The distinction is straightforward: differential scanning calorimetry (DSC) measures heat flow, while thermogravimetric analysis (TGA) measures mass change. DSC helps explain transitions and reactions within a polymer. TGA helps determine what leaves the sample as it is heated and what remains. Choosing the right technique starts with the question you need the data to answer.
Two Different Measurements
In DSC, a small sample follows a controlled temperature program while the instrument records heat flow associated with changes in the material. A glass transition appears as a change in the heat-flow baseline. Melting typically produces an endothermic peak; crystallization and many curing reactions produce exothermic peaks. The test can be run through heating, cooling and reheating steps to distinguish the condition of the supplied sample from its behavior after a controlled thermal history.
In TGA, a sensitive balance records the sample’s mass as temperature changes or as the sample is held at a set temperature. The resulting curve shows when mass is lost and how much remains. A low-temperature loss may be consistent with moisture or other volatile material; subsequent losses may reflect decomposition of organic components. The final residue may provide information about inorganic content, depending on the formulation and test conditions. TGA does not directly identify the chemical substance responsible for each loss.
That difference matters because many important polymer transitions involve little or no mass change. A polymer can pass through its glass transition or melt without losing a measurable portion of the sample. TGA will not measure those transitions in the way DSC does. Conversely, a DSC peak does not, by itself, quantify how much moisture, solvent or filler a compound contains.
| Engineering question | More relevant technique | What the result can tell you |
|---|---|---|
| At what temperature does the amorphous phase change mobility? | DSC | Glass transition temperature, Tg |
| When does a semicrystalline polymer melt or crystallize? | DSC | Melting temperature, crystallization behavior and associated enthalpy |
| Has a thermoset completed its cure? | DSC | Residual cure reaction, when detectable under the selected conditions |
| How much mass is lost on heating? | TGA | Mass-loss steps and their temperature ranges |
| Is a compound’s inorganic residue consistent with its specification? | TGA | Residual mass under a defined temperature and atmosphere program |
| When does substantial thermal decomposition begin? | TGA | A method-dependent mass-loss onset or specified percentage mass-loss temperature |
DSC and TGA are therefore complementary, not interchangeable. The most useful report is not necessarily the one containing the most tests; it is the one that measures the property behind the engineering decision.
When DSC is the right choice
Glass transition and melting
The glass transition temperature, or Tg, describes a change in molecular mobility in a polymer’s amorphous regions. On a DSC trace it generally appears as a step in the baseline, rather than a sharp peak. Tg can help engineers compare materials or investigate processing history, but it does not directly measure how much load a finished component can carry at that temperature.
For a semicrystalline polymer, DSC identifies melting temperature, or Tm, and crystallization events. These results can reveal differences between batches or processing conditions. The first heating scan reflects the sample as received; cooling and reheating show its behavior after a controlled thermal cycle.
For example, if two batches of an injection-molded appliance component show different dimensional behavior, DSC can compare their melting and crystallization profiles. The results can guide the investigation, although processing records and other tests may still be needed to establish the cause.
Crystallinity and cure
The heat absorbed during melting can contribute to an estimate of crystallinity. That estimate requires an appropriate reference value and must account for crystallization during the scan and, in filled compounds, the proportion of polymer in the sample. DSC does not provide a universally valid crystallinity percentage without those inputs.
For thermosets such as epoxy systems, a residual exothermic peak can indicate that further cure remains possible. DSC can also follow heat released during an isothermal cure. These measurements can help investigate inconsistent electronics encapsulation, for example, but cure time and degree of cure must be interpreted against the specific formulation and test program.
Oxidative induction time
Under a specified method, DSC can measure oxidation induction time, or OIT: the time to a detectable oxidation event at a defined temperature and atmosphere. It is useful for comparing the oxidative stability of suitable materials or production lots, but it is not a direct prediction of service life.
When TGA is the right choice
TGA is the more direct choice when the question concerns mass loss or residual mass. It can investigate volatile content, decomposition and the residue remaining after heating. Results depend on the test program: the same material may behave differently in an inert gas and an oxidizing atmosphere.
Moisture and volatile material
If an incoming compound appears to contain unexpected volatile material, TGA can quantify mass lost in a selected temperature range. That loss should not automatically be called “moisture”: water, solvent and other volatiles may overlap, and identifying them can require further analysis. DSC may detect a related thermal event, but it does not replace a mass-loss measurement when the specification asks for a percentage.
Fillers, fibers and ash
For a glass-fiber-reinforced thermoplastic, TGA can measure residue after the polymer matrix has decomposed. In a simple formulation, that result may provide an estimate of glass-fiber loading. In a more complex compound, residue is not necessarily glass fiber: other inorganic ingredients may remain, while some fillers change mass during heating. Interpreting the result requires knowledge of the formulation and test conditions.
If an automotive supplier suspects that a reinforced housing contains the wrong inorganic loading, TGA addresses that question more directly than DSC. If the concern is a changed melting or crystallization profile, DSC is the better starting point.
Decomposition temperature
TGA shows when measurable mass loss begins or reaches a specified percentage. The reported “decomposition temperature” therefore depends on the heating rate, atmosphere and criterion used to read the curve. A DSC event may indicate a reaction, but without a mass measurement it cannot establish how much material was lost.
How to specify the test
Before requesting thermal analysis, describe the decision, not just the instrument. “We need a thermal test” gives a laboratory too little information. “We need to compare Tg between accepted and rejected molded parts” or “We need to check whether the inorganic residue matches the compound specification” points toward a useful method and a meaningful sampling plan.
A practical request should identify the polymer or suspected formulation, the sample form, the production batches to compare and the property of interest. Relevant processing history also matters. A DSC first heating scan can retain evidence of that history, while heating, cooling and reheating can help separate it from behavior under a controlled cycle. Moisture or other volatile material can itself affect a DSC result, so sample storage and preparation should be discussed before testing.
The limitations should be explicit in the report and its interpretation:
- DSC does not measure filler loading directly. A changed melting enthalpy in a compound could reflect several factors, including polymer fraction or crystalline structure.
- TGA does not measure Tg or Tm directly. Those transitions generally do not require a change in sample mass.
- Neither curve identifies every constituent on its own. A mass-loss step or thermal peak is evidence to interpret, not a complete chemical identification.analyzing-testing.netzsch+1
- Neither DSC nor TGA substitutes for a loaded-part test. If the decision concerns deflection under heat and mechanical load, HDT addresses that behavior more directly. Vicat addresses penetration-based softening under its specified conditions.
This last point is especially relevant to component design. A material’s Tg, Tm, HDT and Vicat temperature describe different responses to heat. Using a transition temperature as a stand-in for the operating limit of a loaded part can lead to a poor material decision. Plastanalisi’s thermal testing portfolio includes DSC, OIT determination, HDT and Vicat, allowing the test choice to follow the performance question.
Choosing the next step
If you need to understand how a polymer changes as it is heated or cooled, start by discussing DSC: Tg, melting, crystallization and, where relevant, cure or an appropriate OIT method. If you need to quantify what the sample loses or leaves behind, TGA is the more relevant technique. Some investigations call for both, particularly when a formulation change may have affected both composition and thermal behavior.
If your team is unsure which property will resolve a material-selection, quality-control or failure-investigation question, Plastanalisi can help define the DSC or other thermal tests it does perform and clarify where a separate TGA result would be needed.