Types of Silicone Rubber
The organic groups in silicone rubbers may be methyl, vinyl, phenyl or other groups. According to ASTM D1418 standard, which covers a system of general classification or nomenclature for rubber and rubber lattices, silicone rubbers are classified as:
Methyl Group – Also known as dimethylsilicone elastomer/rubber or simply methyl silicone rubber. It is also referred by MQ.
Methyl and Phenyl Groups – Also known as methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber. It is referred as PMQ and it has excellent low temperature performance
Methyl and Vinyl Groups – Also known as methylvinylsilicone elastomer/rubber. It is referred as VMQ as well.
Methyl, Phenyl and Vinyl Groups – It is referred as PVMQ as well and known for its excellent low temperature performance.
Fluoro, Vinyl and Methyl Groups – Also known as fluorinated rubber or Fluorosilicone rubber. It is referred as FVMQ and they are highly resistant to chemical attach (fuel, oil, solvent…)
Among the families of silicone rubber variations, there are three main forms of silicone rubber, depending on the processing method: liquid, room temperature vulcanized, and high temperature vulcanized.
Liquid silicone rubber (LSR) contains polymers of lower molecular weight and hence shorter chains. It can be extruded or injection molded with specialized machinery. This type of silicone rubber is more weather resistant and transparent.
RTV (room temperature vulcanized) silicone rubber is a type of silicone rubber made from one-part (RTV-1) or two-component (RTV-2) systems where their hardness range of very soft to medium. It’s generally used for potting, sealant, and encapsulations. Silicone rubber molds are also made from RTV silicone rubber.
Solid Silicone Rubber or High Temperature Vulcanized (HTV) has long-chained polymers with the heaviest molecular weights. They are available in uncured form and required traditional rubber processing techniques.
Curing Method of Silicone Material
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.
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.[1] 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.[3] 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.
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 (compare cumene hydroperoxide), has principal breakdown products of acetophenone and phenyl-2-propanol. The other is dichlorobenzoyl peroxide, whose principal breakdown products are dichlorobenzoic acid and dichlorobenzene.
