Nitrile Failing in Hydraulic Oil

[kritter]

I am using a die cut nitrile bumper .25" thick inside of a shock absorber for an internal extension cushion. Some have failed some have not. When the part does fail its catastrophic destroying all the internals of the shock absorber due to contamination which plugs the ports, bends/breaks shims and makes a non damping unit.

I am trying to figure out why nitrile is failing in oil where it is supposed to be compatibile with and when the seals sealing the shock are nitrile...

Temps range from 60F to 150F..180F rarely so it is within the temp ranges of the material and the seals, sealing the shock are nitrile and we have never seen a failure.

Any help is appreciated. The bumper supplier and seal supplier are not the same.

 

[swall]

Could you better describe the failure of the die cut part? Does it swell, shrink or just break up?

 

[kritter]

It swells and breaks up. Failure looks identical to rubber submerged in oil that is agitated...nothing but pieces/chunks left and the oil is compleltly contaminated/black.

The cushion sees a compression loading of roughly 2000lbs everytime the tire unloads.

 

[swall]

First verify that it is in fact made from nitrile rubber. An FTIR analysis would tell you this. If it is, you will have to dig deeper. Look at state of cure. Find out what plasticizers are used in the rubber compound.

 

[tom1953]

I agree about verifying that the rubber is indeed nitrile rubber. If it is, there can be significant differences between the quality of different nitrile rubber compounds. I'm assuming you're buying the nitrile rubber from a supplier and it's not your formula.
Nitrile rubber, the base rubber I'm talking about here, not compounds, comes in slightly different compositions, varying in the levels of acryloNITRILE and butadiene that are used to polymerize into nitrile (or NBR) rubber. Higher acrylonitrile (ACN), the better the oil and probably hydraulic fluid resistance. Higher butadiene, better low temperature properties, but poorer oil resistance. Also, compounds can be cheapened by adding higher amounts of fillers and plasticizers, giving poorer strength properties. Some compounds that don't need as good oil resistance can have other rubbers (e.g., polybutadiene or SBR rubbers) blended with it, as these are generally less expensive than nitrile rubber.

Regards,
tom