Elastomeric Materials based on Hydrogenated Nitrile Rubber for Seals in Carbon Dioxide (R 744) High Pressure Service – Improving the Resistance against Explosive Decompression

The “Explosive Decompression” phenomenon is regarded as a consequence of hindered diffusion of gases that are developing suddenly when the internal pressure drops in a high pressure hydraulic system.
Difficulties of that kind are reliable if carbon dioxide (R 744) is potentially replacing R 134a in air conditioning circuits due to the EU – regulation 2006/40/EG. Conditions requiring high pressure around 57 atm to 75 atm at ambient temperatures are necessary to establish a liquid-to-gas equilibrium (see: phase diagram of carbon dioxide).
Focussing on Explosive Decompression attempts are made to understand the interaction of CO₂ with a typical seal polymer such as hydrogenated nitrile rubber (HNBR).
The resistance against Explosive Decompression is assumed to be high if the

• permeation rate of CO₂ is appropriate and if the
  
• swelling of CO₂ (liquid) is low as well.
  

The measurement of the CO₂ –permeation according to DIN 53536 results in the fact that high acrylonitrile polymers and the use of special fillers such as  diatomaceous  earth are required to reach K_D ~ 2 g/m²*d at RT and K_D ~ 20 g/m².d at 70°C ,respectively.
The determination of the swelling of elastomer materials in CO2 (liquid) is not possible to be determined exactly. However, a rough estimation can be achieved when the test pieces are removed from an autoclave after being exposed for, e.g. 4 h. The volume swell is roughly estimated at DV ~ 5 % (base polymer of 43 % acrylonitrile).
The Explosive Decompression properties are tested with an common laboratory sized autoclave which was decompressed rapidly after the storage time was over. The development of blisters could be optically followed by the aid of microscope.
High acrylonitrile based  85 -90 Shore A vulcanizates , the composition of which is based  on  diatomaceous earth and silica to achieve TS of > 11 %,  are found blister-free in this  decompression test.