[phi]

Hi there! I have a question regarding silicone oil. Apparently is a huge number of pure silicone oil lubricants that differ in viscosity. From what I understand, and please correct me if I am wrong, the difference in viscosity arises from how long the molecules are. My question is: If I mix 20 wt and 10,000 wt silicone oils in the correct proportion to produce a viscosity of, say, 1000 wt, would the product be exactly the same as what I would get by buying straight 1000 wt silicone oil? Or is it maybe the case that the straight 1000 wt lubricant will have lots of "medium size" molecules, but not many very short or very long ones, whereas my product would have fewer medium sized molecules, but lots of very short and lots of very long ones? If so, what would be the practical implications, in particular when using the product to lubricate plastic and metal parts at room temperature?

Thanks in advance for your *delete me*

[Enthalpy]

Hi Phi, welcome here!

Mixing lubricant, including silicone oil, is commonly done - BUT I would not do it with compounds as varied as 20 and 10,000. With a factor of 1 to 2, yes. One producer (Wacker I believe) gives diagrams and formulas for the resulting viscosity; if memory serves, they combine the log of the viscosity as the mix proportions.

What can happen? First, I'm not quite sure that 20 and 10,000 are both liquid. But one unwanted result is that the lighter oil may evaporate, especially in vacuum, leaving only the paste. I can't exclude neither that shear segregates the short and long molecules locally, leaving an unexpected viscosity at the most critical location, the contact point.

Silicone oil isn't that expensive. You could get a few oils around your target viscosity and mix only similar ones.

You already know that silicone oil is a bad lubricant, don't you? It's a "non-newtonian" fluid, whose viscosity drops at high shear number, that is, near the contact point. It gets thinner exactly where a lubricant shoud bring its viscosity, so silicone doesn't separate the moving parts. Hydrocarbons are nearly irreplaceable as lubricants. I can't tell how a mix behaves for that.

If you can, take a true lubricant. Silicone oil is more or less mandatory in vacuum and such very special cases. If you need silicone, maybe you find some with graphite or other powder that will lubricate your parts at the contact point.

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Most silicone oils and greases are just poly(dimethylsiloxane) of varied length which determines the viscosity, boiling point if any, density and so on, yes. Though, some silicones (oils?) differ: you also get poly(diphenylsiloxane) and other variants. But within the oil families I know, the oils differ just by the length.