Thermogravimetry (TG) is a technique for measuring mass change of a sample with
temperature. A sample to be measured is placed in a furnace and its mass change is monitored
by thermobalance
Analyze the material decomposition and thermal stability through mass change as a function of
temperature in scanning mode or as a function of time in the isothermal mode. TG curves are
plotted as mass change expressed in percent versus temperature or time.
Instrument Set up
The Cahn microbalance senses the vertical displacement of a
sample due to mass change using an optical system.
The optical system includes a light source, a flag, a light tube
and a photodiode.
The flag beneath the balance arm interferes with light
propagating from the source to the light detector (photodiode)
when the mass change is sensed by the balance beam.
A feedback control system adjusts the current in a magneticcoil system and maintains the balance beam in its original
horizontal position even if the sample mass keeps changing.
 sample is connected to the microbalance
 Furnace designed as cylindrical tube with heating elements
on its wall
 Diameter of the tube accommodates the sample with
relatively little open space.
 Thermocouple location eliminates temperature gradient
among the sample, thermocouple and heating elements
 Protective gas will flow into and out of the furnace tube to
maintain an inert atmosphere during heating
Sample Dimension
• Endothermic decomposition: high heating rate will increase the starting and finishing temperatures of
decomposition.
• Temperature range from start to finish will be wider at a higher heating rate than a lower heating rate
• High heating rate :temperature difference between the sample and thermocouple junction.
• Real sample temperature may lag behind that of the thermocouple
• Decomposition with volatile products: products take time to diffuse out of the sample and be carried away
by flowing gas. A low heating rate is more likely to generate thermal equilibrium and give a reproducible
result for the analysis
horizontal line :no decomposition with mass loss of volatile products
indicates a rapid mass loss at the initial stage of a TA curve (Drying or desorption)
one-stage decomposition curve: Stability Limit of Sample
multistage decomposition with stable intermediates
multistage decomposition but with no stable intermediates: High heating rate
version of (iv)
indicates that a chemical reaction with mass gain: say oxidation of metal
mass-gain reaction occurs and then a mass-loss reaction occurs
Derivative Thermogravimetric Curve(DTG)
 A peak in a DTG curve represents a maximum of
mass change rate.
 DTG does not contain any new information
other than the original TG curve
 however, it clearly identifies the temperature at
which mass loss is at a maximum
The temperature at which decomposition
starts is defined as the intersection of
initial line tangent and tangent of line
portion when the slope changed.
The
finishing
temperature
of
decomposition is defined in a similar
manner, here midpoint temperature
between the starting and finishing
temperature can be defined as TB
• Crystal water loss occurs at separated temperatures.
• Correspondingly, the structure of a sample goes through
several stages of change during crystal water loss.
• TG curves do not always
decomposition temperatures
show
obvious
• desirable to plot DTG curves with TG curves to
reveal the decomposition temperatures of
polymers.
• Only DTG curves could clearly indicate the
decomposition temperature of natural rubber, as
well as the higher decomposition temperature of
butadiene rubber.
Composition Analysis
Demonstrates that the amount of ceramics (hydroxyapatite) in composites can be
accurately determined
Measure the weight differences in composite
samples before and after the polymer
(ultrahighmolecular weight polyethylene)
Kinetic Analysis : How to obtain a DTG equation
Here α
For a linear heating rate
Eq. 1
Taking derivative of Eq 1, we get
Integrate both sides to get the TG curve (theoretical)
Kinetics of most reactions under isothermal conditions can be summarized by the general
equation:
The various mathematical equations that relate with the rate of some solid-state reaction as
given in the previous slides. g(α) and f(α) uses TG data in their integral or differentiated forms,
respectively.
The former uses data directly from the TG curve, and the differentiated form uses data from the
DTG curve.
Each equation is associated with a particular model for the progress of the reaction, such as gas
diffusion into a solid particle or the appearance of product nuclei with subsequent growth of
those nuclei.
The shape of the TG curve will vary with the particular mechanism involved, as will the
temperature at which dα/dt is a maximum
 The traditional method of conducting kinetic studies is the isothermal method: loading the
sample into the TG apparatus, rapidly heating it to some specific temperature, and
monitoring the mass change.
 The experiment is repeated with a new sample at another temperature, and a family of
curves is generated
 The total mass loss for the reaction is found by heating the sample in a rising-temperature
experiment until the mass loss is complete.
 A series of α and dα/dt values can then be calculated for a given temperature
 data are fitted to the expressions as given earlier, and the best fit suggests the model that
is likely to apply to the reaction under test.
 From this expression, the average rate constant k can be calculated at each temperature. A
plot of log k vs. 1=T should provide a straight-line graph of slope E/R from which E can be
calculated, and the intercept gives the preexponential factor A
1. It is difficult to determine exactly the
time and temperature of the beginning of
the reaction.
2. It requires several experiments and so is
time consuming.
3. Each experiment requires a new sample,
which must react in exactly the same way
as all the other samples. This requires
careful attention to standardization of the
experimental procedures, which includes,
e.g., the use of the same sample pan with
the same sample packing and gas
exchange.
Decomposition of a solid material :Affected by the partial pressure of
product gas
Using values of the Gibbs free-energy change (ΔG°), the enthalpy change (ΔH°), and the entropy change (ΔS°), are
the decomposition temperature T can be calculated for any partial pressure P of CO2:
Hence, under equilibrium conditions, the decomposition temperature is lowered by 138 C. The temperatures
found by thermal methods are always higher because these are dynamic techniques, although for small sample
sizes and slow heating rates the correspondence is close.