Silicone rubbers are rubber compounds with both
organic and inorganic properties, as well as highly pure fumed silica as two main
components. They possess many characteristics, which are not present in other organic
rubbers and have importance roles in numerous industries such as electrical, electronics,
automobiles, food, medical, household appliances and leisure products. Silicone rubber is
uniquely different from conventional rubber in that the molecule structure of the polymer
consists of long chains of alternating silicone and oxygen atoms. This polymer therefore
has an organic and inorganic nature, the inorganic part make the polymer very resistance
to high temperature, good electrical insulting properties and chemical inertness while the
organic components make it extremely flexible.
Characteristics
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Heat Resistance | |
Compression Set |
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Cold Resistance | |
Fatigue Resistance |
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Weathering Resistance | |
High Tensile and Tear Strength |
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Electrical Properties | |
Incombustibility. |
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Electric Conductivity | |
Gas Permeability |
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Thermal Conductivity | |
Physiological Inertness |
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Resistance to Radioactive Rays | |
Transparency and Colouring |
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Resistance to Steam | |
Non-stickiness Properties Non-corrosive |
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Resistance to Oil Solvents And Chemicals |
Silicone rubbers are extremely heat resistant as compared to normal organic rubbers. There is almost no change in properties at 150°C and thus they can be used almost permanently. For their excellent heat resistance, they are widely applied as material for rubber parts that are used at high temperature.
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Silicone rubbers are extremely cold resistant. The brittle point of normal organic rubbers is about –20°C to –30°C. In contrast, the brittle point of silicone rubbers is as low as –60°C to –70°C.
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Silicone rubbers have excellent weathering resistance, Under the ozone ambience that are produced due to corona discharge, normal organic rubbers deteriorate tremendously but silicone rubbers remain almost unaffected. Furthermore, even under long term exposure to ultraviolet and weathering, their properties remain almost unchanged.
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Silicone rubbers are excellent in electrical insulating properties and stable under a wide range of both frequency and temperature. Furthermore, no significant deterioration in characteristics is observed when silicone rubbers are immersed in liquid. Thus, they are best to be used as electrical insulators. In particular, silicone rubbers extreme resistance to corona discharge or electric is at high voltage, and therefore widely used as insulating materials for high voltage portions.
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Electric conductive silicone rubbers are the rubber compounds with electric conductive materials such as carbon being incorporated. Various products with electric resistance ranging from a few ohms-cm to e+3 ohms-cm are available. Moreover, other properties are also comparable to those of normal silicone rubbers. Therefore, they are widely used as contact points of keyboards, around heaters and sealing materials for anti-static components and high voltage cables. In general, electric conductive silicone rubbers available in the market are mostly those with volume electric resistivity ranging 1 to e+3 ohms-cm.
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The thermal conductivity of silicone rubber is about 0.5 e+3 cal.cm.sec. C, this value shows excellent thermal conductivity for silicone rubbers, thus they are used as heat sink sheets and heating rollers.
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Normal silicone rubbers (dimenthyl silicone rubbers) do not show excellent resistance to radioactive rays in particular as compared to other organic rubbers. However, methyl phenyl silicone rubbers, with the phenyl radical being incorporated into the polymer, possess good resistance to radioactive rays. They are utilised as cables and connectors in nuclear power stations.
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Silicone rubbers have low water absorption of about 1% even when they are immersed in water for a long duration. Mechanical tensile strength and electrical properties are almost unaffected. Generally, silicone rubbers do not deteriorate when the are in contact with steam, the influence becomes significant when the steam pressure in increased. Siloxane polymer breaks under high-pressure steam above 150°C causes the deterioration in properties. This phenomenon can be rectified by silicone rubber formulation, selection of vulcanising agents and post cure.
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Silicone rubbers have excellent oil resistance at high temperature. Silicone rubbers are slightly inferior at temperatures below 100°C but offer superior resistance to oil above 100°C. In addition, resistance to solvents and chemicals are also excellent.
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When silicone rubbers are used as rubber materials for packing which undergo compressive deformation under heating condition, the capability to recover is especially important. The compression set of silicone rubbers is table over wide range of temperature from –60°C to 250°C.
Generally silicone rubbers require post cure. Especially in the case of the manufacturing products with low compression set. Post cure is desired and the selection of optimum vulcanising agents is necessary.
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In general, silicone rubbers are not superior to normal organic rubbers in terms of strength in dynamic stress like fatigue resistance. However, to overcome this defect rubbers, which are 8 to 20 times better in fatigue resistance, are being developed. These products are widely applied in many aspects such as keyboards of office automation machines and rubber parts of transport vehicles.
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In general, the tear strength of silicone rubbers is about 15kgf/cm. However, high tensile and tear strength products (30kgf/cm to 50kgf/cm) are also made available by improving the polymer as well as selection of fillers and cross-linking agents. These products are best utilised to manufacture complicated mouldings, which require greater tear strength, mould cavities with reverse tapers and huge mouldings.
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Silicone rubbers do not burn easily even though they are drawn closely to the flame. However, once they catch fire, they burn continuously. With the incorporation of minute flame retardant, silicone rubbers can possibly acquire incombustibility and ability to extinguish. The combustibility classification standards of UL94 and UL94V-0 standard are being developed.
These products do not release any smoke or toxic gases when burn, as they do not consist of any organic halogen compounds which are present in organic rubbers. Therefore, they are of course, used in household electrical appliances and office machines as well as materials for the closed space in aircraft, subways and building interiors. They become the indispensable products in safety aspects.
UL94 Combustibility Classification Standards |
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| Classification | Standards |
| 94V-0 | Continuous combustion time: average below 5 seconds and maximum below 10 seconds |
| 94V-1 | Continuous combustion time: average below 25 seconds and maximum below 30 seconds. |
| 94HB | No combustion up to 4 inches standard line under horizontal combustion test. |
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The membranes of silicone rubbers have better permeability for gases and water vapour as well as better selectivity in comparison to organic rubber. Their applications as gas-water separating membranes for artificial heart-lung and oxygen enriching devices are being examined.
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Silicone rubbers are generally inert to physiology. They also have interested properties such as they do not cause the coagulation of blood easily. Therefore, they are being utilised as catheters, hollow fibres and artificial heart-lung, vaccines, medical rubber stoppers and lenses for ultrasonic diagnosis.
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Normal organic rubbers are black due to the incorporation of carbon. As for silicone rubbers, it is possible to produce highly transparent rubbers by incorporating fine silica which do not deteriorate the original transparency of silicone.
Silicone rubbers with excellent transparency and tensile strength are also being developed and utilised as medical, food processing tubes and various products. Due to the excellent transparency, coloration by pigments is easy. Thus, colourful products are possible.
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Silicone rubbers are chemically inert and possess excellent mould releasing property. As such, they do not corrode other substances. Due to this property, they are used as fixed rolls of photocopy machines, printing rolls, sheets and (lost-wax) etc.
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