Silicones oxidative stability

Polysiloxanes, also called silicones, are characterized by combinations of chemical, mechanical, and electrical properties which taken together are not common to any other commercially available class of polymers. They exhibit relatively high thermal and oxidative stability, low power loss, high dielectric strength, and unique rheological properties, and are relatively inert to most of the ionic reagents. Almost all of the commercially utilized siloxanes are based on polydimethylsiloxane with trimethylsiloxy end groups. They have the widest use... 

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

Little is known about valence-shell expansion reactions among still higher row elements, but the considerable oxidative stability of silicones and of tetraalkyl derivatives of group IV elements suggests that valence shell-expansion doesn t provide an easy path for oxygen attack in this group. 

The thermal stability, as well as structure-related properties, such as resistivity and elasticity, of polysiloxanes is dependent on the nature of the pendant groups on the silicon atoms. Thus high-molecular-weight polydimethylsiloxanes are attacked at temperatures near 200 °C in the presence of oxygen, but substitution of a phenyl group for one methyl group raises the oxidative stability to 225 °C. 

The greater stability of sterically hindered siloxanes indicates that oxidation occurs at the silicon atom. Stability toward oxidative cleavage is dependent on both the nature of the organic groups and the backbone structure. 

In addition to pure thermal stability, if the polymer is to be heated in air, one must also consider oxidative stability. In the carbon series oxidation always leads to more stable species and tends to occur, blit in the silicon series there is a much higher tendency towards reaction with oxygen. This is the principal reason for the low utility of the silane polymer. Finally, a third factor in polymer stability is the ease of attack by solvents, acids, bases, etc. This is largely determined by the ionic nature of the bonds involved. The silica based polymers should be more susceptible to such attack than carbon, since they have a higher percent of ionic nature. 

The thermo-oxidative stability of a silane where silicon is linked to imide nitrogen by an aromatic radical (CA-25) was greater than that of a compound where silicon is linked to nitrogen by an aliphatic group (CA-11). Upon... 

Besides organic polymers, inorganic polymers also are susceptible to thermo-oxidative degradation. Nielsen describes the stabilization of silicone fluids with redox systems of iron and cerium by a free radical mechanism. When stabilized, these fluids show oxidative stability up to 700°F. 

Improvement of the oxidative stability of silicone fluids is needed to extend their usefulness. It was recognized early (30) that some improvement could be obtained by using conventional hydrocarbon antioxidants—the aromatic amines and phenols. Such can be useful with DMS and MPS and are especially useful with alkyl (C H2n+i, n > 1) silicone fluids which are more comparable with alkanes in susceptibility to oxidation as n increases. 

Silicone rubber offers a set of unique properties to the market, which cannot be obtained by other elastomers. The Si-0 backbone provides excellent thermal stability and, with no unsaturation in the backbone, outstanding ozone and oxidative stability. The very low glass transition temperature, combined with the absence of low-temperature crystallization, puts silicones among the materials of choice for low-temperature performance. The fluoro-substituted versions provide solvent, fuel, and oil resistance along with the above-mentioned stability advantages inherent with the silicone backbone. 

Silicon is the second most abundant element in the earth s crust, next to oxygen. Due to the stability of silicon oxide compounds, elemental silicon does not occur in its free state in nature, but occurs as oxide (e.g., sand, quartz, amethyst, flint, opal, etc.)... 

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