_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

The main Groups IV and V
anes) are stable up to at least 150°C in air for unlimited periods. In closed or
vacuum systems they can be used up to 300°C. Antioxidants are often added to
increase their resistance to degradation in the atmosphere. The ignition
temperatures of the high molecular weight polymers are above 400°C.
Over the range 0-200°C the temperature coefficient of viscosity of silicone
fluids is only about o ne tenth of that of mineral o ils. In contrast the isoelectronic
polymers (Me 2 SiCH 2 )" have normal viscosity characteristics. This means that
silicones can be used over very much wider ranges oftemperature. Some can still
be poured well below - 50°C. Polysiloxane chains are very flexible: as noted
above for hexamethyldisiloxane. the bond angles in the chains are readily
deformed. Moreover there are two mutually perpendicular (2p-3d)rr systems
which together have approximately cylindrical symmetry about the Si-O
bonds. This means that there is little resistance to torsional motion within the
molecule. There is also essentially free rotation of methyl groups about the
carbon -silicon bonds. (Barriers to rotation (kJ mol- 1 ): About Me-Si. 6. 7:
Me-C,l5.1: About Si-O. 0.8: C-0.11.3.)
The viscosity of silicone fluids is also little affected by pressure. A pressure of
2000 atm causes the viscosity of mineral oii to increase between 50 and 5000
times, whereas the viscosity of a typical silicone fluid increases only about 14
times under the same conditions. Even when subjected to extreme pressures,
which would make mineral oils become solid, silicone fluids remain in the liquid
state.
As a result of these properties, silicone fluids find application for example as
heat transfer media, hydraulic fluids, brake fluids, lubricants and vacuum pump
oils. One of their earliest uses was as dielectrics: the dielectric constant changes
very little with tempera ture and is also essentially independent of frequency over
the range 10 2 -lWHz. On thermal breakdown organic polymers have the
disadvantage that they commonly produce carbon, which conducts electricity.
Organosiloxanes are more robust and also tend to decompose or oxidize ta
products which are themselves insulators, such as silica or volatiles of low
molecular weight.
The surface effects of silicones also lead to useful applications. They derive from
the presence ofboth polar Si-O and non-polar hydrocarbon groups in the same
material. Textiles or papers treated with silicone oils become water repellent
because the polar groups orient themselves clase ta the fibres, presenting a
hydrophobic hydrocarbon exterior. Water penetration into bricks, concrete and
other building materials is also greatly reduced by impregnation with silicones.
Glass vessels drain more efficiently after coating. Dimethyl and phenylmethylpolysiloxanes
are used as paint additives as they improve the water repellent
properties of the painted surface as well as reducing the surface tension of the
paint.
In order to make the silicone adhere more strongly to the surface which is being
treated cross links are often introduced which anchor the two together. This can
be achieved by including some MeSiH groups in place of Me 2 Si in the polymer
114