T E C H S P O T L I G H T

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 temperatureAs 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:

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 temperatureIn 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

α = ΔLLxΔT

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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.

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 ConsiderationsEach 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.

Figure 1: In DSC, the Tg is defined by observing changes in the heat capacity of a polymer as a function of temperature.

Figure 2: In TMA, the Tg is identified as the onset temperature of the change in expansion behavior of a polymeric material.

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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 reliabilityFor 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.

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

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.

ConclusionDue 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.

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.

High Tg epoxies are frequently used in downhole applications.