Material detail – Silicone Rubber
Silicone rubber is an elastomer (rubber-like material) composed of Silicone–itself a polymer—containing Silicon—together with Hydrogen, Carbon and Oxygen. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties. Silicone rubber is generally non-reactive, stable, and resistant to extreme environments and temperatures from -55 °C to +300 °C while still maintaining its useful properties. A material of choice in industry when retention of initial shape and mechanical strength are desired under heavy thermal stress or sub-zero temperatures¹.
Properties such as elongation, creep, cyclic flexing, tear strength, compression set, dielectric strength at high voltage, thermal conductivity, fire/temperature resistance and tensile strength is superior to other rubbers. Silicon based rubbers degrade far more slowly than many other other rubbers when subjected to aging factors.
Silicone rubber is highly inert and does not react with most chemicals. In addition it is also a reliable solution (as opposed to rubber and thermoplastic estomers) for migration or interaction problems between the main active ingredients. Its chemical stability prevents it from affecting any substrate it is in contact with (skin, water, blood, active ingredients, etc.)²
There are many special grades and forms of silicone rubber, including:Stem resistant, metal detectable, high tear strength, extreme high temperature, extreme low temperature, electrically conductive, chemical/oil/acid/gas resistant, low smoke emitting, and flame-retardant. A variety of fillers can be used in silicone rubber, although most are non-reinforcing and lower the tensile strength.
Silicone rubber is available in a range or hardness levels, expressed as Shore A or IRHD between 10 and 100, the higher number being the harder compound. It is also available in virtually any colour, and can be colour matched.
Once mixed and coloured, silicone rubber can be extruded into tubes, strips, solid cord or custom profiles according to the size specifications of the manufacturer. Cord can be joined to make O-rings and extruded profiles can be joined to make seals. Silicone rubber can be moulded into custom shapes and designs. Manufacturers work to set industry tolerances when extruding, cutting or joining silicone rubber profiles. In the UK this is BS 3734, for extrusions the tightest level is E1 and the widest is E3.
Non-dyed silicone rubber tape with an iron-oxide additive (making the tape a red-orange colour) is used extensively in aviation and aerospace wiring applications as a splice or wrapping tape due to its non-flammable nature. The iron-oxide additive adds high thermal conductivity but does not change the high electrical insulation property of the silicone rubber. This type of self-amalgamating tape amalgamates or fuses to itself, so that when stretched and wrapped around cables, electrical joints, hoses and pipes it bonds into a strong seamless rubbery electrically insulating and waterproof layer, although not adhesive.
With the addition of carbon or another conductive substance as a powdered filler, silicone rubber can be made electrically conductive while retaining most of its other mechanical properties. As such it is used for flexible contacts which close on being pressed³.
Mechanical properties of silicone rubbers
|Hardness, Shore A||10–90|
|Tensile Strength||11 N/mm²|
|Elongation at Break||100–1100%|
|Maximum Temperature||+300 °C|
|Minimum Temperature||-120 °C|