Microporosity

The term 'microporous' is often applied to specialist paints and stains for exterior wood: to describe a coating that acts as a barrier to liquid water, but allows water vapour to pass through. The implication being that there is something special about the coating that keeps rainwater out of the timber, but lets moisture that has found its way into the timber escape as a vapour. Although almost all coatings will act as a barrier to liquid water, it is not correct to imply that coatings can be made so that moisture vapour can only travel through them in one direction. Moisture vapour will be able to pass through a coating equally well in either direction: from places where there is more moisture, to places where there is less moisture. The rate at which moisture passes through a coating is controlled by the permeability of the coating, the moisture content gradient, the film thickness, and the temperature.

ICI Paints has avoided using the term 'microporous' to describe its products, regarding it as a somewhat misleading sales gimmick, because nearly all paints and stains offer some protection against rainwater, and all paints and stains will to a greater or lesser extent allow moisture vapour to pass through them in either direction. Instead scientists at ICI prefer to speak in terms of appropriate levels of 'moisture vapour permeability': too little can result in paint blistering or worse still wood rot can begin to occur, too much allows moisture to pass into and out of the timber too easily, leading to splitting and cracking of the wood. So it is not just a question of the more the better. Moisture vapour permeability is one of a number of important attributes that must be considered when formulating an exterior coating - others include: adhesion, cohesion, flexibility, mould resistance and photostability.

Microporosity is therefore not some miracle technology, it is an invented word that has been used to describe paints and stains that are usually quite high in moisture vapour permeability - sometimes excessively so. Often such wonder products are single paints said to do the whole job in one go. We believe that this approach is grossly over simplistic, and prefer to take a much more systematic approach to achieving lasting durability with coatings for exterior wood, particularly where redecoration is concerned. For example, each part of the Weathershield Exterior Gloss System is formulated to do a specific job, and has a carefully balanced set of properties, including a suitable level of moisture vapour permeability, working together they help to keep the moisture content of the wood below levels at which timber decay can become a problem.

The basics - Water vapour permeability

'Microporous' - 'Breathing' - 'Permeable'

All paints can claim to be microporous.

What is more important than microporosity is the following properties:

• Mechanical Properties - flexibility
• Adhesion
• Biological resistance
• Chemical resistance
• Resistance to degradation by sunlight.
• Early shower/water resistance
• Resistance to soiling/dirt pick-up.
• Compatability with previous coatings and where appropriate glazing seals.