CO2-Permeable Oxygen-Blocker Wanted

If I could guess I would say...

Chlorophyl

Plankton

Plant leaves absorb CO2 and release oxygen.

What is the mixture makeup (Wt%O2 / Wt%CO2 / Wt%Other)?

What are the flow rates you are trying to achieve?

Have you thought of other ways to do the separation?

Thanks for your reply. A package contains a food product that, over time, produces CO2. Oxygen causes the product to loose flavor. The material has to be incorporated into the wall of the package, allow CO2 to pass thru, and prevent Oxygen from entering the package. If the package experiences a slight pressure difference due to shipping by road, or air; Oxygen shouldn't enter the package. Elastomeric seals of any kind are prohibited. Sincerely,

I'm no chemist but it strikes me as difficult to consider permeation when the molecule you want to keep out has a lower atomic weight than the one you want to escape... Have you considered a CO2 absorbant material (dessicant) that stays within the package?

Guest,

I am a chemist, and you got it right. The CO2 molecule is larger than the O2 molecule, so it is difficult to imagine a membrane that allows the larger molecule through but blocks the smaller one. A proven approach would use instead a chemical reaction to remove CO2, for example the reaction of sodium oxide with carbon dioxide produces a thermally stable solid sodium carbonate:

Na2O + CO2 --> Na2CO3

So I would suggest incorporating sodium oxide (or something similar) into a plastic film that is impermeable to all gases. No oxygen gets in, and the doped film removes the CO2. I have no idea if anyine has already attempted this. If not, and if it works, sounds like a good idea for a patent.

Yes, but the material is corrosive. I intend to contact one of the manufacturers. Thanks for your response

Hasn't this already been done? I seem to remember that there is a special type of plastic that was designed specifically to stop the spoilage of foodstuffs by out-gassing byproducts. I don't know the name or who manufactures it but I was under the impression that it has been in production and use for some time.

Typar, Tyvek, etc, Dupont cie could help. those paper allows moisture out, but not in.

Can you freeze the product so it doesnt create co2 ? If you add chemicals to food packaging, you may have a surprise in the taste of it. Vaccum bag it ?

not so. These materials allow moisture vapor in or out. They simply block water vapor fom coming in.

Tad

"not so. These materials allow moisture vapor in or out. They simply block water vapor fom coming in."

What is the difference between "moisture vapor" and "water vapor"?

sorry, meant to say vapor goes in and out, but liquid water is blocked

Despite the comments above, membrane material are routinely used to selectively pass heavier gases, while retaining lighter ones. Pemeration through solid membranes is not based on molecular weight or size. Permeation is based on the gas solubility in the membrane material. The gas essentially "dissolves" into the membrane on one side and evaporates on the other. So, in many cases higher molecular weight compounds are much more permeable than lower weight compounds.

Look at http://www.sspinc.com/membranes_spec.cfm for permeabilities of two types of membrane. Note that the permeation rate for CO2 is about 5X that of oxygen, (and higher than H2 or He). Look at the m213 material at the end of the chart. It is much less permeable to either CO2 or O2, but has a somewhat higher CO2/O2 permeation ratio.

That being said, you are fighting the rule of partial pressures. In a static system that you describe, eventualy both sides of the package will come to equilibrium with the outside air, although this will take some time.

Since air has about 400 ppm of CO2, the packaging will only pemeate out levels above 400 ppm of CO2 taht build up on the inside. As for O2, well air is about 19%, so even though it's permeation rate is low, it will slowly enter the package.

Good luck!

"Despite the comments above, membrane material are routinely used to selectively pass heavier gases, while retaining lighter ones. Pemeration through solid membranes is not based on molecular weight or size."

"That being said, you are fighting the rule of partial pressures. In a static system that you describe, eventualy both sides of the package will come to equilibrium with the outside air"

Tad's first statement is partially correct. Separation of gases via membranes can rely on adsorption/desorption mechanisms to favor the passage of larger molecules (but as he pointed out, this selectivity fails if the system reaches equilibrium). But membranes sometimes do use size effects to pass smaller molecules while blocking larger ones (and this approach remains effective even at equilibrium). Unfortunately, size-exclusion membranes will not help with Guest's goal since O2 is smaller in size than CO2.

Tad's second statement above is correct. At *equilibrium* a semi-permeable membrane will not be able to both remove CO2 from inside the package, and prevent O2 from diffusing into the package. As I stated before, a chemical reaction will be needed to accomplish this goal.

It is true that there are size exclusion membranes, but these operate by diffusion, not permeation. That is, there are tiny holes in size exclusion membranes, and so are not solid. This is why I said permeation through solid membranes. So, I think my first statement is correct as stated (unless you disagree with my nomenclature).

Actually, even permeation membranes have some diffusion component mechanism, because is anything completly solid? For instance, I would suspect that the "permeation" rate stated for He and H2 are actually due more to diffusion than solvation in the membrane.

All in good fun :)

Tad