Application Note
In general, two different principles are described for the determination of the permeability rate of films. These are defined in several standards:
- A quite common method is a setup with the film exposed to an atmosphere of very high relative humidity at one side with a flow of dry gas passing the other side of the film. Water molecules passing the film are carried by the dry carrier gas and detected by a NIR sensor. Although this method provides fast results, calibration of the setup with reference materials with given permeability rates is required.
- The second method uses dishes covered with the film material under test. By filling those dishes either with a desiccant, a saturated salt solution or water, a difference to the partial water vapor pressure of the environment is generated, resulting in a migration of water molecules through the film. This migration is determined gravimetrically by weighing the mass of the dish. Although being more time- consuming than the NIR determination, this method provides very precise results, allowing even reference films to be generated for the calibration of NIR instruments.
