Architects memo no.
52: June 1998
humidity, temperature, water - waterworld ll
Water - this chemical oddity dominates our lives. It supports
life and destroys it. It shapes and reshapes our environment. It fills
our oceans, lakes, and rivers. It is in the very air that we breathe,
and, in the variability of the amount present in the air, causes headaches
for the paint chemist.
Everyone who has hung washing out to dry will be familiar with the
fact that some days are good drying days and some are poor. At any specific
temperature, air has the ability to hold a specific amount of water
in it. For example, at 15°C one cubic metre of air can hold exactly
12.8 gms of water as vapour - and no more. Washing, or paint, put out
to dry under such conditions simply will not dry as the air can hold
no further water. Air saturated with water like this is referred to
as having 100% relative humidity.
At 5°C, air can only hold 6.8 gms of water and so if the air,
referred to above, cools down to 5°C, 6 gms of water vapour will
condense from it as dew. Conditions such as this regularly occur during
spring and autumn as warm day temperatures fall sharply in the late
afternoon leading to a dramatic increase in relative humidity, up to
and past the dew-point. This imposes disciplines on painters regarding
how long they can keep painting on a beautiful late autumn afternoon.
Many a litre of roof paint has ended up literally down the drain due
(no pun intended) to the painter trying to squeeze an extra hour of
productivity from the day.
High temperatures and low humidities have the opposite effect and
lead to very rapid evaporation of water.
Under still conditions the evaporated water can lie like a blanket
over the surface, raising the relative humidity locally, and slowing
the evaporation of the remaining water.
A breeze however, will disperse this layer and allow the rapid evaporation
of the water to continue unhindered.
Waterborne paints, in the main, are particulate materials carried
in a medium of water. They require some time to achieve their optimum
packing during film formation and too rapid drying can lead to less
than optimum film properties. Too rapid drying can also lead to wet
edge or open time problems where the leading edge of the applied paint
sets up before the painter can get back onto it with his next brush
load. This prevents smooth melding and leads in turn to aesthetically
Too rapid drying of waterborne paints can be mitigated by avoiding
painting in the full sun; pre-dampening porous surfaces; and by the
use of temporary humectants like hot weather additives. Wet edge or
open time problems can be further alleviated by technique. Most waterborne
paints set after they lose a specific percentage of their water. A leading
edge of paint that is feathered out to a thickness of 10 microns will
set ten times faster than one that is left at 100 microns. Therefore
when applying waterborne paints, the thicker one keeps the leading edge,
the better will be the final appearance.
Formulating paint to cope with our temperate climate can be very taxing
but, as ever, for the paint chemist water has a role - either to wash
down the aspirin or to dilute (a little) the Scotch.