Originally Posted by
jemoryl
There is a lot of false information being thrown around here. CO2 is not a 'smaller' molecule than N2, it simply diffuses through butyl rubber at a faster rate due to various subtle interactions.
The reason why CO2 is used in a cartridge is simple: when compressed to a certain pressure at room temperature it condenses into a liquid - this is what is present inside your little cylinder. Air (or nitrogen) would still be a gas at the same pressure/temperature; since a liquid is more dense than a gas, you can get 12 or 16 grams of CO2 in a small package. You could not do this were the substance to remain gaseous. When expanded to a lower pressure the liquid changes into a gas to inflate the tire (and this absorbs heat, which is why the inflator gets cold).
If you have taken chemistry or physics you may have been exposed to 'phase diagrams' for various substances: these show the state of the substance as a function of various variables (e.g. pressure, temperatue, etc.). CO2 is also common, non-toxic and will not harm rubber.
jemoryl has it right; the CO2 is stored as a liquid at high pressure. The other issue is that liquid CO2 is a commodity (and by the way there's a shortage of high purity CO2 in the West). It's used for carbonated drinks, beer, etc. It's stored in large tanks (~250 tons) at -17 degrees F and at 220 psig. Since your cartridges are at ambient temperature the pressure in the cartridge is significantly higher. This is similar to the concept of the propane tanks in your BBQ. It's a liquid in the tank under pressure. When you release the pressure by opening the valve, it turns into a gas.
Cliff