@Susan Deats posted:It appears to be this stuff.
High-Shrink-Ratio Heat-Shrink Tubing
Dielectric Strength: 455 V/mil
Shrink Temperature: 660° F
Temperature Range: -450° to 500° F
This tubing shrinks down to a quarter of its original ID, so it creates a tight seal over irregularly shaped objects. Made of PTFE, it withstands temperatures up to 500° F and resists abrasion and chemicals.
Dielectric strength is the maximum voltage a material can handle before it starts to breakdown.
I followed Susan's link and then went down the rabbit hole of "What is PTFE?" Turns out PTFE is a carbon and fluorine synthetic polymer – Polytetrafluoroethylene. Also known by the brand name, Teflon.
Bob