This property is desirable when comparing candidate films during the material selection process. Good flex crack resistance will reduce the propensity of the package developing holes during the distribution cycle thereby destroying the package integrity.The sample (size 200 by 280 mm) is attached to the Gelbo flex-tester mandrels. The flexing action consists of a twisting motion combined with a horizontal motion (compression), thus repeatedly twisting and crushing the sample. For polymeric film, the number of predetermined strokes is typically set at 1000, after which the sample material is examined. The formation of pinholes is determined by use of a blue dye solution (Toluidine Blue) and allowing it to stain through the pinholes onto an absorbent white backing. The number of pinholes is a measure of the flex crack resistance of the material and is reported as, for example, 3 pinholes/1000 flexes.
|ASTM F392 Standard Test Method for Flex Durability of Flexible Barrier Materials