Silicone rubber offers good resistance to extreme temperatures, being able to operate normally from -100°C to +250°C. At extreme temperatures, the tensile strength, elongation, tear strength and compression set can be far superior to conventional rubbers. Organic rubber has a carbon to carbon backbone which can leave them susceptible to ozone, UV, heat and other aging factors that silicone rubber can withstand well. This makes it one of the materials of choice in many extreme environments. Silicone rubber is a highly inert material and does not react with most chemicals. Once milled and colored, silicone rubber can be extruded into tubes, strips, solid cord or custom profiles. Cord can be joined to make O-Rings and extruded profiles can be joined to make seals. Silicone rubber can be molded into custom shapes and designs.
- Incredible resistance to extreme temperatures
- Better fire resistant properties and excellent electrical insulator compared to natural rubbers
- Thermal stability making it possible that properties such as volume resistivity, dielectric strength and power factor are not affected by changes in temperature
Silicone rubber is well suited for a variety of applications, including:
- Shaft sealing rings
- Spark plug caps
- Radiator and automotive heating silicone hoses
- Window and door silicone seals
- Expansion joints
- Oven door seals and gaskets
- Silicone rubber tubing
Silicone Rubber Properties
|ASTM D-2000 Classification||FC, FE, GE|
|Military (MIL-STD 417)||TA|
|Durometer Range (Shore A)||30-90|
|Tensile Range (P.S.I)||200-1500|
|Elongation (Max %)||700|
|Resilience — Rebound||Good|
|Abrasion Resistance||Fair to Poor|
|Oil Resistance||Fair to Poor|
|Low Temperature Usage (F°)||-60° to -150°|
|High Temperature Usage (F°)||Up to 480°|
|Aging Weather — Sunlight||Excellent|
|Adhesion to Metals||Good|
Comment — Silicone rubber material can be compounded in different ways to meet any number of applications. Silicone is able to be compounded to have tensile strength in the area of 1500 PSA and tear resistance up to 200lbs. It also has a low compression set and good resilience in addition to being moderately resistant to solvents. Silicone has excellent heat resistance and good release characteristics. It also exhibits extreme low temperature properties and can be highly resistant to oxidation and ozone attack. Generally, silicones are attacked by most concentrated solvents, oils, acids and dilute sodium hydroxide.
Key Properties that have made silicone rubbers an important engineering material include:
- Good thermal stability — allows for properties such as volume resistivity, dielectric strength and power factor to be unaffected by temperature changes
- This allows for a large operating range (-100 - 250°C)
- Ability to repel water and form tight seals
- Good electrical insulation
- Low chemical reactivity
- Excellent resistance to oxygen, ozone and sunlight
- Anti-adhesive properties
- Low toxicity
- Some grades of silicone have poor hydrocarbon, oil and solvent resistance
- High gas permeability
- Relatively high cost
Heat and Cold Resistance
Silicone rubber withstands high and low temperatures far better than their organic rubber counterparts, with the ability to be used indefinitely at 150°C with almost no change in its properties. Certain silicone compounds can withstand use even at 200°C for 10,000 hours or more and some silicone rubber products can withstand heat upwards of 350°C for short periods. This makes silicone an excellent material for rubber components that are exposed to high temperature environments.
Silicone rubber also exhibits excellent resistance to cold temperatures. The embrittlement point for some silicone parts is between -60° to -70°C whereas typical organic rubbers experience embrittlement somewhere between -20° to -30°C. When exposed to temperatures at which organic rubbers turn brittle, silicone rubber remains elastic. Some silicone products are able to withstand extremely low temperatures, keeping their elasticity at -100°C and below.
Silicone rubber exhibits exceptional weatherability properties, and can be exposed to wind, rain and UV rays for long periods of time with virtually no change in physical properties. In addition, ozone created by corona discharge, has almost no effect on silicone rubber, where it will rapidly deteriorate most organic rubber counterparts.
Moisture and Steam Resistance
Silicone rubber can be immersed in cold, warm, or boiling water for long periods of time and only experience water absorption of about 1%. Furthermore, being immersed in water has virtually no effect on the electrical properties or mechanical strength of the material.
Under ordinary pressure, contact with steam typically causes little or no deterioration in silicone rubber parts. As steam pressure increases, however, the deterioration effects on the silicone increases. High pressure steam exposure at temperatures over 150°C can result in a breakdown of the siloxane polymer which, in turn, will lead to a decline in the properties of the rubber. This polymer breakdown in the presence of high pressure steam exposure can be avoided by adjusting the silicone rubber formula, selecting the proper curing agent and/or post-curing the compound. Numerous silicone rubber products can be customized to your specific needs with an improved resistance to steam and hot water.
Resistance to oils, solvents, and other chemicals
Certain silicone rubbers show an outstanding resistance to oil at high temperatures. At lower temperatures (below 100°C) organic rubbers such as nitrile rubber and chloroprene will exhibit a higher resistance to oil. However, at temperatures exceeding 100°C, silicone rubber proves to be a superior material.
Silicone is essentially unaffected by polar organic compounds such as aniline and alcohol as well as dilute acids and bases, showing an increase in volume due to swelling in the range of only 10%-15%. Silicone will show signs of swelling when exposed to non-polar organic compounds such as benzene, toluene, and gasoline - unlike organic rubbers, however, silicone will not decompose or dissolve and will instead return to its former state when the solvent is removed.
