Silicone is a synthetic compound with inert properties, available in various forms such as oil, rubber, and resin. It is commonly heat-resistant and exhibits rubber-like characteristics, making it widely used in sealants, adhesives, lubricants, medical applications, cookware, and insulation. Silicone is a polymer composed of silicon, carbon, hydrogen, and oxygen, and sometimes incorporates other elements.
Here is a summary of the most popular types of silicone:
Silicone is classified based on the organic groups
Silicone can contain various organic groups, such as methyl, vinyl, phenyl, or others. The ASTM D1418 standard provides a general nomenclature system for rubber and rubber lattices, which includes the following classifications:
Methyl Group: Also known as dimethyl silicone elastomer/rubber or methyl silicone rubber. It is sometimes referred to as MQ.
Methyl and Phenyl Groups: Also known as methyl-phenyl silicone elastomer/rubber or phenyl silicone rubber. It is also referred to as PMQ. This type of silicone rubber demonstrates excellent performance at low temperatures.
Methyl and Vinyl Groups: Also known as methyl vinyl silicone elastomer/rubber. It is referred to as VMQ.
Methyl, Phenyl, and Vinyl Groups: Referred to as PVMQ. This type of silicone rubber is recognized for its excellent low-temperature performance.
Fluoro, Vinyl, and Methyl Groups: Also known as fluorinated rubber or fluorosilicone rubber. It is referred to as FVMQ. These silicone rubbers exhibit high resistance to chemical attacks, including fuels, oils, solvents, and more.
Silicone Is Classified Based On The Molecular Structure
Room Temperature Vulcanized (RTV)
RTV Silicone is a type of liquid silicone that undergoes a curing process at room temperature, transforming into a flexible rubber material. The term RTV means Room Temperature Vulcanising.
There are two main forms of RTV silicone available: one-component and two-component (a base and a curative). Manufacturers offer RTV silicone in a variety of hardness options, typically ranging from very soft to medium, with a hardness measurement of 15 to 40 Shore A. To initiate the curing process of RTV silicone, manufacturers provide a catalyst, which can be either platinum or a tin compound like dibutyltin dilaurate.
RTV-1 silicone is supplied and ready to use. Once applied, a stabilizing agent in the silicone quickly reacts with moisture in the air, curing the outside. Various types of crosslinking agents are used, some of which emit small amounts of acetic acid, amines, or alcohol during the setting process, while others are odorless. RTV-1 silicones are highly regarded for their user-friendliness, good adhesion, and stability at high temperatures. Typical applications as sealants, bonding glues, and coatings.
RTV-2 silicone comes in two separate parts that need to be mixed just before use. Once mixed, it cures faster compared to RTV-1. RTV-2 silicone offers a broader range of products, exhibiting exceptional mechanical properties over a wide temperature range. Condensation-cured RTV-2 silicone is generally less expensive than its platinum-cured counterpart. The absence of byproducts in platinum-cured silicone is particularly important for certain applications. RTV-2 silicone finds diverse uses, including 3D printing, coatings, and molds.
Liquid Silicone
“Liquid silicone” refers to the initial state of the silicone material used in the production of silicone-based products. In the past, silicone was supplied in a semi-solid, sticky form that required compression molding. However, modern liquid silicone rubber (LSR) is a two-part material cured with platinum, possessing excellent fluidity. This characteristic enables it to capture intricate details in injection-molded components and conform to various surface shapes when used as a coating. Moreover, the platinum-curing cross-linking system of LSR does not generate any byproducts during the curing process.
LSR offers numerous advantages depending on its intended application or industrial use. Firstly, it exhibits high biocompatibility, making it suitable for manufacturing parts that come into contacts with humans, such as organ components or prostheses. Secondly, LSR is highly durable, ensuring long-term stability and resistance to chemicals. Thirdly, it remains compatible with a wide temperature range, from -60 °C to +250 °C, while maintaining its superior mechanical properties. Fourthly, LSR possesses desirable electrical properties, making it ideal for insulation and precise protection against conductivity issues. Lastly, it is transparent and can be pigmented to match various color requirements, including human skin tones.
