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A closer look at low gloss powder coatings
AFM measurements clarify the modification effect of
incompatible polyester-β-hydroxyalkylamide resin blends.
For the defined matting of TGIC-free polyester β
-hydroxyalkylamide powder coatings, dry blends of
formulations based on resins that are incompatible due to
their different reactivity have been shown to have a high
potential. To clarify the effects that govern the gloss
reduction, the correlation of coatings performace data with
Atomic Force Microscopy results can be extremely helpful.
Louis T. Germinario, Damiano Beccaria, Andrea Capra, Imir
Bejko.
Many outdoor applications in powder coatings specify a low
gloss finish. Coating systems based upon polyester resins
and beta-Hydroxyalkylamide as the curative play an
increasingly important role in this arena. Dry blending of two
coatings of different reactivity has been a standard method
for achieving low gloss in powder coatings made with TGIC.
However, the substitution of TGIC with β-Hydroxyalkylamide
(β-HAA) has required the design of a new generation of
polyesters. As we have shown in the preceding paper [1],
new formulations based on dry blends of powder coatings
made with six different polyester resins with different ratios
of β-HAA polyester-curing agent provide coatings with
different levels of gloss, chemical and mechanical
properties. For such blends, a strategy was found, which
allows consistent, well defined target gloss values to be
formulated. This study has shown that the relative difference
in acid values of two resins is a key parameter and can be
used as a measure to control the final gloss values of the
formulated coating.
In an effort to better understand the structure-property
relationships found for the blends, Atomic Force Microscopy
was used to characterize the surface roughness and surface
mechanical properties of these systems.
Experimental details
Typical properties of the six resins used for the primary
powder coatings are reviewed in Table 1. Only white coating
formulations were used for the AFM studies reported
here.These formulations based on the six resins are given in
Table 2. They were prepared by pre-grinding the resins,
curing agents and additives, together. Formulations were
then made in an "APV MP-30" extruder, followed by grinding
in a strand mill and classified using a 106 µ sieve.
The resulting coatings were dry blended in various
combinations, always at a 50/50 weight percent ratio. The
blended coatings were sprayed on "QD-36" panels using a
60KV corona spray gun. All coatings used for the AFM
analyses were cured at 200°C for 10 minutes and
characterized using the following ASTM test methods:
- Method D 523 for testing the gloss, using the Byk Gardner
"Micro-Tri-Gloss".
- Method D 2794 for testing direct and reverse impact values
Measuring surface stiffness via AFM
For the AFM measurements, the cured blended powder
coatings were analyzed directly on the panels. In order to
accurately characterize the coating's surface morphology
and topography, all surface images were obtained with a
commercial atomic force microscope, "Dimension series
D3000" AFM (Digital Instruments, Santa Barbara, CA).
Probes used for analysis are from Nanodevices, "Multi 75",
with spring constants of 5 N/m, resonant frequencies of 75
kHz and a tip radius below 10nm. AFM was used to
measure both the topological surface roughness and the
surface stiffness using the force modulation imaging mode.
The latter is a very useful imaging mode, which identifies
and maps differences in surface stiffness and also provides
the surface topography. Force modulation is a contact
imaging mode, where the probe's movement perpendicular
to the sample surface (in the z-direction) is modulated or
oscillated. As the tip is scanned over a surface, under the
same applied force, a stiff area on the sample will deform
less than a softer area. Thus, stiffer areas will resist vertical
(z) oscillations of the tip and lead to greater tip deflection
and consequently appear as higher image brightness.
Image data were recorded as both height and force
modulation modes, which were recorded simultaneously.
Surface roughness analysis was also performed on film
surfaces by determining the root-mean-square (RMS)
surface roughness. This is based on the following equation:
RMS = [y12 + y22 + y32 + ... + yN2]1/2 / N
where y is the height value of each image point and N is the
total number of points for a given image.
To improve the sampling statistics, surface roughness
measurements were collected from 10 areas on each
sample, each area 100 by 100 microns wide and randomly
chosen on each panel.
beta-HAA content affects the roughness
The left-side image in Figure 1 is a top view or height image
that contains topographic information from the resin 1/resin
4 white blend, while the image on the right is a force
modulation image. Overall, this system contains 3.75% β
-Hydroxyalkylamide in the binder. The average RMS
roughness for this system is 0.44 microns, while the average
RMS force modulation value is 1.86 nm.
Height images can also be displayed as 3-D images, where
the white peaks are more easily recognizable from the
darker, low-lying "valleys". Figure 2 provides an example of
such a 3-D view of the resin1/resin 4 white coating. This
view clearly displays the gradual undulations that are
characteristic for the surface topography of this formulation.
Upon increasing the concentration of β-HAA to 6.75% (resin
3/resin 6 white coating, Figure 3), the morphology is
significantly changed. An increase in peak heights and
surface roughness is found. The average RMS roughness
increases to 0.72 microns, while the average RMS force
modulation value also increases to 6.13 nm.
These data provide some direct evidence for a
concentration effect of the curative on the coating's surface
roughness and stiffness. As the amount of β-HAA is
increased, the surface RMS roughness increases
significantly, as does the average surface stiffness. The data
support the hypothesis that higher β-Hydroxyalkylamide
levels, and higher crosslinking densities, lead to an increase
in both surface roughness and stiffness. Figure 4 provides a
3-D view of the surface topography of the resin 3/resin 6
white coating. The presence of both 'stiff' and 'soft' domains,
a few tens of microns in diameter, that are visible in the
force modulation images, suggests that the film leveling is
minimized due to a lack of mixing of the individual resin
components. In contrast, the resin 1/resin 4 white coating
systems, that contains 3.75% β-HAA (Figure 1), shows no
evidence of separate domains in the force modulation
image, suggesting an intimate mixing of the resin
components.
