Planetary nebulae (PNe) are ideal probes of dust formation and evolution in low- and intermediate- mass stars. The analysis of dust emission in the 5 to 40 micron range, however, has concentrated mainly on a relatively small number of nearby Galactic PNe. The analysis has been recently extended to fainter sources located in the direction of the Galactic Bulge, and in the Galactic Halo. In addition, a significant number of Magellanic Cloud PNe was also studied by us with Spitzer/IRS. This has allowed us to study the dust properties and derive preliminary conclusions which suggest that indeed there are strong differences in the characteristics of the dust observed from source to source which seem to be mainly dependent on the mass of the progenitor star and the metallicity. To complete the picture, we propose to study with Spitzer/IRS the mid-IR spectra of all Galactic Disk PNe smaller than 4 arcsec (excluding duplications) to reveal the relations between nebular dust, gas abundance, and metallicity, at early evolutionary stages, as most compact PN are expected to be dynamically young. With the proposed observations we will solve some key questions on the formation and evolution of dust in PNe, such as the dependency of mass-loss with metallicity and progenitor mass. The statistical properties inferred from the proposed observations will be analyzed relatively to those of Magellanic Clouds, Galactic Halo and Galactic Bulge PNe, to asses the main characteristics of the particular solid state features detected in C-rich and O-rich PN as a function of the stellar population, with a large metallicity baseline. In addition, the proposed observations will yield alpha-element abundances with lower uncertainties than previosuly calculated. The Galactic distribution of our targets are such that metallicity gradients will also be derived from this program. The large and complete sample will fill a gap in the Spitzer program, with a strong legacy value.