Page 283 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.75

which can be used along with the dimethylsiloxane repeat units. A third type of repeat
unit, which contains a vinyl group (VMQ), can be used to improve curing characteristics:

The starting material for the production of polydimethylsiloxane (MQ) is dimethyldi-
chlorosilane, which, in the presence of acid, is hydrolyzed and subsequently condenses
with the elimination of water to give a mixture of straight-chain and cyclic oligo-dimethyl-
siloxanes. A second step is needed to convert the reaction mixture to high-molecular-
weight polymer.
High-temperature-vulcanizable grades of MQ have average molecular weights in the
range of about 300,000 to 700,000. (The room-temperature-vulcanizable grades used for
calking and adhesives have much lower molecular weights, in the range of about 10,000 to
100,000. These polymers are not discussed here.)
The high-temperature-vulcanizable grades have broad molecular weight distributions
and high viscosity-average molecular weights. Yet, they have very low viscosities in the
unvulcanized state.
Properties of Silicone Rubbers. Gum vulcanizates have essentially no tensile strength.
Fillers are therefore essential. Reinforcing silicas are frequently used. Even then, the ten-
sile properties and abrasion resistance of silicone rubber vulcanizates are poor in compari-
son with of other types of elastomer. However, the properties change very little with
increasing temperatures. These polymers excel in high-temperature applications.
Silicone rubber vulcanizates withstand long-term exposure to hot air at temperatures as
high as 180 to 250°C. They remain elastic for 1000 hr at such temperatures. However,
high-temperature (120 to 140°C) steam attacks silicone rubbers. The vulcanizates are very
resistant to weathering and ozone attack and be used in hoses to convey ozone gas. These
materials are also resistant to high-energy radiation.
MQ and VMQ elastomers stiffen only below –50°C, but some PVMQ vulcanizates are
flexible even at temperatures below –100°C, and this is without plasticizers.
The oil swelling resistance of silicone elastomer vulcanizates is similar to that of chlo-
roprene rubber vulcanizates. Silicone elastomers are resistant to certain heat transfer fluids
but not to motor fuels, chlorinated hydrocarbons, esters, ketones, and ethers.
Silicone elastomers are very permeable to gases—generally about 100 times more so
than are butyl or nitrile rubbers.
Silicone rubbers are good electrical insulators and remain so at temperatures up to
180°C.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.