July 20, 2019, 12:06:36 PM
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Topic: Soap thickenner in mineral oil  (Read 226 times)

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Offline pcm81

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Soap thickenner in mineral oil
« on: June 22, 2019, 08:52:12 AM »
I have an application that needs to have mineral oil staying between a moving surface and a stationary graphite pad on the surface. Oil impregnates with graphite dust and provides electrical connection in a vibrating environment.
I am trying to increase viscosity of the mineral oil to determine optimal viscosity for this application.
I am using Sodium stearate as the thickenning agent. I am having trouble dissolving Sodium stearate in oil. I am using a heated magnetic stirer, but soap just stays in clumps that swirl in oil. If i add water to the oil it dissolves the soap and creates uniform mixture that kind of looks like light grease; but if i boil the water out the Sodium stearate falls out of the mineral oil.

Any ideas or recommendations would be appreciated.

Offline AWK

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Re: Soap thickenner in mineral oil
« Reply #1 on: June 22, 2019, 09:53:17 AM »
Try nonionic surface active agents
AWK

Offline Enthalpy

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Re: Soap thickenner in mineral oil
« Reply #2 on: June 24, 2019, 05:37:09 AM »
An oil thickened with soap is called a grease in mechanical engineering. Typically using sodium, calcium or lithium stearate. My suggestion is to start with the grease and dilute it with the oil.

Many such greases contain already graphite.

If it's only a matter of vibration, I'd prefer a flexible conductor. Copper braids exist for that use. Flexible printed circuits (typically 35µm copper on 25µm Petp) have good endurance.

Offline pcm81

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Re: Soap thickenner in mineral oil
« Reply #3 on: June 27, 2019, 09:14:33 AM »
An oil thickened with soap is called a grease in mechanical engineering. Typically using sodium, calcium or lithium stearate. My suggestion is to start with the grease and dilute it with the oil.

Many such greases contain already graphite.

If it's only a matter of vibration, I'd prefer a flexible conductor. Copper braids exist for that use. Flexible printed circuits (typically 35µm copper on 25µm Petp) have good endurance.
The problem is with contact between stationary graphite brush and rotating platter being lost due to vibration.

I tried mixing grease and oil before posting this thread, but grease just clumps up in oil, instead of actually mixing. This is why I thought about mixing sodium stearstearrate with oil to basically thicken the oil. It does not dissolve in oil even at elevated temperature. It does dissolve if I add some water and essentially dissolve soap in water while mixing it in oil. If I were to then boil water off the soap precipitates out of solution.

Offline Enthalpy

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Re: Soap thickenner in mineral oil
« Reply #4 on: June 27, 2019, 03:56:35 PM »
If I may suggest:

I'm not convinced at all that graphite powder will make a contact. Conductivity needs so much graphite that the suspension is a paste, or even, it lacks liquid.
So before you invest time in mixing graphite in grease, oil, mixing oil with grease, and so on, I'd feel reasonable to mix graphite powder in any runny liquid and check at what consistency conductivity appears. If this fail, you won't have lost too much time.

To suspend much solid in a liquid, the general trick is to have several widely different grain sizes of the solid(s). Like concrete comprises stones, pebbles and sand. The smaller grains fill the voids between the bigger ones. With one grain size, less the 0.4 to 0.6 volume fraction of liquid makes a paste that doesn't flow. But if you mix three grain sizes differing by a factor of 10, the minimum volume fraction of the liquid drops to (0.4 to 0.6)3.

Whether this helps conductivity? I doubt it. Conductivity needs the grains to touch an other, and then the paste is immobile.

Did you consider completely different solutions? Like a spring that pushes the graphite brush against the platter? Or transmitting contactless with AC?

Generally speaking, this problem is difficult, solving it reliably is a matter of decade experience. Maybe a manufacturer of DC motors like Maxon would design a brush and ring for you.

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