EPDM Compound

[klemend]

Hello i have an problem with EPDM rubber diaphragm (membrane).
I use this membrane in pressure application sensor. When pressure increase, the diapragm make an movement and i measure this movement.
The problem is that with time the EPDM rubber chanhe her characteristic. After 2 or 3 days i see that at first day calibrated value is changed.

I also do not know what to measure on rubber diaphragm, is it hardness of rubber the right data?

Do you have any idea how cen i improve this?
Is silicon rubber bether in stability with time?

 

[Demon3]

Yes, silicones should be way more stable. You can buy silicone ware that you cook cakes in directly in the oven. The price will be way higher than EPDM though.

Measureing hardness may mislead you because it only reflects the modulus at the surface of the part and is more likely to change quickly due to UV, heat, oxidation.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

 

[btrueblood]

Er. EPDM is more stable in water and water-based compounds than silicone, and has better flex life (silicone is also porous to water vapor, thus makes a poor(er) seal). Silicone can withstand higher dry temperatures as Demon3 noted.

You may want to look into details of the compound (i.e. is it pure EPDM, or is there other stuff mixed in?). A large amount of carbon black in your compound, or nitrile rubber mixed in, or some sulfur-type cure agents, can cause the compound to absorb a fair amount of water, and change the way the diaphragm will respond. Try specifying a peroxide-cured EPDM.

 

[berti]

Hi klemend and all,

I am not sure if the problem you describe is maybe a more general problem.
Any elastomer takes its elasticity (that is: it returns to old shape after deformation) from internal network of crosslinks. Unfortunately nearly nothing in this world is perfect and so is any rubber. If you apply a stress and keep it constant (as with your diaphragm) you will see a certain relaxation over time and depending mainly from temperature and chemical environment.

So you have to choose rubber (of any type) with a high level of crosslink density (look for low value of Compression set) or separate the two functions of your diaphragm: sealing with (any) rubber sheet and controling the force by steel spring below.

Hope that helps a bit.

Berti

 

[Compositepro]

The way the application is described it sounds like the elasticity of the diaphragm is used as a force balance against the measured force. This will not work. Diaphragms are used as a fluid separator to prevent leaks and to transmit force without introducing any additional force. Fluid pressure is usually ballanced against a spring force on the other side of the diaphragm. Elastomers will not act like metal springs, except crudely and for short periods of time. Their properties are strongly affected by temperature and creep.