1. Relationship between phenyl content and thermal oxidation stability
The introduction of phenyl (Ph) significantly improves the thermal oxidation stability of silicone oil, and its mechanism of action can be reflected by the following quantitative relationship:

Thermal decomposition temperature (Td): The Td of methyl silicone oil (Me-Si) is about 316℃, and Td can be increased by about 30-50℃ for every 10% increase (molar fraction) in phenyl content. For example, the Td of methylphenyl silicone oil (MePh-Si) with a phenyl content of 30% can reach more than 400℃, and the Td of silicone oil with a high phenyl content (>50%) can exceed 450℃.
Oxidation induction period (OIT): At 250℃, the OIT of pure methyl silicone oil is about 24 hours, while the OIT of silicone oil with a phenyl content of 30% can be extended to more than 14 hours, and the OIT of silicone oil with a high phenyl content (>50%) can reach 44 hours or even longer.
Gel Time: At 300°C, the gel time of methyl silicone oil is 1 hour, while the gel time of silicone oil with 30% phenyl content is extended to 14 hours, and the gel time of silicone oil with high phenyl content can exceed 44 hours.

2. Quantitative influence mechanism of phenyl content
The improvement of phenyl on the thermal oxidation stability of silicone oil mainly comes from the following three aspects:

Steric hindrance effect: The rigid structure of the benzene ring can effectively prevent the chain reaction of oxidative free radicals and reduce the probability of Si-C bond breakage. For every 10% increase in phenyl content, the steric hindrance effect can reduce the oxidation rate by about 20-30%.
Electronic effect: The conjugation effect of the benzene ring can disperse the electron cloud density generated during the oxidation process and reduce the activity of free radicals. For every 10% increase in phenyl content, the quenching efficiency of free radicals can be increased by about 15-25%.
Crystallization behavior: Silicone oil with low phenyl content (<10%) has a low crystallization temperature (about -70°C), while silicone oil with high phenyl content (>30%) has a significantly higher crystallization temperature, forming a denser molecular arrangement, further hindering the penetration of oxidizing media.

3. Trade-off between phenyl content and other properties of silicone oil
The increase in phenyl content will not only improve thermal oxidation stability, but also affect other properties of silicone oil:

Viscosity-temperature performance: For every 10% increase in phenyl content, the viscosity index (VI) of silicone oil can be reduced by about 5-10 points, resulting in increased high-temperature viscosity and decreased low-temperature fluidity.
Mechanical properties: An increase in phenyl content will improve the compression resistance of silicone oil, but at the same time reduce its demolding effect. For example, the viscosity change rate of silicone oil with a phenyl content of 30% under pressure is about 15-20% higher than that of methyl silicone oil.
Cost: The price of phenyl monomer is about 3-5 times that of methyl monomer. For every 10% increase in phenyl content, the production cost of silicone oil can increase by about 20-30%.

4. Optimization suggestions for phenyl content
Depending on application requirements, the selection of phenyl content should follow the following principles:

High-temperature lubrication field: It is recommended that the phenyl content be controlled at 20-30% to balance thermal oxidation stability and viscosity-temperature performance. For example, silicone oil used for aircraft engine lubrication usually has a phenyl content of 25-30%.
Extreme thermal environment: It is recommended that the phenyl content be increased to 40-50% to achieve a thermal decomposition temperature above 450°C. For example, silicone oil used for heat exchange fluid in nuclear power plants usually has a phenyl content of 45-50%.
Cryogenic application: It is recommended that the phenyl content be controlled at 5-10% to maintain a low crystallization temperature. For example, silicone oil used for ultra-low temperature vacuum diffusion pump oil usually has a phenyl content of 5-8%.