Silicone fluids, oxidative stabilization

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. 

It has been observed (a) that aeration plays an important role in prolonging the life of a redox metal stabilized silicone fluid by sweeping away any partially oxidized volatiles to prevent their return to the bulk fluid (b) that aeration is needed to keep some redox antioxidants from... 

Features Reactive mercapto functionality high affinity for metal surfs. better high temp, and oxidative stability than conventional silicones solv-free compat. with aliphatic/aromatic hydrocarbon soivs., dimethyl silicone fluids... 

Features Reactive mercapto functionality high affinity for metal surfs. better high temp, and oxidative stability than conventional silicones solv.-free compat. with aliphatic hydrocarbon soivs., dimethyl silicone fluids Properties Colorless to It. straw clear to si. hazy liq. sol. in aliphatic hydrocarbon soivs. m.w. 3700 sp.gr. 0.98 dens. 8.0 Ib/gal vise. 120 cSt flash pt. (PMCC) > 200 F 100% act. 

The oxidative stability of fluids may be improved by the incorporation of an antioxidant such as an iron soap. When silicone fluids are heated in the absence of air, chain scission occurs and the viscosity falls. 

Nielsen J.M. Oxidative stabilization of dimethyl silicone fluids with iron between 70 and 370°C. J. Polym. Sci. Symp. 1973 40 189-197... 

Diffusion pump fluids are high molecular weight materials, such as many oils and mercury, which vaporize at a reasonable temperature. A concern is the thermal and chemical stability of the fluid. Hydrocarbon oils tend to break down under heat to form low molecular weight fractions, or they may oxidize and polymerize into a vamish-like material, and therefore are not desirable for many applications. Silicone oils are much more stable with respect to temperature and oxidation and are the fluids most often used in DPs for vacuum deposition processes. 

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