Silicone & Its Applications

Silicone rubber

Silicone rubber is an elastomeric material derived from silicone, a polymer comprising silicon, carbon, hydrogen, and oxygen. The term “silicone” was first coined in 1901 by Kipping to describe compounds with the generic formula R₂SiO. These compounds were soon identified as polymers, specifically polydialkylsiloxanes, with the general structure:

R₂SiO, where R may represent methyl, phenyl, vinyl, or trifluoropropyl groups.

In 1871, Ladenburg discovered that diethyldiethoxysilane {(C₂H₅)₂Si(OC₂H₅)₂} produced an oil resistant to decomposition at high temperatures when exposed to a dilute acid. Kipping later advanced organosilicon chemistry by developing silanes via Grignard reactions and hydrolyzing chlorosilanes to form large molecules.

The most common silicones are polydimethylsiloxanes, which are trimethylsilyloxy-terminated polymers with the structure:

(SiMe₂O), where “Me” represents a methyl group. These polymers are linear and liquid, even for large molecular weights (n). The main chain unit, (SiMe₂O), is often abbreviated as “D” due to its ability to expand within the polymer structure. Similarly, M, T, and Q units correspond to specific polymer configurations.

The unique properties of silicones arise from their dual nature: organic groups attached to an inorganic backbone. This combination results in a set of exceptional characteristics:

 • The Si–O bond energy is significantly higher than the C–C bond energy, imparting enhanced stability and resistance.

 • Silicones exhibit excellent thermal and thermo-oxidative resistance.

 • They resist electromagnetic and particle radiation (UV, alpha, beta, and gamma rays) far better than organic plastics.

General Properties of Silicone Rubber

Structure and Stability [1]

Silicone rubber features siloxane (Si–O) bonds, offering higher binding energy (106.0 kcal/mol) compared to carbon bonds (84.9 kcal/mol). This provides superior heat resistance, chemical stability, and electrical conductivity. The helical molecular structure and low intermolecular forces contribute to high elasticity, compressibility, and cold resistance. The methyl groups on its outer structure enable water repellency and good release properties.

Heat and Cold Resistance [2]

Silicone rubber excels in temperature extremes, maintaining properties at 150°C for extended periods and tolerating up to 300°C for short durations. It remains elastic at temperatures as low as -60°C to -70°C, unlike organic rubbers that become brittle at -20°C to -30°C. Adjustments to formulations and curing can enhance its performance in sealed high-temperature conditions.

Weatherability and Moisture Resistance [3]

Highly resistant to ozone, UV, wind, and rain, silicone rubber retains its properties even after prolonged exposure. It absorbs minimal moisture (1%) and resists degradation from ordinary steam, though high-pressure steam above 150°C can reduce its performance, mitigated by tailored formulations.

Tear, Tensile Strength, and Flex Fatigue [4]

Silicone rubber has tear strengths of 9.8 kN/m to 49.0 kN/m with specialized grades available for demanding applications. Though weaker against dynamic stress compared to organic rubbers, advanced formulations now offer up to 20 times better flex fatigue resistance.

Compression Set [5]

Silicone rubber maintains a consistent compression set across a wide temperature range (-60°C to 250°C), outperforming organic rubbers at elevated temperatures when cured properly.

Thermal Conductivity and Flame Retardancy [6]

With thermal conductivity of ~0.2 W/m·K, silicone rubber can be modified with fillers for higher conductivity. It resists ignition but can be made flame-retardant without toxic byproducts, making it suitable for electronics and confined spaces.

Electrical Conductivity

Conductive silicone rubber, infused with materials like carbon or silver, is used for applications such as keyboards and high-voltage cable shielding. It offers stable insulation across temperatures and frequencies, with excellent resistance to corona discharge and arcing.

Chemical and Oil Resistance

Silicone rubber resists high-temperature oils and polar organic compounds, with limited swelling in non-polar solvents like gasoline. However, it is vulnerable to strong acids and bases.

Gas Permeability and Radiation Resistance

Silicone rubber’s high gas and vapor permeability enables use in oxygen enrichment systems. While standard grades lack radiation resistance, methylphenyl silicone variants are effective in nuclear environments.

Physiological Inertness

Silicone rubber is biocompatible and used in medical and leisure products such as baby bottle nipples, stoppers, swimming caps, and goggles due to its inertness and pleasant texture.

[1] Amin M, Akbar M, Amin S (2007) Hydrophobicity of silicone rubber used for outdoor insulation (an overview). Rev Adv Mater Sci 16:10–26
[2] Ghanbari-Siahkali A et al (2005) Investigation of the hydro- thermal stability of cross-linked liquid silicone rubber (LSR). Polym Degrad Stab 90(3):471–480
[3] Tan J, Li X, Zee JWV (2007) Degradation of silicone rubber under compression in a simulated PEM fuel cell environment. J Power Sources 172(2):782–789
[4][5] Saleem A, Frormann L, Soever A (2010) Fabrication of extrinsically conductive silicone rubbers with high elasticity, analysis of their mechanical and electrical characteristics. Polymers 2:200
[6] Hamdani S, Longuet C, Lopez-Cuesta JM, Ganachaud F (2009) Flame retardancy of silicone-based materials. Polym Degrad Stab 94(4):465–495

Applications of Silicone Rubber

Automotive Industry

Silicone rubber is integral to automotive manufacturing due to its heat, chemical, and weather resistance, electrical insulation, and tear strength. Applications include gaskets, sealants, connectors, spark plugs, tires, radiators, heat exchangers, and engine components like valve covers and oil pumps.

Aviation and Aerospace

Its stability under extreme temperatures and environmental stress makes silicone rubber ideal for sealants in doors, windows, and panels, as well as fuel control diaphragms, hydraulic lines, and cable clamps. It is also used in keypads for aerospace computers.

Bakeware and Cookware

Silicone rubber enhances bakeware with its flexible, non-stick surface that resists odors and flavors. Products include baking mats, molds, cake pans, ice cube trays, utensils, and spatulas, all capable of transitioning between freezer, oven, microwave, and dishwasher.

Semiconductors and Toys

Silicone rubber encapsulates semiconductors in industrial motors, power supplies, and high-temperature applications like aircraft engines and oil drilling. Its durability, weather resistance, and sterilization ability make it ideal for toys and play equipment.

Cable Accessories and Electronics

In the cable sector, silicone rubber is used for terminations, insulators, and surge arrestors. It seals and protects circuits, gaskets, and electronic components, while its heat stability and resistance to kinks make it ideal for hoses and coolant lines.

Medical Devices and Veterinary

Silicone rubber’s inert nature, free from plasticizers or latex, makes it safe for medical and veterinary applications, ensuring reliability without harmful by-products.

Molding

Silicone rubber is perfect for flexible, reusable molds in industries, art, and crafts. Its ease of processing and excellent release properties suit prototypes, small production runs, and reproductions of materials like wax, plaster, concrete, and resin. It’s widely used for reproducing archeological findings, architectural elements, and museum artifacts.