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. Silicone rubbers are often one- or two-part polymers, and may contain fillers to improve properties or reduce cost. 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. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including: 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 with products such as silicone sealants.

During manufacture, heat may be required to vulcanize (set or cure) the silicone into its rubber-like form. 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.

History

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 polydimethylsiloxane commercially.

Corning Glass formed a joint venture with Dow Chemical 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. Wacker Chemie also started production of silicones in Europe in 1947. The Japanese company Shin-Etsu Chemical began mass production of silicone in 1953. The companies mentioned above are now still the main competitors in the oligopoly that comprises the silicone industry.

Properties

Silicone rubber offers good resistance to extreme temperatures, being able to operate normally from −55 °C to +300 °C. At the extreme temperatures, the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers although still low relative to other materials. Organic rubber has a carbon to carbon backbone which can leave them susceptible to ozone, UV, heat and other ageing factors that silicone rubber can withstand well. This makes it one of the elastomers of choice in many extreme environments.

Compared to organic rubbers, however, silicone rubber has a very low tensile strength.^[1] For this reason, care is needed in designing products to withstand even low imposed loads. The material is also very sensitive to fatigue from cyclic loading. Silicone rubber is a highly inert material and does not react with most chemicals. Due to its inertness, it is used in many medical applications and in medical implants.

Structure

Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units unlike many other polymers that contain carbon backbones. Polysiloxane is very flexible due to large bond angles and bond lengths when compared to those found in more basic polymers such as polyethylene. For example, a C-C backbone unit has a bond length of 1.54 Å and a bond angle of 112˚, whereas the siloxane backbone unit Si-O has a bond length of 1.63 Å and a bond angle of 130˚.

The siloxane backbone differs greatly from the basic polyethylene backbone, yielding a much more flexible polymer. Because the bond lengths are longer, they can move further and change conformation easily, making for a flexible material. Polysiloxanes also tend to be chemically inert, due to the strength of the silicon-oxygen bond. Despite silicon being a congener of carbon, silicon analogues of carbonaceous compounds generally exhibit different properties, due to the differences in electronic structure and electronegativity between the two elements; the silicon-oxygen bond in polysiloxanes is significantly more stable than the carbon-oxygen bond in polyoxymethylene (a structurally similar polymer) due to its higher bond energy.

Mechanical properties
Hardness, shore A	10–90
Tensile strength	11 N/mm²
Elongation at break	100–1100%
Maximum temperature	+300 °C
Minimum temperature	−120 °C

(Polymax 2005)

Special grades

There are also 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 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.

Liquid Silicone

Liquid silicone rubber is a high purity platinum-cure silicone (not to be confused with silicone oil).

Medical device manufacturers favour silicone rubber because of its biocompatibility and excellent part quality. Medical grade silicone combined with a lack of human contact reduces risk of contamination, especially when manufactured in a clean room environment. However, failures of medical implants have occurred due to both poor manufacture and poor design, especially breast implants.

It is typically supplied in two parts with one of the parts containing the platinum catalyst. These are then automatically mixed with any colours and ingredients which may be required. The mixing produces a very homogeneous material that leads to products that are not only very consistent throughout the part, but also from part to part.

Applications

Once mixed and coloured, silicone rubber can be extruded into tubes, strips, solid cord or custom profiles according the size restrictions 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 BS3734, for extrusions the tightest level is E1 and the widest is E3.

Becoming more and more common at the consumer level, silicone rubber products can be found in every room of a typical home. From automotive applications; to a large variety of cooking, baking, and food storage products; to apparel, undergarments, sportswear, and footwear; to electronics; to home repair and hardware, and a host of unseen applications.

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 delivers a lack of brittleness below freezing and excellent tolerance of high temperatures of over 150 °C. Also, its hygienic 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-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 tape self-fuses or amalgamates without any added adhesive.

Self-healing

Recently, silicone rubber formed the matrix of the first autonomic self-healing elastomer.^[2] 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.^[3]

References

^ Compare Materials: Natural Rubber and Silicone Rubber
^ Keller et al., A Self-Healing Poly(dimethyl siloxane) Elastomer, Advanced Functional Materials, v. 17, p. 2399–2404 (2007).
^ Keller et al., Torsion Fatigue Response of Self-Healing Poly(dimethyl siloxane) Elastomers, Polymer, v.49 p. 3136–3145 (2008).

Compare technical properties of Silicone Rubber grades and types