High Vacuum Sealing a Pouch

Hi John

Thanks for your reply.

After a period of time there is some minor loss of vacuum. The evidence suggests that whatever is causing the significant drop in vacuum is happening immediately the pouch is sealed. There is little evidence to suggest that any inserted material is out gassing and causing the loss of vacuum as the same happens when an stainless steel is inserted in the pouch.

The barrier material is high grade with very low permeability.

Yes, we are hunting for an RGA to detect the gases given off during the sealing process into the chamber with a view of designing a getter for them.

DT

as the same happens when an stainless steel is inserted in the pouch.

Did you mean to say "as the same happens when no stainless steel is inserted in the pouch."?

If not, have you tried this?

While I don't have a huge amount of experience with vacuum systems, my guess is that the plasticizer is coming out of the plastic. The main purpose of the plasticizer is to keep the plastic from becoming brittle, so a little is helpful for the long run. To get rid of it, you could try putting the bag in a vacuum oven at as high of a temp as you dare for a few hours. I would guess that high vacuum for 2 hours at 120C would greatly reduce your problems.

I had though it to be out gassing so I left the complete package baking out over 24 hrs at high vacuum with the same results.

I get the impression that your sealing process is outgassing something from the pouch material.

I'm no vacuum-head either, but I'm interested in your use of polyethylene... On a molecular/atomic scale, polyethylene is very porous. In fact, I was experimenting with batteries a while ago, and I was looking for a material that would keep the liquids separate, but would allow the ions to pass freely. Turned out polyethylene was the best material I could find for doing this.

So is it possible that polyethylene works as a vacuum barrier up to a point, but is too porous for the vacuum you're trying to maintain?

Hi, that is an interesting thought. Are you suggesting the seal would rapidly leak until the internal pressure had reached an equilibrium at about 1mbar?

Did you also consider Polyethylene terephthalate for your purpose which is also commonly used as a sealing layer in barrier film structures although it melts at 175C rather than Polyethylene's 135C.

Do you think Polyethylene terephthalate could offer a more robust barrier at this level of vacuum?

DT

Most of the vacuum sealed pouches I've seen are aluminized Mylar (aka Polyethylene terephthalate). This is what was used for the old passive communication satellite Echo. To not leak gas for years in space is a testament to how good of a seal one can get. That says nothing about out gassing though.

Hi Redfred

Mylar is indeed one of the materials we are using in this application.

DT

Just a question... By heat treating the stainless, aren't you running a risk of changing its characteristics?

Depending on the material specification a metallurgist will be able to answerer this sensible question.

Most likely an atmospherically controlled heat treatment should take care of this concern.

When we use stainless in vacuum applications we stove the parts at 1000°C for 1 hour in a reducing nitrogen atmosphere to ensure that no outgassing occurs later.

Hi Krishnan

Thanks for your reply, the stainless steel is a bit of a side track to the issue, I only used stainless steel as a control it is not part of our process, and I believed it to be fairly inert at 10-3 to 10x-8 mbar. At a greater vacuum I can appreciate the need to bake out as you describe.

Trying to evacuate an 'empty' pouch results in the same anomaly I described in the original post - we cannot seal below about 1 mbar.

DT

Presumably this is to keep the components clean until used. The alternative would be to purge the bag with components in using dry nitrogen then seal with the bag slightly inflated with nitrogen.

Thanks RHABE

Some useful suggestions there.

My barrier material supplier is a specialist and assures me this is the correct material but they normally use it to seal at 1mbar not 10-3mbar. The aluminum-oxide is coated on layers within laminated polyester and 'should' not be exposed to oxidation but I will check with them.

We do try and keep the material clean, handling with latex gloves.

We tried baking out the packages at vacuum over night - they will only sustain a temperature of 50C. There was no noticeable change and it was this that rather ruled out outgassing from the component parts in my mind.

I am already looking for a cheap RGA - if any one has one available (2 1/4 inch con-flat flange) let me know!

DT

In a different context, we had problems with oligamers (lower molecular weight polyester) on the surface of finished biomedical implants creating tissue overgrowth. Extracting the oligamers in a Soxhlet extractor removed the anomaly and has been introduced into the process. It is likely that a similar situation may exist in this case as well.

Hi,

"aluminum-oxide" is a whole bunch of different materials with many modifications and nearly all will absorb large amounts of water.

Only the material deposited as thin-film in high vacuum by evaporation or sputtering or other plasma deposition is nearly non-absorbing (and thus also desorbing moisture).

So try a material with pure aluminum as the barrier, this will be much better.

I have two RGAs for sale, one was in continuous operation from 1998 to 2003 (Balzers), then only in storage, may need new filaments and cleaning at filament site. The other one was purchased as "used, in working condition, but displays: permanent fatal error in the RS232 data transmission, MKS make. Location: Germany. Price to be discussed.

RHABE

I agree with Vermin. I think you need to change materials. I'm not sure what your requirements are but I would use a Buna or Viton material, these "rubbers" perform much better under vacuum and are used in almost all vacuum pumps. Also how are you sealing the pouches? after the pouch is sealed how are you checking the vacuum inside? The actual sealing process could cause a poor vacuum reading depending on how your getting the reading in the first place.

We are an engineering company marketing vacuum pumps and systems for variety of applications. I will review your requirement and respond.

I would like to have schematic of your process so that I can help you resolve your difficulty.

Best regards,

Hari Rambhia

Avni Enterprises
6/306, Jogani Industrial Complex,
V. N. Purav Marg, Chunabhatti,
Mumbai - 400 022, Maharashtra, India

Tel : 0091 22 3296 8812
Fax : 0091 22 2405 6329
e-mail : avnient@vsnl.com

I don't know anything about this sort of thing so please forgive (and ignore) me if I'm being stupid.

Is it possible to carry out an experiment something like this.

1.) Insert a wire or something similar in the "mouth" of the pouch.
2.) Reduce the vacuum to 5 x 10^-3 mbar.
3.) Seal the pouch as shown:-

4.) Remove the wire, and, reduce the vacuum to 5 x 10-3 mbar for a while (with the pouch flat).
5.) Make the second seal as shown:-

6.) Remove vacuum.
7.) Re-apply the vacuum, and, check result.

I'm simply trying to reduce the outgassing caused at the edge of the heat sealing process to a minimum.