Glass Transition Temperature (Tg)

Glass transition temperature of epoxies and their important role in selecting the right epoxy

When a polymer system such as an epoxy adhesive, potting compound or sealant is heated, significant changes occur in mechanical strength properties, thermal and electrical properties, coefficient of thermal expansion and chemical resistance. These properties do not necessarily recover upon cooling. A considerable change in these properties can be seen above the glass transition temperature (T_g) of the epoxy. Polymeric compounds which are exposed to a temperature below the T_g, exhibit remarkably higher physical strength and stiffness, as well as better electrical insulation properties, dimensional stability and chemical resistance than above the T_g.

The polymer segments below the T_g are stiffer and stronger than at higher temperatures above the T_g. Performance upon structural loading is therefore much enhanced. It is convenient to consider the T_g as the maximum sustainable operating temperature especially for high tech structural applications.

The T_g is measured by methods such as Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analyzers (DMA) or Differential Thermomechanical Analyzers (DTA). It is greatly affected not merely by the chemical composition of the epoxy compound, but by its cure conditions such as time, temperature, specific response to heating, amount of load applied, degree of orientation and the rate of testing. For many applications, the so called Heat Deflation Temperature (HDT) (ASTM D648-21, 1978) is used in place of the T_g. This method employs castings with an applied load of 1.8 MPa (264 psi).

The T_g of available adhesives, sealants and potting compounds varies from as low as 50°C to above 250°C. Table 1 shows the T_g that is typically obtainable with widely used Master Bond epoxy compounds.