Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Martin Thorn on 19/03/2009 10:30:01

Title: Why does cold air keep balloons inflated?
Post by: Martin Thorn on 19/03/2009 10:30:01

Martin Thorn  asked the Naked Scientists:
   
Shortly before Christmas my daughter was given an inflated balloon in a shop and we left it in our unheated conservatory.  Two months later it had still not shrivelled nor deflated.

If we had kept it a heated room it would not have lasted little more than a week;

why does the cold air preserve the balloon please?

What do you think?

Title: Why does cold air keep balloons inflated?
Post by: daveshorts on 19/03/2009 10:49:36

Rubber is stretchey because it is made up of long polymer molecules which straighten out when you stretch them and then vibrations make them become wiggley as it shrinks again

see two kitchen sciences for how rubber works:
Heat shrink (http://www.thenakedscientists.com/HTML/content/kitchen-science/exp/heat-shrink/)
Rubber Fridge (http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/make-a-rubber-fridge/)

I have noticed something similar with rubber gloves recently, they are very stiff if kept below about 5C for a long time, but become flexible once you warm them up. If you cool rubber to very low temperatures it becomes very brittle and will shatter, as the molecules do not have enough energy to move past one another, but we are talking about something more subtle.

I have read about rubber crystalising very slowly at low temperatures when stretched - the long thin polymer molecules line up aproximately when you stretch the rubber. This means they can bond to one another forming a weak crystal which is fairly stable , so it doesn't  shrink again until you warm it up enough for the thermal vibrations to break the weak bonds in the crystal and then the rubber molecules can become more wiggley and shorter again.

Title: Why does cold air keep balloons inflated?
Post by: yor_on on 19/03/2009 13:30:12

Dave?
Are you saying that the primary reason is that the balloon under low temperature becomes rigid and therefore do not shrink? If one took that balloon into a heated room, will it 'shrink' as the balloons surface molecules looses their 'crystaline' structure, and as the balloons density inside it still will have became less?

Title: Why does cold air keep balloons inflated?
Post by: Vern on 19/03/2009 16:24:05

Dave's post seems reasonable but I have never seen the effect. If I glean correctly from the post, it means that the air inside the balloon may leak out and stabilize with the pressure of the air in the room, but if the balloon is no longer elastic, it keeps its form.

Title: Why does cold air keep balloons inflated?
Post by: syhprum on 19/03/2009 17:44:30

Toy balloons are normally filled with a mixture of expensive Helium and cheap Nitrogen with the proportions adjusted so that they will just float.
If the balloon was well inflated when kept cool although the small Helium molecules will have snuck thru the rubber it may well have retained most of the Nitrogen.
I think you will find it will no longer float.

Title: Why does cold air keep balloons inflated?
Post by: Don_1 on 20/03/2009 09:31:49

Could higher pressure in a warm room result in more molecules being forced out of the balloon than would escape under the lower pressure in a cold room?