Page 95 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.35

2.2.17 Polyimide (PI)
Thermoplastic polyimides are linear polymers noted for their high-temperature proper-
ties. Polyimides are prepared by condensation polymerization of pyromellitic anhy-
drides and primary diamines. A polyimide contains the structure -CO-NR-CO as a part
of a ring structure along the backbone. The presence of ring structures along the back-
bone, as depicted in Fig. 2.27, gives the polymer good high-temperature properties. 255
Polyimides are used in high-performance applications as replacements for metal and
glass. The use of aromatic diamines gives the polymer exceptional thermal stability. An
example of this is the use of di-(4-amino-phenyl) ether, which is used in the manufacture
of Kapton (Du Pont).

FIGURE 2.27 Structure of polyimide.

Although called thermoplastics, some polyimides must be processed in precursor
form, because they will degrade before their softening point. 256 Fully imidized injection-
molding grades are available, along with powder forms for compression molding and
cold forming. However, injection molding of polyimides requires experience on the part
of the molder. 257 Polyimides are also available as films and preformed stock shapes. The
polymer may also be used as a soluble prepolymer, where heat and pressure are used to
convert the polymer into the final, fully imidized form. Films can be formed by casting
soluble polymers or precursors. It is generally difficult to form good films by melt extru-
sion. Laminates of polyimides can also be formed by impregnating fibers such as glass or
graphite.
Polyimides have excellent physical properties and are used in applications where parts
are exposed to harsh environments. They have outstanding high-temperature properties
and their oxidative stability allows them to withstand continuous service in air at tempera-
tures of 260°C. 258 Polyimides will burn, but they have self-extinguishing properties. 259
They are resistant to weak acids and organic solvents but are attacked by bases. The poly-
mer also has good electrical properties and resistance to ionizing radiation. 260 A disadvan-
tage of polyimides is their hydrolysis resistance. Exposure to water or steam above 100°C
may cause parts to crack. 261
262
The first application of polyimides was for wire enamel. Applications for polyim-
ides include bearings for appliances and aircraft, seals, and gaskets. Film versions are used
in flexible wiring and electric motor insulation. Printed circuit boards are also fabricated
263
with polyimides.

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