Satas D., Tracton A.A. (ed.). Coatings Technology Handbook
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82
KENDALL
If changes with time are to be studied, it may be possible to apply the coating to the ATR
crystal and then watch changes with time. The evaporation
of
solvents. curing reactions,
and migration
of
additives
to
the surface are some of the possibilities. Films of soluble
components can be cast on the ATR crystal. ATR has been used to study polymer blend
films.
I
'
3.4
Infrared Microscopy
Infrared microspectroscopy is increasingly being used
to
identify very small samples,
or
to examine a small region of a coating.13"" This might be useful to find the cause of a
failure or blemish. Infrared microscopes can be accessories to IR spectrometers,
or
they
can be stand-alone instruments. There is usually provision for the visual observation of
the sample
so
that the analyst can correctly position a small sample
or
select the portion
desired from a larger sample. Apertures can be adjusted
so
that the IR beam strikes only
the desired portion of the sample. Either transmission or reflectance spectra are possible.
For transmission measurements the most common problem is excessive sample path length.
The considerable pressures achieved by a diamond cell can reduce many samples to the
thickness required for useful spectra. Forensic applications, which require such analyses
as the identification
of
small paint chips, rely heavily on infrared microscopy. Many
analysts use IR microscopy for most of their solid samples.
3.5 Infrared Photoacoustic Spectroscopy
Photoacoustic accessories to IR spectrometers have features attractive for coatings applica-
tions. This means of obtaining an infrared spectrum requires little sample preparation.
After the sample is placed in a sealed cell, it is irradiated with
a
modulated or varying
infrared beam. At frequencies at which the sample absorbs, it alternately heats and cools.
This heating and cooling is transferred to the gas in contact with the sample, causing it
to expand and contract. A microphone in the cell is able to detect the alternating waves
of expansion and contraction as sound waves. Wavelengths of infrared radiation which
are more strongly absorbed will cause more intense sound waves, thus leading to
a
spectrum
much like an absorption spectrum in transmission mode.
There are several advantages to using IR photoacoustic spectroscopy
to
study coat-
ings. It can be used for insoluble or otherwise intractable coatings, since the sample has
only
to
fit in the cell. As a surface technique, it lends itself readily to coatings. The
application of IR photoacoustic spectroscopy to polymers has been reviewed.'"'') It is
possible to
do
depth profiling. By varying the conditions under which the IR photoacoustic
spectrum is obtained, it is possible to change the depth at which the photoacoustic signal
originates. Information on the composition of the coating and the substrate to a depth of
tens of microns is obtainable. Depth profiling is especially informative with step-scan
FTIR photoacoustic spectroscopy.20-" With this type of instrumentation the wavelength
dependence of the depth information is almost eliminated.
3.6
Other Sampling Methods
There are numerous additional means
of
obtaining infrared spectra of coatings if the
standard methods are impractical. If a coating
or
a coating component proves to be insolu-
ble, pyrolysis of the sample is a possible approach." Pyrolysis equipment is available
from a number
of
vendors,
or
a test tube can be used. Many types of polymers can be
INFRARED SPECTROSCOPY
OF
COATINGS
a3
analyzed by
IR
spectroscopic examination of the pyrolyzate.” It is necessary to have
access to reference spectra of pyrolyzates, as the pyrolysis products often differ from the
polymer and the monomers.
Reflectance techniques can be used to examine coatings. By comparing the reflec-
tance from
a
sample to that of
a
nonabsorbing reference material, it is possible to determine
the absorptions due
to
the sample.’’.’‘ Diffuse reflectance and specular reflectance can
be used to obtain the spectra
of
coatings on
a
variety of substrates. Reflection measurements
have been used to study coatings on ~teel.”.’~ Diffuse reflectance is useful for forensic
samples.’”
4.0
DATA INTERPRETATION
Once the infrared spectrum is obtained, it must be interpreted. This depends on the skill
and experience of the interpreter and the availability of reference information. Search
software and an electronic database of reference spectra make the task easier,
as
does
complete information about the sample and its history. The Federation of Societies for
Coatings Technology publishes an excellent book on infrared spectroscopy for the coatings
industry,‘ which includes instructional text and excellent reference spectra. The spectra
are now available in digital form from Nicolet Analytical Instruments.3”
An infrared spectrum is an almost unique fingerprint of
a
particular molecule or of
a
coating formulation. The spectrum is characterized by the number, shape, and intensity
of absorption bands. This pattern can be compared to
a
collection of known patterns, either
manually or with
a
computer. Extensive collections of reference spectra are available.
