Dimensional changes due to water absorption

[PeterSl]

I found this old thread that relates to my problem but unfortunately it never got answered.
thread334-73835
I'm fairly sure that the % volumetric change of a component isn't the same as the % water absorption but I assume there must be some relationship even if it's material specific. I would be interested to know what it is, if it exists.
Secondly can anyone tell me what effect pressure has on the water absorption of polymers, and what happens when the pressure is removed?
I'm working on ROVs that are deployed to 3000m and have several components made from a Nylon 66 (probably not a good choice of material) based bearing material and I'd like to be able to quantify the affect of the sea water pressure on the dimensions.
Thanks,
Pete

 

[Pud]

Lots of graphs for you: Here

And you are correct - PA6.6 would not be my first choice material either.

No idea of pressure implications though, sorry.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[PeterSl]

Thanks Pud, that will at least get me started.

Pete

 

[rickfischer51]

If you have access to Moldflow, take a look at the PVT curves for your material. I make 3000 meters to be 4371 psi for seawater with a apecific gravity of 1.025. That should be in the range of PVT data in the program. ALso, I think the bulk modulus can be used to determine volume change from a hydrostatic pressure. The bulk modulus K = E/[3*(1-2n)] = V*dp/dV, where E is Youngs modulus, V is volume, and n is Poisson's ratio.

 

[PeterSl]

Thanks for the reply Rick, a valid point about the compressibility of the plastic. But the point I'm having problems with is what effect does pressure have on the water absorption.

 

[chicopee]

There seems to be a fair amount of technical publications on the internet about pressure vs. water absorption of plastics and rubber.

 

[Demon3]

My best guess is that pressure will have very little effect on moisture adsorption amount but higher pressure may may it adsorb faster.

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting

http://www.phantomplastics.com

Plastic Training Seminars

http://www.plastictraining.com

 

[PeterSl]

Chicopee, I'd be interested in what you found I failed to turn up anything

 

[CheFEM]

Hi PeterSI,

You might also look at a free trial of brand new CheFEM prologue simulation software. This program quickly tells you the dimensional change of engineering polymers (including Nylon 6,6) and composites when exposed to a wide range of chemicals (including Water on 3000 meter depth), and a lot more...

Check the program at: CheFEM prologue simulation.

Hope this helps.

Kind regards,
CheFEM development team

 

[rickfischer51]

Chris, would not increasing the pressure reduce free volume, therby inhibiting moisure from migrating deeper into the plastic? Just a thought.

 

[Compositepro]

Rick, that might be a very transient effect. Once the water is absorbed there is no pressure difference between the water inside the bulk polymer and the water outside, so the polymer is not compacted. The pressure effect on absorption would be due to the higher concentration of water under pressure (higher density). This effect would be small.