Chemistry Reference
In-Depth Information
d
dt
0
.
5
(1
=
k
1
(1
−
η
)+
k
2
η
−
η
)
,
(5.9)
where
k
2
= 10
8
.
5
exp
s
−
1
28000
R
T
−
(5.10)
for temperatures lower than or equal to 493 K and
k
2
= 10
12
.
8
exp
s
−
1
38000
R
T
−
(5.11)
for temperatures higher than 493 K.
The kinetic constants for low-temperature decomposition in Eqs. (5.10) and
(5.11) significantly differ from that for high-temperature decomposition, as deter-
mined from experimental ignition data (here
Q
= 2
.
08 kJ g
−
1
, from [17], was used):
10
16
exp
s
−
1
.
48000
R
T
k
high
−
temp
≈
2
·
−
(5.12)
An analytical expression for
k
1
could not be derived in [18] due to the extremely
strong autocatalytic effect (
k
1
/
k
2
1). The approximate order of magnitude for
k
1
was found from the results of specially designed experiments (that measured the
weight decrease under isothermal conditions) to be 10
−
5
s
−
1
at
T
= 493 K. Extrapo-
lation to
T
= 493 K using Eq. (5.12) gives a similar constant value of 1
.
5
10
−
5
s
−
1
.
Taking into account the long range of the extrapolation, one can consider this agree-
ment between the data to be quite satisfactory.
Thus two preliminary conclusions can be drawn:
(a) The kinetic constants of the limiting stage of the fast high-temperature decom-
position of composite solid propellants can be evaluated (for a certain range of
t
ign
) by the ignition method using the flash-block device.
(b) The satisfactory agreement between the
k
1
data for high-temperature and low-
temperature decomposition of the polyurethane-based (P) material obtained for
high
E
(determined by the ignition method) may not be just a random coinci-
dence. In this case, reactions with such high values of
E
are assumed to occur
at low temperatures as well. Due to high activation barriers, the contributions
from these reactions are not noticeable at significant degrees of conversion in
the low-temperature region, while at higher temperaturesthey become more im-
portant. Correspondingly, the effective constants of high-temperature decompo-
sition significantly differ from those in the low-temperature region.
Thus the ignition method can be used to evaluate the kinetic constants for reactions
that dominate in the high-temperature region (if the decomposition mechanism is
constant over a wide temperature range). The low-temperature kinetics of these re-
actions cannot be studied by traditional techniques due to their negligibly small
contributions to the overall process.
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