Nicolet Analytical Instruments”’ and the Sadtler Division of Bio-Rad” offer extensive
collections in computer ready form. Hummel has prepared several excellent collections
of
infrared spectra useful for coatings applications, including polymers and related materi-
als.’2-3J
Computer searches work best for pure compounds or mixtures that happen to be
in the library. Mixtures may not be satisfactorily matched by search software. If
a
computer
search fails to find
a
good match, then it may be worthwhile to simplify the interpretation
by separating the coating into components and scanning each. Even if
a
computer search
fails to find
a
good match, the presence of group frequencies in infrared spectra (explained
below) often means that the best matches have functional groups similar to those
of
the
unknown. It is
a
good idea carefully
to
inspect the conclusions reached by the software
before accepting them
as
legitimate.
An infrared spectrum contains significant structural information besides being
a
unique fingerprint.3s Infrared absorption bands can be divided into characteristic bands
and fingerprint bands. The latter are unique
to
a
particular molecule. Characteristic bands
or group frequencies are caused by
a
particular functional group, such as an ester or an
amide. These group frequencies are almost independent
of
the rest
of
the molecule in
which they occur and can be used to determine which functional groups are present in
a
molecule and which are absent. Their slight dependence on the rest of the molecule can
be used to gain additional information. For instance, unsaturated and saturated esters
have slightly different carbonyl stretching bands. Compilations of group frequencies are
available to guide the analyst through interpretation.36 Once the functional groups present
in the unknown are identified, and once alternatives have been eliminated, the analysts
can search the available reference spectra for compounds or polymers with the same
functionality
as
the unknown.
84
KENDALL
5.0
APPLICATIONS
There is an extensive literature on the application of infrared spectroscopy to coatings.
Books on IR applications include coatings.37-") Every odd-numbered year there is
a
review
in
Analytical Chernistry
of the analysis of coatings.'" In even-numbered years the review
issue of
Analytical Chemistry
covers specific techniques, including IR spectroscopy.4'
The major infrared spectrometer manufacturers, such
as
Nicolet, Pkrkin-Elmer, Bio-Rad,
Mattson, and Bomem, have applications laboratories staffed with experienced analysts.
Laboratories in the coatings industry have experienced infrared spectroscopists. All
of
these can be sources of valuable help.
A principal application of
IR
spectroscopy to coatings is chemical analysis. IR is
unique in being able to identify almost the full range of coating components, including
volatile solvents, resins and polymers, inorganic and organic pigments, and
a
wide variety
of additives. The use
of
solid sampling accessories for the analysis of coatings has been
reviewed:
as
has the analysis of polyester and alkyd resins.'j The use
of
FTIR in the
analysis of coatings and surfaces in packaged foods has been reviewed."' Forensic chemists
rely
on
IR, particularly infrared microscopy, to identify paint chips and other coatings.
Their focus is matching their sample to a particular source, such
as
an automobile. The
diamond anvil cell is useful for forensic analysis.'' Identification of coatings and their
components can be required in many different contexts. The manufacturer or formulator
can use IR to identify contaminants and check the quality of raw materials. Products can
be checked with IR. The analysis
of
competing products often relies on IR.
IR is very useful for studying changes that occur with coatings. Examples include
following polymerization reactions that occur during the production
of
a coating or after
its application." Other examples include the curing of epoxy-based
coating^,'^
automotive
clear coats,4x optical fiber coatings,J9 and powder coatings.'" The evaporation of solvents
can be monitored. Defects in coatings can be studied." The degradation of
a
coating with
time, temperature, or adverse conditions can be monitored." The degradation products
can be identified and the mode of degradation elucidated. The application of IR to
UV
durability predictions has been reviewed." It may be possible to monitor the coating in
situ. Otherwise, the coating must be applied in such a way that it can be observed by IR.
For example,
a
coating can be applied
to
an ATR crystal.
