Ultra Low Vacuum Adhesive

[gwolf2]

I am trying to find a sealing material or an adhesive which has a permeability about 10x lower than butyl rubber. I am trying to hold a vacuum of 10-4 mbar/torre for 1-5 years and I know from my geometry that butyl rubber won't do the trick. 10-4 torre is about as good as you will get without installing additional absorbers for the lighter gases which pass through metal.

I made an extensive product search 2 years ago and came up with nothing. I thought that I would try again now as new products appear quite often. Here are my requirements.

Either:

A pre-formable seal material with strength and stiffness similar to rubber which will fit between a well-prepared steel or aluminium frame and toughened glass plate.

Or:

1) An adhesive which will bond aluminium or steel to glass.
2) Has a strength similar to common epoxy ie around 10MPa
3) Can be outgassed at a temperature less than 50C, and/or by storage under vacuum prior to final charging.

I realise that this material probably doesn't exist yet but it's worth asking from time to time. I know of one newish polymer which will go to 2-3x better than butyl rubber but that still isn't enough, I need 10-20x better.

Regards,

gwolf

 

[gwolf2]

p.s. It can't be too expensive. I looked at indium and it's too expensive and rare for my application.

 

[btrueblood]

gwolf,

How close do novolac and standard epoxies get you, and have you tried the various fillers for these.

Can you name the new-ish polymer?

What gas(es) are you trying to form a barrier to?

An approach a colleague used, to some limited success, was multiple layers of alumized kapton film, not sure if the approach will work for you or not, not knowing your geometry.

Good luck!

 

[CoryPad]

If an elastomeric seal (i.e. similar to butyl rubber) could work, then have you tried engineering elastomers? Fluorinated ones from DuPont (Viton), 3M (Dyneon) or Solvay Solexis (Technoflon)?

 

[unclesyd]

Take a look at the white paper (June 8, 2009) on this page concerning the Low Volatility Silicone adhesive developed for space applications

http://www.nusil.com/index.aspx

If you can use an o-ring type seal you might want to check out the seals by Helicoflex. I've used these seals in numerous application for both pressure and vacuum applications.

http://www.helicoflex.com/ViewProductMain

 

[jedward]

If your geometry is "tight", meaning a small flat gap that need to be closed, you could consider using FEP teflon sheet compressed with a moderate knife edge. I used this for high vac cryo applications for a glass to metal seal. It should work at higher temperatures to over 100C

 

[gwolf2]

Thanks a lot. I'll digest this at the weekend. I'll add this intermediate post based on your responses to maybe gather a few more ideas.

The gas I am trying to keep out is air. I do not want to try keeping out the light gas (i think it's helium, I don't have my notes with me) which will diffuse through even metal and render even the best seal only partially effective. I know that I can use a lump of stuff inside which will absorb the gas which leaks through, but unless you can get a huge surface area by spraying this material on, you will not get a very effective mop-up. The spray process is very expensive, patented, and would blow the economics of the device out of the water.

I forget the name of the newish elastomer which I found because it was good but not good enough. It was some sort of particle impregnated polymer. The particles block the path of gas molecules through the material by making the gaps too small as I understand it.

Epoxies with similar particles seemed to give superior gas blocking properties but suffered from outgassing after cure. You had to cook the parts at about 140C for 24 hours to drive the remaining volatiles out of the epoxy. With my geometry (which I can't disclose too much about) the components are large and made of aluminium and glass. FE models which I have run suggest that the cooking process would generate stresses high enough to crack the epoxy.

I have to watch high temperatures for welding type processes because the glass is toughened and anything too hot will destroy the toughening.

What I can say of the interface is that it is flat, square, 1mx1m ish, glass plate, aluminium or steel, seal all around, probably 1-3cm wide seal by 1-2mm thick if polymer, 0.5mm thick if adhesive.

I take note of some of the material suggestions already made.

*ANY ADVICE WITH PERMEABILITY AND DIFFUSION EQUATIONS GREATLY APPRECIATED. I have some simple equations from corporate web sites like DOW, 3M etc but it is often difficult to equate stated properties on the web sites with the actual process which I am simulating. I suspect much of the data is confidential.


Thanks again, gwolf

 

[btrueblood]

Gwolf,

There are some formulations of epoxies that can be outgassed at lower temperatures, I think - but it sounds like you are going to the right sources anyway (try Master Bond too), so you might be correct about low-perm epoxies needing a high bakeout.

Filled butyl is going to be tough to beat for your application. You should talk directly to a seal supplier about what material choices you have, possibly speak to a rubber compounder directly.

