CO2 in tires

SUMMARY: CO2 is *not* bad for your tires. It *does* leak out 11x faster than air, assuming your leaks are not just from the valve.

DETAILS: I heard it said to never fill your tires with CO2 because it’s bad for them. “You should deflate your tires and reinflate them with air when you can.” I wasn’t sure, but I could imagine possibilities, since CO2 is used for oil well secondary reclamation. Maybe it did something to the rubber?

I did research, and many anecdotes, justifications, etc abound. Reasons stated include: “Degrades the rubber”, or “it’s a more dry gas”, or “CO2 forms carbonic acid with the moisture in the tire which degrades it”, or “CO2 expands so much that it will rupture your tire.”, or “Diffuses out of the tube faster”.

In researching actual scientific literature, most of this is completely false.
* Butyl rubber is very resistant to all atmospheric gasses, refined or not.(1) There is no degradation from using CO2.
(1)Encyclopedia Of Corrosion Technology edited by Philip A. Schweitzer, P.E.

* ALL refined gasses are very if not completely dry due to the procedure of separating them. Even so, a bike tire takes about 16g of gas to fill up(2). At most, air can contain 4.25% water vapor(3) (think Atlanta summers). That’s 680mg of water vapor. That’s not going to hurt a butyl tire because water is polar, and butyl is non-polar(1). Butyl rubber is used for radiator hoses, and last for decades under heat and pressure. Plus, water vapor permeates rubber 200 times faster than air (or liquid water 400x), so you don’t have to worry about it for very long.
(2) (3)Michael B. McElroy “The Atmospheric Environment” 2002 Princeton University Press p. 34 figure 4.3a

* Any carbonic acid, a) it would be a minute amount, and b) butyl rubber is resistant to carbonic acid up to 150F. (1)

* Over the entire operating temperature range of tires, you would see at most 2 psi difference between gasses(4). At the temperatures and pressures used for tires, no one will even notice the difference between these gasses.

There is truth in diffusion rates being higher.
* CO2 diffuses through rubber much faster than air(5). Hydrogen is the standard “1” at roughly 11L/m^3, gas. Nitrogen is 0.16. Air is 0.22. Argon, 0.26. Oxygen, 0.45. Helium, 0.65. Carbon Dioxide, 2.9. Ammonia, 8.0. Water Vapor 47-110, depending on thickness.
(5)Scientific Papers of the Bureau of Standards, Vol. 16, 327-362 (1920) Scientific Paper 387 (S387), “Permeability of Rubber to Gasses”, July 12, 1990.

* The relative diffusion rates between natural rubber and butyl rubber are as follows(6):
Helium = 27%, Hydrogen = 15%, Oxygen = 5.6%, Nitrogen = 5%, Air = 4.8% and CO2 = 4%.
(6)Exxon Chemical, quoted by Timco Rubber

* Combining these, Butyl lets through this many litres per m^3: CO2 = 1.276L; Air = 0.116L; Nitrogen = .088L.

While butyl rubber (IIR) is less permeable than natural rubber by far, CO2 diffuses through rubber 11x faster than air. If you have to air up your tires every 9 days, then expect CO2 to last you a day. If you have to air up your tires daily (latex tubes), then expect CO2 to last you a few hours.

On a related note, as you keep airing up the same tube, eventually it becomes higher concentration of Nitrogen, because all of the other atmospheric gasses diffuse out of the tube at a faster rate.(5)

Note that Nitrogen is 3/4 of Air(7)(8), and is within 3% of the same Gas Expansion Constant(9). Really hot rubber, has a higher permeability than cold rubber(5). So it is easy to see using Nitrogen in tires is not because of expansion differences, but because it leaks out more slowly.


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