Recent Trends in Architectural Coatings
Deck Restoration Coatings
One of the trends in the last few years
has been the appearance of deck restoration coatings. These are opaque, solid
color materials designed for application
to a wood deck at a thick film, typically
eight to twelve mils. As wood decks age,
they can develop cracks and splinters as
well as a visually unsatisfying appearance. Replacing a wood deck can be very
expensive; therefore lengthening the service life by a few years with a restoration
coating can be desirable. By utilizing a
deck restoration coating, the customer
can make an older deck functional while
at the same time delaying the cost of replacing it. Figure 3 shows a sample of
a deck restoration coating applied to a
weathered board with cracks.
To be effective, the coating must have
the ability to bridge cracks, cover up
splinters and hide visual defects. As is
the case with any coating on a horizontal
surface, the substrate must be properly
prepared before application is attempted.
Exterior Paint Properties
Keeping the exterior of a house looking
fresh and new can be challenging. Many
homes experience extremes of tempera-
ture, precipitation, sunlight and particu-
late matter from the environment. One
area of emphasis in the last ten years has
been dirt pickup resistance. This is the
ability of the coating to retain its origi-
nal color even after exposure to dust and
smog particles. The best way to evalu-
ate dirt pickup resistance is two expose
painted panels to an outdoor environ-
ment. However, waiting for differen-
tiation of coatings by natural exposure
can take a year or longer. Accelerated
methods involving wet or dry exposure
to dirt have been employed in labora-
tories in order to predict dirt pickup re-
sistance. This is a difficult property to
quantify because there is great variation
in the type of dust or dirt present in any
given environment, as well as variation
in temperature and rainfall.
In general, coatings that are higher in
gloss, smoother and harder at the surface have better dirt pickup resistance.
The more porous, rough surface of a flat
coating allows for dirt to get stuck in the
valleys between the pigment particles.
A lower Tg coating can allow for dirt
to embed itself in the surface when the
weather is warm and the coating is soft.
Even so, within a given sheen/hardness
of coating, one can see great variation
in resistance to dirt pickup. There is no
magic formula for a coating with good
dirt pickup; we have arrived at most of
our formulas by testing individual ingre-
dients in each class of material (binder,
thickener, additive, etc.). In Figure 4,
you can see a demonstration of the vari-
ation in dirt pickup resistance of four
exterior satin formulations.
In this test, dry dirt was applied to the
bottom half of the coated panel. After
10 minutes, the panel was turned vertical so that most of the dirt fell from the
surface of the film. As you can see, one
coating retains significantly less dirt than
the others.
Exterior elastomeric coatings represent a particular challenge for achieving
dirt pickup resistance. These coatings are
designed to be highly flexible so that they
can bridge cracks in the substrate as they
form. However, this results in a rather
soft, low Tg film that can allow dirt to
stick at the surface and embed itself in
the coating. Future material development efforts will be aimed at making flexible coatings with some hardness at the
coating/air interface to prevent dirt from
sticking. Currently, ASTM subcommittee
D01.42 is working on publishing a new
test method for measuring dirt pickup resistance of architectural coatings.
Another important property for exterior paint is the development of integrity
against rain early in the life of the coating,
or “early rain resistance.” Exterior painters cannot predict exactly when rainfall
will occur. It is desirable to have a coating that will not be damaged or washed
away by a rain shower one or two hours
after it is applied. This property can be
easily tested by exposing painted panels
to water streaming from a shower head at
a controlled flow rate (Figure 5).
Early rain resistance is achieved by
creating a coating that coalesces quickly
and accelerates water leaving the film.
Usually this property comes at the expense of dry time: a coating that has early
rain resistance will become dry to touch
very quickly.
VOC-Compliant Oil-Based
Materials
Despite the recent trend to waterborne
coatings, there are some applications in
which oil-based materials are still preferred. For those who want the look and
ABOVE:
Figure 4: Variation in dirt pickup
resistance of four exterior coating formulations.
LEFT:
Figure 5: Early rain resistance
test.
