ASTM D3418 covers transition temperatures and the enthalpies of fusion and crystallization of polymers by differential scanning calorimetry. One run on a 5 mg to 10 mg sample provides the glass transition, melting point, crystallization temperature, and the heat associated with each. MatX Lab runs the method on resin pellets, powders, films, molded parts, and compounded material, aligned to ASTM D3418, ASTM E1356, and ISO 11357.
What Is ASTM D3418?
ASTM D3418 is the standard test method for transition temperatures and enthalpies of fusion and crystallization of polymers by DSC. The instrument holds your sample and an empty reference pan on the same temperature program and records the difference in heat flow between them.
Three features emerge from that trace. A step in the baseline marks the glass transition. An endothermic peak marks melting, and its area gives the heat of fusion. On cooling, an exothermic peak marks crystallization and gives the heat released.
When heat of fusion is divided by the value of a completely crystallized reference polymer, it gives the percent crystallinity. Heat of fusion for polyethylene is around 293 J/g, for polypropylene around 207 J/g, and for PET around 140 J/g, and therefore, if heat of fusion for polyethylene is 90 J/g, its crystallinity is about 31%.
ASTM D3418 Scope and Applications
The method applies to thermoplastics, thermosets, and polymer compounds where a measurable thermal event exists across the instrument’s range. Highly filled and heavily crosslinked materials give weaker signals and need more sample or a complementary technique.
Applications:
Material identification. Verifying the resin in terms of its melt temperature and crystallinity, which can distinguish different polyethylene, nylon, and polyester resins.
Lot acceptance testing. Ensuring that the incoming resin is the same as the accepted material before molding.
Process assessment. Reading crystallinity to understand how the cooling rate in a molding or extrusion process affected the finished part.
Contamination and blend checks. Finding a second melting peak that should not be there, which usually means regrind, cross-contamination, or an unintended blend.
Elastomers and rubber formulations use the glass transition as the desired pathway to ASTM D7426, which requires a quicker heating rate appropriate for this procedure. Thermogravimetric analysis determines the composition, loading, and thermal stability of the materials.
ASTM D3418 Test Procedure
The heat-cool-heat cycle carries the method. The first heating removes processing history, and the second heating yields the polymer.
| Step | What happens |
| Sampling | A representative piece is taken, avoiding surface skin, sprue, and any region with a different thermal history from the bulk. |
| Encapsulation | 5 mg to 10 mg is sealed in an aluminum pan with flat contact against the pan base, since poor contact broadens every feature. |
| Calibration | Temperature and heat flow calibration are verified with indium, and a second reference is added when the range demands it. |
| First heating | The sample heats at 10 °C/min through all expected transitions, erasing molding and storage history. |
| Controlled cooling | The sample cools at a defined rate. The crystallization peak is recorded here, and cooling rate changes both its position and its size. |
| Second heating | The sample heats again under identical conditions, and this trace supplies the reported transitions. |
| Peak integration | Melting and crystallization peaks are integrated against a defined baseline to give enthalpies in J/g. |
| Evaluation | Transition temperatures are read with the basis named, and crystallinity is calculated where a reference enthalpy applies. |
Limitations: The heating rate influences all results, so a test at 10°C/min and a test at 20°C/min using the same material will differ, and comparison between labs is only possible if the heating rate is equal. The filler reduces the amount of polymer in the sample; therefore, enthalpies must be normalized to the amount of polymer before drawing any conclusions about crystallinity. The choice of baseline affects peak area, and for a wide, flat melt, two different operators can obtain different results through integration. A sample amount less than 5 mg gives a poor signal, while a sample greater than 10 mg causes a thermal delay that shifts the peaks upward. Recycled and blended materials produce overlapping peaks that require interpretation.
ASTM D3418 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Sample mass | 5 mg to 10 mg per pan |
| Material supplied | 5 g minimum so sampling is representative and repeats are possible |
| Sample type | Pellets, powder, film, molded sections, machined chips |
| Pan type | Sealed aluminum, hermetic where volatiles are expected |
| Heating rate | 10 °C/min standard |
| Purge gas | Nitrogen at constant recorded flow |
| Replicates | Two per material, three where crystallinity feeds a specification |
Send material that represents the part, not the runner or the surface. Skin and core in a thick molding crystallize differently, and a sample taken from one will not describe the other.
Let us know what the polymer is, if you can identify it. The anticipated melting range lets us create a temperature profile that completes the test in one run, without realizing at the end that we missed a transition.
ASTM D3418 Test Results and Reporting
Your report gives the thermogram for every specimen with transitions tabulated alongside.
Glass transition appears with the evaluation basis named. Melting and crystallization temperatures are reported as peak values with onset temperatures supplied, and enthalpies are given in J/g with the integration baseline shown on the trace. Where a reference enthalpy exists for the polymer, percent crystallinity is calculated, and the reference value used is stated.
Every trace is associated with running conditions: sample weight, pan type, heating/cooling rate, gas and flow used, and the specific heating step where these values were obtained. Unexpected peaks are always explained and not simply ignored on the graph.
ASTM D3418 FAQs
Should I use the first or second heating result?
The second, in practically all cases. The first includes the history of molding and storage, whereas the second reflects the sample itself. If the process history is the focus of our investigation, we provide both.