Silicone rubber does experience adverse effects when it is exposed to strong acids and bases, therefore it should not be used in applications where it will come into contact with such chemicals. Typically, the effects of solvents on silicone are evidenced through the swelling, softening, and reduced strength of the rubber part.
Silicones thermal stability stems from the Si-O and Si-CH3 bonds which are thermally stable themselves. The partially ionic nature of these bonds (51%), however, makes them vulnerable to concentrated acids and alkalis at ambient temperatures.
Silicone materials are generally flexible at low temperatures due to their low glass transition temperature (Tg). Silicone tends to stiffen up at higher temperatures.
At 25°C silicone rubber exhibits gas permeability that is approximately 400 times that of a butyl rubber counterpart. This allows silicone to be useful in applications such as oxygen permeable membranes in medical devices.
Silicone parts can provide excellent electrical insulation, with grades available that exhibit volume resistivity as low as .004ohm.cm. The thermal stability of these materials also allows for properties such as volume resistivity, power factor, and dielectric strength to be unaffected by changes in temperature. There is no decline in insulation performance even when the silicone is immersed in water, making it an ideal insulating material. Silicone material also displays arc and corona resistances surpassed only by mica.
When brought close to a flame, silicone rubber will not ignite easily. However, once it is ignited, it will continue to burn. However, even at extreme temperatures when silicone rubber burns, the by-products will be nontoxic and the residual ash will continue to provide electrical insulation properties. It is possible to improve flame retardancy and/or give the material self-extinguishing properties by adding a small amount of flame retardant to the rubber compound. Many closed cell silicone sponge and silicone gaskets have flame retardant properties making them suitable for gasket applications in Mass Transit systems, telecommunications equipment and analytical instrumentation.
Some silicone rubber products have received UL94 V-0 certification according to the UL94 (USA) standards for flammability classification. When they do burn, these materials produce almost no black smoke or noxious gas since they do not contain the organic halogen compounds typically found in organic polymer rubbers. UL94 V-0 silicones are used in consumer electronics and business equipment and in closed spaces such as aircraft, subways and building interiors.
When a silicone rubber products application causes it to be under compression in heated conditions, the recovery of these materials from compression deformation becomes a crucial consideration. Unlike most organic elastomers such as EPDM and neoprene, the compression set of silicone rubber is consistent over a wide range of temperatures from -60° to +250°C. The selection of a proper curing agent and post-curing is particularly recommended when using silicone material to make molded products that require a low compression set.
Classes of Silicone Rubber
The various classes of silicone rubbers according to ASTM D1418 Rubber classifications are as follows:
|MQ||Silicone rubbers having only methyl groups of the polymer chain (polydimethyl siloxanes)||Not commonly used|
|VMQ||Silicone rubbers with methyl and vinyl substitutions on the polymer chain||General purpose|
|PMQ||Silicone rubbers having methyl and phenyl substitutions on the polymer chain||Extremely low temperature applications
Not commonly used
|PVMQ||Silicone rubbers having methyl, phenyl and vinyl substitutions on the polymer chain||Extremely low temperature applications|
|FVMQ||Silicone rubbers having fluoro, methyl and vinyl substitutions on the polymer chain||Applications that involve fuel and oil exposure and solvent resistance.|
- Liquid Silicone Rubbers (LSR) — also called heat curable liquid materials. Processed on specially designed injection molding and extrusion equipment.
- High Temperature Vulcanizing (HTV) — Also called heat curable silicone, these materials usually are in a semi-solid gum form while in the uncured state. Rubber-type processing is required to produce finished items.
- Room Temperature Vulcanizing (RTV) — Usually comes in the form of a flowable liquid. Commonly used for sealants, mold making, encapsulation and potting. RTV materials are generally not used as conventional rubbers.
Components of Silicone Compounds
Aside from RTV and liquid curing systems, silicone rubbers are typically cured using peroxides such as benzoyl peroxide, 2,4-dichlorobenzol peroxide, dicumyl peroxide, and t-butyl perbenzoate. Vinyl containing silicones have also been successfully cured using alkyl hydroperoxides and dialkyl peroxides.
To improve the otherwise poor tensile strength of silicones, reinforcing fillers are added. The most preferred filler is silica fume with particle sizes ranging 10-40nm. Carbon black has also been used as filler. These fillers interact with the vulcanizate, forming a pseudo-vulcanization which can occur either during mixing (creep hardening), or in storage (bin ageing).
Compared to natural rubbers, silicones exhibit better fire resistant properties. These properties can be further improved through the addition of flame retardant additives such as carbon black, aluminum trihydrate, platinum, zinc or ceric compounds. The addition of carbon black as an additive will also increase the silicones electrical conductivity.
- Spark plug caps
- Shaft sealing rings
- Radiator and automotive heating hoses
- Silicone O rings
- Corona and embossing roller gaskets
- Window and door seals
- Expansion joints
- Silicone gaskets
- Cables and cable terminations
- Keyboards and contact mats
- Corona-resistant insulation tubing
- Conductive profiled silicone seals
- Tubing for dialysis and transfusion equipment
- Bellows for artificial respirators
- Dummies for babies