Interpreting TGA Curves

Artifacts

Artifacts are effects observed on the measurement curve that are not directly caused by the sample, i.e. effects that do not relate to the properties you want to measure. The main types of TGA artifacts are:

• Buoyancy effects caused by the density of the surrounding gas decreasing on heating. Typically this results in an apparent gain in weight of 50 µg to 200 µg. Since buoyancy effects are reproducible, the curves can be corrected by performing an automatic blank curve subtraction. This also applies to buoyancy effects due to gas switching, which is often used in TGA analysis. All the curves shown in this article are blank curve corrected.
• Fluctuations of the purge gas flow rate can also affect the measurement curve. The flow rate should therefore be kept as constant as possible and not be changed during a measurement.
• Sudden loss of mass arising from the ejection of part of the sample. This often occurs when a sample decomposes with the formation of gas. You can prevent this by covering the sample with coarse grain Al₂O₃ or a crucible lid with a hole.
• Apparent weight gains caused by samples that foam and make contact with the furnace wall. You can avoid this problem by using smaller samples.

Measurement conditions 

In TGA, the exchange of materials between the sample and the immediate surroundings must be possible. The crucible is therefore either open or covered using a lid with a hole. The measurements are normally performed with a dynamic temperature program at rates between 0.5 and 50 K/min, often at 20 K/min. A starting temperature of 25 °C to 30 °C is used in order to detect possible drying of the sample. The final temperature is usually relatively high, e.g. 600 °C for organic substances or ≥1000 °C for inorganic samples because in most cases the decomposition of the sample is also of interest. Organic samples are often pyrolyzed under nitrogen. The carbon black formed during the pyrolysis process or present in the sample as an additive (e.g. in polymers) is then burnt in an air atmosphere. 

Final remarks  

The information presented above on the origin and physical and chemical background of typical TGA effects should help you interpret your own measurement curves. If the interpretation of the results is still unclear, you might well consider using other techniques such as:

• DSC and TMA measurements
• the analysis of any gaseous compounds evolved (EGA)
• the observation of the sample under the hot stage microscope.

_{Interpreting TGA curves | Thermal Analysis Application No. UC 131 | Application published in METTLER TOLEDO Thermal Analysis UserCom 13}