Permeability

The term "permeability" is used to indicate the rate at which water (usually as vapour) can pass through a coating. It is expressed as the weight of water passing through a given area in a given amount of time, the most common unit being grammes per square metre per day (g/m²/24 hours). This can refer to the whole coating system and is then known as film permeability or sometimes the permeance. Some published data on permeability makes the assumption that it is inversely proportional to film thickness. In this case the results of a permeability test are converted to a constant film thickness and are expressed as a specific permeability, usually assuming a 100 micron film.

It is clearly important to know which units are being used. Film permeability is a more practical expression than specific permeability for the following reasons:

• It can be related to end use immediately, whereas specific permeability must be calculated back to film thickness.
• Although more scientifically accurate, specific permeability obscures the large practical differences between systems which arise from film thickness. Therefore, although the specific permeability of stain and varnish might be very similar, the permeability of the varnish will be much lower because the film is much thicker. Permeability should be measured using the film thickness which will be achieved under service conditions.
• The assumption that permeability is in inverse proportion to film thickness is often incorrect.
• In practical terms permeability must refer to the whole coating system. But each layer will have a different individual permeability and it will be the least permeable layer which will control the rate rather than the whole film thickness.

There are other pitfalls in the use of permeability data. The results of tests depend on temperature and relative humidity as well as on method. Laboratory tests often use a cup across which the film is supported; the cup contains either water or a dessicant and is weighed at intervals to determine the rate of kater gain or loss. But water-filled cups give different results to those filled with dessicant and both may be different from results obtained on a wooden substrate. For example, one set of tests showed a particular paint system to have a film permeability of 50g/m²/24 hours when tested on a wet cup but the same system applied to three different pieces of Scots Pine showed film permeabilities of 5, 25 and 172! The properties of the wood and its interaction with the coating have a major effect on practical moisture transmission.

Despite these problems, standardised permeability measurements are a useful guide to coating performance. It is usually sufficient to divide coating systems into low, medium and high categories. Of major importance is the ability of a coating through its permeability to influence the rate at which a substrate reaches an equilibrium with its surroundings. High permeability leads to rapid fluctuation, which can cause splitting, warping and excess movement. Low permeability can allow moisture to be trapped, greatly increasing the chance of biological attack.

The extremes of permeability are represented by a low-solids wood stain on the one hand (which exerts no moisture control after a period of weathering) and several coats of traditional undercoat and gloss over an aluminium primer on the other. The best solution usually lies between these extremes, though there is no agreement on an actual figure, which will vary according to the nature of the wood and the intended end use. As a guideline, however, a figure between 35 and 60g/m²/24 hours will be adequate to control moisture movement in joinery. But for fencing, and some cladding, a much higher figure is allowable and will ensure that water levels do not build up to a dangerous level.

The permeability of a coating is dependent on many factors, including the nature of the binder; the type, level and dispersion of pigments; film thickness; temperature; moisture gradient and age. Its value may be changed by altering the composition, but this will also change other properties.

There has been a growing tendency to equate permeability with terms such as 11 micro-porous", "breathing" and "ventilating". This has created the illusion of a specific type of paint and contributes to confusion in nomenclature. These paints do not form a coherent group: they may be solvent- or water-borne, part of a system or self-priming, flat or glossy. Even their permeability varies up to an order of magnitude. A further misconception is the idea that breathing paints permit the passage of water in one direction only Most coatings prevent the passage of water as liquid, but vapour is still able to permeate through. Differences in the rate of vapour permeability in and out of a film can be observed depending on how the test is carried out. Nonetheless, a coating which lets water out, rapidly will also let it in.