Chemistry Reference
In-Depth Information
A set of experimental data on the pyrolysis of crosslinked PMMA (which does
not soften or melt during decomposition under experimental conditions) is the most
useful information for analyzing linear pyrolysis kinetics over wide ranges of
T
0
and
U
. The expressions of linear pyrolysis theory can be used to analyze the data
covering the full temperature and the decomposition rate ranges.
For fast linear pyrolysis (
U
>
0
.
1mms
−
1
), the shapes of the thermoplastic poly-
meric samples do not change due to the small thickness of the preheat zone and
the high viscosity of the melt (the characteristic time of the “pressing-out” of the
softened material is much higher than its pyrolysis time).
For slow linear pyrolysis (
U
<
0
.
1mms
−
1
), the thickness of the preheat layer in
the PMMA sample becomes comparable with its size (diameter). Under these con-
ditions, the shapes of crosslinked PMMA samples do not change, while for linear
PMMA samples, expansion of the hot end is observed when decomposition begins
(the change in the sample shape remains constant during the linear pyrolysis pro-
cess). This phenomenon was ignored in previous studies of linear pyrolysis.
Thus, data on the fast linear pyrolysis of linear PMMA (that is, for a constant
sample shape throughout the process) should be used for quantitative analysis.
Let us consider the dependence of the crosslinked PMMA decomposition rate on
the surface temperature (square points in Fig. 3.12). Three regions characterized by
different dependence patterns can be distinguished.
In region I (
T
S
<
700 K,
U
<
3
10
−
2
mm s
−
1
), the relationship between the
decomposition rate and the surface temperature can be described as
U
×
.
In region II (
T
S
= 700-860 K,
U
= 0
.
03-0
.
7mms
−
1
), the slope of the straight line
lg
U
= lg
U
0
−
43000
R
T
S
∼
−
R
T
S
.
In region III, the increase in
U
caused by the growth in the heater-generated heat
flux density is not accompanied by a noticeable increase in the pyrolysis surface
temperature.
Since PMMA decomposition occurs in the volume (see Sect. 3.2.1), the
expressions derived in Sect. 1.5 can be used to analyze the fast linear pyrolysis
of semi-infinite samples. For cylindrical samples (
d
=(6-8)
21000
A
/
T
S
decreases by approximately a factor of two:
U
∼
−
10
−
3
m) at
U
and
T
S
characteristic of regions I and II, the Bio number was found from direct heat-flux
density measurement data [25] to be 0.3-0.4. According to Eq. (1.32), the transfer
of the linear pyrolysis macrokinetics from the inner kinetic mode to the inner diffu-
sion mode should take place at
U
×
10
−
2
mm s
−
1
, which is in a good agreement
with the boundary between regions I and II (Fig. 3.12).
Analysis of the data shown in Fig. 3.12 using Eqs. (1.1) and (1.37) resulted in the
following values for the volumetric decomposition rate constant:
≈
4
×
s
−
1
for crosslinked PMMA
(650
<
T
S
<
700 K, the inner kinetic mode; 700
<
T
S
<
860 K, the inner diffusion
mode) and
10
12
exp
43000
R
T
k
= 3
.
3
×
−
