On the other hand, the CO2 concentration measured at day 0 was already far below the set value of 50% in unpressurized and HP-treated samples with a CO2 concentration similar and close to 22% in both samples. Hence the high pressure treatment did not induce any immediate change in gas composition in comparison with non pressurized samples. The permeation of gas through a polymer can be described by a diffusion model, using Henry and Fick's laws to obtain the expression that relates the permeation rate with the area and thickness of the film