Flame Retardant Mechanism of Silicone Resin: A Scientific and Industrial Perspective

Silicone resin stands as a cornerstone of modern flame-retardant materials, offering a unique combination of safety and performance. Its inherent organic-inorganic hybrid structureprovides a fundamentally different approach to fire safety compared to conventional halogen-based retardants. This article delves into the science behind its flame-retardant mechanism, its validation through international standards, and its practical applications across high-demand industries, highlighting how advanced manufacturers like Biyuan leverage this technology to provide innovative solutions.

The Inherent Flame Retardancy of Silicone Resin

Unlike many organic polymers that readily combust, silicone resin is a thermosetting polysiloxane with a high cross-linking density. Its molecular backbone consists primarily of strong silicon-oxygen (Si-O) bonds, which have a dissociation energy of approximately 108 kcal/mol, higher than that of Si-C bonds (~78 kcal/mol) . This robust structure is the foundation of its thermal stability.

When exposed to heat, silicone resin does not fuel the fire like ordinary plastics. Instead, it undergoes a complex degradation process. The initial step involves the oxidation of lateral organic groups, such as methyl groups. As the temperature increases further, the material forms a protective silica-rich residue (silica ash)on its surface . This transition from an organic polymer to an inorganic layer is the core of its flame-retardant action, which operates primarily in the condensed phase.

Detailed Flame Retardant Mechanism

The flame-retardant mechanism of silicone resin is multifaceted, providing a physical barrier that effectively shields the underlying material.

1. Formation of a Protective Silica Layer

Upon exposure to high temperatures, the organic components of the silicone resin gradually degrade. The remaining silicon and oxygen reorganize to form a continuous, coherent, and thermally stable silica (SiO₂) layer. This layer is not a fragile ash; it acts as a robust, insulating blanket that evenly covers the material’s surface.

2. The “Shielding Effect” in Action

This silica layer functions through a powerful shielding effectwith three critical outcomes:

• Diffusion Barrier:It acts as a mass transport barrier, dramatically delaying the volatilization of organic decomposition products from the underlying polymer. This reduces the amount of flammable gas available to feed the flame in the gas phase .
• Thermal Insulation:The layer has excellent heat insulation properties, protecting the virgin polymer beneath from the external heat flux and slowing down the thermal degradation process .
• Oxygen Barrier:It limits the access of oxygen to the unburned fuel, thereby suppressing the combustion process .

The integrity and coherence of this char layer are paramount to its effectiveness. Research indicates that the strength of this silica ash layer directly governs its efficiency as a flame retardant .

3. The Condensed-Phase Mechanism and Synergistic Effects

Silicone resin operates predominantly through a condensed-phase flame-retardant mechanism, meaning its primary action occurs in the solid phase of the material rather than in the flame itself . This is an environmentally favorable approach as it typically minimizes the release of toxic gases.

The performance can be significantly enhanced through formulation. For instance, manufacturers like Biyuan utilize synergistic additives to improve the char layer’s strength and coherence:

• Transition Metal Oxides:Incorporating oxides of iron (Fe), cerium (Ce), titanium (Ti), or zirconium (Zr) can positively influence high-temperature resistance. These compounds promote the formation of a more compact and robust protective layer .
• Carbon and Halloysite Nanotubes:These nano-additives help dissipate heat and reinforce the char structure, further reducing heat release rates .

Industry Standards and Certifications: Validating Performance

The flame-retardant efficacy of silicone resins is rigorously evaluated using standardized laboratory tests, which are critical for industrial acceptance and certification.

• UL 94 Vertical Burning Test:This test classifies materials based on their burning behavior. Silicone resin formulations, especially those optimized with synergistic fillers, can achieve the highest rating of V-0, indicating self-extinguishing properties .
• Limiting Oxygen Index (LOI):LOI measures the minimum oxygen concentration required to support combustion. Materials with an LOI greater than 21% (the oxygen level in air) are considered self-extinguishing. High-performance silicone resins can achieve very high LOI values, sometimes up to 42%in intrinsically modified systems, highlighting their exceptional flame retardancy .
• Cone Calorimeter Test:This is one of the most comprehensive methods, simulating real-fire conditions. It measures parameters like Heat Release Rate (HRR) and Total Heat Released (THR). Silicone-based materials consistently show low HRR values, typically between 60-150 kW/m², which minimizes their contribution to a fire .

These performances are benchmarked against international standards from organizations like ASTM, ISO, and SAE (e.g., SAE AMS3368for high-temperature cure elastomeric silicone resins) , providing engineers and designers with verified data for material selection.

Application Scenarios and Biyuan’s Solutions

The unique properties of flame-retardant silicone resin make it indispensable in industries where safety and reliability are non-negotiable.

• Aerospace and Aviation:Used in seals, coatings, and flexible ablation materials for spacecraft, where components must withstand extreme heat fluxes. The formation of a coherent char layer ensures circuit integrity and thermal protection for dynamic structures .
• Construction and Building Safety:In curtain walls and concrete expansion joints, non-corrosive RTV silicone sealants like Biyuan’s BAIYUN SS911 seriesprovide not only excellent weatherproofing but also crucial fire resistance, with a joint movement capability of +100/-50% and warranties extending up to 15 years .
• Electronics and Electrical Engineering:As a key material for circuit boards, insulating sleeving (e.g., GB/T 41593-2022), and encapsulants, silicone resin protects sensitive components from heat and prevents flame propagation, meeting stringent standards like UL 94 V-0.
• Transportation and Automotive:In electrical insulation for cables and components under the hood, silicone resins ensure fire safety and long-term stability.

For a silicone resin factory, advancing material science is key to solving industry pain points. A manufacturer like Biyuan addresses common challenges such as balancing flame retardancy with mechanical properties, processability, and environmental compliance. By developing advanced formulations—such as intrinsic flame-retardant SR containing phosphaphenanthrene structures or hyperbranched polysiloxanes—Biyuan provides solutions that offer enhanced performance without sacrificing other critical properties like transparency or flexibility . This focus on innovation ensures that Biyuan’s products meet the evolving demands of high-tech industries, providing reliable, certified, and high-performance flame-retardant solutions.