Why is PDMS stable over such a wide temperature range?
Because its backbone is silicon and oxygen rather than carbon. The silicon-oxygen bond is considerably stronger than carbon-carbon and is not attacked by oxygen in the same way, so PDMS remains serviceable from below minus 50 to above 200 degrees Celsius, where a hydrocarbon oil of similar viscosity would oxidise, thicken and char.
2
Why does silicone fluid viscosity change so little with temperature?
Because the silicon-oxygen backbone is unusually flexible and the chain rotates freely, so the molecules do not become dramatically more entangled as they cool. The result is a remarkably flat viscosity-temperature curve compared with mineral oils, which is why silicone fluids are used in damping and hydraulic applications across wide temperature swings.
3
How does PDMS break foam?
By destabilising the liquid films between bubbles. Its very low surface tension means it spreads spontaneously across the foam lamella, and as it spreads it thins and ruptures the film, collapsing the bubble. That requires the antifoam to be insoluble in the foaming medium and finely dispersed in it, which is why antifoam products are emulsions or compounds rather than neat fluid.
4
How is PDMS grade specified?
Primarily by viscosity, which spans from a few centistokes to over a million and determines the application entirely. Low-viscosity fluids are used as spreading agents and carriers, mid-range as damping and release fluids, and very high viscosity gums as the base for elastomers. Volatiles, refractive index and flash point are also specified.
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