Thermogravimetric Analysis (TGA)

What is Thermogravimetric Analysis?

Thermogravimetry (TG) or Thermogravimetric Analysis (TGA) belongs to the thermoanalytical methods.

TGA is based on continuous recording of mass changes of a solid or liquid sample as a function of a combination of temperature with time, and additionally of pressure and gas composition.

TGA is mainly used in material characterization (R&D) but also for QC/QA and failure analysis in industry.

Definition

Thermogravimetry (TG) or Thermogravimetric Analysis (TGA) is a well proven Thermal Analysis method. TGA is used in the research & development of various substances and engineering materials – solid or liquid – in order to obtain knowledge about their thermal stability and composition.

What Information Can Thermogravimetric Analysis Provide?

  • Thermal stability (TOE)
  • Mass loss (Δm) due to physical processes such as desorption, evaporation, sublimation
  • Mass loss (Δm) due to decomposition to evolved gases
  • Mass loss (Δm) due to reaction such as reduction, dehydration
  • Mass gain (Δm) due to reaction such as oxidation
  • Decomposition rate (Δm/dt; DTG)
  • Thermokinetics (for irreversible processes)
Measuring Principle

Measuring Principle

A thermobalance is used to measure the mass change of a sample as a function of temperature or time, under a defined and controlled environment with respect to heating rate, gas atmosphere, flow rate, crucible type, etc.

Measurement Result

This TGA plot shows the decomposition of calcium oxalate monohydrate, CaC2O4. H2O, in air at a heating rate of 10 K/min. The decomposition occurs in three mass-loss steps (black curve) with the release of water (12.3%), carbon monoxide (19.2%) and carbon dioxide (30.1%). The corresponding 1st derivative of the TGA curve (dottet line, DTGA) provides the decomposition rate and is helpful for evaluating the mass-loss steps accurately.

Super-Res® – for the Separation of Overlapping TGA Steps

Super-Res® is a software extension for thermobalances where the mass- change rate (1st derivative of the TGA signal; dTGA/dt) is used to modify the temperature program. 

To achieve a predefined mass-change rate, the furnace temperature is controlled;
⇒ no more fixed heating rate during the experiment.

Objective: improved separation of overlapping effects

According to ASTM E 473: controlled-rate thermal analysis (e.g., mass loss)

Operation Modes

Three modes are available:  

  • start/stop  
  • step-iso  
  • dynamic

The objective of Super-Res® is the improved separation of effects.

Within one measurement, up to 12 different Super-Res® segments can be programmed. The nominal heating rate is always the maximum. By using Super-Res®, the effective heating rate is reduced, if necessary. The minimum heating rate is always 0.

To determine proper Super-Res® parameters, it is recommended to carry out a general measurement in the “normal” TGA mode at first.

Start/Stop Mode

The measurement starts with the pre-selected heating rate. As soon as a given threshold value (blue) is reached, heating stops and starts again when the mass-change rate drops below that threshold. The mode can be used to achieve a constant mass-loss rate, e.g. for binder burn out.

Example:

Measurement on a rubber mixture in the start/stop mode with a threshold value of 1 %/min

Stepwise Isothermal (Step-Iso)

Represents an extension of the Start/Stop Mode. There is an upper and a lower threshold. Heating is stopped as soon as the mass change rate exceeds the upper threshold (blue) and starts again when the mass change rate has fallen below the lower threshold (pink). In between, the temperature is kept isothermal. This means, the heating rate is either the nominal (pre-selected) one or zero; no intermediate values are taken.

This mode usually causes oscillation of the mass-change rate.

This mode can, for example, be used to adapt to a zone furnace.

Dynamic Heating Rate

This mode allows for the highest flexibility including dynamic change of the heating rate.
The closer the mass change rate comes to the predefined threshold value, the more reduced is the heating rate. Smooth crossover from one heating rate to another. Preferred mode for TGA experiments.

The control factor used for the dynamic mode (number between 1 and 99) regulates how fast the system reacts with a change in the derivative signal. Control factors near 99 result in a sharp change of the current heating rate and finally in Super-Res® control similar to the start/stop mode.  Control factors near 1 result in a very soft change of the current heating rate. The default value for the control factor is 20.

Examples

Examples

NBR Rubber Mixture

  • E1 = 100 kJ/mol, E2 = 200 kJ/mol

NR/EPDM Rubber

  • E1 = 100 kJ/mol, E2 = 200 kJ/mol

Caolin (21%) and Montmorillonite (78%)

Natural products underly a variability in composition. This is most probably the reason for the differences in the step heights observed.

Advantages and Limitations

Super-Res® can help increase the resolution of TGA measurements within an acceptable time interval. However, the kinetics of the reactions involved are pivotal and dictate the separation of overlapping steps by decreasing the heating rate. For better optimization of the dynamic mode, activation energies have to be taken into distinct consideration. Only in cases where consecutive reactions occur in which the activation energy of the 1st step (E1) is lower than the activation energy of the 2nd step (E2) does rate-controlled mass change work well. However, for independent reactions, it is heavily dependent on the sample‘s properties.

