Microporous insulation materials

Microporous insulation materials consist mainly of highly dispersed silica with a particle size of only 5-30 nm. The highly dispersed silica powder is pressed to plates, which receive heat treatment up to 800 °C, after which the plates are self-supporting and possess a micropore structure with pore diameter of 0.1pm. The addition of opacifiers to the highly dispersed silica starting material reduces the loss of heat by radiation. The dates for such insulation boards are shown in Table 18. 

Since the 1980s a new microporous insulation material based on silicic aerogels is in a test phase. About 60 years ago Kistler from Stanford University developed a method to produce aerogels from silicic material without shrinking them He reacted waterglass with hydrochloric acid, washed out the sodium and chloride ions from the gel, substituted the water with alcohol and dried the gel in an autoclave above the critical point. Other methods begin with tetramethoxysilane for the production of monolithic aerogels. 

The structure of aerogel is similar to the structure of a compressed fumed silica The diameter of the Si02 sphere is about 5 nm, the diameter of the pores in the aerogel is about 50 nm. Aerogels are, therefore, also excellent microporous insulation materials. 

The uses of microporous insulation materials are almost unlimited. This material can be used to protect specific areas from high temperature. It can increase the effective volume and the productivity of heated devices for the same energy consumption. 

Table 18. Microporous powder pressed to platens as a thermal insulating material...

For a long time the technical application of this microporous insulation was limited, as the material made from pressed powder was hardly workable and, for example, had to be protected through a covering made from glass fiber. 

Table 2. Properties of microporous thermal insulation materials before and after hardening reaction.

In general, acid hydrolysis and condensation results in linear or weakly branched chains and microporous structures in silica sols [48] and the resulting gelation times are generally long. In contrast, uniform particles are easily formed in base catalysis (i.e., mostly NH4OH based) and lead to a broader distribution of larger pores, which is less favorable for thermal insulation materials [49]. 

The sharp decline in the thermal conductivity in the range 1000-100 mbar shows that the pore diameter of this material is of the same order as the mean free path of the gas molecules. Based on the above, fumed silica is excellent for use in the production of the so-called "microporous" thermal insulation. 

The formation of usually coarsely disperse systems during gaseous phase evolution is of importance in the industrial production of various solid foams with valuable mechanical, thermal insulating and sound insulating properties. Examples of such materials include various types of foam concretes (production of these usually involves the evolution of C02 gas in the reaction between CaC03 and HC1), foam plastics, and microporous rubber. In nature the degassing of magma leads to the formation of pumice stones and tuffs. 

Standard specifications for the implantable PTFE are given by ASTM F754. PTFE also has an unusual property of being able to expand on a microscopic scale into a microporous material which is an excellent thermal insulator. PTFE cannot be injection molded or melt extruded because of its very high melt viscosity and it cannot be plasticized. Usually the powders are sintered to above 327°C under pressure to produce implants. 

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