I have a bolt of warm and natural and the paper that is wrapped around the bolt says "Only the highest grade, 100% cotton fibers are selected to be punched with hundreds of needles through a strong, thin base material." So to me that says the thin base is more than likely the polyester fabric. The box that the bolt comes in only says 100% cotton. I have used this batting for hot pads, but I wouldn't use it in the Microwave.
Good Luck
Suzy