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The flex resistance or flex durability of a plastic film is a measure of its toughness, specifically its resistance to pinholing when subjected to flexing abuse. A cylinder of the film is repeatedly flexed under specified conditions using a Gelbo flex tester and the sample inspected for pinholes after a set number of full or partial flex cycles.
Alternatively, in the case of barrier films, the oxygen or water vapour transmission rates can be tested after flexing. A breakdown in the metallization, oxide coating or foil layer will often compromise the barrier performance of a plastic film laminate long before a pinhole through the structure is seen.
Smithers test using a Gelbo flex tester in accordance with ASTM F392.
A less aggressive abuse test, where the film is creased before testing for barrier testing, is described in BS 3177 appendix A.