Study on Flame Retardancy of Aerogel

Research and Application Progress on Modification of Silica Aerogel

Silica aerogel (SiO₂ aerogel), the earliest inorganic aerogel material, has drawn significant attention for its unique three-dimensional network structure and intrinsic flame-retardant properties since Kistler first prepared it in 1931. Composed of a Si–O–Si framework, it can form a dense silica layer at high temperatures, providing an effective barrier against heat and flame.

However, its inherent drawbacks limit practical use: high brittleness (compressive strength < 0.1 MPa), strong moisture absorption (contact angle < 30°), and long-term structural degradation due to hydrophilic hydroxyl interactions. To address these issues, researchers have introduced hydrophobic groups (e.g., methyl, phenyl) via surface modification, increasing the contact angle above 150°. Yet, residual organics (Si–OR, Si–R) from the modification process can release flammable volatiles during pyrolysis at 200–400 °C, raising fire risks.

To resolve this contradiction, teams have optimized preparation methods to improve flame retardancy. Silica aerogels prepared by supercritical drying (SA_sd) showed a 36.45% reduction in total heat release (THR = 1.02 MJ/m²) compared with ambient-dried samples, thanks to a more complete network with fewer organic residues. Li’s team replaced conventional hydrochloric acid with phosphoric acid (PA), exploiting phosphorus-based flame-retardant chemistry: above 300 °C, PA decomposes to phosphoric acid, which promotes dehydration and char formation. This creates a dense carbon layer (42% char residue), reducing THR by 49.3% compared to traditional TEOS/HA systems. The combined gas- and condensed-phase effects significantly delayed combustion, achieving a peak heat release rate (PHRR) of 18.7 kW/m²—meeting UL-94 V-1 standards.

As a functional filler, silica aerogel also enhances composite flame retardancy. Deng’s team developed a PVA/SA/GF ternary system with a hierarchical porous structure via freeze-drying; adding 20 wt% SA increased residual carbon to 75% and reduced HRR by 61.2%. Lee et al. used pore-repair technology to incorporate 15 vol% SA into PDMS, increasing the oxygen index from 25.2% to 26.4% and significantly reducing molten drips during burning. This dual “structural barrier + chemical synergy” approach offers promising directions for next-generation flame-retardant materials.

Current studies confirm that flame retardancy and mechanical strength of SiO₂ aerogels can be improved through innovations in preparation, phosphorus–nitrogen flame-retardant systems, and nanostructure engineering. Future research will target organic–inorganic hybrid systems capable of UL-94 V-0 performance while maintaining superhydrophobicity (contact angle > 160°), with potential applications in aerospace and advanced battery systems.

Thermal Insulation Mechanisms of Aerogels

Aerogels exhibit ultra-low thermal conductivity (0.012–0.024 W/m·K) due to their distinctive microstructure. By forming a uniform nanopore network with multi-level fractal pores, aerogels achieve triple suppression of heat conduction, convection, and radiation, delivering insulation performance 2–3 orders of magnitude better than traditional materials.

1. Gas Confinement Effect (Convection Suppression)

Pores range from 2–50 nm. When pore size is smaller than the gas molecule mean free path, gas molecules are confined, eliminating convective heat transfer.

2. Radiation Scattering Effect (Thermal Radiation Shielding)

The ultrafine porous network, with ~10¹⁸ pore walls per cubic centimeter, scatters and reflects infrared radiation, reducing radiative heat transfer by two orders of magnitude.

3. Phonon Barrier Effect (Solid-State Thermal Conductivity Suppression)

Low-conductivity SiO₂ skeletons create ultra-long heat paths (10–20× apparent thickness) and enhance phonon scattering via quantum confinement, reducing solid thermal conductivity to 1/100 of bulk materials.

This multi-scale insulation mechanism allows aerogels to achieve aerospace-grade thermal protection while maintaining ultra-lightweight density (as low as 3 kg/m³), and they have been successfully applied in Long March launch vehicle thermal protection systems.