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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. Due to these properties and its ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products, including: In its uncured state, silicone rubber is a highly-adhesive gel or liquid.
In order to convert 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. 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. In a platinum-based silicone cure system, also called an addition system because the key reaction-building polymer is an " Addition reaction " , two different chemical groups a silicone hydride and a vinyl react in the presence of platinum which serves as the catalyst.
In this reaction, an ethyl group [C H2 - C H2 ] is formed and there are no byproducts. Two separate components must be mixed to catalyze the polymers: Such silicone rubbers cure quickly, though the rate of or even ability to cure is easily inhibited in the presence of elemental tin , sulphur , and many amine compounds.
Condensation curing systems can be one-part or two-part systems. The silanol condenses further with another hydrolyzable group on the polymer or cross-linker and continues until the system is fully cured. Such a system will cure on its own at room temperature and unlike the platinum-based addition cure system is not easily inhibited by contact with other chemicals, though the process may be affected by contact with some plastics or metals and may not take place at all if placed in contact with already-cured silicone compounds.
The crosslinkers used in condensation cure systems are typically alkoxy, acetoxy or oxime silanes such as methyl trimethoxy silane for alkoxy-curing systems and methyl triacetoxysilane for acetoxy-curing systems. In many cases an additional condensation catalyst is added to fully cure the RTV system and achieve a tack-free surface. Organotitanate catalysts such as tetraalkoxy titanates or chelated titanates are used in alkoxy-cured systems. Acetoxy tin condensation is one of the oldest cure chemistries used for curing silicone rubber, and is the one used in household bathroom caulk.
The smell of vinegar in the form of acetic acid is the usual indicator that the curing reaction is taking place. Non-acid-producing formulations also exist which have a shorter shelf-life, however, and reduced adhesion when finally cured. Mixing of the two parts causes the curing to take place. Once fully cured, condensation systems are effective as sealants and caulks in plumbing and building construction and as molds for casting polyurethane, epoxy and polyester resins, waxes, gypsum, and low-melting-temperature metals such as lead.
They are typically very flexible and have a high tear strength. They do not require the use of a release agent since silicones have non-stick properties. Peroxide curing is widely used for curing silicone rubber.
The curing process leaves behind byproducts, which can be an issue in food contact and medical applications. However, these products are usually treated in a postcure oven which greatly reduces the peroxide breakdown product content.
One of the two main peroxides used, dicumyl peroxide, has principal breakdown products of acetophenone and phenylpropanol. The other is dichlorobenzoyl peroxide, whose principal breakdown products are dichlorobenzoic acid and dichlorobenzene. 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 it commercially.
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. In fact, silicone is an inorganic polymer , and the technically correct term for the various silicone rubbers is polysiloxanes or polydimethylsiloxanes. Corning Glass in a joint venture with Dow Chemical formed Dow Corning in 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 now Momentive Performance Materials. Wacker Chemie also started production of silicones in Europe in The Japanese company Shin-Etsu Chemical began mass production of silicone in The companies mentioned above are now still the main competitors in the oligopoly that comprises the silicone industry.
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. This makes silicone rubber one of the elastomers of choice in many extreme environments. Silicone rubber is highly inert and does not react with most chemicals.
Due to its inertness, it is 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.
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. 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 farther 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 also because polyoxymethylene decomposes formaldehyde, which is volatile and escapes driving decomposition forward, but Si-containing decomposition products of silicone are less volatile.
There are many special grades and forms of silicone rubber, including: 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 , 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. Becoming more and more common at the consumer level, silicone rubber products can be found in every room of a typical home. It 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.
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.
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-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 pressed, used in many devices such as computer keyboards and remote control handsets.
In , silicone rubber formed the matrix of the first autonomic self-healing elastomer. Additionally, this material had improved fatigue properties as evaluated using a torsion-fatigue test. From Wikipedia, the free encyclopedia. L and Pizzi, A. Retrieved from " https: Elastomers Sculpture materials Silicones Siloxanes. All articles with unsourced statements Articles with unsourced statements from January Views Read Edit View history. This page was last edited on 28 April , at By using this site, you agree to the Terms of Use and Privacy Policy.
Polymax [ citation needed ].