Gasket Absorbing Oil?

[joshpjost]

Utilizing a 3/32" thick Buna-N gasket between cover and housing that contains food grade mineral oil. Gasket is machined to conform to shape of cover and for screw thru holes. Noticing that during disassembly, the gasket loses its form and becomes very wavy. Suspect it is absorbing oil. Is this possible for Buna-N? Is this a produce of machining the rubber?

 

[Metten]

It is not normal for a standard NBR to react that way. (Buna-N is NBR)

When it's properly vulcanized, it should maintain its form even when machined. You cannot easily change the oil absorbance of an NBR once vulcanized.
I propose you take the oil/grease from the machine and put it on a gasket that was machined + one that was not. Preferably you measure volume and weight of a smaller sample and you submerse it in the oil for fe 24h and than measure again. (ideally you compare with another NBR to roughly rule that your oil is the culprit). If after a short period of time you noticed the wavy behaviour for your gasket material, there is something wrong with the material, at least for your application.

 

[johnnymat]

NBR comes in a variety of polymer grades and of course mix recipes. The major factor affecting oil compatibility is the acrylonitrile content. Generally speaking NBR comes in 3 grades of Low, Medium and High acrylonitrile content (ACN).

The higher the ACN content the better the oil resistance, but the low temp flexibility is worse. and vice versa for low ACN grades.

I suspect the food grade oil is quite low molecular weight and if this is combined with a low or medium/low ACN content you could quite easily get the swell/ and distortion you are seeing.

However the swell tendancy in oil is also very dependant upon the material being fully vulcanised ( cured) it is not out of the question that this may not have happened at the manufacturing stage. and therefore there could be a batch quality issue.

( 1 hour at 150 deg C in an oven would finish off any resdual undercure.

But I would look for an NBR with a High ACN content ( about 40%) as long as you dont need good flexibility at about -20 deg C

Another possibility to explian the distortion might be the recipe. if you are using a soft rubber grade then it will be filled with processing oil and depending on the application conditions( temp and chemical environment) you might be losing some of this which would lead to distotion. if you can accurately weigh before and after samples you will be able to determine if this is a oil swell or oil loss situation

Johnnymat

 

[joshpjost]

What is the most appropriate method to specify NBR material on a gasket fabrication layout? Currently, the drawing identifies Buna-N, however given the fact that there are a multitude of "recipes". I want the most oil resistance. I was thinking of specifying the grade and class, for example:

ASTM D2000 BG (where G is the class and is based on the resistance of the material to swelling in oil)

What is the industry standard for specifying the rubber on mechanical drawings?

 

[GrahamBennett]

I use the ASTM D2000 line call out system.