Oxidative stabilization of dimethyl silicone fluids with iron between 70 and 370°C

Abstract

Long term oxidation tests below 150°C on dimethyl silicone (DMS) fluids with controlled iron content, in conjunction with previous work, have provided a broad concept of the processes of oxidation in DMS fluids between 70 and 370°C. A nonfree radical, iron‐catalyzed oxidation of DMS fluid occurs both below and above 150°C. A diverging, free radical chain reaction which is inhibited by iron is initiated by direct attack of oxygen on the methyl groups of DMS above 150°C. It is believed that a ferric iron complex is the key mechanistic species both in the catalyzed oxidation and in the free radical reaction inhibition. When temperature, iron dispersion, and aeration are held constant, the oxidation rate of a DMS fluid below 150°C varies directly with its iron level. Above 150°C the oxidation rate is still dependent on iron level, but an ‘optimum iron level’ for a minimum oxidation rate exists for each temperature of oxidation. This optimum iron level increases with the temperature. A conceptual diagram has been developed to unify and summarize these effects over the temperature range 70–370°C, providing a basis for the most effective use of iron to stabilize DMS fluids.

The Thermal Stabilisation and Ceramifying of Silicone Rubbers

“…Nielsen [18] has suggested that inhibitors of the oxidation of polydimethylsiloxanes may be divided into two groups. The first group includes compounds whose inhibiting capacity is used up in the process of oxidation -organic salts of iron and cerium, and the second group includes compounds whose inhibiting capacity remains constant -organosiloxy vanadates and certain metal oxides.…”

“…Investigation of the mechanism of oxidation of the methyl group in polyorganosiloxanes in the presence of variable-valency transition-metal compounds [3,18] showed that by reacting with molecular products they initiate a chain radical process, and by reacting with free radicals they rupture the chains. Depending on the conditions, one of these functions of the catalyst may predominate, as a result of which it may act now as an initiating agent, now as an inhibitor.…”

Mechanism of the antioxidation effect of α-Fe2O3 on silicone rubbers at high temperature

“…These results are in agreement with the XRD analysis, and further conrm that Fe 3 O 4 is generated in the process of antioxidation. Moreover, the Fe 3+ (complex) is regarded as the composite of Fe 3+ on the surface of the a-Fe 2 O 3 complex with oxygen on the backbone of SR 13 (Fig. 5).…”

“…9 Another simple and efficient method involves the use of antioxidation additives such as ferric oxide (Fe 2 O 3 ) and cerium oxide. [10][11][12][13] Fe 2 O 3 is a common and most effective heat-resistant additive, which signicantly improves the antioxidation of SR. Many studies have reported that Fe 2 O 3 could enhance the thermal stability of SR, [13][14][15][16] nevertheless, there is still no consensus about the mechanism of the antioxidation effect of Fe 2 O 3 on SR. Nielsen 13 suggested that free radicals are destroyed as a result of the reduction of the metal ion as follows:…”

“…However, direct evidence supporting this mechanism has rarely been reported. 13 Kuzmunskii et al 16 claimed that the mechanism of antioxidation did not involve the reduction of Fe 3+ by free radicals, but rather involved the absorption of Fe 2 O 3 , which causes the oxidation of active molecules or adsorption and inactivation of free radicals, thus decelerating the oxidation of SR.…”