Page 179 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.49

TABLE 3.46 Kapton Polyimide Films
Density 1.42
Tensile modulus, kpsi, 23°C 430
200°C 260
Tensile strength, kpsi, 23°C 25
200°C 17
Elongation, %, 23°C 70
200°C 90
Impact strength, J/mm 23
Folding endurance, cycles 10,000
Initial tear strength, g 510
Tear propagation, g 8
Volume resistivity, Ω-cm 10 15
Dielectric constant 3.6
Dissipation factor 0.0025
Dielectric strength, V/mil 5,400

(Table 3.48). Monsanto (Skybond) and American Cyanamid (FM-34) used m-phenylene
diamine and benzophenone tetracarboxylic dianhydride (Fig. 3.43) to produce glass cloth
laminates (Table 3.49). General Electric silicone polyimides (SiPI) are block copolymers
of benzophenone tetracarboxylic dianhydride with methylene dianiline and bis(aminopro-
pyl) tetramethyl disiloxane (Fig. 3.44), designed primarily for high-temperature electrical
insulation (Table 3.50).
3.1.6.3 Second-Stage Addition Polymerization Cure of Polyimides. To cure thermoset-
ting polyimides without the problem of volatile by-products, the cross-linking reaction is
based on addition polymerization instead of condensation polymerization. This again is a
two-stage process. In the first stage, a low-molecular-weight oligomer is prepared contain-
ing finished imide groups; since it is low-molecular-weight, it is still easily processable,
even though it contains aromatic and heterocyclic rings. Then, in the second stage, reactive
groups in the oligomer are polymerized by addition reactions, building to high molecular
weight and a high degree of cross-linking as well. Several types of reactive groups have
been developed.

FIGURE 3.43 Monsanto and American Cyana-
mid polyimides.

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