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, T_g.

Although the glass transition temperature of an epoxy (or any other thermosetting adhesive) is generally reported as a single value, 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.

1. Physical properties of a material are directly affected by deviations above and below the Tg. 

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 T_g. Physical changes observed above T_g are generally reversible as long as the excursions above T_g are limited in time and temperature. A polymer will return to its original state once the temperature dips below T_g. Long exposure to above T_g temperatures, however, may have a permanent effect on the polymer properties.

Due to the clear change in properties above the glass transition temperature, T_g is considered an extremely useful yardstick for the reliability of epoxies at elevated temperatures. When choosing an epoxy for a high-temperature application, T_g is typically the primary property to consider.

As mentioned above, it is important to carefully select an epoxy with suitable T_g values for a proposed application to optimize performance. Changes in physical properties at above T_g temperatures can be substantial.

The coefficient of thermal expansion increases by a multiple of 3-5 above the T_g. 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.

2. Bond strength gets affected and we see that in lap shear results.  This will affect end performance of the adhesives.

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.

3.Modulus and resulting stiffness of the adhesive get affected.  This could lead to failure modes such as high internal stress creation which leads to visible signs of stress releases commonly seen as cracks, breaches, or fissures.

As the temperature of an epoxy rises above its T_g, the storage modulus 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.

4. Loss of tensile strength at excursions above Tg.

At temperatures higher than the T_g, the adhesive begins to soften and loses some tensile strength. Brief temperature excursions above the T_g do not permanently alter the physical properties of the material. As the material returns to lower temperatures, its strength profile is restored.

Higher T_g epoxies tend to be very rigid which can make them unsuitable for certain applications. Cryogenic applications, for example, may require a low T_g epoxy to ensure that the material is not too brittle at typical operating temperatures.

5. Processing can affect Tg and alter subsequent physical properties.

Additionally, if the epoxy application involves rigorous thermal cycling with short periods above the T_g, a more flexible, lower T_g epoxy may be the right choice. Lower T_g epoxies generally exhibit higher flexibility. Curing at low temperatures or room temperature results in the lowest possible T_g for the given epoxy system.

How Does Cure Schedule Affect T_g?

T_g 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 T_g values are not achievable via room temperature curing. Curing the same material at higher temperatures will result in a higher T_g. In many adhesive tables, the T_g is specified as a parameter together with the curing schedule. As a general rule, an epoxy’s T_g cannot be significantly higher than the highest temperature reached during the curing process.

How is T_g Measured?

T_g 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. T_g 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.

Choosing An Epoxy Based On T_g

The following Resin Design products exhibit high T_g 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 T_g 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 T_g is an excellent metric to quickly compare epoxies for a high-temperature application, there are some limitations in relying on T_g 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.

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