Effect of delamination angle on delamination energy

Different tests have been traditionally used to evaluate the delamination energy of paper and board. The values of delamination energy differ quite a bit. We propose that this is related to differences in the angle that opens between the delaminated faces of the specimen. For example, the STFI Z-toughness test forces delamination at zero delamination angle while the ordinary peeling test leads to ca. 180 degrees delamination. In addition to these two, we considered a third test, a "nip peeling" test where the specimen is pulled through a nip formed by two rolls. The resulting delamination angle is ca. 90 degrees.

In experiments with LWC base paper the STFI Z-toughness test and the ordinary peeling test gave roughly equal delamination energies, but the nip peeling test gave approximate one half smaller energy values. The minimum energy at an intermediate delamination angle can be explained to be the result of two competing effects when the delamination angle increases. One is the decrease of outer plastic zone, and the other is the increase of bending energy consumption. Analogy can be drawn to a similar phenomena observed in in-plane tear tests.

Unlike the three z-directional fracture energy measurements, the Scott bond test measures tensile energy absorption in the thickness direction of paper and no delamination angle can be defined. However, even in the Scott bond test energy consumption is related to the spatial extent of microscopic damage. The comparison of Scott bond with in-plane fracture energy suggests that roughly 70% of all inter-fiber bonding was broken in the Scott bond test of our specimens.