An open bottle was used as the control (n = 5). Put the bottles (known weight M1) into the drying oven at 37 ◦C. Evaporation of water through the test membrane was monitored by measurement of loss of weight of the bottles. After 24 h weighed the bottles on an electronic balance (M2). The water vapor transmission rate (WVTR) was then calculated from the formula:
A
= 4(M1 −
M2)
D2 ,
where A is air permeability 24 h/m2.