In the field of construction, there are many types of thermal insulation materials, which are mainly divided into two categories: inorganic cotton thermal insulation materials and organic polymer thermal insulation materials. Inorganic cotton thermal insulation materials have excellent fireproof and high temperature resistance, but they have strong water absorption capacity and the thermal insulation effect of the product itself deteriorates after absorbing water; organic polymer thermal insulation materials have the characteristics of light weight, low thermal conductivity, and low water absorption. They once occupied a mainstream position in the market of building thermal insulation materials, but with the improvement of national requirements for product fire resistance, their application in domestic construction has been greatly restricted. Aerogel composite products have excellent thermal insulation performance, fire resistance and high temperature stability, and meet the thermal insulation and fire resistance requirements of thermal insulation materials in the field of construction. They have become a new type of material that has attracted much attention in the field of building thermal insulation. Aerogel is a highly dispersed solid material composed of colloidal particles or polymer molecules aggregated with each other, with a nanoporous network structure and filled with gaseous dispersion medium in the pores. Its solid phase and pore structure are both nanoscale. Aerogel is known as one of the world's lowest density solids, with a density of 3.0kg/m3, which is only 2.75 times the density of air. It is called "frozen smoke". It is precisely because of its microstructure showing a "honeycomb" porous morphology that it has the characteristics of large specific surface area, high porosity, low density, and low thermal conductivity.


The synthesis process of aerogel generally goes through two processes: sol-gel polymerization and supercritical drying. There are generally three methods to achieve the sol-gel process: 1) gelation of colloidal powder sol; 2) hydrolysis and polycondensation of alkoxide or nitrate precursors to form gel; 3) polymerization of polymer monomers in solution or copolymerization of several polymer monomers to form gel. Although aerogel is light, has high porosity and low thermal conductivity, its low strength and brittleness limit its promotion in the field of building exterior wall insulation. Therefore, the application of aerogel in the field of building insulation is mainly to combine it with inorganic fibers, which not only retains the excellent properties of aerogel itself, but also makes up for the defects such as poor mechanical properties, so that its products are widely used in engineering. At present, aerogel composite products are widely used in thermal engineering, mainly SiO2 aerogel composite products.


Aerogel composite products retain the excellent thermal insulation, fire resistance and high temperature stability of aerogel, and can be widely used as excellent thermal insulation and fireproof materials in the construction field.


Thermal insulation performance


In the construction field, the thermal conductivity of insulation materials is an important parameter to measure the thermal insulation performance of materials. Among conventional thermal insulation materials, the thermal conductivity of inorganic wool materials such as glass wool, rock wool, and aluminum silicate wool at room temperature is generally in the range of 0.036 to 0.050 W (/ m·K), and the thermal conductivity of organic thermal insulation materials such as polystyrene boards, extruded boards, and polyurethane at room temperature is in the range of 0.021 to 0.041 W (/ m·K). The thermal conductivity of aerogel composite products at room temperature is usually lower than 0.023 W (/ m·K), and even better products will be lower than 0.017 W (/ m·K), which is unattainable by traditional thermal insulation materials. The main principle is the special nano three-dimensional pore structure of the material itself: 1) The high porosity and low thermal conductivity can greatly inhibit heat conduction; 2) The 50-60 nanometer aerogel pores prevent air molecules from flowing freely, and convective heat transfer will be greatly inhibited; 3) 1 mm thick aerogel material contains tens of thousands of pore walls, and these pore walls can be regarded as radiation reflection and refraction surfaces. This structure maximally inhibits heat transfer (heat conduction, heat convection and heat radiation), making its thermal conductivity very low.


At room temperature, the thermal conductivity of aerogel composite products is already lower than that of conventional thermal insulation materials, and as the temperature rises, the increase in its thermal conductivity is much lower than that of other materials, that is, the higher the temperature, the more obvious the thermal insulation advantage of aerogel composite materials. If aerogel composite products are used in building exterior wall insulation, roof insulation or passive houses, they can effectively avoid heat loss inside and outside the building space, and can also reduce the thickness of the insulation layer and the laying area, reducing the project cost; directly solving the risks of hollowing, cracking, and falling off caused by the thickness of the insulation layer.


Combustion performance


With the gradual implementation of mandatory measures for building energy conservation in my country, building exterior wall insulation technology has been widely used in buildings, but the important condition for its application is its fire safety. In the application of building insulation materials, the combustion performance of insulation materials is of great concern. When a fire occurs, external insulation materials with poor combustion performance have a strong promoting effect on the vertical spread of the fire, so in order to reduce the risk of fire, strict requirements must be placed on the combustion performance of insulation materials. Aerogel composite products not only have good thermal insulation performance, but also have excellent fire resistance. Since aerogel composite products are formed by compounding reinforcing materials with sol by sol-gel method, and then using a certain drying method to replace the liquid phase in the gel with gas, the reinforcing materials used for compounding are usually inorganic fiber materials, and the aerogel itself has excellent fire resistance, so the aerogel composite products have excellent fire resistance.


Temperature stability


In addition to having good thermal insulation and fire resistance, aerogel composite products can also have long-lasting stability in high temperature environments, and almost no volume shrinkage, melting, sintering, degradation and other phenomena occur. Aerogel composite products have excellent thermal conductivity at high temperatures. In an environment below 600℃, the linear shrinkage of aerogel composite products is extremely small and almost negligible, and the internal structure of aerogel will not change in a high temperature environment, and it is still an amorphous structure, indicating that the high temperature stability of aerogel composite products is very excellent compared with conventional insulation materials.


Against the background of increasing energy scarcity in the world, my country has attached more and more importance to social energy conservation and emission reduction, especially in the field of construction. Both conventional building insulation materials and new insulation materials are required to meet the standards of energy conservation and emission reduction. Although the production cost of aerogel composite products is relatively high, they have excellent performance in thermal insulation, fire prevention and high temperature stability, and can adapt well to the country's energy conservation and emission reduction policies, and can play an important role in the construction field in the future. As long as the industry continues to innovate and improve the production process of aerogel composite products and reduce production costs, aerogel composite insulation products will eventually occupy an important position in the future building insulation field.