Inside Silicone Rubber: The Chemistry, Performance, and Engineering Behind Its Extreme Versatility

Silicone rubber is one of the most chemically stable and high-performance elastomers used in modern engineering. From aerospace insulation to medical tubing and high-voltage electrical tapes, its ability to retain flexibility, resilience, and dielectric strength over an exceptionally wide temperature range sets it apart from conventional organic rubbers.

 The Molecular Backbone — Why Silicone Is So Stable

Silicone rubber is composed of long polymer chains known as polysiloxanes, built from alternating silicon (Si) and oxygen (O) atoms. Each silicon atom is typically bonded to organic side groups such as methyl, vinyl, or phenyl.This molecular structure — Si–O–Si–O — is fundamentally different from the carbon–carbon (C–C) backbone of organic rubbers like EPDM or NBR. The Si–O bond energy (≈ 451 kJ/mol) is substantially higher than the C–C bond (≈ 356 kJ/mol), giving silicone its superior resistance to thermal oxidation, UV radiation, and ozone degradation.

In practical terms, this means silicone doesn’t embrittle, crack, or lose elasticity when exposed to high heat, sunlight, or oxygen over long periods — a key advantage in outdoor and high-temperature environments.

Polymer Types and Structure Control

Different silicone rubbers are engineered by altering the side groups attached to the silicon atoms or by modifying the polymer end groups.

1. Polydimethylsiloxane (PDMS): The most common base polymer, known for exceptional flexibility and low glass-transition temperature (Tg ≈ –125 °C).
2. Methyl Phenyl Siloxane (PMPS): Adds phenyl groups for improved radiation and low-temperature performance.
3. Vinyl-Terminated or Hydroxyl-Terminated Silicones: Used in peroxide- and platinum-cure systems for precise control over crosslink density and mechanical properties.

The choice of backbone and cure chemistry allows engineers to fine-tune silicone for specific mechanical, thermal, and electrical performance.

Thermal Endurance and Mechanical Properties

Silicone rubber remains elastomeric from –60 °C to +230 °C, and specialized formulations can withstand brief exposure up to +300 °C.

At such extremes, most organic elastomers would soften, char, or crack. Silicone, however, maintains its integrity because it forms a thin silica (SiO₂) layer at high temperature — a self-passivating barrier that prevents further degradation.

 Even after prolonged aging, silicone maintains compression set resistance, ensuring that seals, gaskets, and tapes retain their shape and elasticity over time.

Crosslinking Systems — Chemistry in Action

Silicone gum alone is soft and weak; its true elastomeric properties arise after crosslinking, which creates a three-dimensional network.

Three primary curing systems are used in industry:

• Peroxide Cure: Generates carbon–carbon crosslinks through free-radical reactions. This method provides strong, thermally durable vulcanizates ideal for industrial and automotive parts.
• Addition (Platinum) Cure: A hydrosilylation reaction between vinyl and Si–H groups, catalyzed by platinum. Produces high-purity, non-yellowing silicone ideal for medical, food-grade, and electrical applications.
• Condensation Cure: Involves silanol–crosslinker reactions that release small by-products (e.g., alcohol or oxime). Used for RTV (Room-Temperature Vulcanizing) sealants and coatings.

Reinforcement and Functional Additives

Unfilled silicone rubber is mechanically weak, so it is compounded with reinforcing and functional fillers to enhance performance:

1. Fumed Silica: The most common reinforcing filler; forms a hydrogen-bonded network with silanol groups, improving tensile and tear strength.
2. MQ Resin (Methylsiloxane–Q units): Increases cohesive strength and improves self-fusing behavior — critical in high-performance silicone tapes and sealants.
3. Alumina Trihydrate (ATH): Provides flame retardancy and improves dielectric properties.
4. Iron Oxide or Ceramic Fillers: Used in conductive or heat-dissipative silicone grades.

These formulations allow silicone compounds to achieve precise hardness, thermal conductivity, and surface adhesion characteristics.

Electrical and Environmental Resistance

With a dielectric constant of 2.9–3.1 and volume resistivity exceeding 10¹⁴ Ω·cm, it ensures stable insulation even under high-voltage or corona discharge conditions.

Chemically inert and hydrophobic, silicone resists moisture absorption, maintaining its dielectric integrity in humid or outdoor environments. Combined with its non-carbon backbone, this makes it virtually immune to tracking, arcing, and weathering — crucial for power cables, insulators, and stress-control mastic tapes.

The unmatched performance of silicone rubber lies in its chemistry .

At Centroid Polymer Technologies, silicone remains at the heart of our innovation in self-fusing tapes, extruded profiles, and high-performance sealing solutions