Curve Interpretation Part 4: TGA Measurements - METTLER TOLEDO

Curve Interpretation Part 4: TGA Measurements

In Part 4 (UserCom 41) of this series, we discuss different questions that arise in connection with TGA measurements.

We also show how measurements with other thermal analysis techniques such as differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), thermooptical analysis (TOA) can aid the interpretation of TGA measurements.

Influence of heating rate 

 

Introduction

A TGA measurement is usually performed to determine the temperature at which a change in mass occurs in a material when it is heated using a defined temperature program under defined atmospheric conditions.

We also want to know how large the mass change is. This gives us information on the composition of the material or about chemical reactions that occur during the heating ramp. Typical questions that arise in connection with TGA measurements are:

  • How can we determine the initial content of volatile substances (moisture, solvents)? 
  • Processes that are associated with mass changes can overlap. How can we separate them? 
  • What happens during a mass change? What substances are evolved during the loss of mass? How can we identify them? 
  • We expect certain effects for example on the TGA/DSC curve but we cannot identify them. What can we do in this situation? 
  • These questions will be discussed in the following sections

 

Initial Content of Volatile Substances  

Volatile substances (moisture, solvents) can strongly influence the properties of materials. If we want to determine their initial content, nothing must be allowed to escape between taking the sample and performing the actual measurement.

Likewise, with hygroscopic materials, we must prevent the uptake of moisture by the sample between sampling and measurement. Whether these requirements can be satisfied or not depends on the configuration of the TGA.

a. The TGA is not equipped with a sample changer

In this case, we recommend that you note both the mass of the crucible and the initial mass of the sample. This allows any changes in sample mass to be determined between taking the sample and beginning the measurement. 

b. The TGA is equipped with a sample changer

In addition to the method mentioned above under a), there are two other possibilities:

1. If the TGA is equipped with a lid piercing kit and if the maximum temperature during the measurement is below 640 °C, the samples can be prepared in an aluminum crucible sealed hermetically using a special lid. In contrast to the normal aluminum lids, the special lid is higher. The lid is pierced by the needle of the lid piercing kit immediately before the measurement without the needle coming into contact with the sample. This technique minimizes the time a sample can release volatile substances or absorb moisture.

2. In all other TGA crucibles (aluminum oxide, platinum, sapphire), the loss of volatile substances and the absorption of moisture can be largely prevented by placing an aluminum lid over the sample in the crucible. The sample changer removes the lid immediately after inserting the sample.

 

Overlapping Changes in Mass

In many cases, different processes associated with changes in mass overlap. Several strategies can be used to improve the separation of overlapping changes in mass and thereby quantitate the mass changes more accurately.

  • Change experimental parameters such as the heating rate, sample mass, atmosphere or the gas exchange conditions between the sample and the atmosphere using different crucible lids (e.g. crucible lids with holes of different size, see [1]).


Summary

The main difficulties in the interpretation and evaluation of TGA measurements are due to overlapping mass changes and the fact that TGA measurements alone cannot identify the evolved substances. Identification is however possible if the TGA is combined with a system for gas analysis such as mass spectrometry, Fourier transform infrared spectroscopy or gas chromatography/mass spectrometry is used.

The separation of overlapping mass losses can often be improved by varying experimental parameters such as sample mass, heating rate, or the atmosphere. Finally, other thermal analysis techniques such as DSC, TMA, DMA or hot-stage microscopy can lead to a better understanding of materials investigated by TGA.

 

Curve Interpretation Part 4: TGA Measurements | Thermal Analysis Application No. UC 411 | Application published in METTLER TOLEDO Thermal Analysis UserCom 41