|CO2 solubility in Polycarbonate|
Posted by Diffusion Polymers on October 31, 2006 at 20:44:48:
In Reply to: CO2 solubility in Polycarbonate posted by Kathy Fagan on October 31, 2006 at 20:42:57:
Dear Kathy Fagan,
In the initial syringe situation the partial pressure of carbon dioxide CO2 is approximately 0.65 bar and N2 is 0.35 bar. Realize that the atmospheric outside N2 pressure is 0.78 bar: the chemical potential of the outside nitrogen N2 is higher than in the syringe (the gas mixture in the syringe can be assumed to be ideal). There will be a netto flux of nitrogen N2 gas and oxygen O2 gas into the syringe, and a flux of carbon dioxide CO2 out of the syringe in the first stage. Therefore the relative concentration of CO2 gas drops down fast.
When the Nitrogen and Oxygen approach equilibrium - the partial pressure of Nitrogen in the syringe becomes around 0.78 bar and Oxygen approaches 0.21 bar, the Carbon Dioxide relative concentration drops more slowly as the partial pressure of Carbon Dioxide gas is getting lower and the "dilution" effect by Nitrogen and Oxygen cancels out.
A good polymeric material gives a sufficient time lag before the mass transfer situation as described will occur. Furthermore Carbon Dioxide gas has a high solubility in Polycarbonate which is making the loss situation worse than when you would for example use a good grade of PET (highly crystalline).
The phenomenon can be solved by use of the figures in the gas diffusion section and the appropriate (Maxwell-Stefan) mass balances.
PS An interesting case story on diffusion of CO2 (carbon dioxide) through the wall of a PET: (Polyethylene Terephthalate) bottle that contains soft or alcoholic drinks. You can think of cola or beer. Reason for putting gaseous CO2 in drinks is taste. The gas gives a sparkling feeling and a reduction in tasted sweetness. One can imagine that diffusion of CO2 through the wall is a problem because of loss of these features in time; in other words, a loss of quality in time. In this case story, the CO2 diffusion from a bottle as a function of time is demonstrated in real-life circumstances. Read more: Carbon Dioxide Diffusion from PET