Thermoanalytical Techniques - Thermal Analysis of Construction Materials - page 19

After all these technical considerations in the variable heating rate
approach, Sichina [49] highlighted the following:
• In the variable heating rate approach, the heating rate
controlled at any given time by the instrument is dependent
upon the rate of sample volatilization, but the decompo-
sition is dependent upon experimental factors such as
initial sample mass, geometry and physical nature of the
sample, surrounding atmosphere, purge gas, flow rate,
heating rate, etc. Therefore, this may affect the precision
of the resulting data because the experimental variables
associated with the variable heating rate approach may have
a larger effect on the decomposition kinetics as compared
to experiments performed at constant heating rates.
• Decomposition of a material is a kinetically controlled,
time-based phenomenon. Hence, resolution of any ana-
lytical experiment should be properly defined on a time
basis rather than a temperature basis because time is
always the factor in any experiment. Changes in the
heating rate during a decomposition event may result in
artifacts in the TG data when plotted as a function of
temperature.
• Separations of decomposition events plotted on a time
basis are always real, but resolution of events plotted on
a temperature basis may not necessarily be real.
• Since the time-based quantity is always equivalent to the
rate of mass loss, the derivative of weight loss should be
displayed on a time ( dc / dt ) rather than a temperature
basis.
The stepwise isothermal approach was first introduced by
Sorenson. [51] In this approach, a maximum heating rate and two weight loss
per minute thresholds are defined by the operator. The instrument ramps at
the maximum heating rate until the sample starts to lose weight and reaches
the maximum specified threshold, stops and then goes to the next segment
where the temperature is held isothermally until the rate of decomposition
falls below the minimum threshold. The method is repeated until all the
weight losses have been observed.
The approach has a kinetic treatment similar to Eq. (6), but the term
( E a / RT ) is constant during an isothermal experiment. Hence, if n is equal