This is a Fiberglas Tape Coated with Non-stick PTFE (polytetrafluoroethylene).  PTFE is a synthetic fluoropolymer of tetrafluoroethylene -- the most well known brand name of PTFE is Teflon® by E. I. DuPont Co. Temperature and chemical resistant, it is used as a release surface on heat sealers and other packaging equipment.  It ilso used as protective sheets for hot press transfer applications. The adhesive tape has a plastic release liner to enable ease of application.