The need for measuring the throughput of water vapour through packaging films or barriers has numerous drivers. The obvious one of typical food packaging is now well served by readily available permeation measuring instruments. The more demanding requirement for films used for the present commercial generation photo voltaic solar cells is arguably within the limit of present day commercial instruments. However the need for extreme hermeticity to water vapour in the construction of organic LED displays has set the latest challenge to the instrument manufacturers. Reputedly a water vapour transmission rate (WVTR) of less than 10-5 g/m2/day will endow OLED displays with a lifetime of better than 1 year1 after which time the internal electrodes will be increasingly compromised. Barrier manufacturers are now looking towards WVTR performance of 10-7 g/m2/day, to provide commercial products with more useful lifetimes. These materials can only be currently produced in multi-layered form, alternating traditional organic polymer film with inorganic metallic materials. In this way, despite defects in the individual layers, labyrinthine paths2 are created to hold back gas and vapour throughput.

Techniques for measurement

Traditionally a system to measure WVTR comprises a wet and adjacent dry chamber separated by a sheet of barrier material under test. Strict control of the wet and dry environments, combined with moisture measurements can, using Fick’s Law, be used to calculate the water transmission through the barrier. The law simply states that diffusion through a barrier is a linear function of the concentration gradient across the barrier and its thickness. So with knowledge of the water vapour concentration on both sides, and the dimensions of the barrier, WVTR can be found. So the task becomes one of measuring the moisture levels on both sides of the barrier under test. To put this in perspective the 10-5 g/m2/day WVTR mentioned above with respect to OLEDs, is the equivalent of less than 0.2 ppbv water vapour content in the dry chamber of a typical permeation instrument configuration. There are a number of candidate sensing methods for moisture measurement, and the low moisture dry chamber trace level task will be considered exclusively. The most common techniques for trace water are infra-red (IR) light spectral absorption, mass spectrometry, metallic Calcium oxidation rate and Coulometric electrochemical cells. Of these the IR techniques can be further divided into four readily identifiable contenders, Fourier transform IR, cavity ring down spectroscopy (CRDS), tuneable laser diode (TLD), and non-dispersive or filtered IR. FTIR, based on multiple peak identification in the spectrum, has long been the research grade technique of choice, but is not a leader in the commercial arena, being complex and expensive. CRDS is based on the concept of measuring the decay time of a very short laser pulse fired into resonant cavity containing the gas to be measured. The IR pulse spectrally selected for water will decay slower with less water present. It has a performance close to the present requirement for film WVTR testing and indeed has been used in this capacity3. This study at the NPL, UK appears to be a proof of concept, and it’s likely a commercial implementation would be expensive with no clear path for measurement enhancement.