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          The current position: NEWS > FAQ > Why not many manufacturers of nano aerogel insulation felt on the market?

          Why not many manufacturers of nano aerogel insulation felt on the market?

          Date:2016-10-4 source:www.rodrigojames.com Number of visits:1156

          The invention relates to a nano aerogel thermal insulation felt, which is a flexible thermal insulation felt which takes nano silica aerogel as a main material and is compounded with glass fiber cotton or preoxidized fiber felt through a special process. Nano aerogel insulation felt is the lowest thermal conductivity solid insulation material in the temperature region of about 400 ℃ at present ( the thermal conductivity in the high temperature region of 400 - 1000 ℃ is much higher than the micro-nano insulation series ), which has the characteristics of softness, easy cutting, small density, inorganic fire prevention, overall hydrophobic, green environmental protection and the like, and can replace the traditional flexible insulation materials such as glass fiber products, asbestos insulation felt, silicate fiber products and the like which are not environment-friendly and have poor thermal insulation performance. The nano aerogel insulation felts sold on the market are blocky, of course, can be made into any desired shape according to the needs of customers. Although the nano aerogel thermal insulation felt is not added much aerogel, but does not affect its use effect, although the thermal insulation effect of nano aerogel thermal insulation felt is very good, but it is not popular in the market, the reason is not popular still need to start with nano silica aerogel, this paper first introduce the preparation process of nano silica aerogel.
          The preparation of SiO _ 2 aerogels can be divided into two processes: sol gel process and drying process.
          1. preparation of SiO _ 2 alcohol gel by sol-gel method
          The sol-gel process refers to the process that solid particles with a linear degree of 1 - 100 nm are uniformly dispersed in a proper liquid to form a single-phase solution, and the single-phase solution is converted into a porous glue stick block ( gel ) with a certain strength under certain reaction conditions ( temperature, humidity, ph value, pressure and the like ). Therefore, the sol-gel process is the premise and basis for the formation of SiO _ 2 aerogel nanoporous structure.
          At present, tetraethylorthosilicate ( TEOS ) is widely used as the main raw material for the preparation of SiO _ 2 aerogels at home and abroad. The preparation method comprises the following steps of: acidifying tetraethylorthosilicate ( adding HCl ) to prepare sol, adding an alkaline substance ( such as nh4oh ) into the sol to adjust the ph value to 5 - 7, strongly stirring, and aging for several days to fully connect the micro-regions of the network structure to obtain the gel with higher strength. Repeatedly soaking and washing the gel saturated with water in water, removing various soluble ions, soaking in ethanol ( ethanol does not take part in the reaction, and is used as a diluent for adjusting the density of a network so as to finally adjust the density of the aerogel ) for a long time, mutually dissolving water and ethanol in the soaking process, and finally replacing most water molecules in the gel body by ethanol to obtain the SiO 2 alcohol gel. 
          2, drying SiO 2 aerogel process
          In order to obtain SiO _ 2 aerogel, the solvent and reaction residue in the pores of the gel network must be removed. Since the pore size is on the order of nanometers, it is extremely difficult to remove these liquid materials without damaging the nanostructures, so it is very important to select the appropriate drying process.
          Because there are a large number of oh and other hydrophilic ion groups on the surface of the gel, the micro-pores of the gel have strong capillary action. Due to the strong capillary force, pores on the order of nanometers of the gel tend to disappear. therefore, the traditional drying method, namely, under room temperature or proper heating conditions, the solvent is volatilized naturally or volatilized through decompression, so that the volume of the gel is gradually shrunk and cracked, the nanoporous structure is destroyed, the volume density is rapidly increased, and finally the aerogel which can be used as the nanoporous super thermal insulation material cannot be obtained. At present, the main drying methods are supercritical drying method and non-supercritical drying method. 
          2.1, supercritical drying method
          Supercritical drying technology is a new chemical technology developed in recent years. In supercritical state, the interface between gas and liquid no longer exists, but becomes a uniform fluid between gas and liquid. The fluid is gradually discharged from the gel, and since there is no gas-liquid interface, there is no capillary action, and thus no shrinkage or structural damage of the gel is caused. Until all the fluid is discharged from the gel, and finally a gas-filled ultralight aerogel with a nanoporous structure is obtained.
