Architects memo no.
65: April 2001
hydrophobicity... how repellent is it?
Water, the odd chemical that really should be a gas but isn't,
truly is the great mover and shaper of the world. It lowers mountains,
scours out valleys, shapes coastlines, as well as being provider and/or
host to every known life form on earth.
Wonderful stuff really, but it can also be a damned nuisance: in fact
keeping water out of our buildings consumes a lot of our combined energies.
All of us are familiar with methods used to achieve this:- flashings,
free draining structures, sealants, and membranes to mention but a few.
However even simple contact with water can be detrimental resulting
in dirt and mould build-up along with heat loss.
Fully hydrophobic surfaces, that is surfaces that are simply not wetted
by water, are very attractive in concept and can be achieved by the
application, for example, of a substance such as wax. Wax however is
subject to oxidation, and oxidation renders a hydrophobic surface hydrophilic
(water loving).
Hydrophobic materials that resist oxidation are few and far between
with the silicones being notable exceptions. Use of them both in polishes
and masonry water repellents has been very successful. Modern masonry
repellents based on siloxanes react with silica present in cement, sand,
stone, etc and semi-permanently render the surface hydrophobic.
Incorporation of such materials into a standard paint does not confer
the same sort of repellency as they do to mineral surfaces. The reason
is two-fold, one is because the chemistry of paint resins does not easily
react with siloxanes but the second is more interesting. Studies of
naturally hydrophobic materials, such as the feathers on a duck, have
shown that it is not only the oil that is important but the fact that
the feather has a fine profiled surface.
It appears that a micro-textured hydrophobic surface offers less points
of contact to the water droplet so increasing the water repellent 'ducks
back' effect. This offers the possibility of producing long-term durable
water-repellent paints. The theory is to produce a highly filled paint
using relatively coarse silaceous extenders to produce a porous, slightly
rough surface. Use of silicone resins and reactive siloxanes can render
the surface of the pores hydrophobic which, coupled with a micro-rough
surface, can result in long-term water repellency.
Such paints with an attractive flat surface and very high breathability
may achieve a useful niche in the scheme of things.
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