Page 170 - Handbook of Plastics Technologies

THERMOSETS

3.40 CHAPTER 3

space, appliance, and mechanical construction. The total U.S. market is 600 million lb/yr,
and growth rate has still not reached maturity.

3.1.5 Silicones
Silicone chemistry is a marriage of organic polymers and inorganic ceramics, which has
produced synergistic benefits in abhesion, low-temperature flexibility, high-temperature
stability, flame-retardance, electrical resistance, water resistance, and physiological inert-
ness, leading to a family of elastomers and thermoset plastics with a wide variety of spe-
cialized applications.
3.1.5.1 Chemistry. Silica sand is electrothermally reduced to silicon metal.
SiO + C → Si + CO 2
2
This is mixed with copper catalyst and reacted with methyl chloride at 250 to 280°C to
produce a mixture of methyl chlorosilanes.
9% CH SiCl b.p. 66°C designated T for trifunctional
3
3
74% (CH ) SiCl b.p. 70°C designated D for difunctional
3 2
2
6% (CH ) SiCl b.p. 57°C designated M for monofunctional
3 3
These are separated by fractional distillation.
The chlorosilane Si-Cl bond hydrolyzes rapidly in water to form silanol Si-OH, which
condenses instantly to form siloxane Si-O-Si (Fig. 3.36). Thus, (CH ) SiCl (D) produces
3 2
2
linear silicone rubber. Introducing CH SiCl (T) produces branching and cross-linking; at
3
3
high concentrations, it produces a thermoset plastic. Conversely, introducing (CH ) SiCl
3 3
(M) caps the ends of the growing chains and lowers the molecular weight of the rubber.

FIGURE 3.36 Silicone synthesis.

The most common alkyl group is methyl. Introducing some phenyl groups prevents
crystallization at low temperatures and thus keeps silicone rubber flexible down to lower
temperatures; phenyl groups also increase heat stability at high temperatures, thus creating
a wider useful temperature range for silicone rubber. CF CH CH - and NCCH CH -
2
3
2
2
2
groups are used to increase resistance to fuels, oils, and organic solvents. CH =CH-
2
groups provide reactivity for vulcanization/cure of the rubber. CH O- and CH CO -
3
2
3
groups hydrolyze more slowly than Cl- and are used to provide controlled reactivity for
cross-linking, coating, and adhesive bonding.
3.1.5.2 Properties. Unlike most elastomers, silicone rubber does not contain C=C
groups, so it is much more resistant to oxygen and ozone.
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