Water vapour in the works
Testing for water vapour permeability of materials

Introduction
We are all familiar with the effect that water can have on many of the objects we encounter in our day-to-day lives, the corn flakes have gone soggy because you took a call after pouring in the milk, the car won’t start because of the puddle you went through on the way home, the seat is wet because someone left the sunroof open, and so on. So we develop strategies in our personal lives and our work to protect certain objects from water. We paint wooden objects to protect them from the rain, and wear wellingtons to keep our feet dry, and seal our roofs to keep water out of the house.

Water as a Gas
However, water can exist as a gas, water vapour, and it is in this state that it can pass through many materials that are impervious to water as a liquid. The best analogy for this are the breathable fabrics, such as Goretex, which prevent water as a liquid from passing through, but allow water vapour to pass freely. Similarly, there are many materials we regard as good barriers to water, which actually allow water vapour to penetrate with very little barrier effect.

Films
Most polymers offer very good liquid water resistance, with the exception of a few such as EVOH, PVOH and cellulose. There is little correlation between liquid water resistance and water vapour barrier, however. Some of the best polymeric barriers to water vapour include PVDC (polyvinylidenechloride) and PCTFE (polychlorotrifluoroethylene). The best films are laminates which include a component of aluminium, either as a discrete layer or as a result of a metalisation process.

Sealants and Mastics
One of the widest range of materials chemically, sealants and mastics range enormously in their water vapour barrier, although most are designed to provide liquid water resistance as a primary feature. In this case the manufacturer will formulate a material to meet the physical and chemical properties required of the application. Often the correct thickness to be applied will be specified, and this will provide the appropriate barrier. As always, if water vapour barrier is required, a product which is specifically designed to provide this property should be chosen.

Gaskets, O-rings and Foam Seals
Again there is a wide range of these materials, each appropriate for the designed application. In this case there is usually the requirement to place the material under the specified level of compression to achieve the specified barrier. This requirement also makes testing of the water vapour permeability of this type of material more specialised.

Coatings
Coatings are often required to protect the substrate from water as a liquid and as a vapour, for example to protect wood from swelling and splitting or to protect metals from corrosion. The testing of coatings for water vapour permeability is relatively simple and can typically be completed within an hour. It may also come as a surprise that it is possible to coat paper and board with materials which have poor liquid water resistance, but good water vapour barrier properties.

Tubing
While metal tubing is typically impervious to water vapour, most types of plastic tubing will permit some water vapour to pass through. This may be significant in some dry air applications in the factory, and completely disastrous in a ultra-dry laboratory instrument. The testing of plastic tubing is specialised, and can provide surprising results.

Containers
There are several potential paths for water vapour to take when leaving or entering containers: through the walls, the closure or the seals between the two. There is also the risk of leakage between the seals and the container or closure, and this will often depend on the closure being correctly torqued to the manufacturers specification. Instrumental techniques are commonly used for measuring the water vapour permeability of containers ranging in size from eye-droppers to 25litre drums.

Testing
Generally speaking, the testing process involves applying conditions of high humidity and a set temperature to one side of the material, and measuring how much of the water vapour passes through the object. Traditional techniques depend on measuring the weight gain of a desiccant situated in the dry part of the test rig. However there are a number of problems with these techniques, not the least of which being the time taken to make accurate readings, which is typically measured in days for each sample. Over the last few years, instrumental techniques have been developed which allow for the quick and accurate testing of most materials, including those mentioned above. Testing times start at about 30 minutes for films. Special rigs are required for some of the materials, but once the technique is established, the tests can be repeated reliably.

One caveat is that there are a number of different ways of expressing the barrier to water vapour, and although each industry tends to use a standard set of units, this is not always the case.