Phenyl silicone is an important class of organosilicon materials. By introducing phenyl groups into the siloxane backbone, it significantly improves the performance of conventional silicone rubber. Based on the phenyl content, phenyl silicones can be divided into three categories: low-phenyl, medium-phenyl, and high-phenyl. These differ significantly in performance and application.

I. Classification Standards for Phenyl Silicones
Phenyl silicones are primarily classified by phenyl content (the ratio of phenyl groups to silicon atoms):
Low-phenyl silicone: phenyl content 5-10 mol%
Medium-phenyl silicone: phenyl content 15-25 mol%
High-phenyl silicone: phenyl content 30-50 mol%
The introduction of phenyl groups disrupts the regularity of the siloxane molecular structure, reducing the polymer's crystallinity and glass transition temperature, thereby improving the material's cold resistance. However, as the phenyl content increases, the molecular chain becomes more rigid, and the material's crystallization temperature actually rises.

II. Comparison of the Characteristics of Various Phenyl Silicones
1. Low-Phenyl Silicone
Low-Temperature Resistance: Optimal, with a glass transition temperature of -115°C and maintaining flexibility at -100°C.
Mechanical Properties: High elongation at break and moderate tear strength at room temperature.
Other Properties: Relatively low cost, a potential replacement for standard silicone rubber.
Processing: Relatively easy to process, with good physical and mechanical properties.

2. Medium-Phenyl Silicone
Flame Resistance: Excellent flame resistance and self-extinguishing properties.
Temperature Range: -60°C to 250°C.
Mechanical Properties: Elongation at break decreases rapidly at room temperature, with a slight increase in tear strength.
Processing: Increased processing difficulty, with some decrease in physical and mechanical properties.

3. High-Phenyl Silicone
Radiation Resistance: Excellent radiation resistance
Temperature Range: -100°C to 250°C
Mechanical Properties: High molecular chain rigidity, relatively poor physical and mechanical properties
Other Properties: Excellent ablation resistance

III. Effect of Phenyl Content on Properties
Research has shown that the phenyl content affects silicone rubber properties in the following ways:
Vulcanization Properties: With increasing phenyl content, the cure time (T90) and torque of the compound increase slowly.

Mechanical Properties:
Elongation at break decreases rapidly at room temperature
Tear strength increases slightly
Tensile strength and shear strength show no significant changes.

Low-Temperature Properties:
Glass transition temperature increases
Crystallization performance significantly improves
The tensile cold resistance coefficient of low-phenyl silicone rubber reaches 0.94 at -71°C.

Shear Properties:
The shear modulus of low-phenyl silicone rubber fluctuates slightly over a wide temperature range. The shear modulus of medium-phenyl silicone rubber varies significantly.

IV. Typical Applications
1. Low-phenyl silicone applications:
Automotive parts for high-altitude cold regions (engine seals, door seals, oil seals, etc.);
Electronic equipment seals and gaskets; Building waterproofing and sealing materials; Medical catheters and other medical products.

2. Medium-phenyl silicone applications:
Aerospace high-temperature seals; Aircraft hydraulic seals and silicone rubber tubing; High-temperature cable protection sleeves; Biomedical low-temperature storage equipment.

3. High-phenyl silicone applications:
Irradiation-resistant seals for nuclear power plants and reactors; Radiation-resistant components for spacecraft; High-voltage insulation components; Sealing materials for cryogenic equipment.

V. Summary: Phenyl silicones can be modified to produce materials with varying performance characteristics by adjusting the phenyl content. Low-phenyl silicones are known for their excellent low-temperature resistance, medium-phenyl silicones for flame resistance, and high-phenyl silicones for superior radiation resistance. Increasing the phenyl content increases the material's upper temperature resistance, but also reduces low-temperature performance, increasing processing difficulty and cost. In practical applications, the appropriate phenyl silicone type should be selected based on specific operating requirements.