DSC epoxy curing analysis using dynamic and isothermal methods reveals cure kinetics, degree of cure, and the cure-Tg relationship for thermosetting adhesives
This article explains the factors that control TGA resolution, introduces AMI’s Step Isothermal function — standard on all AMI TGA instruments — as the most effective solution for overlapping events, and demonstrates its performance on barium chloride, where two closely spaced dehydration steps appear as a single broadened event under conventional conditions but resolve cleanly under Step Isothermal control. For a broader overview of AMI’s thermal characterization capabilities, see our thermal properties analysis overview, or explore the full range of thermal analysis instruments.
Thermogravimetric analysis (TGA) is one of the most widely used thermal characterization techniques in materials science, chemistry, and pharmaceutical research. By precisely measuring the mass of a sample as a function of temperature, TGA thermogravimetric analysis reveals the temperatures at which chemical reactions, decomposition, and mass-loss events occur — providing quantitative data on composition, thermal stability, and reaction kinetics that few other techniques can match.
The fundamental challenge in TGA is resolution: when two or more thermal events occur at similar temperatures, their mass-loss signals overlap into a single broadened event that is difficult or impossible to interpret accurately. This resolution problem is one of the primary limitations of conventional linear-ramp TGA, and it occurs with regularity in real-world samples — hydrated compounds with multiple dehydration steps, composites with several polymer components, and cementitious materials with closely spaced decomposition stages all produce this exact problem.
In a TGA experiment, a sample is placed on a high-precision microbalance inside a controlled-temperature furnace. As temperature increases according to a programmed profile — typically a linear ramp — the instrument continuously records sample mass. Any process that involves a change in sample mass produces a step in the TGA curve: evaporation of volatile components, desorption of surface species, dehydration, thermal decomposition, oxidation, or combustion.
The TGA thermogram — mass (or mass %) vs. temperature — is the primary output. Derivative thermogravimetry (DTG), the first derivative of the mass curve with respect to temperature or time, enhances the visibility of individual events by converting mass-loss steps into peaks, making overlapping transitions easier to detect and quantify.
Resolution in TGA thermogravimetric analysis — the ability to separate two events that occur at similar temperatures into distinct, individually measurable steps — is controlled by several experimental parameters:
| Factor | Effect on Resolution | Practical Limit |
|---|---|---|
| Heating rate | MOST IMPACTFUL. Slower rates allow more time for each event to complete before the next begins — improving separation. Faster rates compress events into narrower temperature windows, increasing overlap. | Very slow rates (<1°C/min) dramatically extend experiment time. For closely spaced events, even the slowest practical rate may be insufficient. |
| Sample mass | Smaller samples improve resolution by reducing temperature gradients within the sample and minimizing the heat effects that can shift and broaden decomposition events. | Smaller samples reduce signal strength — below ~1 mg, signal-to-noise becomes limiting for minor components. |
| Purge gas flow rate | Higher flow rates remove evolved gases more quickly, reducing back-pressure effects and improving the sharpness of individual decomposition steps. | Very high flow rates can alter the local atmosphere and affect reaction equilibria — some reactions are gas-atmosphere dependent. |
| Particle size | Smaller, more uniform particles provide more consistent heat transfer and reduce intra-sample temperature gradients that broaden decomposition events. | Grinding can alter sample properties (surface area, crystal structure, moisture content) — sample preparation must not change the material being characterized. |
| The fundamental problem with conventional approaches: Reducing heating rate is the most effective single parameter for improving resolution — but it has a hard practical limit. For events separated by only 20–30°C, no realistic heating rate completely eliminates overlap. Multiple trial runs at different rates waste sample and time. Step Isothermal TGA solves this by dynamically adapting the temperature program in real time based on what the sample is actually doing — not based on a pre-programmed schedule that assumes the sample behaves predictably. |
Step Isothermal is an adaptive temperature control algorithm integrated directly into AMI’s InfinityPro software platform (Figure 1; alt text: InfinityPro software interface showing Step Isothermal option enabled in TGA method setup). Rather than following a fixed temperature ramp regardless of what the sample is doing, Step Isothermal monitors the sample’s actual mass-loss rate in real time and adjusts the temperature program dynamically:
The result is an experiment that adapts intelligently to the sample’s actual thermal behavior — pausing exactly when resolution is needed and resuming when it is not. No manual scripting, no trial-and-error rate optimization, and no multiple experimental runs required.
