Silicone rubber is a synthetic elastomer distinct from organic rubbers because its molecular structure is based on silicon rather than carbon. It is manufactured from silica, a compound derived from sand, which is the second most abundant element in the Earth’s crust. This silicon-oxygen backbone provides exceptional properties, including remarkable heat resistance, flexibility, and resistance to chemical degradation and UV light. These characteristics make it the material of choice for applications ranging from medical devices and automotive seals to cookware and construction sealants.
The Core Building Blocks
The fundamental component of silicone rubber is the polysiloxane chain, which gives the material its properties. Unlike natural rubber and other synthetic elastomers, which have a carbon-carbon chain backbone, silicone rubber features an alternating inorganic backbone of silicon and oxygen atoms (Si-O-Si). This siloxane bond is stronger and more flexible than carbon bonds, accounting for silicone’s superior thermal stability.
The manufacturing process begins by extracting silicon metal from silica through heating in a furnace. This silicon metal is then reacted with methyl chloride to create a mixture of chlorosilanes. These chlorosilanes are distilled to isolate specific monomers, such as dimethyldichlorosilane, which is then hydrolyzed in the presence of water. This hydrolysis forms the siloxane units, which polymerize into long, linear chains known as silicone “gum” or polydimethylsiloxane (PDMS). The final polymer chain length is precisely controlled during this polymerization to determine the initial viscosity of the raw material.
Modifying Ingredients and Fillers
The pure polysiloxane gum is chemically stable but mechanically weak and would be brittle without reinforcement. To transform the material into a strong, resilient rubber, reinforcing fillers are mixed into the polymer compound. The most commonly used filler is fumed silica, a high-purity form of silicon dioxide that is an extremely fine nanoparticle powder.
Fumed silica is added in large amounts, sometimes making up to 50% or more of the compound’s weight in high-consistency rubber formulations. Its tiny particle size and large surface area create a three-dimensional network that strongly interacts with the silicone polymer chains. This interaction dramatically increases the material’s tensile strength, tear resistance, and overall hardness. The amount of fumed silica used differentiates high consistency rubber (HCR) from liquid silicone rubber (LSR), as LSR requires lower filler content to maintain flowability.
The compound is customized further with other additives to achieve specific properties. Plasticizers can be included to increase softness and flexibility, while pigments are added for color. Heat stabilizers or flame retardants may also be incorporated to enhance performance under extreme conditions.
Converting the Compound: Curing Processes
After the polysiloxane gum and fillers are mixed, the material must undergo vulcanization, or curing, to become a stable, elastic solid. Curing creates permanent chemical cross-links between the polymer chains, forming a three-dimensional network that defines the rubber’s elasticity and final shape. The method used for curing determines the final product’s form and application.
High Consistency Rubber (HCR) Curing
HCR curing uses high temperatures (typically 120°C to 180°C) and high pressure in a mold. HCR is supplied as a solid, gum-like sheet and is processed through compression or transfer molding.
Liquid Silicone Rubber (LSR) Curing
LSR curing uses a platinum-catalyzed addition reaction, where two low-viscosity liquid components are mixed and then injected into a heated mold. This process allows for fast cycle times, often 10 to 60 seconds, and is ideal for high-volume, precision parts.
Room Temperature Vulcanization (RTV)
RTV cures at ambient temperatures and is used for sealants, coatings, and adhesives. One-component RTV systems cure when they react with moisture in the air. Two-component RTV systems use a chemical catalyst that initiates the cross-linking reaction upon mixing.