  A sample of 25 IR-bright planetary nebulae (PNe) towards the Galactic bulge
is analysed through 8-13um spectroscopy. The classification of the warm-dust
emission features provides a measure of the C/O chemical balance, and
represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified
dust types, 4 PNe have emission features associated with C-based grains, while
the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe,
<30%, contrasts with that for local PNe, around 80%, although it follows the
trend for a decreasing frequency of C-rich PNe with galactocentric radius
(paper~I). We investigate whether the PNe discussed here are linked to the
bulge stellar population (similar to type~IV, or halo, PNe) or the inner
Galactic disk (a young and super-metal-rich population). Although 60% of the
PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfy
our criteria, and they are probably linked to the inner Galactic disk. In the
framework of single star evolution, the available information on bulge PNe
points towards a progenitor population similar in age to that of local PNe
(type I PNe are found in similar proportions), but super-metal-rich (to account
for the scarcity of C-rich objects). Yet the metallicities of bulge PNe, as
inferred from [O/H], fail to reach the required values - except for the C-rich
objects. It is likely that the sample discussed here is derived from a mixed
disk/bulge progenitor population and dominated by type IV PNe, as suggested by
Peimbert (1992). The much higher fraction of O-rich PNe in this sample than in
the solar neighbourhood should result in a proportionally greater injection of
silicate grains into the inner Galactic medium.
