[kartysam]

Hi All,

Why aliphatic polyisocyanate polyurethane is UV resistant, while other aliphatic compounds like Methane , Ethane are volatile in nature?

and Why benzene is volatile in nature?

Please do reply to the post.

Thank you

[Enthalpy]

Hi kartysam, welcome!

I don't understand the opposition you seem to make between volatile and UV-resistant. Compounds evaporate as a result of a temperature high enough and a pressure low enough. Many compounds stay solid or liquid at warmth but are destroyed by UV.

As for methane, ethane and benzene, they'e volatile because they're small molecules, with few atoms to attract the atoms of the neighbour molecules, so the molecules separate easily to form a gas. This isn't the whole picture however, because some combinations of atoms attract other combinations more strongly.

For instance H₂O is liquid at room temperature while C₂H₆, with more atoms,  is a gas. That's because H linked to O or N gets the ability to make a 'hydrogen bond" with other atoms, especially with other O and N atoms. This "hydrogen bond" is a "van der Waals force", much weaker than a usual C-H or O-H bond, so that a moderate heat breaks it to evaporate the compound, but stronger than the forces between H and H or C and H of different molecules, so that water makes a liquid at room temperature while ethane doesn't.

The resistance to UV isn't trivial to predict. Methane and ethane are among the most resistant. For other compounds, it depends on the presence of weaker bonds, of multiple bonds, and on many details. The UV behaviour of compounds differs also between air and vacuum.