We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a five year period spanning the epochs between days ~6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant and its radiative output is dominated by the interaction of the SN blast wave with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by emission from ~180 K silicate dust, collisionally heated by the hot X-ray emitting gas with a temperature and density of ~5 × 106 K and ~3 × 104 cm-3, respectively. The mass of the radiating dust is ~1.2 × 10-6 M sun on day 7554 and scales linearly with IR flux. Comparison of the IR data with the soft X-ray flux derived from Chandra observations shows that the IR-to-X-ray flux ratio, IRX, is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of IRX is most consistent with the scenario that very little grain processing or gas cooling has occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased by a factor of ~5 with a time dependence of t'0.87± 0.20, t' being the time since the first encounter between the blast wave and the ER. These observations are consistent with the transitioning of the blast wave from free expansion to a Sedov phase as it propagates into the main body
