.0 to 3.0 | |
| Tensile strength ( MPa ) | 70 to 150 | |
| Physical Properties | ||
| Density ( g cm-3 ) | 1.42 | |
| Flammability | V0 | |
| Limiting oxygen index ( % ) | 53 | |
| Radiation resistance | Good | |
| Refractive index | 1.66 | |
| Resistance to Ultra-violet | Poor | |
| Water absorption – over 24 hours ( % ) | 0.2 to 2.9 | |
| Thermal Properties | ||
| Coefficient of thermal expansion ( x10-6 K-1 ) | 30 to 60 | |
| Heat-deflection temperature – 1.8MPa ( C ) | 360 | |
| Lower working temperature ( °C ) | -270 | |
| Specific heat ( J K-1 kg-1 ) | 1090 | |
| Thermal conductivity @23C ( W m-1 K-1 ) | 0.10 to 0.35 | |
| Upper working temperature ( C ) | 250 to 320 | |
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