IR spectroscopy can be used to study surface phenomenan associated with coatings.
The use
of
IR for surface analysis, including depth profiling, has been reviewed."."
Examples include the examination of polymers" and latex-coated paper." Other surface
applications include monitoring acrylate polymerization," observing silicone additives,")
and investigating urethanes6"
IR
spectroscopy is useful for interfacial studies, investiga-
tions of interactions between coatings and substrates, air, liquids, and embedded fibers.
Interactions between air and latex films have been studied."' Diffuse reflectance with IR
spectroscopy has been used to study interfaces.6'
There are many applications of
IR
to
coatings. Most coatings problems can profit
from investigation by infrared spectroscopy.
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INFRARED SPECTROSCOPY OF COATINGS 85
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34
1
(1
994).
Thermal Analysis
for
Coatings
Characterizations
William
S.
Gilman
Gilman
C?
Associates,
South
Plainfield, New Jersey
1
.O
INTRODUCTION
The evaluation
of
substances and finished materials by thermal analysis will be discussed
as a tool that the paint chemist can use to help evaluate coating properties. These properties
are those which change as a function of temperature.
2.0
CHARACTERISTICS
Substances change in
a
characteristic manner as they are heated. Thermal analysis (TA)
monitors these changes. TA procedures are generally used
to
characterize various sub-
stances and materials that change chemically or physically as they
are
heated. These
changes in properties as a function of temperature have been used to help characterize the
interrelationship of a coating’s composition and performance. TA methods or techniques
measure changes in properties of materials as they
are
heated or at times cooled.
A TA evaluation entails subjecting
a
small sample of from a few miligrams to
100
miligrams to a programmed change in temperature. The resulting change in property is
detected, attenuated, plotted, and measured by a recording device.
The instrumentation consists of
an
analysis module, a heating
or
cooling source, a
measuring device, and a system for reporting the results, usually as an
XY
plot. A computer
is used to program and control the procedure, and analyze and store the results.
3.0
TECHNIQUES
The techniques of prime importance in coatings’ characterization and analysis include
differential scanning calorimetry (DSc), differential thermal analysis (DTA), thermogravi-
87
88
GILMAN
metric analysis (TGA), thermomechanical analysis (TMA), and dynamic mechanical anal-
ysis (DMA). Each
of
these will be discussed, with examples of the information derivable
from each procedure.
DSc measures the heat flow and temperature associated with chemical transitions
and reactions. The difference in heat flow between
a
sample and
a
reference material is
measured under precisely controlled conditions. It measures heat flow into or out of a
sample as an exothermic or endothermic change in energy. Generally, the temperature
range of use is
-
180°C
to
+
725°C.
DTA has been replaced by DSc. However, DTA data also address changes in heat
flow and measure the evolution or absorbance of energy in relation to an inert reference.
The temperature detection is somewhat less accurate than DSc, and hence it was replaced
by DSC where better data are obtainable.
It
also measures the temperature excursions of
a sample in the range of
-
180°C to 1600°C.
Thermogravimetric analysis (TGA) measures the change in mass as a function
of
temperature, time and atmosphere. Thus this procedure is limited to applications where a
change in mass occurs. Since only information concerning the changes
in
mass is collected,
nothing is revealed about the nature
of
the transitions. The energy involved, or whether
it is absorbed or released, is not detected or indicated. TGA measures only changes in
mass, usually in the temperature range of ambient to 1200°C.
Thermomechanical analysis (TMA) measures dimensional changes as a function of
temperature and time in materials. Both linear and volumetric changes can be determined.
TMA measures sample dimensional changes, generally, in the range of
-
160°C to
+
1200°C.
Lastly, dynamic mechanical analysis (DMA) is a thermal analysis technique that
measures the properties
of
materials as they are deformed under periodic stress. DMA
employs a variable sinusoidal stress application, and the resultant sinusoidal strain is
measured. Or, stated differently, DMA is a technique that measures the modulus (stiffness)
and damping (energy dissipation) properties of materials
as
they are deformed under
periodic oscillatory stress. DMA detects sample modulus and damping changes in the
temperature range
of
-
150°C to
+
500°C.
