Re: Tyre Pressure.
Tom Anderson
> Simon Brooke <stil...@googlemail.com> wrote:
>
>> On 29 June, 06:55, Ian Smith <i...@astounding.org.uk> wrote:
>>
>>> OK. Modelling balloons selected (because they have higher surface
>>> area : volume ratio and work at higher pressure than 'normal'
>>> balloons). Two different manufacturers. One of each inflated with
>>> CO2 from a cylinder, one with air from track pump.
>>>
>>> http://www.astounding.org.uk/ian/temp/balloon1.jpg
>>>
>>> About two hours later:
>>>
>>> http://www.astounding.org.uk/ian/temp/balloon2.jpg
>>
>> Interesting. That confirms what I have been told about inflating tyres
>> with CO2. But if, as someone claimed upthread (and seems intuitively
>> reasonable to me), CO2 molecules are actually bigger/heavier than
>> average air molecules, what is the physics of this?
>
> The CO2 dissolves (absorbs?) in rubber far more effectively than O2 or
> N2. You end up with a saturated solution of CO2 in rubber & the
> dissolved CO2 can then evaporate from the outer surface of the balloon
> as easily as it does from the inside.
>
> As to *why* CO2 dissolves in rubber more easily than O2 or N2, I've no
> idea!
My guess would be that it's because there's an intramolecular dipole
(which i think might be a quadrupole) between the carbon atom and the
oxygens, which allows it to make dipole-dipole interactions with dipoles
in the rubber. O2 and N2 are symmetric, so don't have a dipole.
The only problem with this guess is that there isn't much in the way of
dipoles in butyl rubber, it being all carbon and hydrogen, which are
roughly equally electronegative.
So, crossposted to sci.chem, where my guess will take a beating.
Sci.chemmers - the backstory here is that tyres inflated with CO2 go flat
faster than those inflated with air, and the working assumption, supported
by Ian's balloon experiment above, is that this is because CO2 permeates
through rubber faster than N2 and O2. We wonder why that's the case.
tom
--
We must perform a quirkafleeg
dlzc
On Jun 29, 1:02 pm, Tom Anderson <t...@urchin.earth.li> wrote:
...
> > As to *why* CO2 dissolves in rubber more easily than
> > O2 or N2, I've no idea!
Keep in mind that CO2 "dissolves" in water much differently than other
gases, and rubber matrices always have some water in them...
> My guess would be that it's because there's an
> intramolecular dipole (which i think might be a
> quadrupole) between the carbon atom and the
> oxygens,
No. CO2 has no dipole (in its "ground state").
> which allows it to make dipole-dipole interactions
> with dipoles in the rubber. O2 and N2 are
> symmetric, so don't have a dipole.
>
> The only problem with this guess is that there isn't
> much in the way of dipoles in butyl rubber, it being
> all carbon and hydrogen, which are roughly equally
> electronegative.
>
> So, crossposted to sci.chem, where my guess
> will take a beating. Sci.chemmers - the backstory
> here is that tyres inflated with CO2 go flat faster
> than those inflated with air, and the working
> assumption, supported by Ian's balloon experiment
> above, is that this is because CO2 permeates
> through rubber faster than N2 and O2. We wonder
> why that's the case.
http://www.bikeforums.net/archive/index.php/t-154847.html
... which leads to:
http://www.madsci.org/posts/archives/may98/895552329.Ch.r.html
... "why" questions don't fare well in science. "How" and "That" it
diffuses out faster is well documented. Even dicussed this same topic
here (sci.chem) about 6 months ago...
David A. Smith
Roger Thorpe
then ammonia (NH3) is a reasonable candidate. I'll see if I can find
some balloons and a ventilated space!
--
Roger Thorpe
Standing on a golf course, dressed in PVC.....
Marvin
Keep in mind that tires are made up of more than rubber.
There is quite a lot of carbon black there to protect the
rubber from oxidation. Perhaps the carbon reduces the CO2
to CO?
Phil Armstrong
> Keep in mind that tires are made up of more than rubber.
> There is quite a lot of carbon black there to protect the
> rubber from oxidation. Perhaps the carbon reduces the CO2
> to CO?
It seems unlikely that ordinary latex balloons contain much carbon
black.
Phil
--
http://www.kantaka.co.uk/ .oOo. public key: http://www.kantaka.co.uk/gpg.txt