Viscosity-temperature coefficient silicone oils

Viscosity index, the customary basis for comparison of change of viscosity in hydrocarbon oils, becomes less satisfactory when applied to silicone oils, because the viscosity index varies with viscosity as well as with the temperature coefficient. In silicone oils the variation of viscosity with temperature is too small in relation to the viscosity itself. A true viscosity-temperature coefficient (VTC) has been proposed as a more satisfactory criterion [see Wilcock, Mechanical Engineering 66, 739 (1944)1. 

Compared with petroleum-based fluids, silicone oils show relatively small changes in viscosity as a result of temperature change (59,328). A common measure of the viscosity change with temperature is the viscosity—temperature coefficient (VTC) (341). Typical dimethylsilicone VTC is 0.6 or less. Phenylsilicones are slightly higher. Organic oils are typically 0.8 or greater. The viscosity—temperature curves follow equations 27 and 28, where a, b, and c are constants. 

Linear short-chain polysiloxanes Me3Si(OSiMe2)nOSiMe3 are used as silicone oils. Such oils are characterized by their high thermal stability and low viscosity temperature coefficient (VTC). A low VTC implies a small change in viscosity vis-a-vis temperature. Thus, many modem silicone oils retain their fluid-like characteristics even at temperatures as low as -80 °C. 

The valuable properties of the silicone oih are their very low coefficient of viscosity with tempera lure (as menticined in Chap. 1), ability fo withstand high temperature without decomposition, and chemical inertness to metals and most reagents. A typical silicone oil inci eases only about seven old in viscosity on cooling from iOO F to —35 F, whereas a hydiocarbon oil with the sanu viscosity at 100 F increases about 1800-fold at —35 F. 

Figure 1 shows the NMR signals of the silicone oils studied at 25 °C. The higher the viscosity Vo, 25 °c, the shorter is the relaxation time Tz- Obviously this is caused by the increasing molecular weight. Table 1 contains the kinetic viscosity, density and the molecular weight data. In Fig. 2 Tz is illustrated as a function of the molecular weight A/ , with the temperature as a parameter. The data were fitted to potential function y = Ax. The correlation coefficients obtained are given in Fig. 2.

The question is whether the viscosity can be predicted also for an unknown sample. This was the motivation for another evaluation of the data of Fig. 4. Figure 5 contains the same data as Fig. 4 but in contrast to Fig. 4 values at the same temperature (-20, 20 and 70 °C) are fitted with a potential function. The results of the fitting and the corresponding correlation coefficients are shown. Figure 5 demonstrates that for a constant temperature Tio correlates with T2. This can be expected as both the flow and the NMR relaxation behavior depend on the ipobility of the molecules in the silicone oils and, thus, on the molecular weight. 

Over the range 0-200°C the temperature coefficient of viscosity of silicone fluids is only about one tenth of that of mineral oils. In contrast the isoelectronic polymers (Me SiCH ) have normal viscosity characteristics. This means that silicones can be used over very much wider ranges of temperature. Some can still be poured well below — 50°C. Polysiloxane chains are very flexible as noted above for hexamethyldisiloxane, the bond angles in the chains are readily deformed. Moreover there are two mutually perpendicular 2p-3d)n systems which together have approximately cylindrical symmetry about the Si—0 bonds. This means that there is little resistance to torsional motion within the molecule. There is also essentially free rotation of methyl groups about the carbon—silicon bonds. (Barriers to rotation (kjmol" ) About Me—Si, 6.7 Me—C, 15.1 About Si—0, 0.8 C—0.11.3.)... 

Silicone oils have the advantage of oxidation stability, low freezing point, and a small temperature coefficient of viscosity [22]. The silicone resins are similarly characterized by chemical inertness, good thermal stability (to... 

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