the impact of water vapour on the modern world
By Christopher Roberts, Director, Versaperm
Water is everywhere, it’s insidious and it’s invasive. Over history man has spent an incalculable time trying to keep it either out of things or in things, on average he makes a pretty good job of it. But only when water is in liquid form. As a vapour the story is very different – and the difference causes wastage running into hundreds of million pounds a year in the UK alone.
Everyone thinks they understand water and the way it works – but vapour doesn’t always work in the way you would expect, and that’s a problem. Materials and products that form a near perfect seal against liquid water can be virtually useless as a barrier to vapour. Sometimes the effect can be highly counter-intuitive – 50 years ago who would have predicted materials such as Gortex – which keeps the rainwater out, but lets sweat evaporate almost as if it wasn’t there.
Water Vapour affects virtually anything, from seals, O-rings, tubing and enclosures on the factory floor through to instrumentation in the lab. And in our day-to day lives it affects food and packaged products through to rust, jammed printers, soggy cornflakes and even missiles going out of control. Water vapour is one of the world’s most damaging contaminants.
To complicate matters still further, a barrier material can lose much of its effectiveness through processing. Even something as commonplace as a blister pack can lose up to 50% of its effectiveness through thermoforming, and another 50% through heat sealing.
There is no really reliable way of predicting the properties of a finished product or enclosure simply by knowing the materials it is made from. This leaves just two options - the “educated guess” and testing.
The testing process involves applying conditions of high humidity and a set temperature to one side of a material, and then measuring how much water vapour passes through. Traditionally this was based on “Gravimetric testing” which required measuring the weight gain of a water-absorbing material in the dry part of the test rig - which is a measure of 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, (typically several days) or the fact that a good technician will often produce better results than a less skilled colleague.
Over the last few years, instrumental techniques, such as WVTR meters, have been developed which give quick and accurate results on most materials. The sensors in these use, for example, Faraday’s law to measure the number of water vapour molecules in the gas on the dry side of the meter. This is directly and absolutely related to the number of water vapour molecules that pass through the material – and a highly reliable and accurate measurement can be taken, often in as little as half an hour.
Testing products, enclosures and components
Techniques developed for testing water vapour can be extremely useful – whether or not the product is sensitive to water, since the vapour acts as a tracer that indicates the route other gases would follow. This is especially useful for enclosures (for example electronics enclosures), threaded or click-fit closures, secondary containers, solid seals or packaging. This method can also be used to test, under a range of temperatures and mechanical pressures, the joints between materials that have different thermal expansion coefficients.
There are two ways to test the permeability of these enclosures. The most reliable technique is to incorporate a water source in the container, and seal it as normal. Where appropriate, this should be done on production scale equipment. The container is placed inside a special chamber, through which dry gas is channeled. Any water vapour that passes through the container walls can be detected easily. Accuracy is typically in the single figure parts per million range, or even, in some cases, parts per 100 million.
The alternative method involves passing the dry gas through the enclosure itself, and placing this in an humidity and temperature controlled chamber.
Any part can be tested for permeability – either by using a specially manufactured jig to hold just that component in the testing environment, or by sealing off the other parts of the container using a non-permeable material.
A common form of testing is on materials – which can be anything from a foil barrier through to plastics, paper or coatings or even an entire structure such as the wall of a house or the blisters in a unitary dose blister pack.
It is useful to look at various commonly used materials and situations to see how they respond to water vapour.
Most polymers offer very good resistance to liquid water, with the exception of a few such as EVOH, PVOH and cellulose. It is surprising but 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. The best films are laminates, which include a component of aluminium, either as a discrete layer or as a result of a metalisation process.
PVC or Polyvinyl Chloride is a widely used material – though it only provides a nominal or zero barrier to moisture and should only be used when the content do not require this protection.
PP or Polypropylene is an economical medium-moisture barrier material, though it is more common in Europe than the USA.
PVC/PCTFE Laminations is a thermoplastic film laminated to the PVC by adhesive or heat sealing, sometimes with an intermediate polyethylene layer to assist lamination. Different thickness of the composite offer average-to-extreme moisture barriers.
PVC/PVdC (or saran) is a film where the PVC is coated with polyvinylidene chloride (PVdC). The coating provides a medium to high vapour barrier depending on the amount used.
COC or Cyclic Olefin Copolymers are a relatively new family of polymers that offer an excellent barrier for applications where halogen free packaging is required. To maintain halogen free status, COCs are normally laminated between layers of PE or PP.
Cold Form Foil is used where products are extremely sensitive to either water or light. It provides an extreme moisture barrier and is constructed using three layers: PVC, aluminium foil, and nylon.
Comparison of Typical WVTR’s - (Relative to Polyamide at 100%)
Best Vapour-barrier materials Relative Permeability based on Polyamide at 100%
Cold Form Foil 0
Polychlorotrifuoroethylene (PCTFE or Aclar) 0.16
Polyvinylidene chloride (PVDC or saran) 0.2
High-density polyethylene (HDPE) 1.2
Polypropylene (PP) 1.6
Good Vapour Barrier materials
Low-density polyethylene (LDPE) 4.8
Polyethylene terephthalate (PET) 5.2
Low vapour-barrier materials
Polyvinyl choride PVC 16
Acrylonitrile copolymer (Barex) 20
Polyamide (PA or Nylon) 100
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.
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 an ultra-dry laboratory instrument. The testing of plastic tubing is specialised, and can provide surprising results.
Sealants and Mastics
Sealants and mastics are both widely used and have a huge range of chemical compositions. Although acting as a barrier to liquid water is one of their main functions, they vary enormously in their resistance to vapour. This can be easily overlooked as the automatic, but fatally flawed first assumption is that if they stop liquid water, they stop vapour as well! A manufacturer will formulate his material to meet the physical and chemical properties required of the application. If the application is sensitive to water then it is vital to check the permeability of the sealant as well.
Containers and enclosures
There are several potential paths for water vapour to take when entering or leaving containers, it can 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. Instrumental techniques are commonly used for measuring the water vapour permeability of containers ranging in size from eye-droppers to 25 litre drums.
Gaskets, O-rings and Foam Seals
Again there is a wide range of these materials, each needs to be appropriate for the designed application. There is usually the requirement to place the material under a specified level of compression to achieve the appropriate barrier. This makes testing of the water vapour permeability of this type of material more specialised. Different compression levels offer different resistance to water vapour – either to much or too little can reduce effectiveness – and the only way to check this is to measure each individual product.
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.
So, although water vapour is insidious and omnipresent, it can be contained with a little knowledge, a little care and a little testing. Testing leads to equipment and instrumentation that works, products that don’t fail or easily “go-off” and a saving of hundreds of millions of pounds. When it isn’t done properly, well – “Water mess!”