Fumed silica (fumed silicon dioxide) does not act as an independent flame retardant in flame retardant materials, but rather significantly enhances the flame retardant efficiency of the system through synergistic effects with phosphorus-based, nitrogen-based, or metal hydroxide flame retardants.  Its core mechanism focuses on condensed phase char formation enhancement and heat release suppression.

Synergistic Flame Retardant Core Mechanism
1. Promoting the Formation of a Dense Char Layer (Charring Enhancement)
Fumed silica nanoparticles undergo interfacial reactions with phosphorus-based flame retardants (such as APP, DOPO) or nitrogen-based components at high temperatures, providing abundant siloxane active sites to catalyze the dehydration and charring of the polymer matrix.
The resulting char layer is dense and continuous, possessing high thermal stability and low thermal conductivity, effectively isolating oxygen, combustible gases, and heat transfer to the interior of the material.
Experiments show that adding ≤1 wt% fumed silica to the polypropylene/APP system improves the integrity of the char layer, increases the oxygen index (LOI) by 2–4%, and consistently achieves UL-94 V-0 rating.
2. Reducing Heat Release Rate and Mass Loss Rate
The high specific surface area of fumed silica (>200 m²/g) can adsorb free radicals generated during combustion, interrupting the chain oxidation reaction.
The nanoparticles form a "thermal shielding layer" at the combustion interface, dispersing local heat flow and significantly reducing the peak heat release rate (p-HRR).
Cone calorimeter tests show that adding 0.5 wt% fumed silica to the polypropylene/APP system reduces p-HRR by approximately 23%, with a corresponding decrease in total heat release (THR).
3. Enhancing the Structural Stability of Intumescent Flame Retardant Systems (IFR)
In intumescent flame retardant systems (such as APP/pentaerythritol/melamine), fumed silica acts as a "skeleton reinforcing agent," cross-linking with carbides to form a three-dimensional network char skeleton, preventing char layer cracking and collapse. SEM observations revealed that after the addition of fumed silica, the carbon layer surface was smoother, had fewer pores, and exhibited significantly better structural compactness compared to the single IFR system.
The synthesized MCA-SiO₂ composite (melamine cyanurate-fumed silica) achieved a UL-94 V-0 rating in polypropylene, with an LOI of 32.4%, attributed to the reinforcing effect of SiO₂ on the char network.
4. Enhanced Thermal Stability and Charring Efficiency
Thermogravimetric analysis (TGA) showed that the introduction of fumed silica increased the initial decomposition temperature of the polymer by 5–15°C and significantly increased the char yield.
In the epoxy resin system, the synergistic effect of DOPO and siloxane increased the char yield by more than 40% compared to the single components, indicating that silicon elements participated in the charring reaction pathway.