Glass Transition (Tg) Temperatures
A glass transition temperature (Tg) describes the point at which a thermoset polymer changes its physical structure to a crystalline state and is an average of the temperature range those changes occurred. The actual glass transition temperature range depends upon the molecular structure of the material, the testing method, sample preparation, the cure schedule, and the degree of cure.
Changes in temperature can have an enormous impact on the performance properties of thermosetting polymer systems.A polymerized (cured) resin and curing agent is an organized crystalline structure sometimes referred to as a “glassy state”. At this “glassy state”, molecules begin to vibrate but remain locked in place. As temperatures exceed the Tg, molecules begin to move more freely and eventually start to soften to a rubbery state at a molecular level. The physical and electrical properties are adversely affected relative to the subjected temperature.
Glass transition temperature is usually a good indicator of a materials resistance to high temperature. Composites containing a silicone based resin, like G7 have a relatively low (Tg) and yet are well suited for applications in excess of 220°C. Materials with lower (Tg) tend to be more flexible than resins with a higher a (Tg) -which may make them more suitable for multiple heat and cooling cycles. An example would be Molybdenum disulfide filled cast nylon which has an average (Tg) of 48°C and superior flexural modulus properties that make them an effective ‘slipper’ bearing in hot roll mills.
The decline of performance properties is not necessarily permanent. This depends on how high a temperature in excess of the Tg the material is and the duration it sees the high temperature. Most thermoset compounds are formulated and tested to a specific temperature rating above the (Tg) that guarantees a minimum 50% of its initial test values at the rated temperature for 20,000 continuous hours. Materials subjected to intermittent periods in above its (Tg) will have a loss of mechanical and electrical properties that return when temperatures return to the (Tg).