Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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112
BRENNAN AND
FEDOR
In addition we should point out that despite many chemists’ fascination with light
energy, the spectrum of
a
test device is only one part of the picture. With any accelerated
tester, there are
a
number of parameters that must be programmed:
UV
spectrum, moisture,
humidity, temperature, and test cycle. Furthermore, the parameters that one chooses are,
to
a
certain extent, arbitrary. No single test cycle or device can reproduce all the variables
found outdoors in different climates, altitudes, and latitudes. Consequently, even the most
elaborate tester is really just
a
screening device. Accelerated weathering data are compara-
tive data. The real usefulness of accelerated testers is that they can give reliable, relative
indications of which material performs best under
a
specific set
of
conditions.
ACKNOWLEDGMENTS
Most
of
the data in this paper were originally presented at the Society
of
Plastics Engineers
Automotive RETEC, November
1987.
The authors are grateful for the cooperation of Kitt
Peak National Observatory, Kitt Peak, Arizona, and Ohio Spectrographic Service, Perma
Ohio.
REFERENCES
1.
N.
Searle, and
R.
Hirt, “UV SED
of
sunlight,”
J.
Opt.
Soc.
Am.,
55
(I
1
)
(1965).
2.
CIE Standard
No.
20, 19.
3.
D.
Grossman, “Know your enemy: The weather,”
J.
Vinyl
Teclrrlol..
3
(I)
(1981).
4.
G.
Zerlaut, “Accelerated weathering
&
UV measurements,” ASTM STP
781.
Philadelphia:
5.
G. Grossman, “Correlation of weathering,”
J.
Cocrtir~gs
Techno/.,
49
(633)
(1977).
6.
J.
Dick, et al., “Weathering
of
pigmented plastics,” SAE Technical Paper
No.
850350,
1985.
7.
R.
Fischer, “Accelerated test with fluorescent UV-condensation,” SAE Technical Paper
NO.
American Society for Testing and Materials,
1982.
84 1022, 1984.
13
Cure Monitoring: Microdielectric
Techniques
Developments
in
the area
of
microelectronics now enable the fabrication of microdielectric
sensors that can analyze drying, curing, and diffusion phenomena
in
coatings.' Several
types of microdielectric sensor have evolved in the past few years, the most sensitive
being based on an interdigitated electrodes and field effect transistors fabricated on
a
3
X
5
mm silicon chip.' The chip sensor is housed in
a
polyamide package and configured
for ease of placement in various processing environments
(Fig.
1).
1.0
THE
DIELECTRIC RESPONSE
The dielectric response arises from mobile dipoles and ions within the material under test.
As
a
coating cures, the mobilities of dipoles and ions are drastically reduced, sometimes
by
as
much
as
7
orders
of
magnitude. Microdielectric sensors are sensitive enough to
follow those changes and are therefore useful for cure monitoring, cure analysis, and
process control.'
The dielectric response is typically expressed by the quantities of permittivity or
dielectric constant
(E')
and
loss
factor
(E):
where
(
E4
-
E")/(
l
+
wt') is the dipole term,
scow
is
the conductivity term, and
E'
=
dielectric constant
F
=
loss
factors
113
114
Connector
DAY
Area
Figure
1
Schematic
diagram
of
microdielectric
sensor.
s
=
bulk ionic conductivity
eo
=
permittivity of free space (a constant)
W
=
frequency
X
2.rr
T
=
dipole relaxation time
E,.
=
relaxed permittivity (low frequency
E')
E,,
=
unrelaxed Permittivity (high frequency
E')
In addition to ions and dipoles, other factors, such
as
electrode polarization4 or
inhomogeneities, may influence the dielectric response; however, these generally play
a
minor role and are usually ignored.
The dielectric
loss
factor is the most useful quantity for monitoring cure reactions.
Either dipole relaxation times or ionic conductivity levels may be monitored during cure.
However, dipole relaxation times are usually difficult to determine because the conductiv-
ity response often dominates the dipole term in Equation
2.
