The first potholders I ever made I used poly batting. You can fold use two of them together and folded in half and it's still not enough to keep from burning your hands! Use all cotton! I used two layers of request weight Quilter's Dream in the next set and they're very nice potholders.
Cut off a small piece of the batt and burn it. If you get all soft gray ash it's 100% cotton. Any hard black lumps and it has some poly.