Page 176 - Handbook of Plastics Technologies

THERMOSETS

3.46 CHAPTER 3

pressure techniques such as vacuum-bag molding. These laminates are 20 to 40 percent
weaker mechanically than epoxy, melamine, and phenolic but superior in electrical insula-
tion properties, especially at high temperatures and in moist conditions (Table 3.43). They
are used in electric motors, terminal boards, printed circuit boards, and transformers. They
are also used for fire-resistance in aircraft firewalls and ducts.

TABLE 3.43 Electrical Properties of Silicone-Glass Cloth Laminates
Matrix resin Phenolic Melamine Silicone

Power factor 0.06 0.08 0.0002
Dielectric strength, V/mil 150–200 150–200 250–300
Insulation resistance, Ω, dry 10,000 20,000 50,000
wet 10 10 10,000

Molding powders are B-stage silicone resin plus glass fiber and catalyst. They are com-
pression molded 5 to 20 min at 1000 to 4000 psi and 160°C and then post-cured several
hours to achieve optimum properties. Electrical insulation and resistance to heat and mois-
ture are outstanding (Table 3.44). Molded parts are used in electric motors and switches.

TABLE 3.44 Silicone Resin Moldings
Specific gravity 1.65
Flexural modulus, 23°C 1,800,000 psi
200°C 900,000 psi
Flexural strength, 23°C 14,000 psi
200°C 5,000 psi
Tensile strength, 23°C 4,400 psi
200°C 1,300 psi
Dielectric constant 3.6
Power factor 0.005

3.1.5.3.4 Coatings. Silicone resin solutions are baked to produce release coatings
that are resistant to heat, water, and weather. These are used in cooking and baking and for
water-repellent masonry. They are also copolymerized with other thermosetting coatings
to increase their heat and weather resistance.

3.1.6 Polyimides
New high-tech industries such as aerospace and electronics have created growing needs
for lightweight, strong materials with increased resistance to heat, oxygen, and corrosion.
Organic polymer chemists have spent the past half century developing new polymers with
higher and higher performance. The guiding general principle has been the use of hetero-
cyclic resonance to provide molecular rigidity and thermal-oxidative stability. There have

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