On the other hand conductivity
usually can be determined throughout the entire cure process, especially
if
low frequencies
(<l0
Hz) are monitored during the end of cure. The inverse
of
the measured conductivity,
resistivity, is often proportional to viscosity of the material under test before gelation and
related to rigidity after gelation.' Figure
2
shows loss factor data during an isothermal
cure of an epoxy resin. Figure
3
compares the
log
(resistivity) calculated from the data
in
Figure
2
to degree of conversion as determined by the fractional generated heat method
using differential scanning calorimetry
(DSc).
Figure
3
shows that the dielectric response
can monitor the entire cure and
is
far more sensitive to last few percent of cure than
DSc.
2.0
CHANGES IN RESISTIVITY DURING
CURE
Figure
4
plots changes in resistivity during isothermal cure versus changes in the glass
transition temperature. These data demonstrate the utility of microdielectric sensors for
monitoring and process control in coatings.
CURE MONITORING: MICRODIELECTRIC TECHNIQUES
115
Y
Figure 2
Dielectric
loss
factor data of isothermal (392°C) epoxy-amine cure. Frequencies range
from
10"
to
IO"
Hz.
Figure
3
Ionic resistivity data from Figure
1
and degree
of
conversion as determined
by
DSc
versus time.
DAY
I
,'C
I.
I
Figure
4
Ionic resistivity data
and
T,
during isothermal epoxy-amine
cure.
Using the previously described techniques, ionic conductivity (or its reciprocal,
resistivity) can be obtained in real time by continuously monitoring a range of frequencies.
It can them be used to control the reaction through variation in either temperature or
pressure. There exist several ways in which dielectric feedback may be used. Some
of
these are
as
follows.
Temperature may be held constant or controlled until a desired viscosity (measured
Viscosity may be held constant
or
varied at will through controlled variation
in
Pressure, vacuum, or mold opening may be activated upon attainment
of
a
critical
Reaction may be terminated when the dielectric reaction rate decreases below some
dielectrically) is attained.
temperature.
viscosity
of
dielectric reaction rate.
critical value.
2.1
Process Control Through Dielectric Feedback
Figure
5
is an example of a process-controlled cure of
a
graphite epoxy using microdielec-
tric feedback. Control
was
achieved using an IBM PC with modified Micromet Instruments
dielectric software and hardware. The cure was carried out in
a
hot press with temperature
controlled from the computer. The process control software sequence was as follows.
I.
Heat and hold at 250°F until
a
log resistivity
of
7.0
is
reached (allows for
2. Hold log resistivity (viscosity) at 7.0 until 350°F is reached (allows for controlled
degassing while preventing premature cure).
curing and prevents second viscosity minimum).
CURE MONITORING: MICRODIELECTRIC TECHNIQUES
117
13
450
Hold
AI
250°F
Hold
lonvisc.
Until AI
7.0
Until
Hold
At
350°F
Cool
Down
-
400
l2
-
lonvisc.
=
7.0
Temp.
=
350°F
Until
Slope
=
0
2-
{
350
i
3
2
250
5
%
300
’.
...
100
I
5
1K
&
10K
Hz
I
I
l
50
0
50 100
150
200
lime
-
min
Figure
5
Process control
of
epoxy graphite cure utilizing microdielectric feedback
3. Hold at 350°F until dielectric reaction rate is near zero (allows reaction to go
4.
Cool
down and notify operator that cycle has been completed.
to
completion).
2.2
Process Control Through Dielectric-Thermal Feedback
The cure in Figure
5
was completely controlled through both dielectric and thermal feed-
back from the microdielectric sensor in contact with the graphite epoxy material. Note
that the second viscosity (resistivity) minimum typical
of
these materials was completely
eliminated through the use
of
viscosity control. This technique is useful for limiting exces-
sive bleed in composites that are prone to such problems. Finally the end point was detected
and the reaction stopped. eliminating unnecessary overcuring time.
