Versaperm Vapour Permeability measurement

Water vapour in the laboratory
Testing for water vapour permeability of materials and containers

Introduction
While water as a liquid constitutes the major part of many chemical solutions, most of us rarely give thought to water as a vapour and the effect on the materials we use daily in the laboratory. As in life where success depends on attaining the correct balance, correct control of water vapour is absolutely critical to success in many laboratory projects. In most cases, this control is achieved by the use of materials that have a specified water vapour permeability, or materials that are intended to act as an absolute water vapour barrier. This article describes the areas where water vapour control may be critical and how permeability is measured.

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 to the other side. Traditionally this required measuring the weight gain of a water-absorbing material in the dry part of the test rig. The weight gain relates to the water vapour that has passed through. However, there are a number of problems with this, not least being the time taken to produce accurate results, which is typically several days for each sample. Over the last few years, instrumental techniques, such as water vapour transmission rate (WVTR) meters, have been developed which give quick and accurate results on most materials, often in as little as half an hour. Some designs are also capable of measuring extremely low leakage levels, which are unrelated to diffusion processes.

One caveat of warning 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, by any means, always the case! Furthermore, the humidity differential and temperature at which the measurement is made has a significant effect on the result. Quoting permeability rates without specifying these conditions is almost meaningless.

So, we can now move on to look at various commonly used materials and situations to see how they respond to water vapour.

Sachets
Sachets are commonly used in laboratories when containing dry powders, sterile objects, or materials that need to be hermetically isolated. A sachet may be designed to retain a certain level of moisture, for example in a gel, or maintain a desiccated powder in that state. The films used in preparing sachets usually have a high polymer content to effect a heat seal. Most polymers offer very good resistance to liquid water, with the exception of a few such as EVOH, PVOH and cellulose. At first sight it may be surprising to learn that there is little correlation between resistance to liquid water and water vapour - so a material that is good in one case might have little effect on the other. 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.

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 laboratory, and completely disastrous in an ultra-dry laboratory instrument. The surface area of tubing quickly adds up to provide a significant source or sink of water vapour. The testing of plastic tubing is specialised, and can often provide surprising results.

Containers
There are several potential paths for water vapour to take when entering or leaving containers. It may flow 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. While glass is typically regarded as practically impervious to water vapour, only ampoules consisting purely of glass are likely to escape the need for closures of another material. Even metal containers suffer the Achilles Heel of requiring a closure and a seal. Instrumental techniques are commonly used for measuring the water vapour permeability of containers ranging in size from eye-droppers to 25 litre drums.

Standards
Special care is appropriate when dealing with analytic grade materials, especially standards. Dry standards that will be accurately weighed during their use may be sensitive to the uptake of moisture, and if this is permitted their accuracy will be jeopardised. Anhydrous standard salts that can exist in different levels of hydration are especially at risk, as the material may appear to be dry as normal, but contain lower levels of active material by weight. In the case of standard solutions the loss of moisture is the primary concern, both during transit and later during storage after the first use. Different closure designs offer advantages at the different stages of the life cycle of the container. It should also be noted that the loss of water from materials does not cease in the freezer, as the low relative humidity below 0°C continues to drive the diffusion process. Furthermore at reduced temperatures seals may not be as effective and the fit of closures may change.

It is often surprising to realise just how many laboratory processes and products are effected by water vapour – but water is omnipresent and can cause everything from electrical and computer failure, through to drugs that become ineffective. It can even make pizzas go soggy!