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Duct tape is a cloth-backed tape that is often referred to as the "handyman's secret weapon".
It is (understandably) often misused by DIYers for sealing ducts, but in fact it has proven to have poor heat resistance and is not suitable for this purposes; instead, there is specialty tape for [HVAC] systems which is made from aluminum foil.
Duct tape or duck tape is cloth- or scrim-backed pressure-sensitive tape often coated with polyethylene. There are a variety of constructions using different backings and adhesives. One variation is gaffer tape designed to be cleanly removed, while duct tape is not. Another variation is heat-resistant duct tape useful for sealing heating/ventilation/air-conditioning (HVAC) ducts, produced because standard duct tape fails quickly when used on heating ducts. Duct tape is generally gray or black but also available in other colors.
In 1942 Revolite, then a division of Johnson & Johnson, originally developed an adhesive tape made from a rubber-based adhesive applied to a durable duck cloth backing. This tape resisted water and was used as sealing tape on ammunition cases during World War II.
The product now commonly called duct tape should not be confused with special tapes actually designed for sealing heating and ventilation ducts. To provide lab data about which sealants and tapes last, and which are likely to fail, research was conducted at Lawrence Berkeley National Laboratory Environmental Energy Technologies Division. Their major conclusion was that one should not use common duct tape to seal ducts (they had defined duct tape as any fabric-based tape with rubber adhesive). The testing done shows that under challenging but realistic conditions, common duct tapes become brittle and may fail.