Architects memo no. 89: November 2007
epoxies can be cured
Your scribe has been asked to address a subject which, on the surface
(first unintended pun) should be relatively straightforward - when does
a paint achieve full cure? After giving some thought to the subject,
I almost gave it away as being too difficult a topic to answer succinctly
in this form of communication. I have a feeling that this still would
have been the wisest course!
I decided to define the term ' full cure' as the time needed
for the applied paint film to reach a plateau of performance properties
(the alliteration was also unintended but I' ll live with it!).
Paints divide into two main categories - those that film form by simple
loss of solvent and those that need a chemical reaction to form a film.
The former category contains materials such as acrylic and nitrocellulose
lacquers, vinyl coatings and chlorinated rubbers. These films quickly
achieve ' full cure' as the solvent evaporates, which generally
occurs in less than 24 hours. In fact small percentages of solvents
can remain in such films for several weeks but generally without impairment
to film properties.
Waterborne, architectural paints also fall into this category because,
after all, they only need to lose water to form a film. These materials
are, however, much more complex. For example, they are critically dependent
on temperature. If one tries to cast a film below the minimum film forming
temperature of the particular paint, a film will not even be achieved,
let alone ' full cure'. If paints are formulated with ' coalescing agents'
to assist film formation these slowly evaporating plasticising solvents
will take at least a week to leave the film and return it to the optimum
Softer polymers can be used that don' t need the assistance of
such solvents and these films achieve their ' full cure'
in much shorter times. Although they may achieve their optimum properties
more quickly, their final film properties may be significantly poorer
than the slower achieving, solvent containing counterparts.
Soft or hard, all of these architectural paints contain water-soluble
thickeners and surfactants which can impact on final film properties.
Take the example of an interior/exterior paint which is applied to a
lounge wall, bathroom wall and an exterior wall exposed to the weather.
In the case of the lounge maximum performance will occur within a few
hours for a solvent-free paint and a few days for a solvent containing
one. The water soluble material will remain in the film and become part
of, and impact upon, the binder.
For the exterior situation a performance peak will occur on the same
time scale as above but the first rains that the paint is exposed to
will wash out the water soluble components and take the film performance
up to another level. ' Full cure' can not be said to occur
until the product has been substantially rainwashed.
In a very well ventilated bathroom the same timetable exists as for
the lounge; condensation however, can be a wild card. Condensed water
can continually soften a film and bring surfactants to the surface.
A regime of wiping off condensation from the surface of a newly painted
bathroom for the first few days of use will result in a permanent increase
of the performance plateau.
Paints that cure by chemical reaction (alkyds, polyurethanes and epoxies)
are governed by a different set of parameters including the type of
chemistry in question and the number of reaction sites. The basic picture
is that these paints are made up of relatively small molecular chains
with selective ' hooks' on these chains. These ' hooks'
react with other chemical molecules, provided by the hardener or oxygen
from the air. Some ' hooks' are much more reactive than others,
and of course, the more ' hooks' there are on a single chain
the more likely is the chance of reaction. These reactions are also
referred to as cross-linking.
All of these products suffer from a common plight in that, as reaction
occurs, the chains get bigger and bigger and more and more sluggish.
Reaction eventually slows right down, well before all of the hooks are
used up. Heat is the great benefactor here, making the chain much more
mobile and increasing the rate of reaction. For every increase of 10°C
the reaction rate is approximately doubled.
The concept of 'full cure' for such systems is therefore a theoretical
concept. In practice, times to achieve desirable plateaus such as block
resistance, abrasion resistance, solvent resistance, overcoating windows
etc are measured by testing at ranging temperatures. These are then
offered as guides in Data Sheets.
A further complication are the waterborne enamels that initially obey
the rules of the first product category but also contain reaction sites
that start cross-linking once the water has left the film. These chemistries
are designed to take these products to another, higher level of performance.
Different resin manufacturers practice different technologies which
can take from a week to a month to fully ' kick in' at ' typical' ambient
temperatures, again with elevated temperatures having a major impact.
The cure of each product on each project varies depending on application,
temperature, paint system and the list goes on. Because these variables
are often uncontrollable, we have a ' rule of thumb' recommendation
that the painted surface be allowed to cure for four weeks before cleaning.
That way no matter what, how or when it was been painted, it will have
had ample time to reach its optimum finish before doing battle with
dusting cloths, paintwork cleaner and overzealous cleaners.