HDT tubing comes with a factory-applied sealant. This sealant is a heat-activated thermoplastic material that remains soft and flexible over long periods under adverse environmental conditions. During heating the sealant softens, bonds to underlying surfaces and fills small voids that might be present. When cool, the sealant forms a barrier against water, moisture, dirt and other environmental contaminants. - 3M™ heat shrink tubing HDT is designed to provide reliable performance as well as excellent mechanical and environmental protection. This flame retardant product combines the important advantage of easy handling and installation with maximum operating reliability and system protection.