Saturday, 11 February 2017

Thermogravitometric Analysis TGA

 “A technique in which the mass of a substance is measured as a function of temperature, while the substance is subjected to a controlled temperature programme.”

Controlled temperature programme” can meanheating and/or cooling at a linear rate (by far commonest)

OR

Thermogravimetric analysis (TGA) measures weight changes in a material as a function of temperature (or time) under a controlled atmosphere. Its principle uses include measurement of a material’s thermal stability, filler content in polymers, moisture and solvent content, and the percent composition of components in a compound.

The measured weight loss curve gives information on


• Changes in sample composition
• Thermal stability
• Kinetic parameters for chemical reactions in the sample

Thermogravimetric Analysis (TGA) basics


·         Test method capable of measuring the mass evolution of a milligram-scale sample.
·         Gas atmosphere is well defined at all times during the experiment.
·         The atmospheric temperature is well-defined and follows a pre-defined program.

Data collected from TGA


·         Mass of sample with respect to Time/Temperature.
·         Properties/Parameters Determined from Data:
·         Heterogeneous Reaction/Thermal Degradation Kinetics,
·         Temperature Range for Pyrolysis

Principle of Analysis


The schematic principle of the TGA measurement is shown in Figure 1. The sample is heated under nitrogen or synthetic air with constant heat rate while the difference of the mass during this process is measured. A mass loss indicates that a degradation of the measured substance takes place. The reaction with oxygen from the synthetic air for example could lead to an increase of mass.

For example:        

                                                      
The TGA curve of two differently cured epoxy resins is shown in Figure 2 and can be used to get first information about the thermal stability of those systems. Other applications in our group are measurements to get the silver content in composite material.

Instrumentation of Thermogravimetry:

TG curves are recorded using a Thermobalance. It consists of:
·         electronic microbalance
·         furnace
·         temperature programmer
·         recorder (instrument connected to thermobalance to record the output/curves


Micro balance:

It is the most important component of thermo balance. A microbalance is used to record a change in mass of sample/substance. There are different types of microbalances viz. beam, spring, cantilever and torsion balances.

Sample Holder or Crucible:

The sample to be studied is placed in sample holder or crucible. It is attached to the weighing arm of microbalance. There are different varieties of crucibles used. Some differ in shape and size while some differ in materials used.

Furnace (Heater/Boiler/Oven):

The furnace should be designed in such a way that it produces a linear heating range. The temperature ranges can affect the internal atmosphere of furnace chamber; therefore, it is necessary to choose specific type of furnace according to temperature ranges.

Temperature Measurement:

It is done with the help of thermocouple. Different materials are used for measuring different ranges of temperatures

Heating Rate:

The heating rate is the rate of temperature increase, which is customarily quoted in degrees per minute (on the Celsius or Kelvin scales). The heating or cooling rate is said to be constant when the temperature/time curve is linear

Data Recording Unit:

The output from the microbalance and furnace are recorded using either chart recorder or a microcomputer (work station). The advantage of microcomputer over chart recorder is that the microcomputer comes with such software which allows data to be saved and plotted while performing mathematical problems.

Recording Of Result:


·         Results are displayed by a plot of mass change versus temperature or time and are known as Thermo gravimetric curves or TG curves.
·         TG curves are normally plotted with the mass change (Dm) in percentage on the y-axis and temperature (T) or time (t) on the x-axis. A typical TG curve has been shown (Figure).
·         There are two temperatures in the reaction, T(procedural decomposition temp.) and Tf(final temp.) representing the lowest temperature at which the onset of a mass change is seen and the lowest temperature at which the process has been completed respectively.


Operational Procedure

·         Conduct baseline experiment with empty sample crucible along a pre-defined temperature program in a well-defined gas atmosphere.
·         Prepare sample crucible by evenly packing sample material into crucible and measure mass of entire crucible.
·         Conduct experiment with sample along the same temperature program in the same gas atmosphere as in the baseline experiment.
·         Allow furnace to cool and clean the sample crucible used in experiment.

Considerations

·         Temperature
·         Heating Rate
·         Sample Size

Limitations

 Only provides meaningful data when a change in mass occurs.
·         Some liquids can be measured, but this is generally very difficult to do.
·         Very small samples are used, so non-homogeneous materials generally cannot be tested

Types of TGA

There are three types of thermogravimetry:

1. Dynamic TGA: In this type of analysis, the sample is subjected to condition of continuous increase in temperature usually linear with time.

2. Isothermal or Static TGA: In this type of analysis, sample is maintained at a constant temperature for a period of time during which change in weight is recorded.

3. Quasistatic TGA: In this technique sample is heated to a constant weight at each of a series of increasing temperature.

Applications

Principle uses of TGA include measurement of a material’s thermal stability and its composition.  Typical applications include:
§  Filler content of polymer resins
§  Residual solvent content
§  Carbon black content
§  Decomposition temperature
§  Moisture content of organic and inorganic materials
§  Plasticizer content of polymers
§  Oxidative stability
§  Performance of stabilizersLow molecular weight monomers in polymers

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