Page 171 - Handbook of Plastics Technologies

THERMOSETS

THERMOSETS 3.41

The Si-O and Si-C bonds in silicones are very stable, giving them high resistance to
heat, electrical, and chemical attack.
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The large size of the Si atom, and the oblique (150 ) angle of the Si-O-Si bonds, give
very little steric hindrance and very free rotation. This makes the silicone molecule very
flexible and rubbery, even down to very low temperatures. On the down side, it also pro-
duces low mechanical strength and low solvent resistance.
The sheath of primary hydrogen atoms, on the methyl groups surrounding the polymer
main-chain, gives low intermolecular attraction, which also contributes to rubbery behav-
ior and low mechanical strength, and especially to low surface energy and low surface ten-
sion, which produce abhesion (nonstick) and water-repellent performance.
3.1.5.3 Rubber. Silicone rubber can be heat-cured by fairly conventional techniques. It
can also be cast and cured at room temperature, producing what is called room-tempera-
ture vulcanized (RTV) rubber.
3.1.5.3.1 Heat-Cured Rubber. High-molecular-weight (500,000) linear silicone rub-
ber is very soft and has no strength or creep resistance. It can be cross-linked by heating
with peroxides (Table 3.37). The reaction of peroxide with the methyl group (Fig. 3.37) is
not very efficient and levels off at 0.4 to 0.7 cross-links per 1000 Si atoms—too low to
give good strength and resistance to compression set. Therefore, the rubber is usually
made with a fraction of a percent of vinyl side-groups; these react readily with peroxide,
giving a 90 percent yield of predicted cross-links and much better strength and compres-
sion-set resistance. If vinyl side-groups are increased up to 4 to 5 percent, silicone rubber
can even be cured by conventional sulfur vulcanization.

TABLE 3.37 Peroxides for Cross-Linking Silicone Rubber
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Bis(2,4-dichlorobenzoyl) peroxide 104–132 C
Benzoyl peroxide 116–138°C
Dicumyl peroxide 154–177°C

2,5-dimethyl-2,5-di(t-butylperoxy) hexane 166–182°C

Most rubber is reinforced by carbon black; silicone rubber is not. Instead, it is rein-
forced by fine-particle-size fumed silica. This definitely improves tensile strength, though
it still cannot equal most other types of elastomers (Table 3.38). Other fillers do not in-
crease strength but may be used to improve processability, increase hardness and reduce
tack and compression set. Carbon black is used to increase electrical conductivity.
Small production runs are processed by compression or transfer molding at 800 to
3,000 psi and 104 to 188°C; mold shrinkage is 2 to 4 percent. Long production runs are
more economical by injection molding at 5,000 to 20,000 psi, 188 to 252°C, and a 25 to
90 sec cycle. Extrusion requires post-cure in a 316 to 427°C hot-air oven, typically
60 ft/min; steam post-cure can run 1200 ft/min. Calendering typically runs 5 to 10 ft/
min.
Specific formulations can aim at various product needs (Table 3.39). Particularly out-
standing is their wide useful temperature range (Table 3.40).

3.1.5.3.2 Room-Temperature Vulcanized (RTV) Silicones. Low-molecular-weight
liquid silicone oligomers, with reactive functional groups, can be poured or spread with

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