Silicone Rubber – Elastic, Flame-Fesistant, Oil-Resistant

First, typical structures of various silicone rubber grades are shown, as described in more detail under "Manufacture".

Several properties of silicone rubber will now be discussed below.

 Elastic Behavior

The best way to assess the elastic properties of a material is to examine its stress-strain diagram. The elastic modulus is a key parameter for describing elastic behavior. It is given by the stress-strain curve as the resistance during strain deformation (N/mm²) expressed in terms of a certain strain value (in %).

A typical stress-strain diagram is shown below.

Silicone rubber possesses an extraordinary property that sets it apart from other elastomers – its elastic behavior changes only slightly with change in temperature.

 Compression set of silicone rubber after 24 hours compression at different temperatures:

 The following diagram shows the extent to which the crosslinking density of the silicone rubber affects the different elastic properties:

Of all the properties shown above, the most important is the low dependence of the compression set on changes in temperature.

 Flame Resistance

Silicone rubber has excellent flame resistance. It has a flash point of 750 °C, an ignition temperature of 450 °C and its LOI values lie well below the critical value of 21 % that approximates the concentration of oxygen in air. (LOI is the abbreviation for limiting oxygen index, which is a measure of the flammability of polymers in oxygen/nitrogen mixtures of different composition).

Only very minor amounts of smoke are evolved during combustion, and no toxic gases such as HCl or sulfur compounds are released.

The principal combustion products are carbon dioxide and water, with the ash consisting of silicon dioxide. Since silicon dioxide is an excellent dielectric, silicone rubber is used in critical sectors to sheathe cables (siliconized cables are employed in shipbuilding, aircraft construction and public buildings).

 To emphasise the extremely positive properties of silicone rubber in the event of fire, the combustion behavior of several elastomers is tabulated below.

Particularly flame-resistant grades of silicone rubber are made by incorporating additives such as Al(OH)₃ hydrate or minute amounts of platinum compounds and TiO₂ (for self-extinguishing products).

 Chemical and Oil Resistance

Silicone rubber is generally very resistant to chemicals. It can be used in applications involving contact with dilute acids and alkalies. However, its chemical resistance can decrease with increase in temperature or concentration.

Its resistance to various oils compares very favorably with that of organic elastomers. The following diagram illustrates how the volume of silicone rubber changes when it has been immersed at 150 °C in hot oil for 14 days or exposed to other chemicals.

Chemicals	Volume change in %
Solvents and fuels (after 7 days at RT)
Acetone	15 - 25
Carbon tetrachloride	Over 150
Ethyl alcohol	0 - 10
Iso-octane	Over 150
Xylene	Over 150
Oils (after 14 days at 150 °C)
ASTM Oil 1	5
ASTM Oil 2	8
ASTM Oil 3	40
SAE Oil 20 W 20	25
Silicone fluid, viscosity 100 mPa s	30

Decomposed, aggressive oil components do the most damage to silicone rubber. Nonpolar solvents, by contrast, cause extensive swelling. However, the original properties are restored when the solvent has evaporated. Silicone fluids also cause reversible swelling of silicone rubber, with the least swelling effected by the phenyl grades.

In contrast, silicone rubbers containing trifluoropropyl groups are extremely resistant to oil.