Silicones economic importance

This chapter is devoted to the chemistry of two of the oldest known elements, tin and lead, and two that are of much more recent discovery, silicon (1824) and germanium (1886). The elements in Group IVA show the trend toward more metallic character in progressing down in the group. Silicon is a nonmetal, tin and lead are metals, and germanium has some of the characteristics of both metals and nonmetals. These elements are all of considerable economic importance, but in vastly different ways. 

Summary The Direct Process discovered by Rochow and Muller around 1940 is the basic reaction used to produce methylchlorosilanes, which are the monomeric intermediates used for production of silicones. An understanding of the elementary reactions, the nature of active sites and the action of promotors does not nearly come close to the performance level of the industrial process and the economic importance. The silylene-mechanism is a useful model to understand the complex product mixture from the reaction of silicon with chloromethane. 

The first industrial production of methylchlorosilanes using the Direct Process started in 1947 in the USA (General Electric, Waterford), in Germany between 1951 and 1955 three companies entered into it. During the intervening five decades the Direct Process became a worldwide utilized process in the fast growing world of silicones and in 1993 the production of methylchlorosilanes passed 1 000 0001 per year in the Western World. In 1995 the production of methylchlorosilanes was about 1 250 000 t, for which 1 000 000 t chloromethane and 300 000 t silicon were consumed. The vale of the silicones market in 1995 was about 6 billion US. These figures emphazise the economic importance of the Direct Process in the silicones industry. 

The metalloids, or semimetals, have properties that are somewhat of a cross between metals and nonmetals. They tend to be economically important because of their unique conductivity properties (they only partially conduct electricity), which make them valuable in the semiconductor and computer chip industry. (Did you think the term silicon valley referred to a valley covered in sand Nope. Silicon, one of the metalloids, is used in making computer chips.)... 

Phosphorus is technologically and economically important in aluminium-silicon alloys. On one hand it regulates the mechanism of solidification of eutectic (12.5 % Si) and nearly eutectic alloys, on the other hand it grain refines the primary silicon in the hypereutectic system (15-25 % Si) When the eutectic or nearly eutectic aluminium-silicon alloys contain less than 5 Mg/g of phosphorus, the alloy solidifies into a lamellar structure. When the phosphorus concentration is above 9 Mg/g a globular structure is obtained. In hypoeutectic alloys with about 7 % of silicon, the solidification is only fine lamellarly at phosphorus contents between 2 ig/g and 4 g/g. When magnesium is present, even below 2 ng/g a globular structure is obtained. 

Waste waters in the production of silicone monomers and polymers is an important economic issue. It is connected with environmental protection from air contamination and pollution of rivers, seas and other water bodies. Especially important in silicone production is the purification of acid waste waters from organic impurities (toluene, benzene, methyl alcohol, etc.). 

Since the spectacular isolation of the first stable disilene by West, Fink, and Michl in 1981 [19] this field has enjoyed a surge of activity [3]. Yet, despite the many important discoveries that have been made, the study of compounds containing double bonds to silicon is still in its infancy. In particular, relatively little is known on the effect of substituents, especially heteroatom substituents, on the properties of disilenes as only aryl-, alkyl-, (Me3Si)2N-, and MeaSi-substituted disilenes have been isolated to date [3]. With experimental progress being relatively slow, ab initio calculations provide a reliable, fast and economical method to extend our knowledge on these novel compounds. 

Primary aminoalkylsilanes are important compounds in application fields like adhesion promotion, surface modifying, and crosslinking, but are also used for endcapping of various substrates. Currently in focus is the recently developed endcapper (Fig. 1) which is the key molecule for the preparation of thermoplastic silicone elastomers (TPSE). Modification of silicone resins and fumed silica with 1 leads also to products with interesting economical potential. 

The polyaromatic mesophase (PA-MP) is a nematic, discotic, chemotropic liquid crystal. Owing to its high density (about 1.5 gcm ), its high carbon yield of about 90 %, and its thermoplasticity, it is unique as a precursor of structure carbons. An important application is the manufacture of high modulus (HM) and ultra-high modulus (UHM) carbon fibers [1]. By alloying with silicon, physical and chemical properties of the materials, such as strength, hardness and oxidation resistance, can be improved. These modified carbons were available by chemical vapor deposition (CVD) processes only up to now. The preparation by liquid phase pyrolysis is novel, economic, and thus opens a completely new field of applications. 

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