I've always been a firm believer in Buna N as the go to O ring material and always wondered at the populist trend to use Silicone in applications where it is quite possibly least well suited.
http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf
The single best educational resource available
I have their hardcover and was unaware that they also publish a PDF version.
The nice thing about the PDF is it's searchable
here's a much smaller guide
http://www.sealdynamics.com/o-ring-reference.pdf
Purusing them you will quickly come to the conclusion that good old black Buna-N (nitrile) is the most useful, cheapest, and most reliable O ring going with a GoZillion (to the tenth power) of duty hours in all industries. Buna N is able to tolerate a wide temperature range under a very diverse set of duty applications from sliding seals to static and is excellent for assemblies which must be disassembled often. Buna N is also very cheap.
Silicone on the flip side is expensive, and when you squeeze it in a static compression it can get a tear started from the ridges in things like tanks and kettles. Once the tear starts, propagation is fast. Silicone takes a set from compression loading it has very low friction tolerance and is a terrible material for frequent disassembly precisely because of it's friction intolerant character.
Silicone leaks gasses faster too. ( though pretty much all O ring gas transpiration rate is so trivial as to be insignificant.)
In fact Silicone's only real advantage is thermal resistance.
But who needs more thermal resistance than Buna N offers?
It seals satisfactorily for five minutes at 538°C (1,000°F) and at 149°C (300°F) for 300 hours.
Buna N is the default material of all generic O ring producers. You can bet, for dead certain, that whenever you see a package of black O rings that they are Buna N, unless the package says otherwise. This is exactly because of Buna N's incredible performance characteristics, extensive range of applications, and enormous history. They want their O rings to have the broadest application capacity for the money and Buna is cheap cheap cheap.
Over 20 years ago I was part of a group touring Parker's O Ring factory. I asked why so many were black. The answer was that rubber-like and rubber products are black is because there's so much carbon through out the process that in order to make the products any other color they'd have to run super clean facilities and take extra measures to exclude carbon from the whole chain of production as is done for other polymer products. He told me that we'd pay a lot more for pretty colored O rings.
Since then, the carbon has been found to slow the rate of gas transpiration through O rings. Prolly because the carbon absorbs the gas. The cost of making pretty colored O rings remains high.
http://www.parker.com/literature/ORD%205700%20Parker_O-Ring_Handbook.pdf
The single best educational resource available
I have their hardcover and was unaware that they also publish a PDF version.
The nice thing about the PDF is it's searchable
here's a much smaller guide
http://www.sealdynamics.com/o-ring-reference.pdf
Purusing them you will quickly come to the conclusion that good old black Buna-N (nitrile) is the most useful, cheapest, and most reliable O ring going with a GoZillion (to the tenth power) of duty hours in all industries. Buna N is able to tolerate a wide temperature range under a very diverse set of duty applications from sliding seals to static and is excellent for assemblies which must be disassembled often. Buna N is also very cheap.
Silicone on the flip side is expensive, and when you squeeze it in a static compression it can get a tear started from the ridges in things like tanks and kettles. Once the tear starts, propagation is fast. Silicone takes a set from compression loading it has very low friction tolerance and is a terrible material for frequent disassembly precisely because of it's friction intolerant character.
Silicone leaks gasses faster too. ( though pretty much all O ring gas transpiration rate is so trivial as to be insignificant.)
In fact Silicone's only real advantage is thermal resistance.
But who needs more thermal resistance than Buna N offers?
It seals satisfactorily for five minutes at 538°C (1,000°F) and at 149°C (300°F) for 300 hours.
Buna N is the default material of all generic O ring producers. You can bet, for dead certain, that whenever you see a package of black O rings that they are Buna N, unless the package says otherwise. This is exactly because of Buna N's incredible performance characteristics, extensive range of applications, and enormous history. They want their O rings to have the broadest application capacity for the money and Buna is cheap cheap cheap.
Over 20 years ago I was part of a group touring Parker's O Ring factory. I asked why so many were black. The answer was that rubber-like and rubber products are black is because there's so much carbon through out the process that in order to make the products any other color they'd have to run super clean facilities and take extra measures to exclude carbon from the whole chain of production as is done for other polymer products. He told me that we'd pay a lot more for pretty colored O rings.
Since then, the carbon has been found to slow the rate of gas transpiration through O rings. Prolly because the carbon absorbs the gas. The cost of making pretty colored O rings remains high.