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THERMAL PROPERTIES
American Society for Testing and Materials procedure D696-70, “Coefficient of
Linear Expansion of Plastics.” A DuPont Model 900 thermal analyzer equipped
with a Model 941 thermomechanical analyzer (TMA) was used for single crystals
and small s,pecimens. They are denoted by ASTM and TMA in the following tables.
2.4 Thermal Decomposition Kinetics. The thermal decomposition rate con-
stants of ex.plosives, discussed in detail by R. N. Rogers,3*4 are found using a dif-
ferential scanning calorimeter at constant temperature.
The kinetic constants were determined using a Perkin-Elmer DSC-1B or DSC-2.
Samples were sealed in Perkin-Elmer No. 219-0062 aluminum cells perforated by a
single O.W:mm-diam hole. Differential and average temperature calibrations of the
DSC-1B were checked before the runs.
The recorder and the DSC with two empty cells on its supports are set at the test
temperature. The sample cell is removed, and the instrument is allowed to
equilibrate. The recorder is started, the instrument range switch is set, and the
sample is d:ropped onto the support. The sharp break on the record is used to mark
zero time. (The absolute position of the zero point on the time axis is unimportant
because rate constants are determined from the slope of the In deflection, b, vs
time, t, plot.)
The DSC. deflection above the base line, b, is directly proportional to the rate of
energy evolution or absorption by the sample, dqldt, which is proportional in turn,
to the reaction rate daldt. Therefore,
ab = pdqldt = daldt = k(1 - a) , (1)
where (Y and /3 are proportionality constants and k is the rate constant. Hence,
In b = In k/a + ln(1 - a) . (2)
For a first-order reaction,
-ln(l -- a) = kt + C , (3)
where C is a constant. Substituting Eq. (3) into Eq. (2) and combining constants
gives
In b = C - kt .
Therefore, rate constants for first-order reactions can be obtained directly from a
plot of In deflection vs time. This provides the rate constant, k, as a function of
temperature, since k is given by
where
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