Tensile Strength

Tensile strength is measured using ASTM D882, where a strip of film of known width is clamped at both ends and pulled apart at a controlled rate on a tensile testing machine until it breaks. The force at break, divided by the original cross-sectional area, gives the tensile strength, usually reported in pounds per square inch or megapascals. The same test also produces elongation at break, which measures how much the film stretched before failing, expressed as a percentage of its original length.

Because blown film typically has some degree of directional bias from the extrusion process, tensile strength and elongation are almost always reported separately for the machine direction (the direction the film traveled down the line) and the cross direction (perpendicular to that, around the original bubble circumference). A film with balanced properties has similar values in both directions, while a film with strong directional bias might be notably stronger or stretchier in one direction than the other, which matters for how the film performs when it's stressed during use.

Tensile strength and elongation together give a picture of how a film responds to a steady, increasing pull, which is relevant for applications like stretch film that rely on controlled stretching, as well as for general film toughness assessments, though it's a different failure mode than the sudden impact measured by dart impact strength.

A stretch film manufacturer cares deeply about both tensile strength and elongation at break, since the film needs to stretch substantially around a pallet load without breaking, which requires high elongation alongside adequate tensile strength.

Tensile strength numbers can be misleading if the machine direction and cross direction values aren't both considered; a film reported with an impressively high tensile strength in one direction might be considerably weaker in the other, which matters a great deal depending on how the finished package will be stressed in actual use.