Glass transition temperature (Tg) is a very useful property for understanding the thermal characteristics of an epoxy resin system. The Tg is the temperature at which the epoxy changes from a glassy (solid) state to a soft, rubbery state. It can be considered the point at which a measurable reduction in physical properties occurs resulting from exposure to elevated temperatures.
Be aware that Tg values can be reported after a second heat. The second heat is the process of testing the sample after it has been exposed to an initial first heat which resulted in an elevated temperature, 200°C, post cured sample. A second heat Tg value is not representative of your sample unless you have replicated the 200°C cure schedule that was used for the first Tg test.
HDT of laminate
The HDT of laminate is the temperature at which a typical 3.2mm epoxy/fibreglass laminate will deform under constant load with the same test parameters as above. The HDT of a laminate is so much higher than a neat resin that it will not deform even at the test’s maximum temperature of 300°C.
Heat deflection temperature
Heat deflection temperature (HDT) is the temperature at which the epoxy will deform under constant load.
A sample is submerged in oil at a carefully calibrated temperature and subjected to 1.82 MPa of bending stress in the centre. The temperature of the oil is then gradually raised until the bar deflects 0.25mm in the centre. This temperature is considered to be the heat deflection temperature.
Tg DMA onset storage modulus and peak tan delta
The dynamic mechanical analyser (DMA) determines the Tg using a mechanical method. The test sample is placed into a 3-point bending fixture and a cyclical load is applied. The temperature of the sample is increased and the change in the deflection is measured. As the temperature is increased during the test, the response of the sample changes. The sample’s response is plotted using three different graphs based on how the bending energy is transferred into the sample: storage modulus, loss modulus, and the tan delta.
- Storage modulus
This is the elastic response. The recovered part of the energy originally put into the sample.
- Loss modulus
This is the energy that is absorbed by the sample due to friction and internal motion.
- Tan delta
This is the ratio of loss modulus to storage modulus, the dampening character of the sample.
When epoxy is below its Tg, the storage modulus is high and the loss modulus is low. The sample releases energy efficiently and does not absorb energy well due to its stiffness. When the sample gets closer to its Tg, the storage modulus decreases. Energy is now absorbed into the sample, driving the loss modulus higher.
- Tg onset storage Modulus
This is a conservative value indicating a measured loss of stiffness.
- Tg peak tan delta
This is the highest measured Tg value.
Tg DSC onset–first heat
While a DMA measures thermal properties of a sample via mechanical means, a differential scanning calorimeter (DSC) machine measures the heat flow in and out of a sample to determine its Tg. This test is conducted by placing a fully cured sample into a small pan in the DSC and heating it to 200°C at a rate of 10°C per minute. The heat flow into the sample is measured and compared to an empty reference pan. The difference in heat flow is measured and plotted. An inflection occurs in the plotted curve at the Tg; the onset is measured at the beginning of this inflection.
Tg DSC ultimate
Ultimate Tg is the highest Tg value that can be attained for a particular epoxy system. In order to achieve this temperature resistance in an application, the epoxy must be post cured at a pre-defined elevated temperature for a specific amount of time.