For those concerned about using a microwave oven to heat chemicals such as propylene glycol, I did the following experiment.
The concern about heating flammable materials is that when the material is heated above its flashpoint, the heated vapor may rise over the side of the containing vessel, then flow down to the source of heat and ignite the liquid in the container. But what if the source of heat is a microwave oven? I heated 8 ounces of water to boiling in a Pyrex measuring cup, which took about 2 minutes, then removed the cup from the oven and felt various surfaces in the oven with my bare hand. There was no surface that was even warm to the touch. I ran the oven for 30 seconds with nothing in it but the plate that rotates. As long as the plate is clean, it does not heat. If it is wet or has something on it from a previous spill, it will get hot, so clean it and dry it before doing this test.
The source of heat in a microwave oven is in fact the material that is being heated by microwave radiation. The surface on which the supposed vapors would fall is much cooler than the surface of the liquid from which the vapors came.
I did not question the wisdom of the assumption that the vapors might rise due to heat in the vessel, then fall in spite of the heat outside the vessel. In any case, if you think the heating might proceed beyond the flashpoint of the material being heated, it appears that it is safer to do the heating in a microwave oven than over a flame or electric hotplate.