Thermal Properties of Composites

Tg: The Glass Transition of FRP Composites

Modern car engine
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Fiber reinforced polymer composites are often used as structural components that are exposed to extremely high or low heats. These applications include:

  • Automotive engine components
  • Aerospace and military products
  • Electronic and circuit board components
  • Oil and gas equipment

The thermal performance of an FRP composite will be a direct result of the resin matrix and the curing process. Isophthalic, vinyl ester, and epoxy resins generally have very good thermal performance properties. While orthophthalic resins most often exhibit poor thermal performance properties.

Additionally, the same resin can have vastly different properties, depending on the curing process, curing temperature, and time cured. For example, many epoxy resins require a "post-cure" to help reach the highest thermal performance characteristics.

A post-cure is the method of adding temperature for a duration of time to a composite after the resin matrix has already cured through the thermosetting chemical reaction. A post cure can help align and organize the polymer molecules, further increasing structural and thermal properties.

Tg - The Glass Transition Temperature

FRP composites can be used in structural applications that require elevated temperatures, however, at higher temperatures, the composite can lose modulus properties. Meaning, the polymer can "soften" and become less stiff. The loss of modulus is gradual at lower temperatures, however, each polymer resin matrix will have a temperature that when reached, the composite will transition from a glassy state to a rubbery state. This transition is called the "glass transition temperature" or Tg. (Commonly referred to in conversation as "T sub g").

When designing a composite for a structural application, it is important to make sure the FRP composite's Tg will be higher than the temperature it might ever be exposed to. Even in non-structural applications, the Tg is important as the composite can change cosmetically if the Tg is exceeded.

Tg is most commonly measured using two different methods:

DSC - Differential Scanning Calorimetry

This is a chemical analysis which detects energy absorption. A polymer requires a certain amount of energy to transition states, much like water requires a certain temperature to transition to steam.

DMA - Dynamic Mechanical Analysis

This method physically measures stiffness as heat is applied, when a rapid decrease in modulus properties occurs, the Tg has been reached.

Although both methods of testing the Tg of a polymer composite are accurate, it is important to use the same method when comparing one composite or polymer matrix to another. This reduces variables and provides a more accurate comparison.