’Silicon’ means silicon simple substance, but ‘silicone’ is the generic name of a siloxane compound having siloxane-bonds or materials consisting of siloxane polymers.
<Form> It is classified silicones into three groups in terms of the main chain structure. First of all, it is an oil form. The silicone oil typically has linear structure and these viscosity is from 0.65cSt degree to 500,000 cSt degree. Secondly, it is a rubber form. The silicone rubber has cross-linked points between poly-siloxane chains. The three types of cross-linked reaction is mainly used for it as follow; radical reaction, condensation reaction, addition reaction. It is classified the silicone rubber into HTV and RTV. In the case of HTV, which is cross-linked with a peroxide by a radical reaction on the temperature condition of 100-180 degrees. On the other hand, RTV is done with a tin-catalyst by the condensation reaction at room temperature. Additionally, the another RTV using the hydrosilylation reaction is also done with a platinum catalyst. In this case, the system of compounds having a SiH group and a vinyl group are used at room temperature or around 100-120 degrees. And finally, it is a resin-form. Silicone resins are typically obtained the network structure by hydrolysis of 2 or 3 functionalized silanes. In addition, it is controlled the various properties by using 4 functionalized silanes.
<Heat Resistance> Silicones have a excellent heat resistance. Why do silicones have such a property ? It is due to the siloxane-bond (Si-O). This bond has thermodynamically stable and it’s the binding energy is 423[kJ/mol]. This is much bigger than the C-C binding energy, which is 349[kJ/mol]. It is one of reasons that the electronegative difference between Si and O combination is much bigger than that of C-C. By the way, the electronegativity of oxygen (O) is 3.44 and that of silicon (Si) is 1.90. The electronegativity is the relative force criterion that an atom draws an electron. Therefore, the difference of the electronegativity between Si atom and the O atom occurs a partial negative charge on O atom and a partial positive charge on Is atom. The Si-O bond is covalent bond, but this polarization of charge results in the having like a ionic bond. Accordingly, the siloxane bond have the thermodynamic stability.
| C-C結合 | Si-O結合 |
|---|---|
| 349 [kJ/mol] | 423 [kJ/mol] |
<Releasability> Next, let’s think about the releasability of the silicones. The surface tension of the silicones is a very small as shown in the following table. As one of reasons, it is a small intermolecular force between poly-siloxane chains. Compared to C-C chains, the Si-O chains is a very flexible because the Si-O-Si bonds are able to forming various bond angles. It is assumed that the various Si-O-Si bond angles makes the bond energy decrease thermodynamically very much. Therefore, the intermolecular distance of poly-siloxane chains is much bigger than that of C-C chains. And as another one of reasons, It is a small polarity of the C-H bond in the poly-siloxane methyl group. As shown in IR-chart of that, the silicone’s C-H signals is smaller than that of C-H such as the paraffin one. This shows that the polarizing of SiC-H is much smaller than the paraffin one. In this way, it is assumed that the small intermolecular force and C-H polarity impact releasability of silicones.
| シリコーンオイルの表面張力[mN/m] | ポリエチレンの表面張力[mN/m] | 水の表面張力[mN/m] |
|---|---|---|
| 20 | 35 | 72 |
<Cold Resistance> The silicones has a great cold-resistance. It is due to a low solidification temperature. The silicone’s one is around minus 50 degrees , but an linear paraffin’s one is around 100 degrees. This means that the silicones so easily doesn’t depend on temperature.
<Gas Permeability> The silicones has a great gas permeability. This is related to large distance between the molecules.
| シリコーンゴムの酸素透過性 [ml/s・m・Pa] | 天然ゴムの酸素透過性 [ml/s・m・Pa] |
|---|---|
| 5 | 0.2 |
<Others> The silicones has other many characterizations, which are defoaming, the insulation, the radiation resistance, the flame retardance, the biopassive.