Page 30 - Handbook of Thermal Analysis of Construction Materials

14 Chapter 1 - Thermoanalytical Techniques

2.5 High Resolution TG

Reactions investigated by TG are, by nature, heterogeneous. There-
fore, experimental results are affected by weight, geometry, and particle
size of the specimen. Moreover, temperature calibration and thermal
gradient in the material can also affect the results. Hence, low heating rates
should be used to alleviate the problem and to obtain good resolution under
non-isothermal conditions.
With complex systems such as polymers and fiber reinforced
composites, good resolution is essential to obtaining reliable results and
kinetic parameters that can be used to compare the stability of different
systems and assess their lifetime. Since, low heating rates lengthen the
experiment time, a novel TG mode, high resolution TG (Hi-Res TM TGA) [42]
was introduced by TA Instrument. This technique provides a means to
increase the resolution while often decreasing the time required for experi-
ments. The technique has two novel non-isothermal modes of operating:
variable heating rate mode and constant reacting rate mode. In the variable
heating rate mode, the heating rate is dynamically and continuously varied
to maximize resolution whereas in the reacting rate mode, an attempt is
made to keep the reaction at a specified constant value by changing the
heating rate.
Using the Hi-Res TM TGA technique, a simplified method has been
developed by Salin, et al., [42] to extract kinetic parameters from variable
heating experiments by using a mathematical function which takes into
account resolution, sensitivity, and initial heating rate. These parameters
affect the overall heating rate and can be controlled by the operator.
As shown by Eq. (8), the kinetics governing a thermal decomposi-
tion event depend on time, temperature, and rate of decomposition. TG
experiments performed at a constant heating rate allow temperature and
time to be interchanged in the case of first order kinetics and one-step

decompositions. [43] Hi-Res TM TGA allows the determination of kinetic
parameters such as activation energy and reaction order for each step in
multiple component materials using four different TG approaches: [44]
constant heating rate, constant reaction rate, dynamic heating rate, and
stepwise isothermal.
As discussed previously, the constant heating rate approach is
based on the Arrhenius Eq. (6) and requires different heating rates. Flynn
and Wall [45] rearranged the equation to obtain Eq. (9)