TECH SPOTLIGHT
Epoxies and Glass
Transition Temperature
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Epoxies and Glass Transition
Temperature
Changes in temperature can have an enormous impact
on the performance properties of epoxies and other
thermosetting polymer systems. Prior to curing, an
epoxy consists of a resin and a curing agent. When
polymerization occurs, the entity becomes an organized
crystalline type structure in what is sometimes referred
to as a “glassy state.” In this state, the molecules are
able to vibrate but are otherwise locked in place. As the
temperature rises, the molecules are able to move more
freely and the material gradually starts to soften. As the
temperature continues to rise, the polymer eventually
experiences a profound state change to a more pliable,
rubbery state. Although this state transition takes
place gradually over a range of temperatures, the glass
transition temperature range (Tg) is often designated
by a specific temperature. The actual glass transition
temperature range depends upon the molecular structure
of the material, the testing method, sample preparation,
the cure schedule, and the degree of cure.
Epoxy properties change with increases in
temperature
As the temperature increases, thermosetting polymers
exhibit changes in their physical properties, including
tensile strength, thermal expansion, heat capacity,
modulus, electrical properties, and others. One significant
change is that of the linear coefficient of thermal
expansion (CTE). The CTE quantifies how much a material
expands or contracts during temperature excursions, and
is approximated as follows:
α = LΔL
xΔT
where α is the coefficient of linear thermal expansion,
ΔL is the change in length of the material, L is the
initial length of the material, and ΔT is the change in
temperature. The CTE is usually reported as ppm/°C. The
higher the CTE of a given material, the more the material
will expand or contract with temperature excursions.
As a material moves through the glass transition
temperature range, its CTE increases dramatically —
ultimately becoming three to five times higher than its
value below the Tg range. After the epoxy passes through
the glass transition temperature range, its material
properties are significantly different from those below the
Tg range. These changes are not necessarily permanent,
however; they depend upon the duration and extent to
which the Tg range is exceeded. Brief excursions above
the Tg will not irrevocably “damage” the material. As an
epoxy returns to ambient temperatures, its strength profile
is typically restored.
It is important that design engineers understand the
nature of this transition so that they can choose the best
system for a specific application.
Understanding the glass transition
temperature
In practice, the glass transition temperature for a given
compound is reported as a single temperature, Tg, which
represents the range of temperatures over which a cured
epoxy transitions from a glassy, hard state to a more
rubbery, softer state.
There are three main methods used to determine glass
transition temperatures: Differential Scanning Calorimetry
(DSC), Thermo Mechanical Analysis (TMA), and Dynamic
Mechanical Analysis (DMA). Each method measures a
different physical phenomenon that is characteristic of the
phase transition, and consequently, each method produces
a slightly different result.
Differential Scanning Calorimetry (DSC)
In DSC, the glass transition is identified by observing the
change in the heat capacity of a polymer as temperature
rises. The underlying principle is that when a material
is undergoing a phase change, more or less heat is
needed to flow to it in order to keep the material at
the same temperature as a reference sample. A small
sample of the material is heated along with a reference
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material in a calibrated thermocel, and the difference in
heat flow between the two samples is observed. A shift
in the differential heat flow occurs as the sample material
transitions from the glassy state to the rubbery state, as
shown in Figure 1. Tg is defined to be the temperature at
the inflection point of this shift.
Figure 2: In TMA, the Tg is identified as the onset temperature
of the change in expansion behavior of a polymeric material.
Figure 1: In DSC, the Tg is defined by observing changes in the
heat capacity of a polymer as a function of temperature.
DSC is a popular method for measuring Tg since it is
less costly than the other methods. However, it has a
number of drawbacks. DSC is more limited in scope and is
sometimes not as accurate as the other methods. In some
cases, the differential heat flow is so small, it is not easily
detected. For polymers with high filler loadings and greater
crosslinking densities, the phase transition is very difficult
to observe using DSC. Because the typical sample size is
in the milligram range, DSC samples may be too small to
adequately represent the polymer material as used in an
application.
Thermo Mechanical Analysis (TMA)
TMA is the technique commonly used to determine a
material’s coefficient of thermal expansion. By observing
changes in the material’s thermal expansion coefficient
as a function of temperature, TMA can also be used to
determine Tg. During a material’s transition from a glassy
state to a rubbery state, changes take place on a molecular
level that result in increased movement. Consequently,
its coefficient of thermal expansion increases noticeably
during the phase transition. TMA involves placing a sample
of a material on a calibrated platform and heating the
sample while an instrumented probe measures dimensional
changes in the sample. Tg is identified as the temperature
at which there is a noticeable shift in the dimensional
change of the sample, as shown in Figure 2. TMA is
considered to be a more sensitive method than DSC for
measuring the Tg, particularly of filled systems.
