ASTM D7426 provides the DSC technique used to measure the glass transition temperature of a polymer and/or elastomeric material. This test involves heating a milligram-sized sample at a controlled rate, producing a step change in heat flow that indicates the transition point. MatX Lab performs the analysis on rubber materials, thermoplastic elastomers, adhesives, and cured thermosets per ASTM D7426.
What Is ASTM D7426?
ASTM D7426 is the standard test method for assigning the DSC procedure for determining the glass transition of a polymer or elastomeric compound. Differential scanning calorimetry compares the heat flow into your sample with that of a reference pan as both follow the same temperature program.
At the glass transition point, the polymer’s heat capacity changes, and the heat-flow curve steps up to a new value. This step indicates the glass transition and is small compared to the melting peak. This is why sample preparation and baseline quality determine whether you can detect the indication.
Elastomers sit well below room temperature. Natural rubber transitions near -65 °C, EPDM near -55 °C, and nitrile rubber moves between roughly -40 °C and -20 °C as acrylonitrile content rises. Silicone runs far colder, near -120 °C.
ASTM D7426 Scope and Applications
This technique applies to polymers and elastomers with a noticeable change in heat capacity. Filled elastomers reduce this heat capacity change, while highly crosslinked or crystalline materials may negate it enough that the mechanical technique would be preferable.
Applications:
- Low-temperature service limits. Establishing where a seal, gasket, or hose compound stiffens, which sets the cold end of its working range.
- Compound identification. Ensuring the incoming rubber is the correct formulation because Tg varies with the polymer and plasticizer.
- Cure and aging studies. Tracking how Tg moves after cure, after heat aging, or after fluid exposure.
- Blend verification. Detecting two transitions in a polymer blend, which tells you the phases stayed separate.
Where the mechanical response matters more than the thermal signal, ASTM D5023 gives Tg by dynamic mechanical analysis in flexure. Transition temperatures on plastics generally fall under ASTM E1356 and ASTM D3418 (both out of V7). Aerospace work under this method covers commercial programs, and MatX Lab analyzes materials at the specimen level.
ASTM D7426 Test Procedure
| Step | What happens |
| Sampling | Cut a representative piece from your material, avoiding surface skin, mould release, and any bloomed additive. |
| EncapsulationSeal | 10 mg to 20 mg in an aluminum pan with good contact to the pan base. Poor contact broadens the step and blurs the transition. |
| Calibration check | Indium and a low-temperature reference verify temperature and heat-flow calibration before the run. |
| Purge and cooling | Nitrogen purges the cell at a steady flow. Liquid nitrogen or a mechanical cooler takes the sample below the expected transition. |
| First heating | The sample heats through the transition at 20 °C/min, which removes molding and storage history. |
| Controlled cooling | The sample cools at a defined rate to a known thermal state, since the cooling rate affects the transition seen next. |
| Second heating | The sample heats again under identical conditions. This trace supplies the reported transition. |
| Evaluation | Extrapolated onset, midpoint, and end temperatures are read from the step, and the reported basis is stated. |
Limitations: Carbon black and mineral fillers lower the polymer proportion in the blend, so a highly filled formulation will show a small peak that is difficult to detect. Plasticizer and oil push the transition down; this means Tg alone cannot characterize a formulation because you need additional information besides Tg. Heating rate affects the reading; for the same formulation, 20°C/min gives a higher Tg than 5°C/miCrosslinkink density pushes the transition up, so an overcured compound appears as a different compound. Two polymers in a mixture can overlap and make two peaks look like one transition, so you may need modulated or mechanical testing to separate the peaks.
ASTM D7426 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Sample mass | 10 mg to 20 mg, adjusted upward for heavily filled compounds |
| Material supplied | 5 g minimum so representative sampling is possible |
| Form | Cured sheet, molded part, pellet, or cut section |
| Pan type | Sealed aluminum, hermetic where volatiles are expected |
| Heating rate | 20 °C/min standard |
| Purge gas | Nitrogen at a constant, recorded flow |
| Replicates | Two per material minimum, three when the step is weak |
Send material that represents the part, not the sprue or the surface. Mould release, bloom, and surface oxidation all sit in the first fraction of a millimeter and can dominate a 15 mg sample.
Tell us the expected transition range if you know it. Starting a run 20 °C above the actual Tg means the step falls outside the trace, and the run must be repeated.
ASTM D7426 Test Results and Reporting
Your report provides the glass transition temperature with the evaluation basis, along with the thermogram for every specimen.
The extrapolated onset, midpoint, and end temperatures are all tabulated so your specification can use whichever it calls for. Each trace includes run conditions: sample mass, pan type, heating and cooling rates, purge gas and flow, and the cycle in which the transition was read.
Where additional features appear in the trace, we describe them. A melting peak, a cure exotherm, or a volatiles endotherm often explains more about the sample than the transition itself, and leaving them out of the report would hide real information.
ASTM D7426 FAQs
Which value do I report as Tg?
Irrespective of which one you have specified. The most commonly used default is the midpoint, while the conservative default for low-temperature conditions is the extrapolated onset, and the report provides all three.
How much material should I send?
At least 5 g. The pan holds milligrams, but representative sampling and repeat runs require more than a single small piece.