On 20 Sep 2006 06:43:32 -0700, "Dirtroadie" <
[email protected]>
wrote:
>I have poked around a bit and have not found any discussion of whether
>the slow air loss typical of latex tubes (or overstretched butyl) can
>be minimized by using a sealant such as "Stan's," "True Goo" or
>"Slime." I realize those are intended for air leaks caused by actual
>breaches in the tube (or tire) but am curious how the leakage due to
>porosity of an intact tube may be affected.
>Does anyone have any experience with this?
>
>DR
Dear DR,
Slime would probably have little effect on air loss in latex.
Overnight, most of the stuff pools at the bottom of the tube, so most
of the inside of the inner tube isn't coated with it. (As soon as you
pedal off, the wheelspin distributes the stuff around the inner tube
again.)
In any case, it's not porosity that causes latex tubes to lose air.
About 80% of the atmosphere is N2, 20% is 02, and a smidgen is CO2.
(Plus milli-smidgens of a few other rare gases.)
These gases are soluble in both butyl and latex rubber:
http://www.madsci.org/posts/archives/may98/895552329.Ch.r.html
As that short article explains, CO2 is about 15 times as chemically
soluble in rubber as N2. That's why tubes inflated with CO2 cartridges
lose "air" so quickly compared to tubes inflated with 80% N2 from a
hand pump.
Just as CO2 is much more soluble than N2, latex is much more
chemically soluble than butyl. So expensive latex tubes lose gases
much more quickly than ordinary butyl tubes. It's not a mechanical
matter of "pores" being larger in latex--it's a chemical process in
which latex is more reactive.
In the process, the gases are attracted to the rubber and are drawn
into it, between its atoms. There's less pressure on the other side of
the tube, so that's where the gas atoms come out of solution.
Eventually, the tire goes flat.
This is why party balloons filled with helium last longer when they
have that shiny metal foil coating. Neither the rubber nor the metal
have "pores" like the pores in our skin, and the size of the gas atoms
doesn't matter. It's just that the gases are chemically soluble in
rubber, but not in metal.
Cheers,
Carl Fogel