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TEMPERlCiURE PC,
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THERMAL PROPERTIES
PBX 9502
2.7 Time-to- Explosion Test. All explosives decompose exothermally at
temperatu:res above absolute zero. When chemical decomposition produces heat
faster than it can be dissipated to the surroundings, the explosive mass self-heats to
explosion. In steady-state conditions, the temperature at which a thermal explosion
is produced is called the critical temperature, Tm. A relatively simple expression
for the critical temperature has been derived7 in terms of the kinetic and physical
parameters.
where
Tm = - E2 ,
R InApQZE
Tm2h&
R = gas constant, 1.987 Cal/mole,
A = radius of sphere, cylinder, or half-thickness of a slab,
p = density,
Q = bleat of decomposition reaction,
Z = pre-exponential factor,
E = activation energy,
X = thermal conductivity,
6 = shape factor (0.58 for infinite slabs, 2.0 for infinite cylinders, 3.32 for
spheres).
The LASL method for determining critical temperatures is based on a time-to-
explosion test that Henkin* developed. The explosive sample, usually 40 mg, is
pressed into a DuPont E-83 aluminum blasting-cap shell and covered with a hollow,
skirted plug. A conical punch is used to expand the plug and apply a reproducible
4OO-lb force. This plug expansion forms a positive seal and confines the sample in a
known geometry. The density, which can be calculated from a sample thickness
measurement, is usually about 90% of the crystal density.
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