Q: What is the difference between tensile and pull-off strength?
Tensile strength, sometimes called chain pull, is measured along the length of the closed zipper: the chain is gripped at both ends and pulled apart until something fails. It reflects how the teeth interlock and how the tape carries load in line with the opening. Values are reported in newtons and are the figure most often quoted in zipper datasheets.
Pull-off strength, or slider pull-off, measures resistance to a force applied perpendicular to the chain at the slider, simulating a wearer yanking the puller outward or a bag strap catching the tab. It tests the slider body, the puller attachment and the bridge, which is a completely different failure path from chain tension.
Both matter, and neither predicts the other. A zipper can be excellent in tension and still lose its puller after a few thousand cycles, or hold its puller while the chain separates under load. Specify both, with the test method and the acceptance value, and add a side-pull or seam-burst test for load-bearing products. Report the failure mode alongside the number so the result can be acted on.