          The medium used for supercritical drying currently has a critical temperature of 374.11 for water, ethanol and liquid CO2 water, a pressure of 22 MPa, a critical temperature of 239 for ethanol, a pressure of 8, a critical temperature of 31.10 for CO2 and a pressure of 7.37 MPa. It can be seen from the above data that the temperature and pressure required to use liquid CO2 as the supercritical drying medium are the lowest and the operation is the safest. On the other hand, the low-temperature drying kept the microstructure of the aerogel basically, which created the conditions for the study of the relationship between the structure and properties of the aerogel, and made the technology closer to practical. Therefore, liquid CO2 is widely used as supercritical drying medium at home and abroad.
          Alcoholized gel is generally used for supercritical drying with liquid CO2. Charging the alcoholized gel into an autoclave, cooling the high-pressure CO2 gas into a liquid in a pipeline, filling the high-pressure CO2 gas into the autoclave, slowly heating the autoclave until the temperature reaches the supercritical pressure, slowly releasing the co medium while slowly heating the autoclave until the pressure in the autoclave is balanced with the external atmospheric pressure, generally lasting for 3 to 7 days in the supercritical drying process, and gradually dissolving ethanol in the pores of the gel in the CO2 to form a single solution system mainly containing CO2 in the alcohol gel and the liquid CO2.
          2.2, non supercritical drying method
          Although the supercritical drying method can ensure that the SiO _ 2 aerogel structure is not damaged in the drying process, the supercritical drying process needs high-pressure equipment and harsh control conditions, the whole drying process is time-consuming and low in preparation efficiency, so that the preparation cost of the aerogel is very high, and the large-scale popularization and application of the massive aerogel are limited, so that the preparation method under normal pressure and below critical conditions has attracted wide attention.
          The aerogel prepare by that normal pressure and subcritical drying method can be roughly divide into two cases: one case is that after the gel is aged, the liquid with large surface tension in the gel is replace by the liquid with small surface tension, and then the aerogel is prepared by drying step by step under the normal pressure or subcritical pressure; In another case, the aged aerogel is subjected to alkylation treatment while water is replaced with an organic solvent and then dried under normal pressure.
          Shen jun, et al. successfully prepared SiO _ 2 aerogels under normal pressure by using relatively cheap polysilica as silicon source, surface modification and reducing the surface tension of the liquid in the pores of the gel. These aerogels are typical nanoporous superadiabatic materials with slightly higher thermal conductivity, but avoid the use of expensive supercritical drying technology, which is beneficial to the large-scale industrial application of SiO _ 2 aerogels.
          Chen longwu, et al. achieved the non-supercritical drying of bulk aerogel by two-step hydrolytic polycondensation of tetraethylorthosilicate ( TEOS ), ethanol solvent replacement and TEOS ethanol solution immersion and aging. the obtained SiO aerogel has certain strength and better morphology, and its microstructure, particle size and pore distribution are completely consistent.
          The bulk SiO _ 2 aerogels were prepared by non-supercritical drying technique using silica sol as the main raw material, doca as the drying control chemical additive in the gel process of silica sol system and selecting the gel process and drying process. The effect of the drying inhibitor can inhibit the growth of gel particles, make the size of particle and network gap of the gel network uniform, also can increase the strength of the gel skeleton, so that the gel skeleton can better resist the action of capillary force, and avoid shrinkage and cracking caused by uneven stress in the drying process. The obtained SiO 2 has a gap rate of about 91 % and an average pore size of 11 - 20 nm. As can be seen from the above, both supercritical drying method and non-supercritical drying method can obtain the required SiO 2 aerogel, but there is still a big difference between them, the comparative analysis of the two are shown in table 1.
          Know whether the preparation process of nano - SiO _ 2 aerogel you feel very complicated, it is also because of its preparation process is still more complex, high cost, and a single SiO _ 2 aerogel strength is very low, also can't scale production, so lead to the nano - SiO _ 2 aerogel as the main material of nano aerogel insulation felt and other nano insulation material price is higher than other insulation materials, but, with the continuous improvement of preparation technology and the continuous reduction of industrial costs, nano aerogel insulation felt will play a more and more important role in national defense, industrial production and other fields, will cause an epoch-making technological revolution in the field of thermal insulation.

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