Dynamic Mechanical Analysis (DMA)
DMA is a procedure that is used to characterize the
viscoelastic properties of materials. The main principle
behind using DMA to define Tg is that the stiffness and
damping (a measure of energy dissipation) of a polymeric
material change significantly at the glass transition
temperature. A controlled oscillatory force is applied to a
sample of known geometry, and the resulting deformation
is measured. The amount of deformation is related to the
stiffness and damping of the material. As the sample is
heated, measurable changes in deformation occur when
the material transitions from the glassy state to the rubbery
state. Tg is determined by observing these changes.
DMA is highly accurate and sensitive, but requires a
precisely machined sample of uniform thickness with
parallel sides and right angles as well as instrumentation
that is properly calibrated for both temperature and
force. It is more complex and expensive to set up and
run compared to DSC and TMA. Additionally, the Tg can
be defined based on three different analysis parameters:
storage modulus, loss modulus, or loss factor. Each
parameter reflects a different component of a material’s
stiffness and damping — and produces a slightly different
Tg value.
Practical Considerations
Each method — DSC, TMA, and DMA — measures a
different physical property of polymeric materials.
Consequently, the Tg results for the same material will
differ depending upon the method used, with variations
ranging from 5°C to 30°C. Curing methodology is another
critical consideration in determining the Tg. The manner
in which the sample system is cured and the ultimate
“completeness” of the cure is absolutely vital in determining
the Tg. That is to say, adding the right amount of heat for
the correct period of time is critical here.
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TMA is often the preferred method of determining Tg
for several reasons. It is far more accurate and reliable
than DSC. Although DMA is the most precise of the
three methods, it is quite involved, whereas TMA offers
a reasonable, straightforward methodology that is
simpler and ultimately the most cost-effective of the
three methods. Another advantage of TMA — particularly
when compared to DSC — is that it better illustrates the
Tg as a range of temperatures rather than as a single
point. When reporting the Tg of a particular compound,
it is very important for manufacturers to specify the test
methodology used.
Generally, the Tg is a good first-order indicator of
the compound’s temperature resistance. One notable
exception is silicones. These polymers have extremely low
Tg values — from -40°C to -100°C — yet they are very
well suited for applications with operating temperatures
reaching 250°C and beyond. For silicones, temperature
resistance is determined by decreases in elongation in air
as the temperature increases. The upper limit is generally
considered to be the temperature at which the silicone has
lost half of its initial elongation at room temperature.
High Tg often enhances reliability
For the most part, the Tg is an extremely useful yardstick
for the reliability of epoxies as it pertains to temperature.
Invariably, a higher Tg material will outperform a lower Tg
material in an application involving elevated temperatures.
However, Tg is not the only consideration for choosing an
epoxy in a higher temperature application. For example, if
the excursion to higher temperature is relatively short term,
a lower Tg material may perform more than adequately.
Additionally, higher Tg epoxies tend to be very rigid,
although this is not always the case (see sidebar), which
can make them less attractive for certain applications.
temperature applications, there is no question that a higher
Tg is a critical parameter. However, it is useful to experiment
with lower Tg epoxies that exhibit higher flexibility,
depending on the parameters of the application. The Tg is
just one of many factors to consider for bonding, sealing,
coating and encapsulation applications.
Exceptional
Epoxies
Typically, adhesives with the best heat resistance
have high Tg values. An exception is the Master Bond
family of EP36 systems, which are B-staged epoxies.
In B-staging, the resin and hardener are mixed, and
a heat cure is initiated, but the reaction is arrested
by quenching or cooling while the adhesive is still
fusible and soluble. The system is partially cured at this
juncture and full cure takes place only after heating
to 350ºF. This results in an epoxy that combines
compliance and superb heat resistance.
The EP36 series has a Tg of 35°C to 40°C and a
service temperature of up to 500°F. Although it
softens at the Tg, it will maintain the same “softness”
until the upper temperature limit is reached, while still
retaining its physical, electrical, and thermal properties.
Its forte is withstanding rigorous thermal cycling at
temperatures of up to 500°F.
Conclusion
Due to the importance of Tg in assessing epoxy
temperature resistance, it is vital for design engineers
to understand what Tg is and how it is measured. There
are limitations in relying on Tg as the sole indicator of
temperature resistance. The importance of testing epoxies
in the specific context of the application is ultimately the
most significant issue of all.
For further information on this article, for answers to any
adhesives applications questions, or for information on
any Master Bond products, please contact our technical
experts at Tel: +1 (201) 343-8983.
High Tg epoxies are frequently used in downhole applications.
If the application involves rigorous thermal cycling with
short dwell times above the Tg, a more flexible, lower
Tg epoxy may actually be suitable. For sustained high
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