Low Creep Elasomer

[edrush]

I am looking for an elastomer of approximately 70A hardness which exhibits very low creep. I am currently using a thermoset urethane. While testing is not complete, preliminary results do not look good.

What type(s) of elastomers typically exhibit the best creep performance?

The application is similar to a suspension airbag except I don't have air as a restoring force. I need the elastomer structure to be the return spring.

 

[polym3]

Creep is usually associated with thermoplastics; whereas "set" or "permanent set" is usually associated with crosslinked elastomers.

At ambient temperatures natural rubber has good low set properties, especially when cured with an EV system. No elastomer is free of permanent set, however. Set can be minimised by elastomer choice, compounding and cure system

 

[edrush]

Thanks Polym3. I have found a thermoset urethane which claims 1% compression set (maybe too good to be true?), so I will give it a try.

Creep and compression set are the same thing in my book. I don't have a good feel for why thermoset urethanes generally have such poor compression set properties. My best guess is that bonds get broken and remade. It would be interesting to run a tesile test after a creep test and to see how much strength is retained.

Another possibility is that there is really very little crosslinking in a typical urethane, so most thermoset urethanes are more akin to thermoplastics than say an epoxy.

 

[polym3]

Hi Ed: I guess we differentiate creep, and set in the "rubber" business, because creep is a time dependant increase in deformation under constant load, (not typical of crosslinked elastomers, but is seen in unvulcanized elastomers). Typically under a constant load, crosslinked elastomers/urethanes will reach a maximum deflection, that does not increase over time.

However elastomers do exhibit stress relaxation under constant load, which IS time dependant. Maybe this is closer to creep than set.

Either way, you are right. The breakdown of crosslinks under compression and new crosslinks forming in the compressed state is the main cause of set.

Neither urethanes nor rubber-like elastomers are heavily crosslinked, especially when compared to epoxies. The ability of elastomers to recover after rapid and short duration deflections is what differentiates them from thermoplastics. This recovery/elasticity is made possibly by the light and rather mobile network of crosslinks.

I'd be interested in seeing your results from the urethane claiming 1% set. ASTM D 395 Method B is about the least demanding compset test (22 hours at 70 deg C) and I've yet to see anything do THAT well.

 

[craigmj]

Ed-

Most Silicones can acheive a low compression set if they are cured for the right amount of time and temperature. By low I mean 5-10%, NOT 1%! That seems impossible to me. What uethane is claiming this?

CJ

 

[edrush]

The product claiming 1% compression set is Cytec RN-3039. While I have not performed compression set, creep, or rebound tests on this material, we did make a prototype part and this material has amazing rebound for a urethane. Unfortunately other properties do not quite meet our requirements. I just can't seem to get past the free lunch effect.