Analysis of The Thermal Insulation Performance And Application Fields of Aramid Fiber Reinforced Aerogel Composites

Aerogels are low-density, primarily mesoporous solids with excellent properties, including low density, high specific surface area, low dielectric constant, and ultra-low thermal conductivity. Examples include graphene or carbon nanotube aerogels, polyurethane and polyimide aerogels, biopolymer aerogels such as cellulose, chitosan, and protein aerogels, as well as their composites and hybrids. Particularly in the past decade, there has been an explosive increase in scientific papers and patents describing new aerogel materials, production processes, and applications, covering areas such as thermal insulation, delivery systems, environmental remediation, catalysis, and acoustics.

Despite the increasing importance of the aerogel field, or perhaps because of it, the definition of "aerogel" remains controversial. Early definitions were often based on the drying techniques used in the production process, such as aerogels from supercritical drying, cryogels from freeze-drying, and xerogels from evaporative drying. However, more recent definitions tend to focus on material properties, particularly the high proportion of mesoporosity. Ultimately, the broadest definition of aerogels refers to any material derived from a gel by replacing the pore fluid with air, without restrictions on pore size or other characteristics. This broader definition primarily includes macroporous materials that do not possess the mesoporosity, high surface area, or ultra-low thermal conductivity typically associated with aerogels, such as freeze-dried cellulose foams.

Silica aerogels are produced through a sol-gel process, with various modifications proposed to enhance resource and cost efficiency. However, most processes still follow the same basic steps. The gelation of silica sols is typically triggered by adding acids or bases to reduce the charge stability of nanoparticles. After gelation, the dissolution-precipitation reaction of silica strengthens interparticle interactions, thereby enhancing the mechanical stability of the gel. The industrial success of silica aerogels is almost entirely attributed to their performance in thermal insulation applications. Their thermal conductivity can be as low as 0.012 W/(m·K), primarily due to the high porosity and tortuosity of the particle network, which restricts solid-phase heat conduction. Additionally, due to the Knudsen effect, the small pore size—below the mean free path length of gas molecules—reduces gas-phase thermal conduction. This ultra-low thermal conductivity (only half that of ambient air and conventional insulation materials) has given rise to a rapidly growing market worth hundreds of millions of dollars.

The total thermal conductivity is closely related to material density, as shown in Figure 1. In conventional insulation materials, radiation plays a significant role, and in cases of large pore sizes, air convection also becomes non-negligible. As density increases, radiative heat transfer decreases while solid-phase heat conduction increases. Due to these competing effects, thermal conductivity exhibits a U-shaped dependence on density. The same influences apply to aerogel materials; however, since aerogel pore sizes are smaller than the mean free path of air, gas-phase conduction is drastically reduced. This decreases the frequency of air molecule collisions, thereby lowering gaseous heat transfer. Consequently, the minimum total thermal conductivity shifts toward higher densities and regions with (multiple) low electrical conductivity.

Silica aerogel nanoparticles construct a multi-network structure through interconnection, but the weak bonding between particles results in poor mechanical properties, low strength, and high brittleness in pure silica aerogels. To address these issues, researchers have explored various reinforcement strategies. Aramid fiber, with its low density, low thermal conductivity, and high mechanical strength, has emerged as an ideal choice for enhancing silica aerogels. With a decomposition temperature of approximately 450°C in air, aramid fiber is particularly suitable for high-temperature insulation applications.

In 2016, aramid fiber-reinforced silica aerogel composites (AF/aerogel) were successfully fabricated. Subsequently, glycidyl propyloxy trimethoxysilane (GPTMS)-grafted aramid fiber and polytetrafluoroethylene (PTFE)-coated aramid fiber aerogel composites were introduced. These composites not only retained low density and low thermal conductivity but also significantly improved compressive and flexural strength.

Further studies have demonstrated that the thermal and mechanical properties of aramid fiber make it highly effective for ballistic protection applications. Compared to aramid fabric alone, aerogel-integrated ballistic test samples exhibited a 72% reduction in fabric perforation rate. In 2021, Almeida et al. compared the reinforcing effects of silica aerogels with aramid fiber and felt, finding that composites incorporating elongated fibers exhibited lower bulk density and greater flexibility, making them well-suited for shape-adaptive and vibration-damping applications.

The combination of aramid fiber and aerogel achieves a complementary enhancement of material properties. As a reinforcing component, aramid fiber provides strong mechanical support to aerogels, improving their mechanical performance, while aerogels contribute their thermal insulation and sound absorption capabilities, working synergistically with aramid fibers.

For example, aramid/aerogel composites prepared using the wet-laid papermaking process not only retain the functional properties of aramid paper but also exhibit improved heat resistance. These composites have broad application prospects in thermal insulation, offering new insights and possibilities for the advancement of materials science.