Glass Transition Temperature (Tg) in Adhesive Materials

Glass transition temperature (T_g) is a measurement of the temperature range where a material shifts from its hard glassy state to a soft flexible state. Glass transition temperature can be measured with Dynamic Mechanical Analysis (DMA) and the ISO 6721-7 standard.

Testing and Simulation of Glass Transition Temperature

Glass transition temperature (T_g) is usually measured in relation to material stiffness, or modulus, and may also be known as the principal relaxation. The glass transition temperature of a thermoset polymer material demonstrates considerable variability; this is usually influenced by factors such as thermal history, state of cure, and moisture content.

Testing an adhesive material’s T_g ensures that it is suitable for use in a specific application and has been cured correctly. Some adhesives have been developed to function in an application below their glass transition temperature, i.e. in a rigid state, while others have been designed to work above their T_g, in an elastomeric or rubbery state.

Image Credit: Huntsman Advanced Materials

Testing Procedure

Various methods can be used to measure the glass transition temperature, but Dynamic Mechanical Analysis (DMA), in accordance with ISO 6721-7 method, is a sensitive and reliable technique.

A specimen of known geometry undergoes torsional oscillation at a consistent frequency (usually 1 Hz) while simultaneously heated at a steady rate. The instrument applies a persistent stress, and the strain is then recorded.

A DMA graph exhibits the variation in response to the adhesive as temperatures change. The storage modulus (G') and loss modulus (G") are shown.

The T_g values most frequently recorded during DMA measurement are:

•  T_g onset - from the inflection of the storage modulus curve
•  T_g - from the peak of the tan-delta (tanδ) curve

Test Parameters

Test specimens are usually rectangular bars or cylindrical rods and the width and thickness of the specimens must remain consistent along the specimen length, deviating no more than 2% of a mean value.

At Huntsman, standard test specimens are milled rectangular bars measuring 50 mm x 10 mm x 2 mm.

Data Provided

As polymers are subjected to heating, molecules slide into the material, which creates several transition points. Below the T_g, the amorphous polymer has a reduced heat capacity due to the relatively static nature of the molecules.

Above the T_g, the flexible, rubbery polymer has a greater heat capacity as molecules start to mobilize. DMA is useful for the separation and measuring up of the elastic and viscous elements of the behavior of a polymer material:

• Storage modulus G’ – highlights the elastic behavior of the material (in Pa)
• Loss modulus G” – highlights the viscous behavior of the polymer (in Pa)
• Loss factor - tanδ represents the ratio between the loss and storage moduli (dimensionless number).

Tips for Modeling and Simulation

DMA measurement is a powerful method for capturing the continuous temperature dependence of the elastic behavior of a material for finite element simulations. This acts as a shape function for elastic components over certain temperature ranges.

The DMA measurements of dynamic shear modulus can be associated with the elastic modulus and Poisson's ratio as follows: G = E / 2(1+ѵ)

It should be noted that the T_g values measured are dependent on the deformation timeframe. While DMA is usually run at 1 Hz, T_g values at increased frequencies could see a transition toward higher temperatures.

Analysis of the relative sensitivity can be conducted by running frequency sweep experiments using DMA and the application of WLF-type analysis.

This information has been sourced, reviewed and adapted from materials provided by Huntsman Advanced Materials.

For more information on this source, please visit Huntsman Advanced Materials.