Thermogravimetric analysis (TGA) is a technique that measures how much mass a material gains or loses as it’s heated. A sample is placed on an extremely sensitive balance inside a small furnace, and as the temperature rises in a controlled way, the instrument continuously records any changes in weight. Those weight changes reveal what the material is made of, how stable it is at high temperatures, and what happens to it as it breaks down.
How TGA Works
The concept is straightforward: heat a material and watch what happens to its weight. As temperature climbs, different things occur inside the sample. Moisture evaporates first. Then volatile compounds burn off or escape. Eventually, the material itself starts to decompose. Each of these events shows up as a distinct drop (or occasionally a gain) on a plot of mass versus temperature.
The instrument tracks these changes with a high-resolution microbalance sensitive enough to detect tiny fractions of a milligram. Thermocouples monitor the exact temperature of the sample throughout the run. The result is a curve that tells you not just how much weight was lost, but precisely when each loss happened relative to the temperature.
What’s Inside the Instrument
A TGA system has three essential components: a microbalance, a programmable furnace, and a temperature sensor. The sample goes into a small crucible or pan, which sits on the balance arm inside the furnace. As the furnace heats up at a controlled rate, the balance continuously records the sample’s weight while the temperature sensor logs exactly how hot things are.
The atmosphere inside the furnace matters enormously. Running the test under nitrogen (an inert gas) isolates pure thermal decomposition from any reactions with oxygen. Switching to air or synthetic air introduces oxidation, which changes the results significantly. Research has shown that using air instead of an inert atmosphere can shift the onset temperature of decomposition by as much as 75 °C. This means the same material can appear more or less thermally stable depending on which gas surrounds it during the test.
Reading a TGA Curve
The primary output is a thermogram: a line plotting mass (usually as a percentage of starting weight) against temperature. A flat section means the material is stable. A downward step means something is leaving the sample, whether that’s water, a volatile additive, or a chunk of the material itself breaking apart. A slight upward step, though less common, indicates mass gain from a reaction like oxidation.
On its own, the thermogram can be hard to read when multiple events overlap. That’s where the derivative curve (called DTG) comes in. Instead of plotting total mass, the DTG plots the rate of mass change. Each decomposition event shows up as a distinct peak, making it much easier to pinpoint the exact temperature where each process hits its maximum. For example, a polymer might show a broad, gradual weight loss on the standard curve, but the DTG reveals that the onset of degradation actually begins around 175 °C, well before the steepest part of the weight-loss step.
Common Applications
Polymer and Composite Analysis
TGA is one of the primary tools for characterizing plastics, rubbers, and composite materials. Polymers lose mass in stages as they’re heated: absorbed moisture leaves first, then low-molecular-weight additives evaporate, and finally the polymer backbone itself decomposes. Each stage produces a distinct step on the curve. Whatever remains at the end, the residue that doesn’t burn off, is typically inorganic filler like glass fiber or carbon black. A single TGA run can quantify all of these components in one shot.
Thermal stability is another key measurement. In nitrogen, a polymer might remain stable up to around 300 °C before its backbone begins to break down rapidly. Comparing curves run in nitrogen versus air shows how much oxidation accelerates that breakdown, which is critical information for engineers choosing materials for high-temperature applications.
Fuel and Biomass Composition
TGA can perform what’s called a proximate analysis of fuels or biomass, breaking a sample into four categories: moisture, volatile matter, fixed carbon, and ash. A standardized method can accomplish this in as little as 25 minutes by stepping through different temperatures and atmospheres. The moisture drives off at low temperatures, volatiles escape at higher temperatures under inert gas, fixed carbon burns away when air is introduced, and whatever remains is ash.
Pharmaceutical Testing
Drug compounds sometimes trap water molecules within their crystal structure, forming what’s known as a hydrate. TGA detects and quantifies this trapped water by measuring exactly how much mass the sample loses as it’s heated. The shape of the weight-loss step even provides clues about how the water is held: a narrow, sharp weight loss suggests water molecules locked in isolated sites within the crystal, while a broad, gradual loss points to water sitting in channels that release more slowly. This information is essential during drug development because hydration state affects a drug’s stability, how it dissolves, and how it behaves during manufacturing and storage.
Quality Control and Composition Screening
ASTM E1131, a widely used standard test method, outlines how to use TGA for compositional analysis. It breaks materials into categories of highly volatile matter, medium volatile matter, combustible material, and ash. Components present anywhere from 1% to 100% by weight can be measured. The method is designed for quality control, incoming material screening, and troubleshooting situations where you need to confirm a material matches a known reference.
Heating Rate and Sample Size
How fast you heat the sample changes the results. Common heating rates range from 5 to 80 °C per minute. Slower rates give the sample more time to fully react at each temperature, producing sharper, more clearly separated weight-loss steps. Faster rates push decomposition events to higher apparent temperatures because the material can’t keep up with the rising heat. Research on thermoplastic degradation has confirmed that lower heating rates yield more accurate measurements of decomposition behavior, particularly for kinetic calculations. A rate of 10 or 20 °C per minute is typical for routine work, while very slow rates (around 5 °C/min) are preferred when precision matters most.
How TGA Differs From DSC
TGA and differential scanning calorimetry (DSC) are often mentioned together, and sometimes even run on the same sample, but they measure fundamentally different things. TGA tracks mass. DSC tracks heat flow. This means TGA excels at detecting any event that involves material entering or leaving the sample: decomposition, evaporation, oxidation, moisture loss. DSC, on the other hand, picks up events that involve energy absorption or release without necessarily changing mass: melting, crystallization, glass transitions, and changes in heat capacity.
Neither technique replaces the other. A polymer melting, for instance, won’t show up on a TGA curve because no mass is lost. And a subtle compositional difference between two batches of the same material might be invisible to DSC but obvious from the weight-loss pattern in TGA. In practice, running both tests on the same material gives a far more complete picture than either one alone.