Fun with Latex and Carbon Dioxide

 

In this post I had originally hoped to answer the question that I know has been on your mind lately:

Does CO2 leak out of a latex tube faster than air?

And I will answer that question. But as I was surfing for information, I came across so much interesting stuff that I’m going to wander all over my topic, like I usually do.

You all know that CO2 leaks out of a butyl tire tube faster than air, right? This is why after replacing a flat on the road and inflating with CO2, if you don’t bleed the CO2 and re-inflate with air, your tire goes soft overnight. Apparently:  “The permeability of a gas through rubber depends mainly on its diffusivity and solubility in rubber. CO2 has a significantly higher solubility in rubber than O2 and N2, whereas the diffusion coefficients differ not that much. The result is that carbon dioxide passes ordinary rubber about 5 times faster than oxygen and about 15 times faster than nitrogen[1, 2].”

https://chemistry.stackexchange.com/questions/54826/why-does-co2-diffuse-through-a-butyl-rubber-membrane-more-readily-than-air

The reference [2] seems to be no longer available, and reference [1] is a cool, very old government research document from 1920 on research performed by Edwards and Pickering for development of dirigible technology. Can you say “Hindenburg”? What exactly they mean by “rubber” is not clear. They say it is made from hydrocarbons (polyprene specifically), not from trees. So I’ll assume it’s something more like butyl than latex, but it can’t be true butyl since that was apparently invented in the 1930s by scientists Sparks and Thomas at the Standard Oil Company.

BTW did you know that according to Edwards and Pickering:

“Ammonia has been considered as a filling gas for balloons. [dirigibles] Its specific gravity is only 0.596, and it offers advantages from the standpoint of freedom from fire hazard and the fact that it can be transported in the liquid form. However, the fact that rubber is quite permeable to ammonia would necessitate the use of a different type of fabric for the balloon [dirigible] envelope.”

Well, at least ammonia’s not flammable.

Where was I? Oh yes, Edwards and Pickering reported that “rubber” is about 13 times more permeable to CO2 than to air. (What they actually reported was that permeability to CO2 is 2.9 times that to hydrogen and permeability to air is 0.22 times that to hydrogen. They were fixated on hydrogen, a popular notion which changed, I’ll bet, on May 6, 1937.)

This doesn’t shed any light on the question of gas permeation through a latex inner tube. For that I want to do some experimentation. I have a few latex inner tubes, lots of air, and a supply of high pressure CO2 from the kegerator. But filling one tire with air and another with CO2 and waiting for them to bleed down is boring and won’t make for very good pictures, so I plan to use party balloons initially. They’re made of latex and their size is a good indication of the internal pressure.  I’ll fill identical balloons, some with air and some with CO2, and monitor how fast they bleed down.

While I’m at it, I’ll test a product called HiFloat, “a patented liquid solution that dries inside latex helium-filled balloons to form a coating that helps hold in the helium”. I have no idea what it is, if it works on helium as advertised, or, and this is key, if it works on CO2 or air. You see where I’m going? If it works in latex balloons, maybe it works in latex inner tubes.

…in self-quarantine waiting for Fedex to deliver my Amazon order of 20 latex balloons and one 5 oz container of HiFloat (Prime one day shipping)…

While we wait, let’s discuss the concept of filling a tire with nitrogen instead of whole air. This became popular a few years back for auto tires. The idea is that tire rubber is less permeable to nitrogen than to whole air, so your tires will maintain proper pressure longer, improving gas mileage. This is also the idea behind tire pressure monitoring systems (TPMS) on motor vehicles, though there is an argument that TPMS has led to people ignoring their tires. Whatever.

From Edwards and Pickering, rubber permeability to nitrogen is 73% of the permeability to whole air. So your tires will bleed down more slowly if filled with nitrogen. I used to think that if I keep filling my tires with air, and the oxygen keeps leaking out preferentially (nitrogen permeability = 36% of oxygen permeability), after a few refills my tires would be filled with essentially nitrogen.

Apparently that’s not entirely accurate because the bleed rate is determined by partial pressure. At some point the partial pressure of O2 in the tire equals the partial pressure of O2 in the atmosphere. At that point no more O2 gets out. By way of example, the partial pressure of O2 in the air around us is about 3 psi (20% of 14.7 psi). At a tire pressure of 73 psi (88 psi absolute) 3.5% O2 would exhibit a partial pressure of 3 psi, same as the air outside the tire. I don’t know if any of this is valid. I could be doing the math all wrong, but it’s fun to think about.

…still waiting for Amazon…

Oh, and nitrogen is dry while compressed air contains moisture from the atmosphere, and that’s supposed to be another advantage of nitrogen. There’s a lot of commercial stuff on the internet about nitrogen in tires. Here’s the most useful objective resource I could find. The guy also has a lot of other fun stuff on his site, and a cool name – MojoTireTools.com.

…balloons arrived, HiFloat is delayed for a few weeks, gonna do CO2 and air in balloons with a followup post next month on HiFloat…

Ok, I finished my testing, and wow, that was satisfying. I could almost see the CO2-filled balloons deflating before my eyes!

In the interest of scientific integrity let me state that I used a hand pump to inflate the “air” balloons, so as not to introduce moisture or excess CO2 from exhalation.

Here are pictures, initial and at one hour, three hours, and seven hours:

And here are two latex tubes, one inflated with air and one with CO2 initial and 16 hours later.
Yes, I did two runs, swapping the tubes, and observed the same result on each run.

So, yes, CO2 leaks out of a latex tube faster than air.

Well, that was fun. I can’t wait to receive my HiFloat and have another balloon party.

Tailwinds

Killa