4.0
APPLICATIONS
The practical applications associated with each technique are numerous, and it would be
impractical even to attempt to list or describe them. Therefore for each technique
of
TA
that we have mentioned, a brief overview will be made with some specific applications
of potential interest
to
the coating chemist.
The information that can be obtained from DTA and DSC is approximately the
same, with the exception that DSC gives quantitative
as
well as qualitative information.
DSC and DTA have been extensively used to study the temperatures and nature of transi-
tions in materials heated under different atmospheres. Oxidation stability
of
various sub-
stances is easily characterized. Evaluations
of
polymer flammability and analysis of subam-
bient phase transition are often conducted.
Coatings generally possess characteristic transitions, including glass transition (a
transition related to changes in specific heat), exothermic reactions caused by physical or
chemical reactions such
as
crystallization or cross-linking reactions, melting, volatilization,
dissociation. a phase change, and oxidation or thermal decomposition. DSC with its ability
to yield quantitative heats of reaction facilitates the determination of the relative heats
of
combustion for flame retardant and untreated materials. The effectiveness
of
the flame
retardant can also be assessed.
Reaction kinetic parameters of chemical reactions are derived from DSC data. Stud-
ies by isothermal techniques as well as rising temperature methods are used. These methods
use the rate
of
heat evolution
as
the measured parameter. The classic epoxy-amine reaction
helps model production methods to minimize raw materials and reaction time
to
obtain a
quality product through first determining reaction kinetics. In addition, since aging or
oxidation is an exothermic reaction, this technique has been used to study long-term
stability
of
coatings.
The applications
of
thermogravimetric analysis (TGA) as an analytical tool center
around determining changes in sample mass
as
a function of time and temperature. Both
isothermal and nonisothermal methods are used. For coatings analysis, nonvolatiles and
thermal stability determinations are usually conducted. However, as in DSc, decomposi-
tion kinetics. accelerated aging, and oxidation stability can be conducted.
Unlike DSc, in which heating is done by conduction, TGA heats by convection and
irradiation. Therefore the temperature sensor is located in the vicinity of the sample
to
obtain
a
true temperature reading.
Applications involving estimating polymer decomposition, polymer lifetime, distin-
guishing flame retardant polymer from nontreated polymer of the same type and kinetics
of
drying, have been reported.
For coatings analysis, compositional analysis may assist in determining why perfor-
mance properties changes have occurred. For example, weight
loss
curves for materials
ostensibly
of
exactly the composition with identical weight losses behaved differently in
a
formulation. Upon further examination it was observed that the weight
loss
of
one
occurred at slightly different temperatures and in different modes, indicating differences
in materials.
With ever increasing control on organic volatiles in coatings, TGA can be used for
volatile organic content determination.
As with DSc, thermal or long term degradation stability
of
coatings based on one
resin or another can be conducted
as
a
formulation tool.
Applications
of
thermomechanical analysis are related
to
a
sample’s dimensions as
a function of time and temperature. Information, such as the compatibility of coating and
substrate, performance of an elastomer in
a
harsh environment, and the adhesion of one
material
to
another, such as multilayered wrapper or baked on coating on a metal substrate,
can be obtained.
The direct application of TMA in coatings technology is found in dilatometry, or
the determination
of
the coefficient
of
thermal expansion.
It
is particularly useful in the
determining if two dissimilar coatings can adhere to each other.
The applications
of
dynamic mechanical analysis provide information about transi-
tion temperatures, curing phenomena, and mechanical properties. These properties include
impact resistance and even sound absorption. The technique measures the properties of
materials as they are deformed under periodic stress. Another way
to
state DMA testing
is that it measures the viscoelastic response of a material under periodic load.
Applications range from viscoelastic measurements to kinetics of cross-linking.
Modulus values, such as flex, Young’s, and shear provide information
on
a material’s
stiffness. while mechanical damping correlates with the amount of energy dissipated as
heat during deformation.
90
GILMAN
In coating technology, DMA is applicable in the study of film properties, such
as
the cure process and film formation. Various properties around the glass transition point
may be studied. For example, in thermoplastic materials, increasing rubbery state modulus
usually means increasing molecular weight. In thermosets, increasing rubbery state modu-
lus values indicate higher cross-link density.
