Chemistry Reference
In-Depth Information
However, the forced high-temperature decomposition of polymeric materials has
been studied to a much lesser degree. This can be explained by some methodolog-
ical difficulties (see Introduction) as well as by the novelty of this research area,
which is mainly connected with space engineering. In this situation, the study of
gross-process macrokinetics is more important than the characterization of reaction
mechanisms for complex transformations of a polymer. Since polymers are used
as components of composite solid and hybrid propellants, studies of the effects of
high-temperature destruction macrokinetic parameters on polymer burning patterns
are of special interest.
In this case, the redundancy of data on the low-temperature kinetics of polymer
decomposition can be illustrated by the following example. At
T
350
◦
C, PMMA
≤
decomposition can be described as a first-order reaction (at
η
>
0
.
2) with activation
165 kJ mol
−
1
. By using corresponding expressions for the rate constant
and Eq. (1.1), one can obtain the temperatures of the polymer surface,
T
S
= 800 and
900
◦
C, for burning rates in the range of 10
−
3
-10
−
2
ms
−
1
(since Eq. (1.1) is valid
for decomposition due to a zero-order reaction, the temperature values obtained are
the lower bound ones). In both cases, the estimated temperature values significantly
exceed the real ones determined from, for example, microthermocouple measure-
ments [20]. The application of the extrapolation method in order to estimate the sur-
face temperature using kinetic equations of thermal decomposition [1, 21, 22, 23]
leads to a similar result. This is because the influence of the effect of the gas-film on
the accuracy of
T
S
determination is neglected.
Thus, studies of the macrokinetics of the forced thermal decomposition of
polymers in the linear pyrolysis mode using improved experimental and theoreti-
cal techniques were of primary importance. In the experiments, samples of linear
(thermoplastic) and crosslinked (thermosetting) PMMA were investigated.
Pieces of standard Plexiglas were used as the linear PMMA samples, while
crosslinked PMMA samples were specially prepared by the block polymerization of
methyl methacrylate with the addition of 2 or 10% triethyleneglycoldimethacrylate
(crosslinking agent). The crosslinked PMMA samples did not dissolve in acetone
and dichloroethane and did not exhibit softening or melting during pyrolysis.
energy
E
≈
3.2.1 Linear Pyrolysis of Thin (“Finite Length”) PMMA Samples
Under Double Thermostating
As was mentioned in Sect. 1.5, the mechanism of decomposition (volumetric or on
the surface) can be directly determined by linear pyrolysis experiments with “finite
length” samples under double thermostating of the hot and cold surfaces. For the
volumetric mechanism, the linear pyrolysis rate should decrease exponentially with
time, while for the surface mechanism,
U
= const
=
f
(
t
). For crystalline samples
(for example AP or salmiac), the decomposition mechanism can be identified from
the shape of the pyrolysis surface: concave in the case of the volumetric reaction of
AP (at
T
S
<
500
◦
C), or convex in the case of the dissociative sublimation of salmac
at the surface.
