Water Vapour Transmission Rate Measurements
One of the major limiting factors in the lifetime of organic electronic devices is degradation due to the ingress of water. The transport of water into the operational parts of the device is governed by the water permeability of the materials in which these parts are packaged. To reduce the water permeability, thin (~10 nm) coatings of inorganic materials, such as oxides, can be applied to the flexible polymeric substrates. To assess the efficacy of such barrier layers, measurements of water permeability are required.
These are generally carried out using the calcium test, radioactive methods with tritiated water or most commonly, the 'MOCON' test (ASTM F1249). The industry goal for water permeation is 10-6 gm-2day-1 and, with recent reports of flexible barriers achieving 10-7 gm-2day-1, the question of how to measure water permeation at such low levels is critical.
NPL prototype facility
NPL is developing a new approach to measuring water vapour transmission rate (WVTR) directly, based on cavity ringdown infra red spectroscopy. This is a significant move away from conventional methods. The facility uniquely provides accuracy and traceablity with a detection limit below 1 x 10-4 g/m2/day.
The system operates with a dry chamber separated from a wet chamber of known temperature and relative humidity by the barrier material under test. The dry and wet chambers make up a diffusion cell in which the test barrier film is sealed. Water vapour diffusing through the film is collected by dry nitrogen and measured by cavity ring down infra red spectroscopy. The measurement cell uses a novel design to ensure optimum sealing conditions in order to eliminate water ingress to the dry chamber. The capability is underpinned by NPL's Trace Water Vapour Facility.
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