2.5.5. Barrier Properties: Water Vapor Permeability

Water vapor permeability (WVP; g/(Pa.day.m)) and water vapor transmission rate (WVTR; g/(m2.day)) were determined according to the standard protocol ASTM E96-00. The films, which had been equilibrated at 23 ± 2 ◦C and 50 ± 5% RH for 72 h, were fixed on the top of equilibrated cups containing a desiccant (15 g of anhydrous CaCl2, dried at 105 ◦C for 2 h before being used). The test cups were placed in a cabinet at 23 ± 2 ◦C and 50 ± 5% RH. The weight changes were monitored for every 2 h over 48 h. The gradient was calculated from the slope of a linear regression of the weight increase versus time [22,23]. WVTR and WVP were calculated according to the following equations:

WVTR = (Δm/Δt)/A, where Δm is the weight changes of test cups (g), A is the test area (m2) and t is test time (day).

WVP = WVTR/Δ*p* = (WVTR/[*p*(RH1 − RH2)]) × e, where *p* is the vapor pressure of water at 23 ◦C (Pa), RH1 is the RH of the cabinet (50%), RH2 is the RH inside the cups (0%) and e is the thickness (m) of the films.
