Mechanical Metric
Mechanical metric quantifies the amount of energy required to create a new surface area by pulling a crack open in a perpendicular direction. For packaging materials, mode I fracture energy identifies the resistance of a paperboard to delamination or splitting during use. This property is measured in joules per square meter and represents the toughness of the fiber network.
A high value indicates that the material can absorb extensive energy before a crack propagates through the thickness.
Seal Strength
Seal strength of a heat seal or a glued joint is often assessed by measuring the mode I fracture energy during a peel test. In flexible packaging, this value determines if a pouch will open cleanly or if the film will tear irregularly. The test records the force required to separate two layers while the crack tip moves at a constant velocity.
Variations in the refining level of the pulp or the addition of dry strength agents directly influence this toughness.
Failure Mode
Failure mode occurs if the energy threshold is too low and the substrate fails internally before the adhesive gives way. This internal ply separation is a common issue in high speed printing where the ink tack exceeds the strength of the board.
