We have some old boards that we'd like to put to use (high voltage switching power supplies/voltage control)). They require 1kv 1uf ceramic capacitors. Unfortunately the price of those went up through the roof. There are some Polypropylene capacitors that seem to fit the purpose like this one: https://www.illinoiscapacitor.com/pdf/seriesDocuments/PPB%20series.pd but priced more reasonable. Do you think it is an ok replacement?

 

Hello there Your concerns would be isolation resistance and temperature coefficient make sure they match and you'd be okay.
If your ceramic capacitor is a NPO ceramic capacitor which is an ultrastable or temperature-compensating capacitor. It is one of the most highly stable capacitors. It has very predictable temperature coefficients (TCs) and, in general, does not age with time.
advantage of polypropylene capacitors are their great tolerance values. Tolerance is about 1 percent, so it's pretty precisely near their nominal values.
polypropylene capacitors exhibit a permanent change in value should they ever be exposed to temperatures much over 70°C; they do not return to their old value upon cooling.

Thank you very much for your help. I now know what to look for