Coatings Clinic
By Clifford K. Schoff, Schoff Associates
Sagging
Sagging is a defect caused by gravity-driven
flow on vertical surfaces. Sags can be subtle or very
obvious, such as the ones in Figure 1. Occasionally,
the defect is not noticeable except for fine pops or
pinholes that occur in the thick area. Sag is caused
by excess flow where a thick section flows faster
than other parts of the wet film. This does not happen if the film thickness is exactly the same across
the entire area. Downward flow would occur, but it
would be the same everywhere and there would not
be noticeable sag. This actually happens occasionally on lab panels hung vertically during application.
A known sag-prone coating will look better than a
normally superior one. Sag in customer plants or in
the field is more apt to happen where there is a hole,
bracket, corner, edge, style line or other place that
catches more paint. In addition, spray application
of paint often produces a pattern of droplets and
the “bumps” may form very small sags (microsags)
on vertical surfaces, but will flow out on horizontal
surfaces. This is one reason why verticals on automobiles, such as doors and verticals on deck lids
(“waterfalls”), look rough in comparison to hoods,
roofs, and deck lid horizontals. Another reason is that
auto companies are so concerned about sagging that
paint applied to verticals often has a higher viscosity,
which prevents sag, but also interferes with leveling.
When sagging occurs on application, it is called
cold sag. When it occurs in the oven, not surprisingly,
we call it hot sag. The latter problem has become a
lot more common as the industry has gone to high
solids coatings with low molecular weight polymers
and oligomers that flow and flow and flow at elevated
temperatures even after all the solvent has gone.
Sag velocity and sag volume are given by the following equations:
v = ρgT2/2η
v = ρgT3/3η
where ρ is the density of the coating, g is the force
due to gravity, T is the film thickness, and η is
the viscosity. These equations really work only for
Newtonian coatings (of which there are very few
these days), but still can provide valuable information. For example, they show the great importance of
film thickness. Doubling the film thickness increases
the sag velocity by a factor of four and sag volume by
a factor of eight.
In preventing sag, it is not necessary to stop
downward flow completely. In fact, some flow is
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October 2014
COATINGSTECH
Figure 1—Sags in an architectural coating (plus a few seeds).
needed for leveling. If the sagging velocity is low
enough, the paint will dry or cure before noticeable
sag will occur. S.Wu [J. Appl. Polym. Sci, 22, 2769,
2783 (1978) (two excellent papers)] estimated that
0.1 cm sag was acceptable and that sag time for
most high solids coatings was about 10 min. This
works out to a low enough velocity, being about 10-2
cm/min. According to Wu, a viscosity of 25–50 Ps
at low shear after application is sufficient. (Such
measurements are done by spraying and scraping
or drawing down, flashing, and scraping to produce
specimens.) In addition, coatings with yield stresses
(which includes most architectural paints) are sagresistant. Unless the gravitational shear stress
exceeds the yield stress, no sagging will occur.
Sagging can be reduced or prevented by raising
the viscosity of the paint at low shear stress or rate
and/or by applying thinner coats of paint. The exact
viscosity needed depends on the paint and the conditions. The film viscosity can be raised by using faster
solvents or by introducing thickeners or thixotropes.
The latter are additives such as fumed silica, treated
clays, microgels, and castor oil derivatives that form
physical networks, which produce yield stresses.
Associative thickeners in waterborne paints serve
a similar purpose. Structures form after application
to reduce flow and prevent sag. Unfortunately, they
also can prevent flow-out and leveling on horizontal
surfaces, so amounts of the additives must be chosen carefully. Ideally, there will be initial flow to allow
leveling, followed by viscosity build to prevent sagging.
Thickeners that produce networks also reduce hot
sag, as do materials such as waxes and certain castor
oil derivatives that melt in to raise dissolved solids.
All of these change viscosity–temperature behavior to
prevent or reduce the initial sharp viscosity drop that
usually occurs on the heating of a coating. CT