Glass Transition Temperature (Tg) in Laminates
The resins used in the manufacture of laminates commonly exhibit a glass transition or Tg. The glass transition is the temperature range in which the amorphous molecules of the resin transition from rigid and brittle to flexible and pliable. Below the glass transition, the amorphous molecules are locked into their position in the molecular matrix. Above the glass transition, they have some freedom of movement which allows them to reorganize into more optimized crystalline state and further cure of the resin.
Because additional cure increases the temperature of the glass transition region, comparison of subsequent measurements on the same sample is a common evaluation of the optimal cure of a resin. Resin systems which do not contain an amorphous component cannot be evaluated by this method.
The glass transition is very complex and causes changes in many measurable characteristics. Differential Scanning Calorimetry will measure a step change in the heat flow. Increase in the coefficient of thermal expansion is detected by Thermomechanical Analysis.
Dynamic Mechanical Analysis evaluates changes in the storage and loss modulus and tan delta.In order to compare values from sample to sample, several methods have been defined to assign a single temperature to the glass transition. Many experimental parameters such as ramp rate, frequency and applied force will also have an effect on the temperature range of the glass transition. If glass transition values are to be compared, experiment parameters and the method used to assign the glass transition temperature must be consistent.