Unlike thermoplastic alternatives such as hot melt adhesives, cured thermosetting epoxies will not re-flow or melt when heated. Instead, epoxies will undergo a transition from a hard rigid state to a more pliable, rubbery state. The temperature range during which this transition takes place is known as the glass transition temperature, Tg.
Although the glass transition temperature of an epoxy (or any other thermosetting adhesive) is generally reported as a single value, the glass transition temperature is really a range. The value presented is typically the midpoint of this transition temperature range.
Glass transition can be understood on a molecular level by considering the change in mobility of the polymer molecules with temperature. At lower temperatures, the polymer molecules are organized in a crystalline type arrangement. This structure is commonly called the “glassy state.” In this state the molecules are locked into position and can only vibrate in place. At higher temperatures, the mobility of the molecules increases and they are able to move more freely. This results in a loss of rigidity and gradual softening of the material to pliable and rubbery state.
There is a marked change in the physical properties of the material below and above the glass transition temperature. Despite these substantial changes, the transition cannot be classified as a phase change. The performance of epoxies generally deteriorates at temperatures above Tg. Physical changes observed above Tg are generally reversible as long as the excursions above Tg are limited in time and temperature. A polymer will return to its original state once the temperature dips below Tg. Long exposure to above Tg temperatures, however, may have a permanent effect on the polymer properties.
Due to the clear change in properties above the glass transition temperature, Tg is considered an extremely useful yardstick for the reliability of epoxies at elevated temperatures. When choosing an epoxy for a high-temperature application, Tg is typically the primary property to consider.
Performance Above and Below Tg
As mentioned above, it is important to carefully select an epoxy with suitable Tg values for a proposed application to optimize performance. Changes in physical properties at above Tg temperatures can be substantial.
- The coefficient of thermal expansion increases by a multiple of 3-5 above the Tg. This means that the solid material will expand or contract much more as a result of temperature changes, resulting in higher thermally induced stress on bonded components.
- The polymer’s ability to hold a strong polymer bond is also reduced as polymer chains become more mobile above the glass transition temperature. This has a significant effect on the lap shear strength of the adhesive system.
- As the temperature of an epoxy rises above its Tg, the storage modulus This is indicative of the change from a rigid to a compliant state. A high storage modulus results in high stiffness, which equates to a low percent elongation and poor energy dissipation when stressed.
- At temperatures higher than the Tg, the adhesive begins to soften and loses some tensile strength. Brief temperature excursions above the Tg do not permanently alter the physical properties of the material. As the material returns to lower temperatures, its strength profile is restored.
Higher Tg epoxies tend to be very rigid which can make them unsuitable for certain applications. Cryogenic applications, for example, may require a low Tg epoxy to ensure that the material is not too brittle at typical operating temperatures.
Additionally, if the epoxy application involves rigorous thermal cycling with short periods above the Tg, a more flexible, lower Tg epoxy may be the right choice. Lower Tg epoxies generally exhibit higher flexibility. Curing at low temperatures or room temperature results in the lowest possible Tg for the given epoxy system.
How is Tg Measured?
Tg can be measured by observing changes in physical properties of the material with temperature. These properties include heat capacity, coefficient of thermal expansion and stiffness. Tg is generally measured using one of the following methods
- Differential Scanning Calorimetry (DSC)
- Dynamic Mechanical Analyzers (DMA)
- Differential Thermo-Mechanical Analyzers (DTA).
Each method measures a different physical property that is characteristic of the transition. Consequently, each method produces a slightly different result with variations ranging from 5-30°C from one method to another.
How Does Cure Schedule Affect Tg?
Tg is determined, not only by the chemical structure of the epoxy resin, but also by cure conditions such as time, temperature, specific response to heating, amount of load applied, degree of orientation, rate of testing, type of hardener and the degree of cure.
Very high Tg values are not achievable via room temperature curing. Curing the same material at higher temperatures will result in a higher Tg. In many adhesive tables, the Tg is specified as a parameter together with the curing schedule. As a general rule, an epoxy’s Tg cannot be significantly higher than the highest temperature reached during the curing process.
Choosing An Epoxy Based On Tg
The following Resin Design products exhibit high Tg values and are excellent choices for high-temperature applications:
- Entex 84251 is a two-part epoxy adhesive. Despite the fact that this adhesive cures at room temperature, this epoxy exhibits a high glass transition temperature of 80°C. This system can, however, be heated during cure to achieve a faster set time and superior high-temperature performance.
- Nexus 84301 is a two-part, toughened epoxy adhesive. Similar to Entex 84251, this product exhibits a Tg of 80°C. However, as a toughened epoxy, Nexus 84301 is also able to provide crack termination properties and a lap shear strength in excess of 2,000 psi (aluminum/ aluminum bond).
- Vivid Cure 86011 is a UV curable epoxy resin. This cationic system is highly cross-linked, resulting in a glass transition temperature of 135°C. Additionally, this product will set on demand with UV exposure. This product performs exceptionally well in high-temperature applications, and is capable of surviving exposure up to 200°C.
Ultimately, although Tg is an excellent metric to quickly compare epoxies for a high-temperature application, there are some limitations in relying on Tg as the sole indicator of temperature resistance. The importance of testing epoxies in the specific context of the application remains the best tool for choosing the right system.