Study on flame retardancy of aerogel

Research and Application Progress on Modification of Silica Aerogel

Silica aerogel (SiO ₂ aerogel), as the oldest inorganic aerogel material, has attracted much attention due to its unique three-dimensional network structure and intrinsic flame retardant properties since Kistler first prepared it in 1931. This material is composed of Si-O-Si skeleton, which can form a dense silicon oxide layer at high temperatures to achieve barrier protection. However, its inherent defects limit practical applications: high brittleness (compressive strength<0.1 MPa), strong moisture absorption (contact angle<30 °), and long-term use can easily lead to structural degradation due to hydroxyl hydrophilic interactions. To this end, researchers have introduced hydrophobic groups such as methyl and phenyl groups through surface modification, which can increase the contact angle to over 150 °. However, the residual organic compounds such as Si OR and Si-R during the modification process pose new safety hazards - these components can produce flammable volatiles during pyrolysis at 200-400 ℃, significantly increasing the risk of material fire.

In response to this contradiction, the research team has started by optimizing the preparation process to enhance flame retardancy. SA sd prepared by supercritical drying method exhibited excellent performance, with a total heat release (THR=1.02 MJ/m ²) reduced by 36.45% compared to the sample dried under normal pressure, attributed to a more complete network structure that reduced organic residues. The Li team innovatively used phosphoric acid (PA) instead of traditional hydrochloric acid catalysts and made a breakthrough by utilizing the phosphorus based flame retardant mechanism: when the temperature exceeds 300 ℃, the phosphoric acid produced by PA decomposition promotes dehydration into charcoal, forming a dense coke layer (with a residual char rate of 42%), reducing the THR of the SS/PA system by 49.3% compared to the traditional TEOS/HA system. This gas condensed phase synergistic effect significantly delays the combustion process, and its peak heat release rate (PHRR=18.7 kW/m ²) reaches the UL-94 V-1 level.

As a functional filler, SiO ₂ aerogel shows unique advantages in the field of composite materials. The PVA/SA/GF ternary system developed by Deng team constructs a hierarchical porous structure through freeze-drying. The addition of SA nanoparticles (20wt%) increases the residual carbon content to 75% and reduces HRR by 61.2%. Lee et al. used pore repair technology to composite SA (15vol%) with PDMS, increasing the oxygen index of the material from 25.2% to 26.4% and significantly improving the phenomenon of combustion dripping. This dual mechanism of "structural barrier chemical synergistic effect" provides new ideas for the development of new flame retardant materials.

Current research has confirmed that the bulk flame retardancy and mechanical strength of SiO ₂ aerogels can be improved through the strategies of preparation process innovation, phosphorus nitrogen flame retardant composite, nanostructure regulation, etc. The future development direction will focus on building organic-inorganic hybrid systems, achieving UL-94 V-0 flame retardancy while maintaining superhydrophobic properties (contact angle>160 °), and promoting their practical applications in high-end fields such as aerospace and new energy batteries.

As a new type of nano porous material, aerogel's ultra-low thermal conductivity (0.012-0.024 W/(m · K)) stems from its unique microstructure. By constructing a uniform and dense nanopore network and multi-level fractal pore structure, this material achieves triple synergistic suppression of heat conduction, convection, and radiation, and its thermal insulation performance is improved by 2-3 orders of magnitude compared to traditional materials. The specific mechanism of action is reflected in three core effects:

Gas confinement effect (convection suppression)

The pore size of the aerogel is distributed in the range of 2-50nm. When the pore size is smaller than the average free path of the gas molecule, the gas molecule is confined in the nano pore cavity and cannot flow freely, thus completely eliminating the convective heat transfer.

Radiation scattering effect (thermal radiation shielding)

The ultrafine porous network composed of three-dimensional nano skeletons forms a massive interface barrier (containing about 10 ^ 18 pore wall structures per cubic centimeter). These micro nano scale interfaces can reduce infrared radiation heat transfer by two orders of magnitude through multiple scattering and reflection effects.

Phonon barrier effect (solid-state thermal conductivity suppression)

The nano skeleton constructed from low thermal conductivity substances such as SiO ₂ forms an ultra long conduction path (the actual heat transfer path can reach 10-20 times the apparent thickness), combined with the phonon scattering enhancement brought by quantum confinement effect, reducing the solid thermal conductivity to 1/100 of bulk materials.

This multi-scale synergistic heat insulation mechanism enables aerogel to achieve aerospace grade heat insulation performance while maintaining ultra light characteristics (density can be as low as 3kg/m ³), which has been successfully applied to the extreme thermal protection system of Long March series launch vehicles.