Materials have Glass Transition Temperatures that help predict the thermal performance characteristics. In material science, most mechanical properties such as adhesion, tensile strength, compressive strength, and modulus (stiffness) are a function of temperature. In many cases, the properties decrease only slightly versus temperature until the glass transition temperature is exceeded and then they decrease significantly. Electrical properties for potting compounds encapsulants are similarly affected and the thermal-chemical resistance of coatings is compromised with higher diffusion rates over the Tg. For dimensional stability concerns, the coefficient of thermal expansion dramatically changes over the glass transition temperature.

Glass transition temp’s are discernible from an inflection point. As the heat flow through the specimen is steadily increased suddenly the sample demands less heat to increase its temperature. This delineation marks a Tg via DSC. In the plot shown here, the heat flow required to raise the temperature of the specimen one degree is constant until about 99°C when a pronounced change in the amount of heat required to elevate the temperature of the sample results in “S” shape. The heat flow becomes linear again afterward at 118°C. The software detects the change in graphical concavity/convexity to be the glass transition temperature at 110°C.

Glass transitions are related to design and the proper combination of ingredients. Too much plasticizer, mix ratio deviations in two component reactive systems and the process temperature and cure schedule duration all affect the Tg. High quality, well controlled processes for thermoset products have reproducible Tg. Out-of-control processes do not.