LSR is considered the material of choice in industries like automotive and medical devices, where the production of small and complex elastomeric parts at high speed and optimal productivity is crucial. In such cases, the liquid injection molding of LSRs becomes one of the most efficient processes for fabricators.
Fluorosilicone
Fluorosilicone is a variation of the polysiloxane (silicone) molecule. Its basic structure consists of a silicon-oxygen chain, similar to conventional silicone. However, in fluorosilicone, some of the methyl (CH3) groups present in the regular silicone molecule are substituted with trifluoro propyl groups. This alteration imparts fluorosilicone with enhanced resistance to various challenging environments, including fuel and oil, mineral spirits, toluene, and other organic solvents. Fluorosilicone achieves this improved chemical resistance while maintaining nearly identical mechanical properties as conventional silicone. However, it performs less effectively in hot air conditions and is significantly more expensive. It is particularly well-suited for aerospace and automotive applications where exposure to fuels and other chemicals is more likely to be encountered.
High-Consistency Rubber (HCR)
Heat Cured Rubber, also known as HCR (High Consistency Rubber), is a type of silicone elastomer that is formed from straight chains with exceptionally high molecular weights. The curing of HCR can be achieved through the use of either an organic peroxide crosslinker or a platinum catalyst. HCR exhibits remarkable resistance to both high temperatures and aging. To enhance specific properties like hardness and heat resistance, HCR can incorporate various fillers. It can be cured using peroxide curing systems or formulations containing a platinum catalyst. The latter method produces no chemical byproducts during the curing process. HCR is an exceptional material suitable for long-term implantable medical devices, automotive engine components, and numerous consumer household products.
What Are the Other Forms of Silicone?
Silicone can be found in various forms, including:
Emulsion: Silicone emulsions are composed of suspended silicone molecules in a stabilized water solution. They are used in applications such as lubricants, release agents, and cleaning and polishing compounds, providing improved spreadability and durability.
Oil: Silicone oils consist of simple, linear polysiloxane chains that offer lubricating properties. They are utilized as hydraulic fluids, lubricating oils, and raw materials for further processing.
Liquid: Liquid silicone rubber has a fluid consistency suitable for precision injection molding. It typically comprises a two-part formulation that is mixed before entering the mold. Liquid silicone rubber finds applications in gaskets, electronics potting, formed parts, and medical devices.
Caulk: Silicone caulk is a viscous, room-temperature vulcanizing form of silicone. It is valued for its superior workability, adhesion, and durability.
Resin: Silicone resins are usually thermosetting products with branching molecules that form densely cross-linked structures. They find use in adhesives, protective coatings, water-repellent coatings, and heat-resistant paints.
Grease: Silicone grease is created by combining fillers with silicone oil. This thermally stable product serves as a lubricant for desired surfaces conducts heat, and provides protection against moisture and chemicals. Silicone grease finds applications in mold release, glass joint seals, and electrical contact protection.
Gel: Silicone gel is produced from a two-part, platinum-catalyzed liquid formulation. Additional oils or softeners may be added to achieve the desired consistency for forming thin sheets or packets. Silicone gel has been used in the medical field for scar reduction.
Foam: Silicone foam is generated by introducing outgassing agents to liquid silicone, allowing the production of open and closed-cell foams with varying densities. The foam can have a smooth skin, suitable for gaskets and spacers, or a rough surface, useful in gap-fillers, pipe insulation, and other applications.
Solid: Solid silicone rubber, also known as “high-consistency rubber” (HCR), refers to a type of silicone with long polymer chains and high molecular weight. Once cured, it becomes relatively dense and hard. Solid silicone rubber is used in the fabrication of final products such as tubing, engine mounts, seals, and gaskets.
Silicones are suitable for a wide range of applications due to their unmatched flexibility and thermal and environmental stability. For more information, contact A1 Silicones