Suitable plastic for -40º, low CTE and isopropanol resistant

[drodrig]

Hi there,

We need a plate where we have some cooling system inside at -40ºF/-40ºC. Covering it we have a glass bell. There will be an o-ring (maybe silicone) on a slot of the perimeter where the bell lays to seal it. We have inside an atmosphere of isopropanol.

The idea was making this plate in plastic (low conductivity), so we need a material that:

- Can resist the lower temperature
- Can be purchased in a thickness of 1 inch/25mm and area of 47 square inches (1200 mm2)
- Is resistant to an atmosphere of isopropanol

My first thought was cheap PVC but it won't withstand the low temperature. Checking in the forum I saw PTFE or PVDF. The second is very expensive and here is the problem with PTFE: The CTE is pretty big (70in/in/R 120mm/mm/K). So because of the big plate dimensions, I'm afraid it will shrink too much (moving the o-ring position).

CTE for plastics is bigger than metals.

Another option would be using a thin stainless steel plate, but with the thermal stress I am afraid it won't stay flat for long

Do you have any recommendation for a suitable material?

thank you

 

[btrueblood]

Plain old polyethylene would work fine, I'd think. Oh, and not silicone for the bell jar seal, you'd want EPDM. And I'd not worry about the o-ring sliding as the plate shrinks, just put some silicone grease on it, it will slide and still seal. Stainless would be fine also, not sure what thermal stress you think would cause it trouble? 80C differential temperatures, on a 6" square plate? Unless your isopropanol is detonating, I'm not seeing the problem. And where are you getting 70 in/in/degF, that's like 4 orders of magnitude (10^4) too large, even for ptfe. The temperature differential will cause a few hundredths of an inch of expansion on a 6x6 in. plate.

 

[drodrig]

Thanks for the answer.

I made a mistake and it was 47inches by 47 inches (1.2m x 1.2m), this is a bit difficult also to find. Also I forgot a 10e-6 in the CTE units

With the thermal stress I meant the possibility of twisting the plate by this change of temperature. Then the bell won't seal anymore. No detonations

thank you

 

[btrueblood]

Ok, so a few tenths of an inch of relative motion. If the pressure is low, a bulb-type seal would likely flex enough - set it up so the seal gets compressed as the plate cools. The plate warping would seem to be solvable with a good insulation layer on the bottom, or cooling applied to both sides, or by using a metal plate - i.e. minimize the temperature gradient through the thickness.