What are Silicone Elastomers?

Silicone elastomers are widely used in a variety of industries due to their unique properties and advantages. As with any material, there are also some drawbacks to consider when using silicone elastomers. This article will examine the key pros and cons of these versatile synthetic rubbers.

What are Silicone Elastomers?

Silicone elastomers are polymeric materials composed of silicone, carbon, hydrogen, and oxygen. The main chain of the polymer consists of alternating silicon and oxygen atoms, with side chains containing carbon and hydrogen. The presence of methyl groups attached to the silicon atoms enables rotation, resulting in silicone’s remarkable flexibility and elasticity.

The two main categories of silicone elastomers are high temperature vulcanized (HTV) and room temperature vulcanized (RTV). HTV rubbers require high heat and pressure during the molding process, while RTVs cure at room temperature using a catalyst. Both provide high temperature stability, resistance to weathering and aging, and low compression set.

Key Benefits of Silicone Elastomers

1. Excellent thermal stability

One of the primary advantages of silicone elastomers is their ability to maintain mechanical properties and elasticity at both high and low temperature extremes. They can withstand temperatures from -100°F to over 500°F (-73°C to 260°C) for extended periods with little degradation. This makes them well-suited for applications like automotive systems, aerospace, electronics, and appliances.

2. Outstanding weatherability and UV resistance

Exposure to sunlight, ozone, and moisture has minimal effect on silicone. They exhibit excellent long-term resistance to outdoor weathering and ultraviolet radiation. Silicone maintains its original properties and does not become brittle or decompose like many rubbers. This stable performance makes silicone elastomers ideal for exterior components.

3. Low compression set

Compression set measures the extent to which a material permanently deforms after prolonged compressive stress. Silicone elastomers have very low compression set compared to other rubbers, meaning they retain their original shape and bounce back after compression. This elastic memory makes them well-suited for seals, gaskets, vibration dampening, and other applications requiring consistent performance over time.

4. High tear strength

The stable molecular backbone structure of silicone lends itself to high tear strength compared to other elastomers. Silicone has excellent cut and laceration resistance, maintaining its integrity even when cut or punctured. This durability allows silicone parts to resist damage in demanding applications.

5. Biological inertness

Silicone is one of the most biologically inert materials available, meaning it does not react with bodily tissues. This makes silicone a popular choice for medical devices and implants that interface directly with the body. Silicone is FDA and ISO 10993 approved for biological safety.

6. Dielectric strength

With a high dielectric strength, silicone can insulate and protect electrical components from current leakage, arcing, and dielectric breakdown. Silicone elastomers maintain dielectric strength across a wide temperature range. This property enables their use for electrical insulation and cable jacketing applications.

7. Design flexibility

The processability of silicone enables significant design freedom and part consolidation compared to other rubbers. Silicone can be readily molded, extruded, or cast into a variety of precise, complex geometries. Silicone elastomers also adhere well to various substrates, allowing multi-material fabrication.

Potential Drawbacks of Silicone Elastomers

While having many positive attributes, there are also some potential downsides to consider with silicone elastomers:

• Surface tackiness – Silicone has an inherent tackiness that can lead to accumulation of dirt and debris. This may require coating or texturing the surface.
• Low tensile strength – While having high tear strength, silicone has relatively low tensile strength compared to other rubbers. Reinforcement is needed for high tension applications.
• Oil and solvent sensitivity – Swelling or shrinkage can occur with exposure to oils and solvents. Use can be limited for applications involving these chemicals.
• Release by-products – Some liquid silicone rubbers release acetic acid during vulcanization. This may limit uses where acidity is undesirable.
• Higher cost – Silicone elastomers are more expensive than other common rubbers like nitrile or EPDM on a per pound basis. However, their performance often justifies the added cost.
• Adhesion challenges – Bonding silicone to itself and some substrates can require specialized chemical primers to achieve sufficient adhesion.

Silicone Elastomer Applications

Thanks to their beneficial properties, silicone elastomers excel in a diverse range of product applications:

• Medical: Catheters, drainage tubes, implants, prosthetics, molded skin contact products.
• Automotive: O-rings, gaskets, seals, spark plug boots, hoses, noise dampening components.
• Electronics: Wire and cable insulation, keyboards, moisture guards, connectors, solid state lighting.
• Appliances: Gaskets, seals, tubing, bakeware handles, kitchen utensils, steamer parts.
• Construction: Sealants, water-repellent coatings, structural glazing, vibration dampeners.
• Aerospace: Gaskets, seals, tubing, wire insulation, vibration dampening pads.
• Food contact: Bakeware, kitchen tools, tongs, spatulas, funnel, jar rings.

Key Considerations When Selecting a Silicone Elastomer

With the wide range of silicone materials available, several factors should be evaluated when selecting the optimal grade for an application:

• Temperature resistance – Maximum service temperature and thermal cycling needs.
• Flexibility and hardness – The required elastomer firmness, compression force deflection, modulus.
• Cure type – Does the part design and production process suit HTV or RTV?
• Resistance – Chemical, oxidation, weathering resistance required.
• Dielectric properties – Dielectric strength and insulating needs.
• Cost – Budget constraints and production volume.
• Appearance – Optical clarity, color, surface finish.
• FDA compliance – Determine if FDA approval is required for the application.
• Strength – Tensile, tear and burst strength needs.
• Lifecycle stability – Expected service life and long-term degradation resistance.

Properly evaluating these parameters helps identify the optimal silicone grade and properties for a given application.

Conclusion

Silicone elastomers offer an exceptional balance of temperature stability, flexibility, tear resistance, and biological inertness unmatched by other rubbers. Their unique properties enable reliable performance across a broad range of demanding industrial and consumer product applications, from medical devices to high temperature mechanical seals. While no material is perfect, silicone comes closest to offering a one-size-fits-many solution. However, the material must be selected properly to align with specific application requirements. By considering silicone’s pros and cons, engineers can make informed design decisions to take advantage of this versatile, high-performance elastomer.