Silicone rubber is an elastomer 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 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. 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. The technically correct term for the various silicone rubbers is polysiloxanes (polydimethylsiloxanes being a large subset), referring to a saturated Si-O backbone.
Curing 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.
Platinum-based cure system In a platinum-based silicone cure system, also called an addition system (because the key reaction-building polymer is an addition reaction), a hydride- and a vinyl-functional siloxane polymer react in the presence of a platinum complex catalyst, creating an ethyl bridge between the two. The reaction has no byproducts. Such silicone rubbers cure quickly, though the rate of or even ability to cure is easily inhibited in the presence of elemental tin, sulfur, and many amine compounds.
Condensation cure system Condensation curing systems can be one-part or two-part systems. In one-part or RTV (room-temperature vulcanizing) system, a cross-linker exposed to ambient humidity (i.e., water) experiences a hydrolysis step and is left with a hydroxyl or silanol group. 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, ester, enoxy, 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. Tin catalysts such as dibutyl tin dilaurate (DBTDL) can be used in oxime and acetoxy-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. Depending on the type of detached molecule, it is possible to classify silicone systems as acidic, neutral, or alkaline.
Two-part condensation systems package the cross-linker and condensation catalyst together in one part while the polymer and any fillers or pigments are in the second part. Mixing of the two parts causes the curing to take place. A typical filler is fumed silica, also known as pyrogenic silica, which used to control the flow properties of the sealant. 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 cure system 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 organic peroxides used, dicumyl peroxide, has principal breakdown products of acetophenone and 2-phenyl-2-propanol. The other is 2,4-dichlorobenzoyl peroxide, whose principal breakdown products are 2,4-dichlorobenzoic acid and 1,3-dichlorobenzene.
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