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PRESS ARTICLE
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!