Don't use Polyester batting for hot pads, period.
I've melted/burned through cotton covered polyester batting filled hot pads when brushed against the heating element in my electric oven, and in a hot oven (425+F). I kept the hot pads because they were large enough to use, even with the scorch marks but have been far more leery about using home made ones ever since that happened.
If you MUST use up your poly batting in hot pads, use them for hot plate carriers or mug rugs, where they are exposed to lower temperature pans/cups, rather than oven direct heat. This will keep your warm plated items from losing their heat while traveling/ sitting on tables, and it will not hurt yourself.