3.0
SUMMARY
Microdielectric sensors are useful for monitoring cures of coatings under actual processing
conditions. The ionic resistivity portions of the
loss
factor data correlate to viscosity during
the early stages of reaction (before gelation or solidification).
As
the reaction progresses,
the slope of the ionic resistivity can be used
to
monitor reaction rate and to detect when
cure is complete. The microdielectric sensors provide
a
unique capability
to
correlate
measurements made
in
the laboratory
to
those
in
the factory and
to
provide the necessary
feedback information for adaptive process control.
118
DAY
REFERENCES
1.
D. R. Day and D. D. Shepard,
J.
Coatings Technol.,
60
(760);57 (1988).
2. (a)
S.
D. Senturia, N.
S.
Sheppard,
Jr.,
H.
L. Lee, and D. R. Day,
J.
Adhes.,
15,
69(1982).
(b)
N.
F.
Sheppard,
Jr.,
D. R. Day,
H.
L. Lee, and
S.
D. Senturia, Sensors Actuators,
2,
263(1982).
3. (a)
W.
E. Baumgartner and
R.
Ricker, SAMPLE
J.,
19(4), 6(1983).
(b)
D.
R. Day,
Proceedings
of
the SAMPE Symposium, Las Vegas, 1986, p. 1095. (c) D. E. Kranbuehl,
Proceedings
of
the SAMPE Symposium, Anaheim, CA, 1988. (d) D. R.
Proceedings
of
the SAMPE Symposium,
Anaheim, CA, 1988, p. 594.
4. D.
R.
Day, T.
J.
Lewis, H.
L.
Lee, and
S.
D.
Senturia,
J.
Adhes.
18.
73 (1985).
5.
J.
Gotro and
M.
Yandrasits,
Proceedings
qf
the
45th
SPE ANTEC, Anaheim, CA, 1987, p.
1039.
14
Test
Panels
Douglas Grossman and Patrick Patton
Q-Panel
Lab
Products, Cleveland,
Ohio
When performing coatings tests, it is important
to
make sure that problems with the
metal substrate do not skew the test results. Test standards exist for
all
sorts
of
coatings
characteristics, including adhesion, flexibility, corrosion resistance, and appearance. These
standards establish test conditions designed to control variables, which can influence test
results. These variables include the method of application, the film thickness, the cure
method, and the test substrate.
In
a
controlled laboratory environment, the application method, film thickness, and
cure method can be controlled with some degree of precision. In many cases, it is not
possible to exercise the same degree of control over the test substrate.
For
this reason,
coatings technicians use standardized test panels when conducting critical tests.
A
standard-
ized panel is produced from carefully specified material and prepared in a tightly controlled
process designed to yield a consistent test surface, which can be relied upon to provide
reproducible results from test to test and from batch to batch.
There are many different types of standardized test panels available. The require-
ments for these panels have been described in both national and international standards.
These include
IS0
1514:
Paints and Varnishes-Standard Panels for Testing, ASTM
D
609: Standard Practice for Preparation
of
Cold-Rolled Steel Panels for Testing Paint,
Varnish, Conversion Coatings and Related Coating Products, and ASTM
D
2201: Stan-
dard Practice for Preparation
of
Zinc Coated and Zinc
Alloy
Coated Steel Panels
for
Testing Paint and Related Coating Products. The following is a general description of
the different types of test panels included in these standards, along with a discussion of
the primary applications and sources of variability for each panel type.
1.0
COLD ROLLED STEEL PANELS
There are a number
of
points to consider when preparing a specification for standardized
cold rolled steel test panels. The type of steel selected should be
of
a standard grade and
119
120
GROSSMAN AND PAlTON
quality. It is important that the steel be widely available.
SAEi
l008
and
1010
are examples
of suitable grades of steel for test panel production. The steel used should also be free
from rusting and staining. Standardizing on a particular grade of steel helps to eliminate
variability in the chemical composition that can influence the results of some types of
testing.
