March 3 - 5,1999, Karlsruhe, Germany - FZK

proceed mostly under heterogeneous conditions,
and it is indicated that full exploration of the
potential modifications would require
documentation of the phase behavior.
Another important area of polymer modification
with subcritical and supercritical water is the
hydrolysis of polycondensation polymers such as
polyethylene terephthalate (PET), polyurethanes,
and nylons for conversion to their monomers [ 37],
Specifically, in supercritical water, 91 % monomer
recovery (terephthalic acid) is achieved at 400 °C
and 40 MPa in less than 15 min reaction times
[38], Studies of these reactions using a
hydrothermal diamond anvil cell to follow the
phase changes during the reaction of PET in water
demonstrate the complexities in terms of
simultaneous reaction and dissolution processes.
Unlike polycondensation polymers, polymers of
addition polymerization such as polyethylene and
polypropylene when depolymerized in inert
atmosphere (39) or in supercritical water (37) do
not convert to just the monomer but, but a
homologous series of oligomers (alkanes and
alkenes). Compared to pyrolysis in argon, for
polyethylene, the portion of the lighter products
increases in supercritical water depolymerizations
conducted at 693 K and water densities of 0.13 and
0.42 g/cm3. The 1-alkene to n-alkane ratio also
increases in supercritical water and with density.
These are attributed to the fact that in argon
pyrolysis, the reaction proceed in the molten state
of the polymer, whereas in supercritical water,
some of degradation products are dissolved in
supercritical water and further decomposes to
smaller fragments, leading to differences in the
reaction phase. Mechanistically, the increased
alkene amount suggests that in supercritical water
beta-sicssion is more prevalent than hydrogen
abstraction reactions in the decomposition paths
[37, 39].
These studies point to the possibilities for using
supercritical fluids as reaction media to control
product distributions.
Another example of polymer modification reaction
is the hydrolysis of cellulose in subcritical and
supercritical water [40], Cellulose is shown to
hydrolyze rapidly (