Page 28 - Handbook of Thermal Analysis of Construction Materials

12 Chapter 1 - Thermoanalytical Techniques

2.4 Thermogravimetry

Thermogravimetry (TG) measures the change in mass of a material
as a function of time at a determined temperature (i.e., isothermal mode),
or over a temperature range using a predetermined heating rate. Essentially,
a TG consists of a microbalance surrounded by a furnace. A computer
records any mass gains or losses. Weight is plotted against a function of time
for isothermal studies and as a function of temperature for experiments at
constant heating rate. Thus, this technique is very useful in monitoring heat
stability and loss of components (e.g., oils, plasticizers, or polymers).
Thermogravimetry is also widely used both in studies of degrada-
tion mechanisms and for methods for service lifetime prediction measure-
ments. [40] TG lifetime prediction routines are available from instrument
manufacturers. The routine calculates the activation energy by using a form
of an Arrhenius equation (Eq. 6).

Eq. (6) dx/dt = A exp (-E /RT) (1-x) n
a

where x = degree of conversion
t = time
dx/dt = reaction rate
n = reaction order
A = pre-exponential factor
E = activation energy
a
R = gas constant
T = temperature (K)
Taking the logarithm of the above equation, the following expression is
obtained:

n
Eq. (7) ln [1/(1-x) ] = (-E /R)(1/T) + ln A - ln (dx/dt)
a
For a given degree of conversion, x , and temperature, T ,
i
i
n
Eq. (8) ln [1/(1-x ) ] = (-E /R)(1/T ) + ln A - ln(dx /dt)
i
i
i
a
Since the reaction rate is constant,
ln A – ln (dx /dt) = constant = β
i