I'm looking for a glue that would allow to seal a large glass window (1 m2) to a metal box and to evacuate the inner volume to about 10-5 Torr. Such glue should hold against thermal expansion mismatch up to bake-out temperature of 120 C. So far I tried TorrSeal (epoxy) that is ok for vacuum properties but is too rigid and cracks when thermal cycling. VacSeal (silicone resin) should be flexible enough but is too thin to compensate for surface imperfections. Also moisture permeation rate could be high. Thanks in advance for any other suggestion...
Your first assignment is to calculate the physical mismatch between the glass and the metal box at 120 C.
This tells you about how much differential displacement that the design for the seal must accomodate.
If the seal is brittle and thin, you can't absorb much differential expansion in a brittle thin seal. However, a much thicker brittle seal could well accomodate the thermal expansion differential.
Thick seals can accomodate more differential expansion via shear in the seal.
The shear capability needed can be estimated from the thermal expansion mismatch, DeltaL, and the seal thickness, t. You can estimate the thermal expansion mismatch from the temperature differential and the expansion coefficients AlphaMetal and AlphaGlass and the window "diameter" Dia.
Mismatch = DeltaTemp * (AlphaGlass-AlphaMetal) * Dia
SealShearStrain = Mismatch / t.
The last equation tells you why a larger seal thickness, t, is pretty important.
People will often elect to use an elastomer gasket of some kind with metal flanges and securements rather than an adhesive.
Good luck.
I take no responsibility for glass vacuum windows shattering. Hope you have followed good guidelines on selection of glass, thickness and freedom from scratches and that sort of thing.
Thanks for the input on thick seals. The window is rectangular (1.5m x
0=2E65m) so I guess that the calculation should be made for the longer side (if it is good for that will be also good for the other...) From the datasheet of Loctite Hysol 1C I have found a value for Lap Shear Strength, ISO 4587 of 14.5 N/mm=B2 , is this the right one to use? Concerning the suggestion to use elastometers (Viton?) do they let water through like many other polymers?
"dr.bakhshi" wrote in news: snipped-for-privacy@v33g2000cwv.googlegroups.com:
Dymax has >1000 different formulations, or could develop something custom for your application. I'm merely pointing the way to a resource which might be of use. You are certainly free to use it, or not, as you see fit. I'd suggest a phone call to their Technical Support group, and talk about your app with someone who knows the Dymax line better than me...
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