Silicone Rubber Chemistry And Properties
Silicone rubber is an inorganic synthetic silicone elastomer made from a crosslinked silicon-based polymer reinforced with filler.
Polymers / Gums
Silicone polymers (also called silicone gums) follow this general structure:
Polymers are identified by degree of polymerization (DP) – the number of repeating Si-O-Si units in the polymer (the sum of subscripts x and y in the above formula).
Rubbers are formulated with specific polymers or blends of polymers of varying weights and functionalities to impart desired performance characteristics. The highly polymerized, high-molecular-weight silicone polymers for high consistency silicone rubber are also commonly known as gums.
||Degree of Polymerization (DP)
||Molecular Weight (MW) of Polymer|
Reinforcing fillers are often included to add strength to the finished rubber. Common fillers are amorphous fumed silicas or precipitated silicas, ranging in size from 5 to 20 nm in diameter. The silica typically exhibits silanol functionality, which enables it to strongly bond with the polymer and increase the strength of the silicone elastomer.
Additives / Extending Fillers
A silicone rubber compound may have one or more additives, or extending fillers, added to impart specific performance attributes.
Pigment color masterbatches enhance aesthetic appeal with a variety of color options.
Silane additives effectively bond the polymer to various inorganic components such as pigments.
XIAMETER® brand rubbers use one of two cure chemistries: peroxide-catalyzed, free radical cure or platinum-catalyzed addition cure. Most XIAMETER® HCR rubbers are peroxide catalyzed. All XIAMETER® LSR materials are platinum catalyzed.
Peroxide/Free Radical Cure
Free radical cure systems require a heat cure to enable the peroxide catalyst to decompose into two free radical-containing components. These components react to transfer the free radicals to the silicone polymer with either alkyl or vinyl functionality.
Crosslinking occurs as the free radicals on one polymer chain react and bond with alkyl species on another polymer chain.
One drawback of peroxide cure in silicone materials is residues in the cured elastomer, including acid byproducts and polychlorinated biphenyls (PCBs). Unless removed, residues can “bloom,” manifesting as a powder on the part surface. Oven cure or post-cure cycles are generally required to remove these byproducts from cured parts.
Addition cure, known as hydrosilylation, involves the addition of a silicon hydride (*SiH) to an unsaturated carbon-carbon bond in the presence of a noble metal catalyst (platinum).
For the cure to occur, the silicone polymers contained in the elastomers must include a vinyl or other alkenyl functionality. Both high consistency and liquid silicone rubber products can employ the addition cure mechanism.
Elastomers featuring this type of cure system are supplied as two-part kits: one part contains the catalyst, the other a silicon hydride-functional crosslinker and an inhibitor to provide working time once the two parts have been mixed.
A major advantage of addition cure rubber is that the cure reaction produces no byproducts. This usually eliminates the need for post-curing, although a post-cure cycle may be used to stabilize or enhance the material properties. Addition cure rubbers are inhibited by amines, sulfur, phosphorous, tin complexes, peroxides and peroxide byproducts, so contamination with these materials must be avoided.
Considerations For Rubber Selection
The wide range of compounded rubbers provides a range of available properties.
Physical Properties – Durometer hardness, tensile strength, tear strength, elongation and modulus are common measurements of rubber performance.
Regulatory Approvals – For food-contact applications, rubber compounds (including base, catalyst and any selected additives) must meet FDA 21 CFR 177.2600 and BfR, XV regulations.
Fluid Resistance – Resistance to oils, fluids, solvents and chemicals can provide performance advantages in many applications. Learn more reading the Silicone Rubber Fluid Resistance Guide (PDF: 495 KB).
For products requiring resistance to solvents and fuels, discover Silastic® brand fluorosilicone rubber (FSR) and fluorosilicone liquid silicone rubber (F-LSR) materials from Dow Corning.