Architects memo no.
47: April 1986
more on durability
In the world of Surface Coatings, people's expectations of durability
naturally vary from situation to situation. Whilst a week would be considered
a useful life for a coat of nail varnish, centuries can be expected
from traditional Chinese lacquer work. However, even such durable material
as Chinese lacquer would have its life drastically shortened if it were
used in a hostile environment for which it was not designed.
Even the notion of useful life varies with the observer.
To an engineer a coating may be considered to be performing perfectly
satisfactorily as long as it is preventing corrosion, even though it
may have lost all gloss and semblance to its original colour. In an
architectural situation however, the most sophisticated system can be
deemed to have failed if the first few exposed microns of film, which
govern gloss and appearance, break down.
The agents working towards breakdown of exposed surface
coatings are UV light, water, and oxygen, in this order of importance.
These agents not only work individually, but can gang up against the
coating creating unholy synergisms. Thus a coating receiving the same
amount of UV radiation will retain its good looks much longer in Central
Otago's dry climate than in Auckland's more humid airs. A useful picture
is of UV light striking the paint films like a rifle bullet, breaking
part of the molecular structure; if water is around it will 'infect'
the break and prevent any possibility of self-healing.
Although there are several components in a paint that
affect durability (some of which have been covered in other memos),
the major influence is the binder, or the material that holds all the
other components together. All paints end up, after application, with
a more or less clear layer of binder right at the very top surface of
the film. The thickness of this layer depends on the style of paint
and the amount of pigment present. Flat paints (containing high levels
of pigment) have a very thin layer of binder over the top, whilst glossy
paints (with low pigment levels) have a thicker layer.
The onset of 'chalking' occurs when this thin top
layer of binder is eroded away by the action of UV light etc. Chalking
will be retarded if the top layer of binder is more 'bullet-proof' and
if the layer is made thicker.
The inherent resistance to degradation is fundamental
to the type of binder chosen. Among the more durable binders available
for architectural coatings are poly vinyl fluorides, silicones, urethane
acrylics, aliphatic urethanes and pure acrylics. Although there will
be some variations within grades and from various suppliers, this list
of high performance binders is in approximate order of durability and,
The aspect of the thickness of the clear binding layer
can be substantially affected by formulation. As has been stated, flat
finishes have a thinner binder layer than gloss finishes and it is a
general rule that, for the same binder system, flat paints will chalk
much more rapidly than their glossier analogues. What happens then if
the clear coat thickness is deliberately increased by the use of glazes
of similar binders? Precisely the same as what happens with high quality
motor cars finished with the latest basecoat/clearcoat systems - much
Let us imagine that a semi-gloss acrylic has a clear
binder layer of 2.5 microns and this layer erodes at a rate of .5 microns
per year. After 5 years, pigment will be exposed and gloss will decrease,
with the onset of chalking very shortly after. If a glaze of 10 microns
were applied over this with the same erosion rate, the onset of chalking
would be delayed, theoretically, 20 years. Results obtaining these theoretical
results have been obtained in practice.
The cost performance benefits of the use of glaze
coats is exceptional and largely without drawbacks. Consideration must
be given to the long-term compatibility of the glaze coats with the
basecoats and this will be dealt with in a later memo.
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