Silicone rubber is an elastomer with a unique molecular structure, consisting of a backbone of alternating silicon and oxygen atoms, with organic groups such as methyl, vinyl, and phenyl attached to the silicon atoms. This distinctive structure grants silicone rubber exceptional properties, making it widely applicable in industries like healthcare, electronics, and aerospace. This article explores its classification, properties, and specialized applications.

1. Classification and Key Properties of Silicone Rubber

1. High and Low-Temperature Resistance
   Silicone rubber excels in temperature resilience, functioning effectively between -55°C and 180°C. With the introduction of phenyl groups, the lower limit can reach -73°C, while the upper limit can momentarily exceed 300°C.

2. Permeability
   Silicone rubber ranks among the top synthetic polymers in gas permeability, particularly for oxygen.

3. Physiological Inertness and Medical Applications
   Its inertness makes silicone rubber non-coagulant, making it ideal for medical devices and implants.

4. Vulcanization Mechanism
   Based on vulcanization mechanisms, silicone rubber is classified into high-temperature vulcanized (HTV) and room-temperature vulcanized (RTV) types. RTV types are further divided into condensation and addition reaction types.

2. Classification and Features of High-Temperature Vulcanized Silicone Rubber

1. Dimethyl Silicone Rubber (MQ)
   One of the earliest types of silicone rubber, MQ retains good elasticity from -60°C to 250°C. However, due to low vulcanization activity and poor processability, it has been largely replaced by modified types.

2. Methyl Vinyl Silicone Rubber (VMQ)
   VMQ introduces small amounts of vinyl into the molecular chain, significantly improving heat resistance, aging resistance, and compression set properties. It is the most widely used silicone rubber type.

3. Methyl Vinyl Phenyl Silicone Rubber (PVMQ)
   PVMQ incorporates diphenyl or methylphenyl siloxane units, offering low-temperature resistance, flame resistance, and excellent radiation resistance. It is suitable for aerospace, electronics, and high-stress environments.

4. Methyl Vinyl Trifluoropropyl Silicone Rubber (Fluorosilicone Rubber)
   By introducing fluorinated alkyl groups, this type achieves outstanding oil and solvent resistance, making it suitable for harsh chemical environments within a temperature range of -50°C to 250°C.

5. Silphenylene and Silphenyloxy Silicone Rubber
   These advanced types withstand temperatures above 300°C and resist gamma radiation, but their low-temperature performance is inferior.

6. Nitrile Silicone Rubber
   This elastomer, modified with methyl-β-cyanoethyl or methyl-γ-cyanopropyl siloxane units, is similar to fluorosilicone rubber, offering excellent oil, solvent, and low-temperature resistance.

7. Silicone-Boron Rubber
   Featuring boron cage structures in its main chain, this high-temperature-resistant rubber can operate continuously at 400°C, though it is costly and less elastic.

3. Unique Properties of Silicone Rubber

1. Resistance to Aging
   Silicone rubber exhibits exceptional resistance to ozone, UV, and environmental aging. Even after prolonged outdoor exposure, its performance remains largely unchanged.

2. Electrical Insulation
   Its insulating properties remain stable under humidity, high frequency, or high temperatures. Post-combustion, the silica residue retains insulating capabilities and resists corona and arc discharges.

3. Low Moisture Absorption and Physiological Inertness
   With a low water absorption rate of about 1%, silicone rubber retains its physical properties even after prolonged immersion in water. It is odorless, non-toxic, and safe for use in contact with the human body.

4. High Gas Permeability
   Silicone rubber has superior permeability to oxygen, nitrogen, and air, far exceeding natural rubber. This makes it ideal for applications requiring selective gas permeability.

4. Conclusion

The unique molecular structure and extensive modification options make silicone rubber an indispensable material in industries like electronics, healthcare, aerospace, and more. With advances in modification technologies, silicone rubber is expected to achieve even greater performance enhancements, broadening its application prospects.