Two parameters control Step Isothermal behavior (Figure 2; alt text: InfinityPro Step Isothermal dialog showing Iso Start Threshold and Ramp Resume Threshold parameter entry fields). Setting them correctly is straightforward:
| Parameter | What It Controls | Recommended Setting | Effect of Wrong Setting |
|---|---|---|---|
| Iso Start Threshold (%/min) | The derivative weight loss rate at which the system switches from ramp to isothermal hold — the sensitivity trigger for detecting a decomposition event | ~1/10th to 1/15th of the expected peak weight loss rate. Run one conventional TGA first to find the peak DTG value, then set the threshold at 10–15% of that value. | Too high: system misses slow-onset events and enters the hold too late, reducing separation. Too low: system triggers holds from baseline noise, creating unnecessary pauses that extend experiment time without improving resolution. |
| Ramp Resume Threshold (%/min) | The derivative weight loss rate below which the system exits the isothermal hold and resumes ramping — defines when an event is considered ‘complete enough’ to advance | ~1/15th or less of the Iso Start Threshold. The system should only resume when the active event has substantially completed. | Too high: system resumes too early, before the event finishes, reducing the resolution benefit. Too low: system waits unnecessarily long after events, extending total experiment time. |
| Practical workflow: (1) Run one conventional TGA at a moderate rate (e.g., 10–20°C/min) to get an overview of the sample’s decomposition profile and find the approximate peak DTG values. (2) Set Iso Start Threshold = peak DTG / 10 to 15. (3) Set Ramp Resume Threshold ≤ Iso Start Threshold / 15. (4) Run the Step Isothermal experiment — the system handles the rest automatically. No further adjustment is typically needed. |
Barium chloride (BaCl₂·xH₂O) provides a clear demonstration of the Step Isothermal advantage. A conventional TGA run at 20°C/min (Figure 3; alt text: conventional TGA of barium chloride at 20°C/min showing two overlapping mass-loss events near 80°C and 110°C appearing as a single broadened step) shows two mass-loss events — one near 80°C and one near 110°C — that appear as a single broadened transition. The events are separated by only ~30°C, and at 20°C/min the heating program advances through the first event before it completes, merging it with the onset of the second.
Running the same material using Step Isothermal (Figure 4; alt text: Step Isothermal TGA of the same barium chloride showing two clearly resolved distinct weight-loss steps at 80°C and 110°C) produces a fundamentally different result. The first dehydration event triggers an automatic isothermal hold. The temperature remains constant until the first event completes, then the ramp resumes — arriving at the second event’s temperature only after the first is fully resolved. The result is two clearly distinct, individually quantifiable mass-loss steps from exactly the same material, at exactly the same temperature program rate, with no additional manual optimization.
| Key result: Two events that appear inseparable in conventional TGA become fully resolved under Step Isothermal control — without changing the heating rate, reducing sample mass, or running multiple experiments at different conditions. The same quantitative accuracy is achieved with a single, automated experimental run. |
Note: for the simultaneous DSC and TGA measurement of the same barium chloride compound — revealing both the mass loss and the corresponding enthalpy of each dehydration step on the same sample — see our article on simultaneous thermal analysis of barium chloride dihydrate. Step Isothermal and STA are complementary approaches to the same resolution problem: Step Isothermal improves TGA-only resolution; STA adds simultaneous enthalpy measurement.
Step Isothermal TGA thermogravimetric analysis is most valuable whenever a sample contains multiple components or phases that decompose or dehydrate in a similar temperature range. The broader the overlap, the greater the benefit:
| Material Class | Overlapping Events That Benefit from Step Isothermal | Related AMI Article |
|---|---|---|
| Crystalline hydrates | Multiple water-of-crystallization steps in monohydrates, hemihydrates, and dihydrates where successive dehydration events differ by only 20–50°C | TGA crystalline hydrates |
| Cement and concrete | Ca(OH)₂ dehydration (400–480°C) and CaCO₃ decomposition onset (500°C) are closely spaced — Step Isothermal improves quantification accuracy of each phase independently | TGA cement hydration analysis |
| Polymer composites | Multiple polymer components, filler burn-off, and char oxidation in complex composite materials often produce overlapping mass-loss events | Nylon degradation analysis by DSC |
| Pharmaceuticals | Residual solvent and water-of-crystallization desorption at similar temperatures; decomposition onset of hydrate vs anhydrate forms | TGA crystalline hydrates; DSC TGA pharmaceutical polymorphism |
| Battery electrode materials | Binder decomposition and active material reactions occurring in overlapping temperature windows during thermal stability testing | Thermal properties analysis |
| Mixed mineral systems | Silicate, carbonate, and sulfate decompositions in natural mineral samples frequently overlap in the 500–900°C range | TGA cement hydration analysis |
Step Isothermal is a standard feature on all AMI TGA 1000/1200/1500 thermogravimetric analyzers — accessible directly from the InfinityPro software interface without additional modules, licensing, or configuration. The feature works with all TGA measurement modes and complements other resolution-enhancing practices (sample mass reduction, optimized purge gas flow) rather than replacing them. In combination with AMI’s 0.1 μg microbalance sensitivity and low-drift thermal design, Step Isothermal enables high-resolution TGA thermogravimetric analysis of complex multi-component samples that conventional linear-ramp TGA cannot adequately characterize.
