The energy consumption of industrial buildings and maintaining indoor comfortable temperature accounts for more than 30% of the world's total energy consumption every year. The use of thermal insulation materials can improve the energy efficiency of buildings and reduce energy consumption. However, the traditional organic thermal insulation materials are generally flammable, the use of organic flame retardants will cause harm to the environment and human health, and the thermal conductivity of inorganic thermal insulation materials is generally high. Although the flame retardancy of general organic-inorganic composite thermal insulation materials has been improved, but still difficult to withstand long-term flame erosion, because the monodisperse inorganic particles gradually fall off with the combustion of polymer matrix, thus losing the protective effect on account of the dropping of the monodisperse inorganic particles gradually with the combustion of polymer matrix, so they lose their protective effect.

    Recently, Professor Yu Shuhong of University of Science & Technology China has developed a new synthetic strategy of phenolic resin (PFR) and SiO2 copolymerization and nano scale phase separation with chitosan as the three-dimensional template and PFR/SiO2 composite aerogel with dual network structure has been successfully developed. Research paper was published in Angew. Chem. Int At. Ed., the first author of the paper is postdoctoral Yu Zhilong.

 Figure 1. Schematic illustration of the synthesis and structural composition of PFR/SiO2 composite aerogel with interpenetrating binary network.

    The composite aerogel with binary network structure has a dendritic microstructure and the size within 20 nm of the fiber, and binary network is homogeneous down to the nanoscale owe to each of the two constituents form a continuous network with a strong interfacial interaction between them.

Figure 2. Microstructure and composition distribution of binary network PFR/SiO2 composite aerogels.

    By adjusting the amount of silicon source, the physical parameters such as density, inorganic content and mechanical strength of the composite aerogels can be controlled. The PFR/SiO2 aerogels with certain mechanical strength and machinability can be compressed by 60% or more without catastrophic collapse. The aerogel has good thermal insulation effect with a lowest thermal conductivity of 24 mW -1m K-1, which is superior to the traditional foamed polystyrene. It displays a thermal conductivity of 28 mW m-1 K-1 at room temperature and low relative humidity

Figure 3. Physical properties of PFR/ SiO2 aerogels. a) Densities b) TGA curves (measured in air) c) HRR d) compressive strain-stress curves. e) The thermal conductivity of PSi-70 as a function of temperature at constant absolute humidity (1 g m-3). f) The thermal conductivity of PSi-70 as a function of relative humidity at temperatures relevant for insulation of buildings in a cold climate.

     This unique binary network structure gives the aerogel excellent fire and flame retardancy. The researchers used propane butane flame lamp (1300℃) and alcohol lamp flame (500~600℃) to detect the fire resistance of aerogels and recorded the temperature changes on the back of the samples with infrared thermography. After 30 minutes of testing, the temperature of the back side of the sample was stabilized at 300 degrees under the blowlamp and that of the alcohol lamp flame was stabilized at about 150℃. Moreover, with the combustion of organic components, the SiO2 network is exposed and attached to the aerogel surface without falling off to playing a role in isolating heat continuously. This material can avoid the failure of bearing structure under the fire, and buy time for evacuation in case of fire accident.

Figure 4. Heat insulation and flame retardancy of PFR/SiO2 composite aerogel under the blowlamp (left) and alcohol lamp flame (right).

The paper links：https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201711717