Silicones: Preparation, Properties and Performance

The objective of this article is to give the curious reader a short but scientific overview about the ways silicones are prepared, their key properties and how these properties allow silicones to perform in many different applications. Introduction By analogy with ketones, the name “silicone” was given in 1901 by Kipping to describe new compounds of the brut formula R2SiO. These were rapidly identified as being polymeric and actually corresponding to polydialkylsiloxanes, with the formulation: R I ( Si O )n I R The name silicone was adopted by the industry and usually refers to linear polymers where R = Me or polydimethylsiloxane (PDMS): Me Me Me Me I I I I ... Si O Si O Si O ... or ( Si O )n I I I I Me Me Me Me The methyl groups along the chain can be substituted by many other groups (e.g., phenyl, vinyl or trifluoropropyl). The simultaneous presence of “organic” groups attached to an “inorganic” backbone gives silicones a combination of unique properties and allows their use in fields as different as aerospace (low and high temperature performance), electronics (electrical insulation), health care (excellent biocompatibility) or in the building industries (resistance to weathering). Historic Background The development of silicone chemistry includes several principal steps. Silicon was discovered by Berzelius in 1824 from the reduction of potassium fluorosilicate with potassium: 4 K + K2SiF6 Si + 6 KF Reacting silicon with chlorine gave a volatile compound later identified as tetrachlorosilane, SiCl4: Si + 2 Cl2 SiCl4 The next step was made by Friedel and Craft, with the synthesis of the first silicon organic compound in 1863, tetraethylsilane: 2 Zn(C2H5)2 + SiCl4 Si(C2H5)4 + 2 ZnCl2 In 1871, Ladenburg observed that, in the presence of a diluted acid, the diethyldiethoxysilane, (C2H5)2Si(OC2H5)2, gave an oil that decomposed only at a “very high temperature.” Kipping laid the foundation of organosilicon chemistry with, among other things, the preparation of various silanes by means of Grignard reactions and the hydrolysis of chlorosilanes to yield “large molecules.” The polymeric nature of silicones was confirmed by the work of Stock. In the 1940s, silicones became commercial materials after Hyde of Dow Corning demonstrated the thermal stability and high electrical resistance of silicone resins, and Rochow of General Electric found a direct method to prepare silicones from Si and MeCl.