Versaperm Vapour Permeability measurement

"O” DEAR
Or why don’t O rings (or seals &  gaskets) always work?

By Chris Roberts, Director Versaperm

The short answer to why seals and O rings don’t always work properly, is water.  Not as a liquid, but as a vapour, where it can flow almost unhindered through many seals - even where they are an almost perfect barrier to liquid water.  The question, and the solution, is more complex and includes some features that, at first sight, are downright odd.

Millions upon millions of O rings, gaskets and sealants are used every year – many of them are used to keep water out (or in), though the story works just as well if you are trying to seal against air, oxygen, hydrocarbons, CO2 or anything else. 

Films & Coatings

 

It sounds an easy job to keep water out – let’s, for example, use a rubber O ring – or even better a silicon rubber O ring? – after all everyone knows they are great at keeping water out.  Wrong!  Silicone rubber lets nearly ten times more water vapour flow through than natural rubber – and 80 times more than the widely used Flurocarbon Dipolymer.  So what about choosing acrylic, or Polyurethane?  Still wrong, I’m afraid.

The short answer to why seals and O rings don’t always work properly, is water.  Not as a liquid, but as a vapour, where it can flow almost unhindered through many seals - even where they are an almost perfect barrier to liquid water.  The question, and the solution, is more complex and includes some features that, at first sight, are downright odd.

Millions upon millions of O rings, gaskets and sealants are used every year – many of them are used to keep water out (or in), though the story works just as well if you are trying to seal against air, oxygen, hydrocarbons, CO2 or anything else. 

It sounds an easy job to keep water out – let’s, for example, use a rubber O ring – or even better a silicon rubber O ring? – after all everyone knows they are great at keeping water out.  Wrong!  Silicone rubber lets nearly ten times more water vapour flow through than natural rubber – and 80 times more than the widely used Flurocarbon Dipolymer.  So what about choosing acrylic, or Polyurethane?  Still wrong, I’m afraid.

O-rings

 
Ref

Chemistry

Hardness Shore

WVTR at 30°C/100%RH g/m²/day

FR10/80

Flurocarbon Dipolymer

80

0.3

GN/W/80

Polychloroprene

80

0.7

FR58/90

Flurocarbon Terpolymer

90

0.8

NR51/50

Natural Rubber

50

2.3

PB80

Medium Acrylonitrile

80

3.3

SIL70/2

Silicone Rubber

70

24.2

 

The problem is that water as a gas acts very differently to water as a liquid, and materials that are an excellent barrier to one can be virtually useless when it comes to the other.  The same is true of hydrocarbons, or anything else you care to mention.

Perhaps from here on in the answer looks simple? – you just go and look up the vapour permeability of the material you want to use, and all will be OK?

Sadly, still wrong - for two main reasons.  First, the vapour permeability of a flat material is different to that of a seal or gasket made from it – the actual manufacturing process can make a huge difference.  Forming a material into a shape will change its resistance, and it can be by 100% or more even before you heat or apply pressure to it! 

The second reason is even more important.  The figures can be downright misleading as different measurement techniques produced wildly different results.  Let me give you a real example.  The fabric Sympatex has a published water permeability of 783.3 g/24hr/m2 using the upright cup method.  Gortex has a published value of 16,612 g/24hr/m2 using the desiccant inverted cup method.  So which is best at keeping water out?  Well, sorry but wrong again! – the difference is in the methods not the fabrics which have very similar permeability - I just chose to tell you results produced by different measurement techniques.  If you use the same methods the figures are very close and both are excellent at their allotted jobs.  The problem is the testing methods – and there are quite a few of them!

This level of difference is not at all uncommon between the methods – the ratio between the two types of test, on the 26 materials we researched, was that one gave an result some 1600% greater than the other for the same thing!

So, unless you do the tests yourself, on your own products, you will need to know a little more about the types of testing.

Testing Techniques

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.  There are two main ways to do this – Gravimetric and Instrumental.

The gravimetric technique dates back several hundred years and is based on measuring a change in weight caused by water vapour passing through the material under test.  A cup of water-absorbing material is covered with the sample and sealed.  The cup is weighed daily as moisture passes through the sample.  In some cases the test is continued until equilibrium is reached, but in others the test is continued for a fixed period or until no more water can be absorbed.

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 become popular as they give quick and accurate results for most materials.  They measure the water that has flowed through the material of the seal into a dry chamber, the sensors in these, for example, use 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. 

Many standards were originally based on the older gravimetric technique, but this is changing as WVTR testing becomes more widespread.

It is vitally important not to mix readings from the two techniques – but this information is rarely given.

Gaskets, O-rings and Foam Seals

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 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.

Common O Ring materials

  .

Aflas FEPM

Resists oils, lubricants and some fuels, it is suitable for use with H2S, amines or high temperature water/steam.

Butyl IIR

Offers low gas permeability and is can be used for vacuum and high pressure applications.  It should not be used with mineral oils

Epichlorohydrin ECO

Provides good resistance to mineral oils, fuels and ozone.  Has poor compression set resistance.

Ethylene-propylene EPM/EPDM

Offers excellent resistance to weathering, ozone, water and steam.

Fluoroelastomers. Dyneon, Viton, Tecnoflon

Works well in severe chemical conditions and high temperatures.

Fluorosilicone FVMQ

Is extensively used for static seals in aerospace as it is highly resistant to many hydrocarbon types.

Hypalon (and other chlorosulphonated polyethylenes)

Offers excellent resistance to weathering and works well with many media for static sealing duties.

Hydrogenated nitrile, Elast-O-Lion

Offers good mechanical and chemical resistance properties.

Kalrez FFKM

These erfluoroelastomer compounds have excellent chemical and temperature resistance (grade dependant).

Natural rubber

Strong with good abrasion resistance, it  is suitable for use with ammonia, ethylene glycol, dilute acids and alkalis, though please note that synthetic rubbers provide greater resistance to heat, weathering and oils.

Neoprene

A general purpose elastomer that stands up well to sunlight and atmospheric ageing.

Nitrile

Widely used with mineral oils water and some solvents, some grades are suitable for food applications.

Polyurethane

Offers good mechanical properties as well as resistance to weathering. hydrocarbon fuels and mineral oils. Stress relaxation may occur above +50°C.

PTFE, Fluolion

Offers excellent resistance to most media and can also be used for 'O' ring back-up rings, though flow characteristics limit its use.

Silicone

Offers good heat and weathering resistance as well as having excellent electrical properties. Sealing ability is limited due to high gas permeability, low tensile strength and poor resistance to tear and abrasion.

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.

I hope that I have highlighted some of the many factors that relate to the problems related to seals and have shown you a few of the answers and a few of the questions you need to ask.  For brevity, I have largely skipped over the problems of temperature and pressure, but I believe that I have covered enough background to understand how to resolve your problems.

According to my dictionary, a seal is a “tight or perfect closure”.  I hope that, if nothing else I have convinced you that tight it may be – but perfect….?  As I said in my title “O dear!”.