Silicone rubber is often marketed as a “flame-retardant material” and is widely used in wire and cable insulation, rail transit components, children’s toys, and other safety-critical applications. However, “flame-retardant” does not mean “non-combustible.” Under extreme fire conditions, silicone rubber—while difficult to ignite, non-dripping, and low-smoke—still undergoes thermal decomposition, forming char and releasing flammable gases. Understanding its true combustion behavior and enhancing fire resistance through scientific modification are essential for public safety.

I. Intrinsic Combustion Behavior of Silicone Rubber

Unmodified vinyl-methyl silicone rubber (VMQ) exhibits the following combustion characteristics:

Limiting Oxygen Index (LOI): ～24–26% (slightly above ambient air at 21%), classifying it as “self-extinguishing”;

Ignition Resistance: Requires sustained open flame (>30 seconds) to ignite;

Burning Process:

Rapidly forms a white, ceramic-like silica (SiO₂) layer on the surface;

This layer acts as a thermal and oxygen barrier, suppressing flame spread;

Virtually no molten dripping (prevents secondary ignition);

Low smoke density (≈90% less than PVC) and minimal toxic emissions (primarily CO, no halogenated acids);

High Char Yield: >60% residue at 700°C, retaining partial structural integrity.

While inherently flame-resistant, unmodified silicone rubber typically fails to meet stringent standards such as UL 9 4 V-0 or EN 45545-2 (railway), necessitating targeted modifications.

II. Mainstream Flame-Retardant Modification Strategies

Metal Hydroxide Fillers

Aluminum trihydroxide (ATH) or magnesium hydroxide (MDH) are the most common halogen-free flame retardants;

Added at 40–60 wt%, they decompose endothermically, releasing water vapor to dilute flammable gases;

Drawback: High loading degrades mechanical properties and complicates processing;

Optimization: Silane coupling agents improve dispersion and interfacial adhesion.

Platinum Complex-Catalyzed Char Formation

Trace amounts (0.1–0.5 phr) of platinum compounds (e.g., modified Karstedt catalysts) accelerate crosslinking during combustion;

Promotes formation of a dense, cohesive SiO₂/carbon composite char layer;

Minimal impact on physical properties; often used synergistically with ATH.

Nano Flame Retardants

Layered double hydroxides (LDHs) and polyhedral oligomeric silsesquioxanes (POSS) create nano-barrier effects at low loadings (5–10 wt%);

Delay heat and mass transfer during burning;

POSS can even be incorporated into the polymer backbone, enhancing thermal stability.

Phosphorus–Nitrogen Synergistic Systems

Although the silicone backbone lacks C–C bonds, methyl side groups can interact with phosphorus-based retardants;

Examples: microencapsulated red phosphorus + melamine polyphosphate promote charring;

Limitation: Red phosphorus imparts color, restricting use in light-colored products.

III. Flame-Retardant Grades and Application Scenarios

表格

Application Field Required Standard     Silicone Rubber Formulation

Consumer electronics cables    UL 94 V-0, VW-1 ATH + Pt catalyst, hardness 50A

High-speed train seals      EN 45545-2 HL3  MDH + nano-SiO₂, low-smoke halogen-free

Children’s toys     ASTM F963, EN 71      High-purity ATH, heavy-metal-free

Firestop sealants (building) GB 23864      Intumescent silicone + ceramic fibers

Note: UL 94 V-0 compliant silicone rubber self-extinguishes within 10 seconds in vertical testing, with no flaming drips.

IV. Key Fire Testing Methods

UL 94: Measures self-extinguishing time in horizontal/vertical orientations;

LOI (ASTM D2863): Determines minimum oxygen concentration to sustain combustion;

Cone Calorimeter (ISO 5660): Quantifies peak heat release rate (pHRR) and total heat release (THR);

Smoke Density (ASTM E662): Evaluates visibility reduction in fire scenarios.

High-performance flame-retardant silicone rubber achieves pHRR <100 kW/m²—dramatically lower than polyethylene (>800 kW/m²).

V. Trends Toward Green Flame Retardancy

Halogen-Free: Eliminating brominated/chlorinated additives to avoid dioxin formation;

Low Smoke & Low Toxicity: Minimizing CO yield to extend evacuation time;

Multifunctional Integration: Combining flame retardancy with thermal conductivity and EMI shielding—for example, in electric vehicle battery enclosures.

Conclusion

The flame resistance of silicone rubber is not an innate guarantee of absolute safety, but a carefully engineered defense line. The white ash it forms in fire marks both the endpoint of material failure and the starting point for human escape. On the tightrope between safety and performance, every formulation refinement is a solemn commitment to the principle of “no ignition, no spread, minimal harm.” Because true flame retardancy has never been about extinguishing fire—it’s about buying humanity one more second of life when disaster strikes.

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