weatherbeaten?

Architect's memo 71: November 2002

Even the most environmentally benign exterior paint is deliberately designed to impact on the environment. Paint is engineered to prevent the natural bio-degradation of many of our building materials by combating Nature's combined battalions of water, UV light, salt spray, acids, oxygen and microbes.

There is a lot of focus on UV levels currently with greater attention being paid to the dangers of skin cancer and concern over the possible dangers of fluctuations in the ozone layer. Australia and New Zealand have been singled out due to a combination of our high levels of UV and our outdoor lifestyles.

Virtually all of the really deadly radiation (less than 300 nanometer wavelength) is absorbed in the stratosphere by a cycle of interactions between UV light and oxygen to produce ozone, which further reacts with UV light to produce oxygen again. However the balance of UV hitting earth (from 300-450 nanometer wavelengths) can still be pretty damaging.

The amount of UV that gets through to the earth's surface varies enormously with Northern Europe only receiving half the levels that we in Australasia do. Distance from the sun has some effect and, as Earth's perihelion occurs in our mid-summer, this will account for some of the difference.

By far the major effect however is how much UV is absorbed passing through our atmosphere. Even a perfectly pristine atmosphere shows some absorption and UV levels at the Equator will be greater than at the poles. This effect is somewhat offset by longer daylight hours.

Major UV absorbers in the atmosphere are clouds and particulate matter. Northern Europe's pollution accounts for a lot of the UV attenuation in that part of the world but natural phenomena such as fine dust in the atmosphere or smoke from bush fires can also reduce UV levels dramatically.

Although UV alone is enough to damage human skin, it is a combination of both UV and moisture that wreaks the most damage on paint films. It seems that while UV can damage paint polymers, the presence of moisture at the instant the damage occurs makes that damage terminal. High humidities, as experienced by almost all of New Zealand and parts of coastal and tropical Australia, greatly increases the stress on paint films. Particularly stressful is the presence of dew on a painted roof.

The Australasian paint industry has generally learned to cope with these conditions and still deliver the required durability. For example, many of the binders successfully used in Europe have had their shortcomings exposed in our climes and have been ignored in favour of more inherently durable polymers.

Many typical 'architectural' pigments have limitations in our exterior environment and Resene in particular lead the way in introducing high performance 'automotive quality' pigments into their architectural paints. The quest continues for greater and greater durability.

Just as the human skin can be protected by sunscreens so there are industrial UV absorbers that can further protect paint films.

Paint degradation is mainly a surface phenomena with only the first 1 or 2 microns of the surface needing to degrade before a loss of gloss or a change in colour is noticed. The addition of UV absorbers into the body of a paint is inefficient as very little ends up on the surface. However if they are formulated into a clear coating and applied as an overglaze significant added protection is achieved.

This added protection will only be specified when more sensitive colours are used or when ultra long-term performance is required. The presence of Resene Sun Defier in the range gives us yet another tool to beat off the effects of our harsh environment.

Tech talk and Architect's memos
Technical information on a variety of topics relating to paint, finishes and coatings.

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