Flame Retardant Properties of Phenyl Silicone Rubber

Phenyl silicone rubber can be divided into three categories according to its phenyl content, with significant differences in their flame retardant properties:
Low-phenyl silicone rubber (phenyl content 5-15%): Curing temperature can drop to -115℃, exhibiting the best low-temperature resistance. It retains flexibility at -100℃, but its flame retardant properties are relatively weak.

Medium-phenyl silicone rubber (phenyl content 15-25%): Exhibits significant ablation resistance and flame retardancy. Increased phenyl content enhances ablation resistance and improves flame retardancy. Vertical burning rating can reach FV-0 (UL94 standard).

High-phenyl silicone rubber (phenyl content above 30%): Exhibits excellent radiation resistance. Can withstand 2.58 × 10⁴ C/kg γ-ray radiation. However, it is difficult to process and has poor physical and mechanical properties. Experiments show that, under the same fire-retardant filler system, methyl phenyl silicone rubber (PVMQ) has the best fire retardant performance. A 1mm thick PVMQ fire-retardant silicone rubber can withstand flame erosion at 1050-1100℃ for 15 minutes without burning through, and a continuous ceramic fire-retardant layer with a volume expansion ratio of 1.24 can be formed on the fire-facing surface.

Flame-retardant Mechanism Analysis
The flame-retardant effect of phenyl silicone rubber is mainly achieved through the following mechanisms:

Molecular Structure Stabilization: The phenyl group forms a π-coordination structure with silicon, inhibiting the oxidation of polymer side groups to form alkoxy radicals under high-temperature conditions. The phenyl group introduces steric hindrance into the silicon-oxygen chain, making it difficult for the polymer to undergo cyclic degradation.

Formation of a Ceramic Protective Layer: A continuous ceramic fire-retardant layer can be formed on the material surface during combustion. This layer effectively isolates oxygen and combustible gases, preventing the spread of combustion.

Free Radical Quenching: The hindered amine structure in some modified phenyl silicone rubbers can generate nitrogen-containing free radicals. These free radicals can capture and quench the flames. Extinguishes OH· and H· free radicals generated during combustion, inhibiting chain reactions.

Improved thermal stability: Increased phenyl content increases molecular chain rigidity, raising the thermal decomposition temperature.
Compared to pure methyl silicone rubber, phenyl silicone rubber exhibits significantly improved thermal stability.

Practical application performance: Phenyl silicone rubber demonstrates excellent performance in various flame-retardant applications:
Extreme environment applications: Temperature resistance ranges from -100℃ to 250℃, and can withstand temperatures above 300℃ for short periods.
Used in aerospace for engine thermal insulation components and aircraft sealing materials.

Nuclear and electric power applications:
Used in nuclear power cables, High-voltage power equipment insulation:
Excellent radiation resistance, capable of withstanding high-energy radiation environments

Electronics and electrical appliances:
As a potting material for electronic components, it meets the FV-0 flame retardant standard.
Room temperature volume resistivity >10¹⁶ Ω·m, breakdown field strength greater than 25kV/mm

Composite material applications:
Blending with ethylene propylene diene monomer (EPDM) rubber can significantly improve flame retardant properties.
Adding 50 phr of magnesium hydroxide can increase the limiting oxygen index (LOI) from 27.6 to 39.2.

Technological development trends:
Phenyl silicone rubber flame retardant technology is developing in the following directions:
Composite modification Technology: Blending with other high-performance rubbers, such as fluorosilicone rubber; Developing novel flame-retardant filler systems, such as modified zirconium phosphate; Environmentally friendly flame retardant development: Developing halogen-free, low-smoke, and low-toxicity flame-retardant formulations; Improving flame-retardant efficiency while reducing environmental impact; Multifunctional integration: Simultaneously meeting multiple requirements such as flame retardancy, radiation resistance, and aging resistance; Developing special silicone rubber materials suitable for extreme environments; Phenyl silicone rubber, with its unique flame-retardant mechanism and excellent comprehensive performance, holds an irreplaceable position in high-end flame-retardant applications and has broad application prospects in new energy, aerospace, and other fields.