Properties of Liquid Silicone Rubber (Part II)

Silicone rubbers exhibit many useful characteristics. The electrical properties are, in general, superior to those of organic rubbers, and the conductivity can be customized using special additives.

Main Properties of Liquid Silicone Rubber

Silicone Rubber Compounds have characteristics of inorganic and organic materials and they offer a number of advantages in comparison to other rubbers.

Here, some of the most important properties of silicone are presented and explained.

Electrical Properties

Silicone rubber has a high insulation resistance, and its electrical properties are stable over a wide range of temperatures and across a wide frequency spectrum. The behavior is the same even if the material is immersed in water.

The resistance to corona discharge is 1,000 times higher than in organic polymers when the operating life is at 3kV-h. The arc resistance is similar to that of a high butyl rubber, polytetrafluoroethylene, or epoxy resin.

Thus, silicone rubber is used extensively as an insulator in high voltage applications.

Some conductive silicone rubbers contain electrically conductive additives such as carbon. The resistance decreases 1010 times, but the other properties remain constant. The applications of these materials are keyboard contact points, antistatic materials, and high-voltage cable shielding.

Thermal Properties

The thermal conductivity of silicone rubber is higher in comparison with common organic rubbers. When the material contains a high amount of inorganic fillers, the conductivity can be improved twice, which makes the material adequate for thermal interface sheets and heating rollers.

Flame Retardancy

In the presence of a flame, the silicone rubber does not ignite easily but, once ignited, it continues burning. It does not produce black smoke or noxious gasses due to an absence of halogen compounds that are common in organic rubbers. If flame retardants are added, retardancy and self-extinguishing are improved. For all of these reasons, silicone rubbers are used in electronics, aircrafts, subways, and building interiors.

Mechanical Properties

One of the most important properties of rubber is its compression set, which is defined as the permanent deformation in the material when a force is removed. The recovery of that deformation is critical in gaskets, for example. This property remains almost constant with the temperature.

Creep is defined as the deformation of the materials under a constant load. Typically, creep increases as temperature rises. Rubbers that are firm and have high thermal stability tend to exhibit lower creep values at higher temperatures.

Silicone rubber exhibits less creep than organic rubbers and the percentage of creep increases slightly with the time in comparison to the exponential behavior of the organic rubbers. To improve this property, post-curing is necessary and the selection of the curing agent is critical.

Unfortunately, one of the biggest disadvantages of silicone rubber is its strength against dynamic stress. To improve it, some additives are included in the formulation and, as a result, the fatigue resistance is 8–20 times higher than in a conventional formulation.

The tear strength of silicone rubber is higher than in organic rubbers and decreases with the temperature. Depending on the application, the property can have higher values using modifiers, special fillers and cross linkers, and the geometrical design of the item is critical to obtain a good behavior.

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