Factors affecting the accuracy of TMA measurements

This study investigates factors that cause differences between reported temperatures in thermomechanical analysis (TMA) instruments and actual sample temperatures. Observations of the melting of indium are made under several sets of conditions. These include varying the indium specimen size, altering the position of the thermal sensor, changing the heating rate, and placing pieces of indium atop and below glass and copper specimens of varying thickness. A TMA instrument with direct-contact thermal sensor and single differential thermal analysis capability yields better sample temperature measurements and allows direct examination of the effects of heat capacity. In a second set of experiments, the glass transition of polycarbonate film is measured in the tension mode using different forces and specimen lengths. The results of this study show that low material thermal conductivity and high specimen heat capacity cause the sample temperature to be lower than that of the surrounding atmosphere. Temperature gradients along the length of the furnace can intensify the temperature difference. Furthermore, stress applied to specimens in tension can lead to erroneous Tg values. Specimen and calibration procedural changes are recommended.