Thermal analysis is constantly undergoing improvements in order to obtain faster,
more reliable, and new information. For example, in most TGA decomposition measure-
ments, the heating rate is maintained constant over the decomposition range, self compen-
sating for internal cooling. Devices are available that will detect both TGA and DSC data
on the same sample simultaneously.
Maximum resolution TGA by rate adjustment addresses weight losses occurring in
overlapping temperature regions. The overlapping can be seen using the first derivative
of the TGA signal.
Thermal analysis, such
as
the techniques already mentioned, can be coupled to
a
mass spectrometer or infrared spectrometer to analyze the effluent output of the TA
instrument.
Modulated
DSC
is an innovation with increased sensitivity, higher resolution, more
complete analysis, and easy data interpretation.
DMA is now available with multiple modes of degradation, including single and
dual cantilever, three-point bend, shear sandwich, compression, and tension. Frequency
multiplexing
of
0.01
to
200
Herz is provided.
We have attempted
to
describe the vast capabilities
of
TA. This technology is at its
best when more then one instrument is used on a research or production problem. In
combination, these devices can supply insight into relationships between fomwlations,
processing, and performance of coatings.
BIBLIOGRAPHY
1.
2.
3.
4.
S.
6.
7.
8.
9.
10.
11.
Robert, Colborn,
Modern Science
crud
Techrlology.
Princeton, NJ: Van Nostrand, 1965.
Robert L. Hassel,
Using
Tetnperuture
fo
Corrtrol
Qutrlitv.
Second Quarter 1991 PI Quality.
Hitchcock, 199
1.
C. Michael Neag,
Coatings
Ckrrrrrct~rizarions
I>J
Thernzcrl
Anccl~~se.~.
ASTM Manual 17. West
Conshohocken, PA: American Society for Testing and Materials, 1995.
Michael Reading, et
al.,
“Thermal Analysis for the 21” Century,”
Americcrrr
Lnborrrtor>l,
Jan.
1998, p. 13.
Rudolf Riesen, “Maximum Resolution in TGA
by
Rate Adjustment,”
Americrrrl
Lrrhortrton,
Jan. 1998, p.
18.
Roger Blain, “A Faster Approach to Obtaining Kinetic Panyneters,”
Americon
Lcrbonrton.,
Jan. 1998,
p.
21.
Bruce Cassel, et al., “Rapid DSC Determination
of
Polymer Crystallinity,”
Anwrican
Lnhortr-
tory.
Jan. 1998, p. 24.
Robert
L.
Hassel, “Thermomechanical Analysis Instrumentation for Characterization
of
Mate-
rials,”
American
krborutoty,
Jan. 1991.
Chang-Hwan Park, et al., “Syntheses and Characterizations
of
Two Component Polyurethane
Flame Retardant Coatings using 2,4-Dichlor Modified Polyester.”
J.
Cncrtirrgs
Techrrology.
Dec. 1997, p. 21.
Jon Foreman, “Dynamic Mechanical Analysis
of
Polymers,”
Anlericnrr
.!homtop.
Jan. 1997,
Mark Kelsey, et al., “Complete Thermogravimetric Analysis,”
Anterictrr~
Lcrborutory,
Jan.
1997, p. 17.
p. 21.
THERMAL ANALYSIS
FOR
COATINGS CHARACTERIZATIONS
91
12.
R. L. Hassel, “Evaluation
of
Polymer Flammability by Thermal Analysis,”
Atnericntl
Lrrborcr-
13. TA Instruments Company has issued a series
of
Thermal Analysis Application briefs that arc
torv.
Jan. 1997.
available from TA Instruments Company, Ncw Castle, DE.
TA-75 Decomposition Kinetics Using TGA
TA-73 A Review of DSC Kinetics Methods
TA-8A Thermal Solutions-Long Term Stability Testing
of
Printing Inks by DSC
TA-l21 Oxidation Stability
of
Polyethylene Terphthalate
TA-123 Determination
of
Polymer Crystallinity by DSC
TA- 125 Estimation
of
Polymer Lifetime by TGA Decomposition Kinetics
TA-l35 Use of TGA
to
Distinguish Flame-Retardant Polymers from Standard Polymers
TA- I34 Kinetics
of
Drying by TGA