Buna-n NBR o-ring cracks industrial atmosphere

[jaapsin]

my Buna-n (NBR) general purpose o-rings crack within 1 year in an industrial atmosphre, not in the sun but in contact with the air around it and some rain. O-ring is used to seal a lid of a box so it is under a bit of stress. Many small transverse cracks almost complete around the ring in contat with outside. Some o-rings from other locations last longer but I not sure if environment/wheater has been the same. My supplier says I made wrong choice have to use chlorobutene CR type in wheatering.
Other sources say they use BUNA-N rubber without problems in cable glands that are used outside for years and I just bought a bad quality?

Jaap Sinte

 

[btrueblood]

I suspect ozone damage, if you have any electrical stuff inside the boxes.

 

[jaapsin]

thanks,
No: the box contains a terminal strip with low 25 VDC power. Perhaps the ozone source comes from lightning or air pollution during warm days? how low ozone amount is critical?

 

[berti]

Hi all,

description of the failure sounds like ozone indeed. Some mechanical stress increases sensitivity. NBR itself is sensitive because of double bond in main chain, good choice of recipe ( black, ozone protecting chemicals like 6PPD and suitable wax transporting the 6PPD to the surface) can help a lot (and explain why other parts from same polymer are better under same conditions)

CR (polychloroprene) has a somewhat better resistance to ozone even it has also double bond in main chain.

Best choice for outdoor applications is EPDM (see the weatherstrips of your car) if you do not need permanent resistance against oil etc.

Regards

Berti