Tech Tip 23 – Understanding Glass Transition Temperature (Tg) in Epoxy Adhesives 

What is Tg?

The Glass Transition Temperature (Tg) is a critical property of epoxy adhesives. It marks the temperature range where the material transitions from a hard, glassy state to a soft, rubbery state. Unlike thermoplastics, cured epoxies do not melt. In fact, they only soften slightly at elevated temperatures. This key difference is explained by the fact that cured epoxies are comprised of chemically crosslinked chains responsible for conceding unique attributes.

How is Tg Measured?

Tg is not a discrete transition, but a temperature range over which the mobility of the polymer chains increases significantly. The ultimate Tg is determined by a number of factors, including the chemical structure of the epoxy resin, the type of hardener, and the degree of cure.

This property is commonly determined via Differential Scanning Calorimetry (DSC) on a cured sample. In this method, the Tg is reported as the midpoint of the transition range, typically identified by the intersection of tangents on the heat flow curve.

Although less common, there are other techniques that can be used to determine the Tg of cured epoxies. These may include Dynamic Mechanical Analysis (DMA), where the Tg is defined as the peak of tan δ or the onset of storage modulus drop.

Thermomechanical Analysis (TMA) can also reveal the Tg by measuring dimensional changes (expansion or contraction) under a controlled load as temperature changes. In this case, Tg is defined as the change in the slope of the expansion curve.

Since this transition is not a discrete event, each methodology above may yield slightly different Tg values. Therefore, it is important to make it clear which method was used when reporting Tg.

Factors Affecting Tg

The Tg values of a cured epoxy can be influenced by different process-related factors and the components used in the formulation. Since this property has a direct relationship with the reaction conversion, the value is strongly dependent on the cure schedule. As an example, an adhesive could have a Tg between 60°C and 110°C, based on the cure schedule. This is why it is important to maintain tight temperature control in any production setting.

In addition, the Tg can be significantly reduced by moisture absorption, a factor which should be considered when designing for humid applications.

Why Tg Matters

Typically, adhesives with the highest Tg have the best heat resistance and therefore deliver the best tensile properties at high temperature. Unless there are significant exotherms associated with the cure process, as a general rule, a Tg cannot be significantly higher than the highest temperature seen during cure.

Tg can also affect the following performance attributes:

• Modulus
  – HigherTg → higher cross-link density → higher modulus (stiffer material).
  –  AboveTg, storage modulus drops as epoxy becomes compliant.
• Lap Shear & Die Shear Strength
  – Strength decreases as temperature rises.
  – Softening aboveTg can allow easier adhesive removal.

• Coefficient of Thermal Expansion (CTE)
  -CTE increases sharply above
  -Tgcan also be identified by the intercept of two CTE curves.

Understanding Tg is essential when selecting and applying epoxy adhesives in temperature-sensitive environments. Because Tg directly influences mechanical performance, dimensional stability, and long-term durability, it should always be evaluated in the context of the full application—considering cure conditions, environmental exposure, and performance requirements.