The permeability numbers you are looking at can be confusing, depending on if you are looking at film barrier material (think sheets of plastic wrap) versus a sealant/caulk/elastomer like you are using. The film materials typically ignore the "per unit thickness" parameter in the diffusion equation for their quoted permeability, and simply infer that you are using one layer at whatever thickness they produce. Elastomer and sealant permeability data is tougher to come by, as the numbers will vary somewhat with the configuration of the joint. There is data in the Parker O-ring handbook, which I presume is what led you to the butyl rubbers.

Oh, and permeability numbers get screwier if they also ignore the pressure parameter, and/or use combinations of SI and other units.

Whatever you use, try and look at multiple seals (multiple layers that the gas has to penetrate). It is not intuitive, but I've seen multiple thin seals perform better than a single thicker seal for gas permeability. YMMV, of course.

FWIW to others reading this string of posts, silicone is about the worst polymer around for permeability. One of the reasons it makes lousy window and bathroom caulk - water vapor permeates right thru the stuff, and allows mold to form in the area behind the bead. Reasonable gas/air permeability can be found with nitrile and neoprene rubbers as well as butyl, and there are more varieties of nitrile around, so your odds of picking one at random to work reasonably well are higher.

 

[unclesyd]

You might want to look at Reactive Solders for your application. I would give Adlera Technology a call with the specific details of your application.

http://www.adheratech.com/Home_Page.html

Here is the Fiber Optic Center where you can the solders. They also have a lineof epoxies.

http://focenter.com/script_0009.asp?tid=193

I have to disagree with the previous post as there are silicone adhesives and sealers going into space on every launch. I do agree that the silicone's from the local hardware leave some something to be desired as to longevity even though I have 2 large aquariums that were sealed with silicone 25 years ago and are still going strong.
If you get the standards mentioned in the white paper at Nusil all the definitions and units are defined. This data is available from all the major manufacturers of silicones.

 

[btrueblood]

Syd,

No, you are not reading that correctly. I said, and will still maintain, that silicone elastomers are very poor at sealing gases, due to their high permeability. This has nothing to do with their durability or suitability in high vacuum (e.g. outer space), but simply that gas will leak thru it much more rapidly (about 100 to 1000x faster) than thru butyl rubber and similar, lower-permeability rubbers.

 

[gwolf2]

Yes, silicone is rubbish at sealing. On the first prototype silicone sealant was used to get some seal, vacuum, and load into the structure. You could see the vacuum dropping at something like 1-20mbar/min even after careful re-sealing etc. I know it was only DIY silicone but it did prompt me to look up the industrial grades and they confirmed my low expectations.

The low temperature solders might be worth a look, I did check some of them out in the initial phase but none looked suitable.

Filled butyl looks interesting, and so by implication any filled neoprenes or nitryles - I didn't know that the rubbers could be particle filled like the harder nylon-like polymers.

I am also considering a two stage seal configuration.

Thanks again for your suggestions, I'll go hunting.

gwolf.

 

[jedward]

We have used DC6-1104 - a clear silicone- for use in our satellite equipment for years
NASA Publication 1124 lists the preferred material for space applications - and can be found on the web

 

[btrueblood]

Sigh.

 

[gwolf2]

jedward,

Please correct me if I am wrong, I would love your NASA silicone to work but I don't think it will. I need to keep air out, if the permeability raises my internal VACUUM from 10-4 mbar to just 1 mbar then my hard vacuum is completely destroyed.

On a space vehicle if I loose 1 mbar of internal PRESSURE to space by diffusion through the silicon then this is of no consequence to the internal space.

Am I right or is the NASA silicone the wonder material which I am looking for?

Still looking by the way, so many filled butyl rubber links turn up as pay per view scientific papers or patents. No companies found yet. It always takes ages..............

 

[unclesyd]

If your application is amenable to a two stage sealant you may want to look at some of the materials offered by SPI in particular #45, VacSeal II, with any other low volatility adhesive. SPI is a very good source of information.

http://www.2spi.com/search/?query=adhesives&start=5&search=1&results=10&type=and&domain=

 

[gwolf2]

unclesyd,

What if I used a conventional butyl seal, maybe cut it short on width to leave a gap to fill, applied a vacuum, then filled the gap with VacSeal?

gwolf.

 

[unclesyd]

We used it a similar fashion as a secondary seal. We also used it as a primary seal on lapped surfaces.

 

[gwolf2]

unclesyd

Interesting, can you remember what vacuum you were going down to and for how long?

 

[unclesyd]

One device we used this approach for was hooked to a helium mass spectrophotomer so it would have been in the range of 10^-8to 10^-9 torr for approximately 1000 hrs. During this time the system was backfilled several times with Hydrogen to 8-10 inchs water column.

 

[gwolf2]

Unclesyd - thanks, looks like the grade of material which I am looking for - I'll give it a try in conjunction with some other tweakes. Thanks again for the tip

gwolf.