The table below summarizes some simulations which clearly show when the Dynamic Heating Rate Mode is successful and when it is not all that helpful.

 

Reaction
Type
Activation EnergyResultExample/
Recommendation
ConsecutiveE1 < E2Improvement in signal separation is
always achieved.
Separation of plasticizer and polymer decomposition
ConsecutiveE1 > E2The signal separation is
always worsened.
Super-Res® is not necessary. Meas-urements should be carried out at high constant heating rates.
IndependentE1 <> E2Improvement in signal separation can be obtained,
if – at a nominal heating rate (HR0) – the signal of
the effect with the lower activation energy is
already at lower temperatures than the signal
of the effect with higher activation energy.
In order to make a decision, test measurements should be run at different constant heating rates.
CompetitiveE1 <> E2With the “Dynamic Heating Rate”,
results can be obtained
which are not only sample-specific,
but also considerably affected
by the presetting of the measurement program.
SuperRes® is not recommended.
Expected behavior of the effects at a dynamic heating rate on the basis of thermokinetic evaluations

 

 

This figure demonstrates the case of a consecutive reaction type with E1 < E2. The simulation proves that in this case, decreasing heating rates lead to an improved separation of thermogravimetric effects.

Simulation with consecutive reactions E1 < E2Simulation with consecutive reactions E1 < E2
Simulation with consecutive reactions E1 > E2Simulation with consecutive reactions E1 > E2

In this figure, the example of a consecutive reaction type with E1 > E2 is presented. Here, the simulation proves that decreasing heating rates worsen separation of thermogravimetric effects.

The simulations clearly show that the decision regarding reasonable use of the dynamic heating rate should be made after preliminary testing at different constant heating rates. There is almost always an improvement in cases such as the separation of plasticizer and polymer decomposition. However, in cases where there are many decomposition stages, it is possible for improved separation to be observed in one stage, but for there to be no visible influence or improvement in another reaction step during the same measurement.

The Super-Res® software extension is available as rate-controlled mass loss (RCM) programming for the NETZSCH TG 209 F1/F3 and STA 449 F1/F3Jupiter®, and as rate-controlled sintering (RCS) programming for the dilatometer systems DIL C/CD/E and thermomechanical analyzer systems TMA 402 F1/F3Hyperion®.

Evolved Gas Analysis

In Thermogravimetry (TG), the mass change (and transformation energetics) of a sample versus temperature or time is measured.   

Evolved Gas Analysis yields additional information regarding the nature (composition) of the gases evolved during a mass-loss step.    

In most cases, a Quadrupole Mass Spectrometer (QMS), an Fourier Transform Infrared Spectrometer (FTIR) or a Gas Chromatograph-Mass Spectrometer (GC-MS) are coupled to a TGA system for evolved gas analysis.  

Possibilities:

  • TG-FTIR Coupling (Transfer Line)
  • TG-MS Coupling (Capillary Coupling)
  • TG-GC-MS Coupling (Transfer Line)

 

TG-FTIR Coupling

  • Yields information on the composition (absorption bands) of the evolved gases (bonding conditions).
    • Easy interpretation (spectra data bases) of organic vapors without fragmentation
    • Symmetrical molecules cannot be detected

TG-MS Coupling

  • Yields information on the composition (mass  numbers of elements and molecules) of the evolved gases.
    • High sensitivity
    • Fast and easy interpretation of atomic/inorganic vapors and standard gases (H2, H2O, CO2, ....)
    • Fragmentation, interpretation of organic molecules sometimes difficult 

TG-GC-MS Coupling

  • Yields information on the composition (mass  numbers of elements and molecules) of the evolved gases.
    • Very high sensitivity
    • Separation of the volatiles using the GC column
    • Interpretation of organic vapors significantly improved
    • Sometimes slow, special measurement processes need to be used or fast GC systems have to be employed
Detailed Insight Into the World of Thermal Analysis

Detailed Insight Into the World of Thermal Analysis

In recent decades, TGA has been used increasingly for the quality control and assurance of raw materials and incoming goods as well as for failure analysis of finished parts, especially in the polymer processing industry.

Various international standards describe the general principles of thermogravimetry for polymers (ISO 11358) or other specific applications, such as compositional analysis for rubber (ASTM D6370) and evaporation loss of lubricating oils (ASTM D6375).

NETZSCH Analyzing & Testing has been manufacturing thermo-microbalances for many years. Our vertical, top-loading design not only provides for easy operation and sample loading, but also allows gases to flow naturally inan upward direction. Evolved gas analyzers such as mass spectrometers, FT-IR spectrometers and/or GC-MS (gas chromatograph-mass spectrometers) can then be coupled directly at the top of the unit. The automatic sample changer (ASC) can also be used to conduct routine measurements around the clock.

Recommended Literature

White Paper

The points outlined in this White Paper are a good guideline for finding a suitable thermobalance – one which is accurate, reliable, easy to operate and equipped for future needs.