What Is Silicone Rubber?
Silicone rubber is an elastomer (rubber-like material) composed of silicone—itself a polymer—containing silicon together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry, and there are multiple formulations.
Liquid Silicone Rubber, uncured
Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost.
Is silicone safe?
Silicone rubber is very safe, is generally non-reactive, stable, and resistant to extreme environments and temperatures from −55 to 300 °C (−70 to 570 °F) while still maintaining its useful properties.
Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including voltage line insulators; automotive applications; cooking, baking, and food storage products; apparel such as undergarments, sportswear, and footwear; electronics; medical devices and implants; and in home repair and hardware, in products such as silicone sealants.
Silicone breast implants are very safe
In its uncured state, silicone rubber is a highly adhesive gel or liquid. To convert it to a solid, it must be cured, vulcanized, or catalyzed. This is normally carried out in a two-stage process at the point of manufacture into the desired shape, and then in a prolonged post-cure process. It can also be injection molded or 3D printed.
Silicone rubber may be cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system. For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure.
The first silicone elastomers were developed in the search for better insulating materials for electric motors and generators. Resin-impregnated glass fibers were the state-of-the-art materials at the time.
The glass was very heat resistant, but the phenolic resins would not withstand the higher temperatures that were being encountered in new smaller electric motors.
Chemists at Corning Glass and General Electric were investigating heat-resistant materials for use as resinous binders when they synthesized the first silicone polymers, demonstrated that they worked well and found a route to produce them commercially.
The atomic structure of silicone rubber
The term “silicone” is actually a misnomer. The suffix -one is used by chemists to denote a substance with a double-bonded atom of oxygen in its backbone.
When first discovered, silicone was erroneously believed to have oxygen atoms bonded in this way. Technically correct term for the various silicone rubbers is polysiloxanes (polydimethylsiloxanes being a large subset), referring to a saturated Si-O backbone.
Corning Glass in a joint venture with Dow Chemical formed Dow Corning in 1943 to produce this new class of materials.
As the unique properties of the new silicone products were studied in more detail, their potential for broader usage was envisioned, and GE opened its own plant to produce silicones in 1947.
GE Silicones was sold to Momentive Performance Materials in 2006. Wacker Chemie also started production of silicones in Europe in 1947. The Japanese company Shin-Etsu Chemical began mass production of silicone in 1953.
Silicone rubber offers good resistance to extreme temperatures, being able to operate normally from −100 to 300 °C (−150 to 570 °F). Silicone rubber has low tensile strength, poor wear and tear wear properties.
Some properties such as elongation, creep, cyclic flexing, tear strength, compression set, dielectric strength (at high voltage), thermal conductivity, fire resistance and in some cases tensile strength can be—at extreme temperatures—far superior to organic rubbers in general, although a few of these properties are still lower than for some specialty materials.
Silicone rubber is a material of choice in industry when retention of initial shape and mechanical strength are desired under heavy thermal stress or sub-zero temperatures.
Compared to organic rubber
Organic rubber has a carbon-to-carbon backbone which can leave it susceptible to ozone, UV, heat and other aging factors that silicone rubber can withstand well.
This makes silicone rubber one of the elastomers of choice in many extreme environments. Silicone is considerably more permeable to gasses than most other rubbers which limits its use in some areas.
Silicone rubber is highly inert and does not react with most chemicals and isn’t available to participate in biological processes allowing it to be used in many medical applications including medical implants.
It is biocompatible, hypoallergenic, which makes it suitable for baby care products, and food contact in general.
Silicone rubber is a reliable solution (as opposed to rubber and thermoplastic elastomers) 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.).
To make silicone, the silicon atoms must be isolated from the silicon dioxide compound silica. This is done by heating large volumes of quartz sand to extremely high temperatures, often up to 1800 °C.
From here, there are several processes where silicon is combined with methyl chloride and heated. It is then distilled into a polymerised siloxane known as polydimethylsiloxane. The polydimethylsiloxane can then be polymerised.
This is done using a variety of techniques depending on the use of the final product. The raw silicone compound is combined with any desired additives, which may include pigments, and the catalyst. It is then injection moulded, extruded or 3D printed.
Curing is the final stage in the production process.
There are many special grades and forms of silicone rubber, including: steam 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 of 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.
Silicone rubber is used in automotive applications, many cooking, baking, and food storage products, apparel including undergarments, sportswear, and footwear, electronics, to home repair and hardware, and a host of unseen applications. It is usually processed and shaped with the following methods.
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 gasket
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.
Silicone rubber can be 3d printed (liquid deposition modelling LDM) using pump-nozzle extrusion systems.
Unfortunately, standard silicone formulations are optimized to be used by extrusion and injection moulding machines and are not applicable in LDM-based 3D printing. The rheological behavior and the pot life need to be adjusted.
3D printing also requires the use of a removable support material that is compatible with the silicone rubber.
The ACE model is 3D printed with silicon
Liquid silicone rubber is also manufactured for life science applications (syringe pistons, closure for dispensing system, gaskets for IV flow regulator, respiratory masks, implantable chambers for IV administration), cosmetic products (Mascara brush, make-up packaging, make-up applicator and lipstick moulds) and optics products (circular lens, collimators, Fresnel lenses and free form lenses).
Silicone mascara brush
Silicone breast pocket
Freeze-tolerant solar water-heating panels
Freeze-tolerant solar water-heating panels exploit the elasticity of silicone to repeatedly accommodate the expansion of water on freezing, while its extreme temperature tolerance maintain a lack of brittleness below freezing and excellent tolerance of temperatures in excess of 150 °C (300 °F).
Its property of not having a carbon backbone, but a chemically robust silicon backbone instead, reduces its potential as a food source for dangerous waterborne bacteria such as Legionella.
Non-dyed silicone rubber tape with an iron(III) 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.
As an electrical insulator, silicone rubber has the added virtue of remaining non-conductive when damaged by heat, reducing the likelihood of runaway arcing.
Silicone rubber conveyor belt
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 pressed, used in many devices such as computer keyboards and remote control handsets.
A silicone rubber pastry brush.
Spoon bowl for kids
In 2007, silicone rubber formed the matrix of the first autonomic self-healing elastomer.
The microcapsule-based material was capable of recovering almost all of the original tear strength.
Additionally, this material had improved fatigue properties as evaluated using a torsion-fatigue test.
Wide application, high efficiency and increasingly researched and developed. Therefore, VIJAYA – VINA can evaluate silicone rubber as one of the ‘materials of the future’.