Thermal conductivity of chlorinated/fluorinated gases

[Urethanesimon]

Hi

 

I'm not sure if this is the right place, but...

 

I am a the editor of Urethanes Technology International and UTECH-polyurethane.com and i would dearly like to know why we use some gases as blowing agents in rigid polyurethane formulations.

 

I understand that they help to make bubbles of the right shape and size. My question is why are materials such as CFCs, HCFCs and now HFOs insulating gases?

 

I guess there could be big dipoles within a smallish fluorinated molecule, but how would that make the gases insulating? Is there some kind of inter molecular attraction in the gas?

 

Any help would be appreciated.

 

Best wishes

 

Simon

[Enthalpy]

Hi Simon, welcome here!

 

Even before the heat conductivity, I'd choose the filling gas of a foam to be nonflammable, especially to insulate a house. Something like butane in the walls is excluded. This limits the choice a lot.

 

Then, the molecules shall be heavy, so they move slowly, and carry slowly the heat they receive from the warmer side and store as rotations, vibrations and so on. They should also be big, so they collide often and diffuse slowly between the warmer and colder faces.

 

Though, heavy and big molecules tend to be liquid or solid. To stay gaseous, they need small sticking force between them. Alkanes bring this but are flammable, haloalkanes are heavier and keep the small intermolecular forces. Good combination.

 

The other attempt would take molecules with as few atoms as possible so the vibrations store less heat and the molecules transport less heat. This attempt is limited though, because heavy atoms tend to make solids. It is the solution where haloalkanes are impossible: in filament light bulbs, where only noble gases withstand the heat (excepted halogens which recompose and have other advantages). Heat conducted away from the filament by the gas is the main competitor to light radiation and efficiency. Argon is the standard choice there, but for efficiency, krypton is better. (Xenon lamps are not filament bulbs).

 

At foams, you may also consider that the gas must not react with the foam despite the huge contact area: again in favour of haloalkanes. Neither shall the gas diffuse out of the foam, and this favours big molecules again.

 

There may (must) be other reasons resulting from the fabrication process.

Edited May 24, 2016 by Enthalpy