Page 173 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.43

TABLE 3.39 Properties of Heat-Cured Silicone Rubbers
High-strength Solvent- Wire and
Grade High.-temp. Low-temp. resistant cable

Shore A hardness 46 63 50 67
Tensile strength, psi 1000 1500 1000 1100
Elongation, % 430 700 220 340
Compression set, % 14 42 50
o
Brittle temperature, C –65 –101 –68
Oil absorption, % 6 10 1
Volume resistivity, Ω-cm 3 × 10 15
Dielectric constant 3.3

Power Factor 0.003

TABLE 3.40 Maximum Use Temperatures of Silicone Rubbers

Temperature, °C Time to 50% retention of elongation
121 10–20 yr

149 5–10 yr
204 2–5 yr
260 3–24 mo

316 1 wk
371 6 hr
461 10 min
518 2 min

densation or addition (Fig. 3.38). In condensation cure, the hydroxyl-terminated silicone
oligomer is cross-linked by tetraethyl silicate, catalyzed by dibutyl tin dilaurate or faster
by stannous octoate, and liberates alcohol, so it can be used only in an open system. In ad-
dition cure, a silicone oligomer containing vinyl CH =CH- groups reacts with a silicone
2
oligomer containing silane Si-H groups, catalyzed by platinum; since no volatiles are lib-
erated, this can be done in a closed system, and it gives higher strength products
(Table 3.42).
More recently, this has led to the development of liquid injection molding (LIM), in
which the reactive silicone oligomer system is injection molded at 200 to 250°C and cures
in a few seconds, a great advance over conventional vulcanization systems.

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