Use of silicone substances

Silicone oligomers and polymers, as well as materials based on them, are increasingly used almost in all industries (Fig. 114) owing to their unique properties they are often implemented where other materials cannot. This wide range of applications is accounted for by the fact that silicone compounds greatly improve the quality of materials, give them long life and produce noticeable techical and economic effects. 

Depending on their chemical composition, molecular structure and molecular weight, silicone compounds are used as liquids, oils and lubricants of various consistency, as elastomers (for sealants, compounds and rubbers), as well as polymers for varnishes, plastic laminates and films. 

Bleaching and polishing compounds Aerosols Anticorrosive coatings 

Heat carriers and coolant fluids Antiadhesion compositions Hardening agents 

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The use of silicon-based antifoams is common in industry (van Bonarius et al., 1993). Flowever, they should be used with care, since these substances can be toxic to the cell above certain concentrations. Furthermore, chemical antifoams can pose problems for the chromatographic purification of the product. Foam traps, which are devices mounted in the upper part of bioreactors to break the foam, have been used successfully at small and intermediate scales, but are not widely used on a large scale. On the other hand, low aeration rates using pure oxygen effectively lead to a significant decrease or even complete elimination of foam, but may result in CO2 accumulation in the medium, which is harmful to the cells (Gray et al., 1996). 

Industrial poisoning. The production of silicone products uses substances harmful for human health. These are inorganic substances (ammonia, chlorine, sodium and potassium hydroxides, sulfuric and hydrochloric acids, hydrogen chloride) and organic compounds of various types, such as hydrocarbons (methane, benzene and its homologues), chlorine derivatives (methyl- and ethylchloride, chlorobenzene), alcohols (methyl, ethyl, n-butyl, hydrosite), acetone, pyridine, etc. The information about their toxicity, explosion hazard, effect on human body, as well as maximum allowable concentrations of gases and vapours in the air at workplace can be found in special references.(Ryabov 1970). A comprehensive description of silicone substances is given in Table 29. 

All this demonstrates that the use of silicone liquids for waterproofing is very wide and is growing wider still. It should be noted that efficient waterproofing of materials requires relatively small amounts of silicone substances e.g., 1 m2 of the facade requires only 5-10 g of the substance fibre materials need about 1 % of the quantity of the material. At the same time, they are very efficient the waterproofing of various materials reduces their water absorption 5-10 times and increases their serviceability by several times. 

Raman spectrometry is another variant which has become important. To quote one expert (Purcell 1993), In 1928, the Indian physicist C.V. Raman (later the first Indian Nobel prizewinner) reported the discovery of frequency-shifted lines in the scattered light of transparent substances. The shifted lines, Raman announced, were independent of the exciting radiation and characteristic of the sample itself. It appears that Raman was motivated by a passion to understand the deep blue colour of the Mediterranean. The many uses of this technique include examination of polymers and of silicon for microcircuits (using an exciting wavelength to which silicon is transparent). 

E.30 The isotope silicon-28 has been proposed as a new standard for the molar masses of elements because it can be prepared to a very high degree of purity. The mass of one silicon-28 atom is 4.64567 X 10-23 g. If silicon were the standard used for molar mass (instead of carbon-12), 1 mol would be defined as the amount of substance that contains the same number of entities as there are atoms in exactly 28 g of silicon-28. In that case, what would be (a) the molar mass of carbon-12 (b) the (average) molar mass of chlorine ... 

The relatively large band gaps of silicon and germanium limit their usefulness in electrical devices. Fortunately, adding tiny amounts of other elements that have different numbers of valence electrons alters the conductive properties of these solid elements. When a specific impurity is added deliberately to a pure substance, the resulting material is said to be doped. A doped semiconductor has almost the same band stmeture as the pure material, but it has different electron nonulations in its bands. 

Even though silicon is extremely abundant, only one silicon-containing compound appears in the list of top 50 industrial chemicals. That is sodium silicate, Na2 Si03, used for the manufacture of silica gel and glass. Nevertheless, with the advent of the electronic age silicon has become an extremely important substance that is the primary ingredient of most semiconductors. Because these are microscale devices, the quantity of production of silicon remains small compared with that of fertilizers and construction materials. Although relatively small in quantity, the value of silicon products is quite high. 

For application of protein-immobilized porous materials to sensor fields, use of an electroactive substance as the framework material is important. DeLouise and Miller demonstrated the immobilization of glutathione-S-transferase in electrochemically etched porous silicon films [134], which are attractive materials for the construction of biosensors and may also have utility for the production of immobilized enzyme bioreactors. Not limited to this case, practical applications of nanohybrids from biomolecules and mesoporous materials have been paid much attention. Examples of the application of such hybrids are summarized in a later section of this chapter. 

The fiber optic refractive index sensor finds use in biomedical applications. It uses a silicon chip with optical waveguides forming ring resonators. When the laser wavelength is scanned, the resonators cause dips in the power transmitted through the device. The wavelength at which these dips occur is a measure of the refractive index of the substance in contact with the chip surface. 

Table 5.1 summarizes the uses of lime. Lime is used as a basic flux in the manufacture of steel. Silicon dioxide is a common impurity in iron ore that cannot be melted unless it combines with another substance first to convert it to a more fluid lava called slag. Silicon dioxide is a Lewis acid and therefore it reacts with the Lewis base lime. The molten silicate slag is less dense than the molten iron and collects at the top of the reactor, where it can be drawn off. Over 100 lb of lime must be used to manufacture a ton of steel. 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

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