Cracking of HNBR diaphragms 1

[proscow]

We have a case of a thin rubber (HNBR ~ 1-2mm) diaphragm in a high pressure CO2 valve failing. It had cracked completely round the flange plus part thickness crack around central support plus a few smaller concentric cracks. It is not ozone cracking. We suspect valve may get cold under some operational conditions (CO2 expansion)and that diaphragm could have cracked when it operated. Any experience? Anyone know how cold HNBR would have to get? Thanks

 

[aholloway]

The stiffening point of the diaphragm will be controlled by the acrylonite content of the base material, the degree of hydrogenation and the specific formulation of the HNBR but generally speaking HNBR has a Tg of about -15 to -40 deg C. That can be lowered by the addition of low temperature plasticizers in the compound. Of course there is always a trade off with high temperature properties with the plasticizers.