What are the reasons for the excellent high - and - low temperature resistance of silicone rubber?

Reasons for the Excellent High and Low-Temperature Resistance of Silicone Rubber

Silicone rubber (Silicone Rubber) is a polymer material primarily composed of siloxane (Si-O-Si) bonds. It exhibits outstanding resistance to both high and low temperatures, maintaining flexibility at extremely low temperatures and enduring prolonged exposure to high temperatures without significant aging or performance degradation. Below are the main reasons for the excellent high and low-temperature resistance of silicone rubber:

1. Unique Molecular Structure

• Stability of Siloxane Bonds (Si-O): The backbone of silicone rubber consists of alternating silicon (Si) and oxygen (O) atoms, forming siloxane (Si-O-Si) bonds. These bonds have a very high bond energy (approximately 450 kJ/mol), much higher than carbon-carbon (C-C) bonds (approximately 348 kJ/mol). This makes the siloxane bonds highly resistant to breaking at high temperatures, contributing to the exceptional thermal stability of silicone rubber.

• Large Bond Angle: The bond angle in siloxane bonds is relatively large (about 140°), which provides the molecular chain with high flexibility. This large bond angle prevents the molecular chains from freezing at low temperatures, allowing silicone rubber to maintain its flexibility and elasticity even in extreme cold.

• Low Glass Transition Temperature (Tg): The glass transition temperature (Tg) of silicone rubber is typically around -120°C, much lower than that of most organic rubbers (such as nitrile rubber or neoprene). This means that silicone rubber remains soft and elastic at very low temperatures, avoiding brittleness.

2. Weak Van der Waals Forces

• Weak Intermolecular Interactions: The van der Waals forces between silicone rubber molecules are relatively weak, allowing the molecular chains to move freely. Even at low temperatures, the molecular chains do not freeze due to strong intermolecular interactions, thus maintaining good flexibility.

• Low Cohesive Energy Density: Due to the weak intermolecular forces, silicone rubber has a low cohesive energy density, which prevents it from sticking together or melting at high temperatures, preserving its mechanical properties.

3. Excellent Oxidation Resistance

• High Chemical Stability: The siloxane bonds in silicone rubber are highly resistant to oxidation by oxygen and ozone, making them less prone to chemical degradation. In contrast, carbon-carbon bonds are more susceptible to oxidation at high temperatures, leading to material aging and performance decline. The superior oxidation resistance of silicone rubber allows it to withstand prolonged use in high-temperature environments without significant degradation.

• Resistance to UV and Ozone: Silicone rubber also exhibits excellent resistance to ultraviolet (UV) light and ozone, preventing degradation or cracking when exposed to outdoor conditions over long periods.

4. Low Thermal Expansion Coefficient

Small Thermal Expansion: Silicone rubber has a low coefficient of thermal expansion, approximately half to one-third that of conventional organic rubbers. This means that silicone rubber undergoes minimal dimensional changes when exposed to temperature variations, reducing stress and deformation caused by thermal expansion and contraction. This further enhances its stability and reliability in extreme temperature environments.

5. Chemical Corrosion Resistance

Broad Chemical Stability: Silicone rubber is highly resistant to a wide range of chemicals, including acids, bases, and solvents, especially at high temperatures. This makes it suitable for industrial applications where it must withstand harsh chemical environments while maintaining its physical and mechanical properties.

6. Excellent Electrical Insulation Properties

High Dielectric Strength: Silicone rubber possesses excellent electrical insulation properties, maintaining stable dielectric strength even at high and low temperatures. This makes it widely used in the power and electronics industries, particularly in applications requiring both temperature resistance and electrical insulation.

Application Areas

Due to these outstanding characteristics, silicone rubber is widely used in the following fields:

• Aerospace: For manufacturing seals, gaskets, and cable jackets, which must perform reliably in extreme temperature environments.

• Automotive Industry: For seals, hoses, and wire harness protection in engine compartments, where it can withstand the high and low temperatures generated by the engine.

• Electronics: For insulating materials, seals, and thermal pads, which must maintain electrical insulation and mechanical performance at various temperatures.

• Construction Industry: For sealants and waterproofing materials, which can be used outdoors for extended periods, resisting climate changes.

Summary

The excellent high and low-temperature resistance of silicone rubber is primarily attributed to its unique molecular structure, weak intermolecular forces, superior oxidation resistance, and low thermal expansion coefficient. These properties enable silicone rubber to maintain excellent mechanical performance, flexibility, and elasticity across a wide temperature range, making it suitable for various demanding operating environments.