| For related reading on how purge gas atmosphere quality affects TGA measurement integrity — a key variable that works in combination with Step Isothermal for complex sample analysis — see our article on oxygen-free TGA analysis and purge gas testing. For TGA applied to the multi-stage dehydration of crystalline hydrate compounds, where Step Isothermal is directly applicable to improve step resolution, see our article on TGA crystalline hydrates dehydration kinetics. |
Resolution of overlapping thermal events is one of the fundamental challenges in TGA thermogravimetric analysis. Conventional approaches — reducing heating rate, reducing sample mass, optimizing purge flow — all improve resolution to a degree, but have practical limits and require multiple trial experiments to optimize. AMI’s Step Isothermal function addresses this directly by adapting the temperature program in real time based on the sample’s actual decomposition behavior, automatically inserting isothermal holds exactly when and where resolution is needed.
As demonstrated with barium chloride, two events inseparable at 20°C/min in conventional TGA become fully resolved under Step Isothermal control — in a single automated experiment, with no manual optimization. This capability is standard on all AMI TGA instruments through the InfinityPro software platform, making high-resolution thermal analysis of complex samples accessible without additional cost or complexity. Explore AMI’s full range of thermal analysis instruments, or visit the AMI Technical Library for further application notes on TGA, DSC, STA, and thermal characterization methodology.
Thermogravimetric analysis (TGA) is a thermal characterization technique that measures the mass of a sample as a function of temperature or time under a controlled atmosphere. As the sample is heated, any process that involves a change in mass — evaporation, desorption, dehydration, decomposition, oxidation, or combustion — produces a measurable step in the TGA curve. The technique provides quantitative data on the temperatures at which these events occur, the mass fractions involved, and the residual non-volatile content. It is one of the most widely used techniques in materials science, pharmaceuticals, polymers, ceramics, and construction materials research.
Overlapping mass-loss events occur when two or more thermal decomposition or dehydration reactions happen in similar temperature ranges — close enough that the conventional linear temperature ramp advances into the second event before the first has fully completed. The result is a single, broadened mass-loss step that blends two distinct events into one apparently continuous transition. This makes it impossible to separately quantify the mass contributed by each component or phase. Overlapping events are common in multi-component materials including composites, hydrates with multiple water-of-crystallization sites, pharmaceutical samples with residual solvents and water, and mineral mixtures with closely spaced decomposition reactions.
Step Isothermal TGA is an adaptive temperature control method that monitors the sample’s real-time derivative weight loss rate during a TGA experiment and automatically inserts isothermal holds when a decomposition event is detected. When the rate of mass loss exceeds a user-defined Iso Start Threshold, the furnace temperature stops increasing and holds constant — allowing the active decomposition event to complete before the temperature advances. Once the rate falls below the Ramp Resume Threshold, the linear temperature ramp resumes. This cycle repeats for each subsequent event, producing a TGA curve where individual events are fully resolved without requiring manual rate optimization or multiple trial experiments.
The recommended starting point for the Iso Start Threshold is approximately 1/10th to 1/15th of the expected peak derivative weight loss rate (peak DTG in %/min). Run one conventional TGA experiment first to find the approximate peak DTG value for your sample, then set the Iso Start Threshold to 10–15% of that value. The Ramp Resume Threshold should be set at approximately 1/15th or less of the Iso Start Threshold — low enough that the system only resumes after the active event has substantially completed. These initial settings typically work well for most materials, with minimal adjustment needed after reviewing the first Step Isothermal result.
Any sample containing multiple components or phases that decompose or dehydrate in overlapping temperature ranges benefits from Step Isothermal TGA. Crystalline hydrates with multiple water-of-crystallization steps, polymer composites with several degradation stages, pharmaceutical formulations where residual solvent and water overlap in desorption temperature, cementitious materials where Ca(OH)₂ dehydration (400–480°C) overlaps with CaCO₃ decomposition onset (500°C), battery electrode materials with binder and active material reactions in similar temperature windows, and mixed mineral systems all benefit substantially. The technique is particularly valuable when the events are separated by less than 50°C and conventional rate reduction alone cannot achieve adequate resolution.
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