It's silicone tape, and is rated to 260C - high enough for an extruder. One of the suggested uses is for repairing car exhaust systems.
I made a temp extruder heater out of two m8 coupling nuts, two wirewound resistors and a few inches of tape. It seems to hold up well, and went up to 220C.
I need to re-drill some welding tips, since I widened them to 3.5mm and made holes in the sides.