In sectors with stringent fire safety requirements, such as rail transit, aerospace, new energy vehicles, and high-rise buildings, the flame-retardant properties of materials can be a matter of life and death. Although silicone rubber inherently possesses certain non-flammable characteristics, it may still char or support combustion under extreme fire conditions. Therefore, enhancing its inherent flame retardancy through modification is essential. Flame-retardant silicone rubber achieves this by employing multiple mechanisms including physical barriers, chemical inhibition, and heat absorption, thereby delaying combustion, suppressing smoke, preventing dripping, and buying precious time for evacuation and firefighting efforts.

Natural Flame Retardancy of Silicone Rubber

The natural flame resistance of silicone rubber originates from its inorganic backbone structure. When burning, Si–O bonds tend to form silica (SiO₂) ash rather than flammable hydrocarbon fragments. This layer of ash forms a dense insulation barrier on the material's surface, blocking oxygen and heat transfer, leading to self-extinguishing behavior. However, pure silicone rubber has a limited oxygen index (LOI) of only around 24–26%, which means it can still burn slowly when exposed to an open flame, failing to meet strict standards like UL94 V-0 and IEC 60695. Thus, effective flame-retardant systems must be introduced.

Mainstream Strategies for Enhancing Flame Retardancy

There are two primary strategies: additive flame retardants and reactive flame retardants.

Additive Flame Retardants:

Metal Hydroxides: Such as aluminum hydroxide (ATH) and magnesium hydroxide (MDH). These decompose at temperatures between 200–300°C, absorbing significant amounts of heat, releasing water vapor to dilute flammable gases, and forming oxide residues that enhance the strength of the char layer. ATH is widely used due to its low cost and good smoke suppression but requires additions of over 60%, impacting mechanical properties.

Platinum-Based Synergistic Flame Retardants: Trace platinum compounds (e.g., derivatives of Karstedt catalyst) can catalyze accelerated cross-linking of silicone rubber during combustion, forming a denser, more stable SiO₂–C composite char layer, significantly improving flame retardancy. This method uses small quantities (<1 phr), minimally affecting physical properties, and is often combined with other flame retardants.

Nano Fillers: Such as layered double hydroxides (LDH), montmorillonite (MMT), or polyhedral oligomeric silsesquioxanes (POSS). These create nanoscale barriers during combustion, impeding heat and mass transfer, and promoting charring.

Reactive Flame Retardants:

Incorporating flame-retardant functionalities directly into the polymer backbone during synthesis, ensuring uniform distribution and permanent integration within the material.

Trends Towards Halogen-Free Solutions

Halogen-free flame retardants represent the current trend due to environmental and health concerns. Traditional halogen-based flame retardants, while highly effective, release toxic and corrosive gases like hydrogen bromide (HBr) upon combustion, leading to their restriction under regulations such as RoHS and REACH. In contrast, inorganic/metal-based flame-retardant systems produce lower smoke density and minimal toxicity, aligning with green safety principles.

Mechanisms of Flame Retardancy

Flame-retardant silicone rubber operates through four key actions:

Cooling through Decomposition: Heat absorption during decomposition lowers material temperature.

Dilution of Oxygen and Fuel: Release of non-flammable gases dilutes oxygen and fuel concentrations.

Formation of Insulating Char Layers: A protective barrier isolates the material from external heat sources.

Termination of Chain Reactions: Some platinum-based systems capture free radicals, interrupting combustion chain reactions.

Practical Applications and Standards

In practical applications, railway cable sheaths must comply with EN 45545-2 HL3 standards, requiring not only V-0 classification but also stringent limits on smoke density (Ds max < 300) and toxicity (CIT ≥ 1.0). Silicone seals within battery packs need to maintain integrity for at least five minutes under 800°C flame exposure. Achieving these criteria depends on precise formulation design and process control.

In conclusion, flame-retardant silicone rubber isn't merely "non-flammable"; instead, it dynamically defends against fire through scientifically engineered mechanisms. It silently reacts at the molecular level to uphold public safety, standing firm amidst flames to preserve order and hope.


Silicone Rubber Compound-Products