CO2 leaks out faster, a LOT faster!

Jon's message above got me to wondering HOW this is possible. After all, since a CO2 molecule is larger than either an oxygen -or- a nitrogen molecule, how could it leak out of a bicycle tire faster? : So with a little digging, I soon found the answer...:swoon:

The rate of loss
Dear Lennard,
Since a CO2 molecule is larger than either an oxygen or nitrogen molecule, why does it leak out of a bicycle tire faster?
Glenn

Dear Glenn,

Upon receiving your question, I put CO2 in a clincher tire with a Michelin butyl inner tube (latex tubes leak air quickly, as you’re probably aware). This particular tire and tube hold air pressure faithfully for weeks on one of my road bikes without needing pumping. And sure enough, within three days after inflating with CO2 to 90psi, the pressure had dropped to 45psi.

Since then, it has not dropped significant pressure in a week. This probably explains why I’ve never noticed this phenomenon before, because, while I often use CO2 inflators, I only use them while mountain biking (I don’t have the patience to pump a fat tire on the trail with a mini pump, and I do with a road tire), and on a mountain bike, I never run pressures over 40psi.

Wanting a bit more detail than my own simple experiment, I consulted my local atmospheric chemist/bike nut Alan Hills.

Answer from Alan Hills
I’ve been told it was True by no less than (our local triathlon legend) Dave Scott. Wading through the web yields some insights on tire pressure loss from tires/tubes inflated with carbon dioxide (CO2) cartridges. Two polymers are used for bike tubes; latex rubber and butyl rubber (isobutylene rubber).

Butyl rubber dominates the market and is used for almost all tubeless tires and bike tubes as its permeability to air is incredibly low — butyl tubes have only 10 percent the leakage rates of natural latex rubber tubes.

 Permeation by diffusion predicts gas leakage rates proportional to the inverse of the square root of their molecular weights. Using air as a reference the predicted leakage rates for common gases are: helium 2.7, air 1.0, nitrogen 1.02, oxygen 0.95, argon 0.85, carbon dioxide 0.81. 

It turns out however that the leakage rate of CO2 is huge, and the reason is that it is actually soluble in butyl rubber and is thus not constrained to normal permeation loss, it can transfer straight through the bulk rubber resulting in severe tire pressure loss on the order of a single day. CO2 is not likely to be replaced by argon or other gases in refill cartridges, however, because CO2 is much more easily liquefied than other gases and can be contained in a moderate-pressure cartridge in a patch kit. An analogous cartridge holding N2 or argon (non-liquified gas) would be dangerous and would require a thick (and very heavy) steel-walled storage vessel. A reference dealing with CO2 transfer through latex rubber sheds light on the loss process.
​

Well, there you have it.
Lennard
​

source VeloNews Technical Q&A

 

The REALLY interesting thing is, I tripped across STAYFILL in my search for the rest of the story. It can keep bike tires inflated for over a year, lasts 100 times longer than CO2, and is Non-toxic, non-flammable, non-ozone depleting.

MTB Tires


Road Bike Tires

STAYFILL cannot leak out
Now for the evolution: STAYFILL can’t pass through the wall of the tire. We have tires in our lab that were filled 4 years ago and they still retain the same pressure today as the day they were filled. Because of STAYFILL's large molecular size, it cannot leak out.

STAYFILL is a drop in replacement for CO2
We designed STAYFILL so that it could be used with standard CO2 threaded cartridge dispensing valves. We’ve been asked if there is any problem mixing CO2 and STAYFILL together. The only problem is that the CO2 left in the tire will still leak out and this will require some topping off with more STAYFILL.
​

Cheap? No it's about $15/tire, but I never even knew it existed or just how fast CO2 leaks through butyl rubber tubes.
​

 

Apparently, per this article STAYFILL is mostly Nitrogen.

And a most excellent 1st comment just after the atricle:

So if I pump my tires with Air, and the non-Nitrogen molecules leak out more readily (O2, CO2, etc.); as I pump up my tires over time (say from 90psi to 115), won't the gas in my tires continually increase in N concentration over time?​

 

Just tripped across yet another interesting article about it...

From STAYFILL

The gas in the Stayfill cylinders is inert so not flammable, toxic, or corrosive; StayFill is a alternative to inflating tires with air, N2, or CO2. It’s a gas mixture we came up with some time ago for a fire sensor system that would hold 120-150 pound-force per square inch gauge (psig) in a plastic line for five years or more. After further thinking, we discovered it worked quite well in bicycle tires. I filled my mountain bike tires up about five years ago to 50 psig and it is still at 50 psig. Finally, we found a vendor to package it up in those cartridges.

We used the standard size cartridges used for CO2 because there are a lot of dispensers already out there.

It will probably take 2.5-3 of our 25-gram cartridges to fill a 26-inch by 2.1-inch mountain bike tire. You can top off the pressure with air. Using 12psig of air in conjunction with the StayFill will actually put the tire in equilibrium. Our new design that should be available in March and will be a larger cartridge with 115 grams of product. This will fill up to a 29-inch mountain bike tire or three road tires to 115psig.

We have one customer that is using it with the tire sealant Slime in New Mexico and the sealant is not drying out like it used to using air. This customer thinks they now have the ultimate tire.

With respect to the first part of that article, using propane (big air) in your rubber/latex based tires is like using Vaseline with your condoms; they don’t last long. Petroleum based products decompose organic substances like rubbers or latex material so I am not sure how long it would take for the propane to begin showing signs of decomposing the inner tire wall.

Here is the link to the material safety data sheet for propane by Air Liquide. Down near the bottom they discuss propane’s use with hydrocarbon lubricants and say, “Non recommended, significant loss of mass by extraction or chemical reaction.”

Propane may not be a lubricant per se, but it is definitely a hydrocarbon. Special rubber compounds can be made to be compatible with hydrocarbons, but it does not strike me as a specification a tire manufacturer would have chosen when they selected the tire materials. We have a division that makes compressed natural gas transports and they ran into a major problem a while back using a rubberized hose with natural gas, which typically has about 8-10 percent propane. The hose began disintegrating after a few months of use under pressure.

Michael Koonce
IGX group, Inc.
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“Proprietary?” Oh really?

Dear Lennard,

Regarding your VeloNews.com article on tire gas that mentioned Stayfill and it being proprietary. The mention of large molecules and special tire gas reminded me of the Ferrari claims of a “proprietary” gas fill used in their F1 car tires and which was part of the McLaren spying saga in 2007. The real story there is that the “secret” gas “formula”, which contains three different hydrofluorocarbon molecules, is in fact identical to a readily available off-the shelf DuPont refrigerant called SUVA 404A (HP62), see DuPont for info and a Materials Safety Data Sheet.

If not the same stuff, Stayfill has to be a similar refrigerant gas mix — there are a very limited number of (ambient temperature) gases with large molecules. Ferrari, for their purpose (lower tire contact temperatures), found that a mixture of Suva 404A and CO2 was best (sorry don’t recall the exact proportions but I recall 80/20 being tried). Of course Stayfill’s real marketing coup is in the packaging, though it’s always possible they’ve mixed two different SUVA product mixtures just to be able to say “proprietary.”

I’ve also been told that Suva 404A had been used in MotoGP and Daytona Superbike racing at least 2 years before Ferrari got the umm, idea.

George

above from VeloNews