low Water vapor transmission rate (MVTR) material

[Mike666]

hi,

I need to design a box that should be totally sealed from water and water vapor.

Although i know that the best materials for that purpose is metals
(for example aluminum (MVTR=0.001 g/m^2/day))it can't be a metal.

The solution I was thinking about was Ceramic ?
Or maybe a plastic with very low MVTR?

What material has the lowest MVTR except metals?
Can I build a composite box from fiberglass (that will give the strength) with a sealing laer made of ceramic material?

thanks

 

[kenvlach]

Hi Mike,
Unless there are special high-temperature requirements, I'd avoid ceramics: brittle, heavy, difficult to form/fabricate, porous (unless glazed or 100% dense).
The usual materials for dessicators are Pyrex glass or acrylic (PMMA = Polymethyl methacrylate, aka PlexiGlass).

Salt spray cabinets and humidity chambers are often made of polypropylene. Windows in chambers can be glass or the lighter weight PMMA).

Clear acrylic and rigid PVC are available in sheet and plate and are easily solvent welded. Polypropylene is heat welded. You can form bends after heating any of these. CPVC is like PVC, but higher temperature limit & a bit better chemical resistance.

As to gaskets for door & windows (if used), look up the gas permeability constants for nrubbers, natural and sythetic, p.10.66 to 10.69 in Lange's Handbook of Chemistry, 14th edn. or elsewhere. Poly(trifluoroethylene) and PVC )soft kind) look pretty good, silicone rubber is terrible (like a sieve for water vapor).

Before engineering anything, take a look at existing equipment closest to your needs (environmental chamber, dessicators, humidity cabinets, salt spray chambers, etc.). I've bought some used stuff from these people:

http://www.lre.com/index-1.htm

Also, check how Water Vapor Transmission Rate (WVTR) is measured per ASTM F1249 at

http://www.ides.com/property_descriptions/ASTMF1249.asp

MIL-STD-810F ENVIRONMENTAL ENGINEERING CONSIDERATIONS AND LABORATORY TESTS may be useful (but it's too long for me to read w/o more purpose!)

http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=35978

Why no metal allowed?

 

[Compositepro]

"Totally sealed" is a meaningless term. When you define what you require you will probably automatically have the answer to your question. Polymers are poor barriers compared to glass or metal but if the material is thick the barrier properties can be more than adequate. A desicant packet can take care of a little permeability for a long time. The barrier in fiberglass composite is polymer but the permeability is usually much higher than pure polymer because of microcracks and porosity. Aluminum foil is a great barrier in this case. I suspect that for your application a Pelican case with some dessicant is what you need.

 

[MechanicalChef]

Out of curiousity, why are we excluding metals?

 

[Joshmo]

I don't see using composites for this design since they have horrible moisture tansmission rates unless a co-cured metallic foil is used.

yes ceramics and metals have the lowest moisture transmission rates....but metals are not transparent to certain radio transmission frequencies which is why perhaps you cannot use them. Ceramics on the other hand are excellent in regards to mositure but are very heavy and hard to manufacture with (as well as many other issues). If you do need to design with ceramics, perhaps a simple slip cast process could be used.

Joshmo

 

[Prabbit]

One way to seal a composite structure is the use of Tedlar (Brand Name). An imepervious film that prevents moisture ingress. This is used extensively in the lower decks of comercial planes. In this region requirements are
Smoke density toxcity
Flammability requirements
Flame through requirements
Etc

In order to meet these requirements phenolics are used