What causes alligatoring in paint films?
We was up to out eyeballs in alligators considering that very question
just yesterday morning. I was at Middlebury College helping the
maintenance crew understand how to "knit" exterior woodwork and paint
materials into an effective weather envelope.
Alligatoring usually begins with cracks in the thick paint film that is
the result of heavy paint buildup. The first cracks usually appear
perpendicular the length of the board. This is often noted in terms like
"perpendicular to the 'grain of the wood'" although this first set of
cracks does not have anything to do with the grain of the wood. With
vegtible-oil binder paint, especially traditional linseed oil, the oil
molecules are in the form of relatively long "strings." When the paint
is brushed onto the surface, the brushing action tends to orient the
stringy molecules in the same direction as the brush strokes, which is
usually along the length of the board. When the paint dries, the oil
molecules combine with some oxegen from the air, which kicks off
polymerization, rapidly forming even longer molecules as the oil changes
from a liquid to a solid. As the paint film ages over the decades and
centuries, the oxidation continues at a very slow rate, and the long
molecules break down into shorter strings that are shrinking slightly.
Stress builds up in the paint film, there is more stress along the
stringy molecules than across the molecules. Since "broken" molecules
can no longer hold the paint film intact, cracks form, perpendicular to
the orientation of the molecules--usually perpendicular to the length of
the board.
This first set of cracks may eventually penetrate down through the
entire paint film to the wood beneath. Water gets into the cracks and
then into the wood. The wood then expands and shrinks more with these
greater changes in moisture content. This rate of expansion and
shrinkage is much greater across of the grain of the wood than it is
along the grain. So, the board is expanding and shringing across its
width, which stresses the paint film forming cracks parallel with the
grain of the wood.
Now there are cracks in the paint file going across the board, and along
the lenght of the board, forming little rectangular plates. With all
these cracks much more water is getting into the wood beneath. The water
wants to get out of the wood, but cannot pass through the extreme
thickness of the heavy paint build up, so the water begins to push the
paint off of the wood, especially beginning right near the crack in the
paint film. Remember how the paint film is shrinking? Well it turns out
that the later layers of alkyd resin paint shrink even more than the
earlier layers of linseed oil, so the film is tending to curl, which
helps pull the paint off the wood. The rectangular plates curling
slightly around their edges completes the visual similarity to an
aligator's skin.
I can't recall if I learned about this from Bill Fiest or Steve Irwin.
--
John
by hammer and hand great works do stand
by pen and thought best words are wrought
www.HistoricHomeWorks.com
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