2.2.4. Water Vapor Permeability (WVP)

The gravimetric method based on ASTM E96/E96M-10 [18] was used to determine WVP at 23 ± 2 °C and 54–100% RH gradient. Prior to the test, the thickness of the samples was randomly measured at four points. Payne permeability cups of 3.5 cm in diameter (Elcometer SPRL, Hermelle/s Argenteau, Belgium) were filled with 5 mL of distilled water (100% RH) and then circular film samples (35-mm diameter) were secured with the outwards-facing side in contact with the air during drying. The cups were placed in pre-equilibrated cabinets at 54% RH using a magnesium nitrate saturated solution (Panreac Quimica, SA, Barcelona, Spain) and they were periodically weighted (±0.00001 g) until the steady state was reached. The free film surface during film formation (air side) was exposed to the lowest relative humidity to simulate the actual application of the films in high water activity products when stored at intermediate relative humidity. Cups with aluminum samples were used as control samples to estimate solvent loss through the sealing. Water vapor permeation rate was calculated from the steady state permeation slopes obtained from the regression analysis of weight loss data vs. time, and weight loss was calculated as the total cell loss minus the loss through the sealing. WVP was obtained by multiplying the permeance by the average film thickness. Four replicates per formulation were made.