Ply Adhesion
Standardized mechanical evaluation of the internal bond strength of paperboard measures the force required to shear the internal fibre plies along a pre-defined plane. The double-notch shear test utilizes a rectangular specimen with two parallel, offset cuts made on opposite sides of the sheet, forcing the shear stress to concentrate in the zone between the notches. By applying tension or compression to the ends of the sample, the instrument determines the maximum load the fibrous structure can withstand before delamination occurs.
This test provides a reliable indicator of ply bond strength in multi-ply board.
Testing Procedure
Precise specimen preparation is necessary to ensure repeatable results during this laboratory procedure. Blades must cut the notches to a depth of exactly half the specimen thickness, or the stress will not concentrate properly in the target plane. During the test, the sample is clamped in a tensile tester and loaded at a constant elongation rate until the fibre plies separate.
Measuring the peak force allows the calculation of the shear strength per unit area, which is expressed in kilopascals.
Industrial Application
Packaging designers employ the resulting shear data to predict the resistance of cartons to bulging and corner tearing. High internal shear strength prevents the plies from sliding or separating when the carton is subjected to stacking loads or compression. If the double-notch shear test reveals low values, the mill must adjust the starch application or the refining intensity at the wet end to improve ply adhesion.
This adjustment prevents structural failures in filled packaging on palletized stacks.
