additives for NBR? 2

[kclim]

Hello all,

Was recently asked to perform a failure analysis on some O-rings, which appear to have suffered from extrusion and cracking damage.

An incorrect material was suspected to be the cause of failure - as a result, two ring samples were sent for analysis, a failed ring and an OEM (presumably correct) one.

The results of the analysis showed them both to be butadiene nitrile rubber, although the failed rubber sample contained aniline and methylaniline as additives, while the OEM sample contained quinolines & nonylphenols.

Now its all well and good to say that the incorrect material was used, but did that contribute to the failure? Some preliminary googling on the additives revealed the following.

- quinoline is an antioxidant, and helps prevent degredation
- nonylphenol is a plasticiser, and reduces 'brittleness'
- the aniline compounds are vulcanisers (make the rubber more resilient?)

I suspect the absence of quinoline might be contributory to failure, but I wasn't sure if the aniline acted as an antioxidant.

Anyone have any further advice?

Thanks in advance

 

[berti]

Hi all,

let me try some remarks.

... some O-rings, which appear to have suffered from extrusion and cracking damage.

Did they tell you more about conditions of failure ?
- hot air
- any chemicals /solvents (which?)
- cold
- ozone /intense sunlight (maybe before use, as e.g. waste water tubes stored on building site)

... The results of the analysis showed them both to be butadiene nitrile rubber.

NBR comes in several types which differ in the butadiene/acrylonitrile ratio.
18 % ACN --> reasonable cold resistance, but bad resistance against solvents (with the result of swelling and loosing mechanical properties)
45 % ACN --> bad cold resistance, but excellent chemical resistance

33 % is common and a good compromise ( as often in rubber and in life ;-))

So if you have some more material check for Nitrogen content (Kjelldal) and calculate approximate ACN content.


...although the failed rubber sample contained aniline and methylaniline as additives, while the OEM sample contained quinolines & nonylphenols.

All the chemicals you mention are not (normally) used as additives themselves but (if you positively identified them) are decomposition products of antioxidants. Quinoline could come from TMQ which is an excellent heat age resistor and Nonylphenol is probably not used as a plasticizer but is the decomposition product of TNPP (tris-nonylphenyl-phosphit.The anilines can come either from accelerators or from another group of antioxidants (substituted diphenylamines)

So if you have time and money enough or things are serious:

- Measure hardness and density
- Do some TGA to find out plasticizer/ Carbon black/mineral filler content
- Extract by boiling ethanol, concentrate and run Thin layer chromatography (TLC) which will show you all antioxidants and most accelerators used.
- Extract a weighed piece by acetone, dry and weigh back, missing weight is sum of antioxidants and plasticizers.
- Take this extracted piece for a.m. N-analysis ( it is still not fully correct because of the filler content !!)

Again: You should know how important are things for you. If it is a legal matter you should better give all these tests to a certified laboratory.

Hope to help

Berti

 

[Demon3]

Great reply berti,

I agree fully with your reply. I was tempted to make some comments but held off as I am more of a plastics guy and was not so sure about the rubber side. I do know about antioxidants and you are right on all those points. Plus I know from plastics that increased acrylonitrile really helps chemical resistance (for example of ABS).

Also you went one step further and suggested things to do to look deeper into the problem. You got a start from me for a really good post.

(In case anyone wonders I am writing this to thank berti but also to let kclim know that the reply sounds plausible.)

 

[kclim]

Berti,

Thanks for the info, very erudite. Now I've got a lot more questions to ask the local chem centre (who performed the analyses).

The rings are used in an oil cooler; they are exposed to both oil (80 C) and water. They aren't exposed to high UV (sunlight) during operation, although I'm not sure about their storage.

Don't know if I can justify the cost of further testing - the failure was a little more than a nuissance and certainly not an OSH issue.

On another note, perhaps you would recommend some literature/texts as a primer to this sort of stuff (rubber additives/effects)?

Thanks again

 

[berti]

Hi all,

thanks for the friendly words

Unfortunately most of the literature I would mention is in German ( as I am German), but I am sure there are similar textbooks in English available.

Application papers of the NBR producers (e.g. Bayer/Lanxess) are probably available from the net.

Berti

 

[kclim]

Just got back some SEM X-ray analysis results back - the failed samples contained calcium (presumably from the water), zinc and sulfur (cross-linkage?) while the OEM sample contained zinc, aluminium, sulfur and silicon.

Anyone care to comment on the zinc/aluminium/silicon? Are they filler materials?