2. Water vapor permeability (WVP)
WVP measurements were determined with the gravimetric
method described in the AFNOR NF H00-030 standard (AFNOR,
1974). The test film was sealed in a permeation cell containing
a desiccant (silica gel) to maintain an RH of 0% in the cell. The
permeation cells were 6.4 cm (internal diameter) by 8.9 cm (external
diameter) by 4.8 cm deep with an exposed area of 26.42 cm2.
The permeation cells were placed in
a controlled temperature
(38
±
1
◦C) and relative humidity (90
± 3%) chamber fitted with
a
variable-speed fan to provide
a strong driving force across the film
for water-vapor diffusion. The water-vapor transport was determined
from the weight gain ofthe cell. Changes in the weight ofthe
cell were recorded as
a function of time. Slopes of weight changes
vs. time (after steady state was reached) were calculated by linear
regression, and the correlation coefficient for all reported data
was >0.99. The steady transfer rate was reached after
∼
1 h. The
water-vapor transmission rate (WVTR) was defined as the slope
(g/d) divided by the transfer area (m
2). After the permeation tests,
film thickness was measured and WVP was calculated as follows,