Polyurethane Conformal Coatings are available as one (most common) or two part formulations.  Most formulations are solvent-based.  Compared to Acrylic conformal coatings, Polyurethane conformal coatings are tougher and more difficult to rework.  They are more chemical resistant.  Single component systems are easier to dispense, but take longer to cure than dual component systems.  Nearly all systems are MIL-I-46058C and IPC CC-830B specified and most have some level of UL acceptance.