Specifying a particular grade of steel may be all that is required for many industrial
applications, but this is not the case when the steel will be used to produce test panels. It
has been well documented that different batches of seemingly identical steel, even when
produced by the same mill, have shown drastic differences in performance under paint
(see Figure
1).
Over the years, several researchers have undertaken to explain
this
problem
of “good steel” vs. “bad steel.”
A
number of these studies have been successful in
isolating the factors contributing to this variability. It
is
important that specifications for
steel to be used in the manufacture of test panels take these factors into account.
Right Rear Fender
Right Front Door
288
hours,
salt
spray
192
hours,
salt spray
Trunk Lid, Center
Hood, Center
288
hours,
salt
spray
288
hours,
salt spray
Left Front Fender
Left Front Door
192
hours,
salt spray
288
hours,
salt
spray
Figure
1
Metal substrates and their effects on paint.
TEST PANELS
121
1.1
Surface Profile
Surface roughness, waviness, and peak count are some of the parameters that make up the
surface profile of a steel panel. Surface profile is an important consideration in designing an
effective test panel specification. Steel mills typically characterize different finishes as
matte, light matte, commercial bright, etc. These characterizations represent fairly broad
ranges in terms of surface protjle. While these characterizations may be perfectly suitable
for most applications, the surface profile for steel used in the production of standardized
test panels must be more narrowly defined.
For
example, if a specification calls for steel
with a “matte” finish, the surface roughness of the steel as received may be anywhere
from
25
to
65
p.
inches, or even higher. Different mills define a matte finish differently.
In addition, depending on the condition and degree of wear of the finish rolls, the “matte”
finish produced by a given rolling mill can vary greatly from shipment to shipment. For
these reasons, a specification for steel used in the production of test panels must call out
a more narrowly defined range for surface roughness (i.e.,
35-45
inches). The actual
range specified is not necessarily as important as the fact that a range is defined.
Some applications in which surface profile is particularly critical include appearance
measurement and testing of phosphate coatings. The surface profile of a steel panel can
have a significant impact on the appearance of a subsequently applied coating. Reliable
evaluations
of
gloss, distinctness
of
image, and other appearance-related properties cannot
be made if the consistency of the surface profile
of
panels used in the evaluations is in
question. Similarly, steel test panels are often used to evaluate phosphate coating processes.
Line technicians expect that a standardized test panel will accept a coating with given
characteristics, assuming the phosphate process parameters are in order. However, normal
mill variability in steel surface profile can contribute to differences in phosphatability.
For example, a rougher steel surface has a greater surface area than a relatively smooth
steel surface. This difference in surface area can lead to differences in coating weight.
Surface profile is by no means the only factor bearing an impact on the phosphatability
of a steel surface. It is, however, an important factor and must be controlled as a Source
of variability.
1.2
Surface Carbon
Over the years, steel suppliers and automotive companies have funded numerous research
projects aimed at determining the source
of
variability in performance between different
lots of seemingly identical steel. The results have
not
been ambiguous. In study after
study, surface carbon has been identified as an important factor contributing
to
variations
in phosphatability and under-paint corrosion resistance
of
commercial cold rolled steel.
When cold rolled steel panels are phosphated and painted under identical conditions
and tested for salt spray resistance
in
accordance with ASTM
B
117, there is a strong
correlation between high levels
of
surface carbon and premature failure in salt spray testing.
Surface carbon is highly adherent and cannot be removed by typical cleaning operations.
In addition. it is impossible to determine the level
of
surface carbon on a steel surface
without sophisticated and expensive laboratory evaluation. For these reasons, it is impor-
tant
to
understand where surface carbon comes from and how it can be controlled.
During the process of cold reduction, a stress is imparted to the steel sheet. resulting
in work hardening. This work hardening must be relieved by annealing, or holding the
steel at an elevated temperature for a specified period of time. Annealing softens the metal,
resulting
in
improved formability. Rolling oils are applied to the steel surface during cold