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Determination of Water Vapour Permeability ASTM D 1653

Scope and Field of Application

If a coating is applied to steel with the intention of preventing corrosion, it is advantageous for it to have minimal water vapour permeability. In contrast, an external masonry coating should be sufficiently permeable to allow water vapour to permeate through it from the interior.

It is important to differentiate between permeability to water vapour and permeability to liquid water. Silicone masonry treatments for example are a good way of ensuring that rain water droplets run off an exterior wall rather than soak in. The treatment however does not result in the formation of a continuous coating film and consequently it has little or no effect on the permeability of the wall to water vapour.

The test method described on this page enables the rate at which water vapour passes through films of paint, varnish, lacquer and other organic coatings to be determined.

There is no pass/fail criteria defined in the standard. This is a matter of agreement between the parties concerned.

The actual standard is a copyright-protected document and we are not able to provide you with copies. If required however, you can easily obtain copies from the American Society for Testing and Materials.

 

Summary of Method

The test equipment consists of a shallow metal cup (known as a Payne cup) which can be sealed using a detached film of the coating under test. In cases where it is not possible to obtain a detached film, a film can be cast on a permeable paper substrate and used as such. Detached films are preferred since although it is possible to obtain paper with very high permeability, there is always the uncertainty of the error caused by the permeability of the coating-paper interface.

There are four ways of carrying out the test.

• The Dry Cup Methods: The cup contains desiccant and is stored at either 50% or 90% relative humidity
• The Wet Cup Methods: The cup contains water and is stored at either 50% or near 0% relative humidity

We normally use the wet cup method at 23°C and 50% relative humidity. The determinations are carried out in triplicate.

The sealed cups are weighed periodically in order to determine the amount of water vapour that has passed through the film. The weight change is plotted as a function of time until a straight line is obtained. This signifies that a steady state has been attained.

The straight line portion of the plot is then used to determine the water vapour transmission rate in terms of grams per hour. Since the area of the circular test film can be calculated, the water vapour transmission rate of the coating can be expressed in terms of grams per square metre per 24 hours.

If required, the permeance of the coating (in metric perms) can be calculated. The standard includes a table showing the relationship between the test temperature and the saturation vapour pressure of water. Using the appropriate value, the permeance of the coating can be expressed in terms of grams per square metre per 24 hours per millimetre of mercury.

 

Sample Requirements

We require no more than 250 g of liquid coating. Alternatively you can supply us with detached film provided it is flexible, smooth and at least 150 x 150 mm.

 

Accreditation

Although PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS), we are not accredited to carry out this particular test.

 

Please contact Peter Collins for further details.

 

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