  We investigate the galactic disk distribution of a sample of planetary
nebulae characterised in terms of their mid-infrared spectral features. The
total number of galactic disk PNe with 8-13mum spectra is brought up to 74 with
the inclusion of 24 new objects, whose spectra we present for the first time.
54 PNe have clearly identified warm dust emission features, and form a sample
which we use to construct the distribution of the C/O chemical balance in
galactic disk PNe. The dust emission features complement the information on the
progenitor masses brought by the gas-phase N/O ratios: PNe with unidentified
infrared emission bands have the highest N/O ratios, while PNe with the
silicate signature have either very high N enrichment or close to none, and SiC
emission features coincide with a range of moderate N-enrichments. We find a
trend for a decreasing proportion of O-rich PNe towards the third and fourth
galactic quadrants. Two independent distance scales confirmed that the
proportion of O-rich PNe decreases from 30+-9% inside the solar circle, to
14+-7% outside. PNe with warm dust are also the youngest. PNe with no warm dust
are uniformly distributed in C/O and N/O ratios, and do not appear to be
confined to C/O~1. They also have higher 6cm fluxes, as expected from more
evolved PNe. We show that the IRAS fluxes are a good representation of the
bolometric flux for warm-dust PNe. The requirement F(12um)>0.5Jy should probe a
good portion of the galactic disk, and the dominant selection effects are
rooted in the PN catalogues.
