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Gas impermeable plastics suitable for ultrasonic welding
- Thread starter dbrando
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btrueblood
Mechanical
Mylar might work. Note that permeability is as much a function of how the product is produced and its form as it is about the base polymer.
Butyl rubber is not a plastic, but has very low gas permeability.
Matweb.com has gas permeability coefficients listed for a variety of polymers, typically films. Some films are less permeable to polar molecules, and vice versa, and thus you may find that a multi-layer composition would work best, depending on the mix of gases you are trying to seal.
Butyl rubber is not a plastic, but has very low gas permeability.
Matweb.com has gas permeability coefficients listed for a variety of polymers, typically films. Some films are less permeable to polar molecules, and vice versa, and thus you may find that a multi-layer composition would work best, depending on the mix of gases you are trying to seal.
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- #4
Compositepro
Chemical
No polymers are "impermeable". The question is how much permeability can you tolerate. Mylar is a brand name for DuPont Polyester films. The films are usually highly oriented so heating them to the melt point will usually result in tearing or very weak seams
And, how much pressure are you going to seal, and whether the rest of your system is up to the task of sealing in pressurized nitrogen?
We once had an interesting discussion, during a design review, about whether we needed a bladder, a pressure relief valve, or some such, to handle the pressure changes from temperature in the pressurized dry nitrogen we were putting into a test instrument. When the system was actually built, the first time we purged the system, it was so leaky that the pressure drop to ambient in about 30 seconds.
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Need help writing a question or understanding a reply? forum1529
Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
We once had an interesting discussion, during a design review, about whether we needed a bladder, a pressure relief valve, or some such, to handle the pressure changes from temperature in the pressurized dry nitrogen we were putting into a test instrument. When the system was actually built, the first time we purged the system, it was so leaky that the pressure drop to ambient in about 30 seconds.
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Need help writing a question or understanding a reply? forum1529
Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
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The pressure of the sealed nitrogen will be 22 psi at room temp and the max operating temp of the part will be 150C. We are essentially attempting to change from using a metal can and epoxy too acheive a hermetic seal, to using a ultrasonically welded plastic container. That being said, the application is in automotive so the permeability of the pastic will need to be low enough so the nitrogen will not leak for the life of the part.
Nylons ultrasonically weld pretty well and are compatible with automotive environment. There are about a million grades, fillers, additives, etc. Permeation is a function of permeability, area, thickness & time. You need to specify how much permeation you can stand. How long is the life of the part? How much of the nitrogen can leak out?
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Note that adding a high aspect ratio grade of a platy filler can help your barrier properties. Look for a the highest aspect ratio mica you can find. Mica filled polymer grades are commercially available.
Chris DeArmitt PhD
President - Phantom Plastics LLC
Consulting, ideas and training on plastic materials
Chris DeArmitt PhD
President - Phantom Plastics LLC
Consulting, ideas and training on plastic materials
Compositepro
Chemical
A 150C polymer pressure vessel?! Very unlikely.
btrueblood
Mechanical
"Would Mylar be conducive with ultrasonic welding? "
I think you can ultrasonically weld the films and have them hold up okay, but as CP said, I think these are multi-layered films where an inner bondable layer for welding is included in the construction. The films are produced with the idea that impermeable bags be made from them, for e.g. food products and/or medical products where oxygen spoilage or contamination with water vapor are issues.
"Is it possible to mold mylar?"
Getting low permeability from a molded part is more of a crapshoot, for reasons the real experts above have noted. It is possible to add metallization, and other barrier coatings, which can affect permeability.
"the permeability of the pastic will need to be low enough so the nitrogen will not leak for the life of the part. "
That is a non starter, in my opinion, but you don't state what the "life of the part is" - I'm assuming something more than 1 year. It might be possible with a fairly heavy metallized polymer, multi-layer construction. That said, I'm not certain that even the metal can with epoxy seal would hold up for multiple years - has that ever been tested? I.e. if this is an arm-wave goal and not a hard spec., and the real requirement is that the unit simply function well over the years, then maybe.
What is the real reason for the nitrogen blanket - is it to prevent moisture intrusion? Oxygen intrusion? There are absorbers or "getters" for these contaminants, which if included inside the capsule could help.
I think you can ultrasonically weld the films and have them hold up okay, but as CP said, I think these are multi-layered films where an inner bondable layer for welding is included in the construction. The films are produced with the idea that impermeable bags be made from them, for e.g. food products and/or medical products where oxygen spoilage or contamination with water vapor are issues.
"Is it possible to mold mylar?"
Getting low permeability from a molded part is more of a crapshoot, for reasons the real experts above have noted. It is possible to add metallization, and other barrier coatings, which can affect permeability.
"the permeability of the pastic will need to be low enough so the nitrogen will not leak for the life of the part. "
That is a non starter, in my opinion, but you don't state what the "life of the part is" - I'm assuming something more than 1 year. It might be possible with a fairly heavy metallized polymer, multi-layer construction. That said, I'm not certain that even the metal can with epoxy seal would hold up for multiple years - has that ever been tested? I.e. if this is an arm-wave goal and not a hard spec., and the real requirement is that the unit simply function well over the years, then maybe.
What is the real reason for the nitrogen blanket - is it to prevent moisture intrusion? Oxygen intrusion? There are absorbers or "getters" for these contaminants, which if included inside the capsule could help.
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- #12
Thank you all for the help. We would like the shelf life of the part to be around 10 to 15 years, however we have yet to define an exceptable amount of nitrogen leak in our spec. The nitrogen within the part is primarily used as a dielectric and arc extinguisher. The main spec involved with the nitrogen is that the voltage must be able to be ramped to 12kV without arcing across the contacts. If this spec is not met, it is assumed the nitrogen level within the part is too low. That being said, the execptable amount of nitrogen leak would still always have to pass this 12kV test.
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