Properties of Liquid Silicone Rubber (Part I)

Liquid Silicone Rubber exhibit many useful characteristics. They are mixed inorganic-organic polymers and the polysiloxanes (silicon-oxygen) bonds are responsible for the final properties of the materials.

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.

General Considerations

Properties of silicone rubbers are related to their chemical structures. The backbone of silicone is formed by siloxane bonds (–Si–O–Si–), which are highly stable. The binding energy is higher than carbon bonds (common in organic polymers) and, for this reason, the silicone rubbers have higher heat resistance, flame retardancy, chemical stability and electrical insulation. In addition, the inter-molecular forces present in the material are lower and, due to the helical form of the silicone molecules, properties such as high stability, compressibility and resistance to cold temperatures are achieved. Finally, the methyl groups can rotate freely; this characteristic gives silicone rubber good interfacial properties such as water repellency, releasability, and better compression.

Temperature Resistant of LSR

In general, silicone rubbers preserve their properties at temperatures higher and lower than organic rubbers. Besides, they can have higher service times at elevated temperatures. As an example, the elongation at the break point of silicone rubbers remains almost constant over time.

Also, silicone rubbers have good resistance to cold temperatures; the embrittlement point is between 30°C and 40°C lower than organic rubbers and, even at these temperatures, silicone rubber remains elastic. The degradation of the silicone rubber can be modified by adjusting the curing agents, adding special additives and, in some applications, taking post-curing into account.

Behavior in the Presence of Humidity and Steam

Silicone rubber remains stable when the application involves immersion in water; that means, they have low water absorption, and there is no effect in electrical properties.

The mechanical properties of silicone rubber remain almost constant. For example, the tensile strength and the elongation slightly decrease with time while the hardness has no changes over time.

Under pressure, at atmospheric values, there is no deterioration; but, with pressurized steam, the negative effects increase with the pressure. Fortunately, the effect can be modified by adjusting the additives and the curing system.

Chemical Resistance of LSR

In general, the effects of solvents on silicone rubber are related to swelling, softening and decreasing in strength. Depending on the solvent, the extent of the effects is higher or lower.

Silicone rubber has an increase in volume of between 10% and 15% due to swelling in the presence of polar organic compounds (aniline, alcohol), dilute acid (acetic acid, formic acid, hydrogen sulfide) or bases (ammonia, pyridine). They do not swell in non-polar organic compounds (benzene, toluene, gasoline), and they return to their initial state when the solvent is removed. However, silicone rubbers are not stable in the presence of strong acids or bases (hydrochloric acid, nitric acid, sulfuric acid, sodium hydroxide, lithium acid, potassium hydroxide).

At high temperatures, the chemical resistance is better than in organic rubbers.

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