Roughness is the cause for gloss reduction
Table 3 summarizes the test results of all white blends,
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including the gloss, impact, and the RMS roughness and
average surface stiffness values calculated from height and
force modulation images, respectively.
The correlation analyses for these measurements are given
in Table 4, expressed as the linear correlation cofficients R2.
Clearly, a high negative correlation is found between RMS
roughness and gloss (R2 = -0.95). This high correlation is
encouraging, because it provides direct evidence for a
sufficiently rigorous sampling protocol used for calculation of
average RMS surface roughness. This relationship between
surface roughness and gloss is also explicable from the
AFM data, because the average size of the surface
imperfections and irregularities, measured from the RMS
roughness calculations, is equal to or larger than the
incident wavelength, supporting the hypothesis that surface
imperfections or irregularities lead to a diffuse scattering of
the incident light and thus to a loss in brilliance and gloss.
A reasonably good negative correlation is also found
between gloss, or RMS roughness, and the average RMS
force modulation, implying that an increase in roughness is
associated with an increase in the stiffness of the coating.
Higher functionality - lower gloss - higher stiffness
There is also a good correlation (R2 = 0.87) between the
RMS roughness and β-HAA content in the blend (Table 4).
This supports the view that a higher crosslinking density in a
coating may lead to increased surface roughness, possibly
due to a restriction in resin flow and lack of leveling during
film formation.
There is also a reasonably high correlation (R2 = 0.82)
between the average RMS Force Modulation and the β-HAA
content. As previously mentioned, height images contain
surface topography information, while force modulation
images differentiate soft from stiff polymer segments. A
higher average value of force modulation intensity is a clear
indicator of a stiffer coating. This relationship is further
validated by the good correlation observed between
Average RMS Force Modulation and the Direct (R2 = -0.77)
and Reverse (R2 = -0.81) Impact (Table 4). β
-Hydroxyalkylamide-polyester systems are a growing force
in powder coatings for exterior applications. Since these
systems cannot be catalyzed, the attainment of high
reproducibility for low-gloss coatings is more challenging
than for their TGIC counterparts.
properties that are relevant to a coating's performance.
Acknowledgement
The authors wish to thank Penny J.E. James, William T.
Sade, Gregory C. Alexander, Donato di Lorenzo, Keith
Middleton, Wayne T Riddick, the Sant'Albano Center of
Excellence and all other people who contributed to the
development of this project.
REFERENCES
[1] D. Beccaria, A. Capra, I. Bejko, L. T. Germinario; ECJ
7-8/2003, p. 21.
The authors:
-> Louis T. Germinario, Ph.D., is Senior Research Associate
in Eastman's Physical Chemistry Research Laboratory in
Kingsport, Tennessee.
-> Dr. Damiano Beccaria is Manager of the Eastman Center
of Excellence Sant'Albano Stura Technical Lab in Italy,
which is in charge of all Eastman Coatings (liquid and
powder) polyester chemistry for the EMEA geographical
area.
-> Andrea Capra is Supervisor of the R&D Laboratory for
liquid and powder polyesters chemistry at Sant'Albano Stura
and responsible for the site Process Support Team.
-> Dr. Imir Bejko is supervisor of the powder coating
application laboratory at Sant'Albano Stura synthetic resins
plant.
Powder Coating Training Course
Organized by the Paint Research Association (PRA), a new
training course, Powder Coating Technology, is to be held
on 4-5 November 2003. It provides an insight into the
formulation and manufacture of coating powders, both
thermoplastic and thermosetting.
The course includes a half-day practical session at a nearby
powder coating installation where there will be an
opportunity to apply coating powders, understand
application parameters, and test coatings in production
conditions.
Contact:
Elisabeth Brown,
The Paint Research Association,
Tel. +44 (20) 86 14 48 15,
e.brown@pra.org.uk
Conclusion
In studying gloss in various dry-blended polyester systems,
using both black and white coatings, it was determined that
the relative difference in acid value (AV) of resins was the
most useful predictor of gloss [1]. This relationship gives the
formulator some options for achieving a specific gloss value,
at a specified price and performance. For instance, for the
formulations listed in Table 2 cured for 10 minutes at 200°C,
the following formula holds:
Gloss 60° = 82 - 0.6 ∆rel(AV) where: ∆rel(AV) = 100 abs(AV
resin 1- AVresin 2) / Average (AVresin 1, AVresin 2)
This relationship has a correlation coefficient that is very
close to minus one, and can be used to predict the level of
gloss, for a two-resins blended system, by judicious choice
of resins and their and acid values. These results further
support the significance of tight acid value control in a low
gloss system.
Scanning probe microscopy and statistical tools have linked
nanostructure and chemical composition with macroscopic
properties such as gloss and impact strength.
The high correlation between the macroscopic gloss and the
microscopic surface roughness provide supporting evidence
for the ability of Scanning Probe Microscopy, and Atomic
Force Microscopy in particular, to measure surface
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Figure 1: Height image (left) and force modulation image (right) of resin 1/resin 4 white
coatings. Average (RMS) roughness is 0.44 microns, average (RMS) force modulation
is 1.86 nm..
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Figure 2: 3D-Topography of resin 1/resin 4 white coatings.
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Figure 3: Height image (left) and force modulation image (right) of resin 3/resin 6 white
coatings. Average (RMS) roughness is 0.72 microns, average (RMS) force modulation
is 6.13 nm..
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Figure 4: 3D-Topography of resin 3/resin 6 white coatings..
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