UL 181B-FX Listed and printed HVAC duct tape used to seal, seam and join Class 1 Flex Duct, even when surfaces are rougher or not fully clean. Offering high holding power, superior shear strength and excellent tear ability, PC 857 is recommended for applications in which the tensile strength and adhesion to stainless steel and polyethylene must meet or exceed the strict requirements of UL, flex duct manufacturers and city and state codes.