Page 116 - Biodegradable Polyesters

94 4 Synthesis, Properties, and Mathematical Modeling of Biodegradable Aliphatic Polyesters

1.0

0.8

Conversion 0.6

0.4
PPSu(D)
0.2 PPSu(T)
PPSu(H)
Model
0.0
0 50 100 150 200 250 300 350
Time (min)
∘
Figure 4.7 Conversion (X ) versus time for the esterification reaction of PPSu at 190 C
w
with different amounts of added catalyst. Experimental data (discrete points) and theoretical
model simulation results [43].
0.5
calculated was: k = 10 −0.91 (mol TBT∕mol SA) . It is very interesting to note
1
that, for PBSu, almost equal catalyst dependence (i.e., 0.51 Ref. [42]) was observed
in literature. Similar results were also obtained by Hu et al. [46].
The effect of the type of glycol used (i.e., EG, PG, or BG) on the esterification
reaction was examined next. It was found that the type of glycol did not affect
much the variation of water conversion with time. Using BG, slightly higher reac-
tion rates were observed compared to those for PG, which in turn were slightly
faster compared to those for EG. Again, the theoretical simulation model fitted
the experimental data very well. The kinetic parameters evaluated are reported in
Table 4.2. As was expected, k increases in the order PBSu > PPSu > PESu.
1
Some typical simulation results from the solution of the theoretical model are
presented next. In Figure 4.8, the normalized weight fraction of SA, PG, and
oligomers during the PPSu esterification reaction is plotted as a function of time.
It is noted that PG is consumed first, while an amount of SA remains finally
unreacted. The amount of oligomers initially increases fast, while afterward it
reaches a plateau. In the same figure, the change in the reaction volume during
esterification is presented as a fraction relative to the initial volume. A decrease in
volume is observed as a result of the continuous removal of water. Furthermore,
the change in the number of moles of all molecular species present in the reactor
as a function of time is plotted in Figure 4.9. It is interesting to note that the moles
of tSA and tPG initially increase with time but then gradually decrease leading
to the production of bSA and bPG which are not initially produced. The amount
of bDPG produced is rather low while it begins to appear only after 20 min of
reaction time.
Theoretical results coming from Equation 4.29 of the model on the aver-
age degree of polymerization as a function of esterification time is shown in
Figure 4.10. It is evident that only oligomers are produced during the esterification