Internal Bond Test (Z-Direction Tensile Test)

For multi-layered materials (duplex paper, coated paper), the internal bond strength of the layers to one another is an important characteristic. It determines the further processability. When testing to TAPPI T 541, the load on the specimen must be perpendicular to the surface of the specimen. In order to eliminate the effects from grips or fixtures to the separation of the layers in a test, the tensile force must be maintained in its vertical direction through rupture. High lateral stiffness and absolute parallelism of the specimen holders is critical for a proper test. Therefore, Zwick developed a special test rig for holding the specimens, which are crossed over at an angle of 90 °, and which accept a paper strip of 5 specimens. Thus, using one single application, 5 tests can be performed without further handling of the specimen.

The specimen holders are mounted on precision compression plates and are parallel and stiff in all directions. How the specimen is applied: using a double sided self adhesive tape, a paper strip is glued onto one specimen holder. The second holder is applied with only a double adhesive tape. During the test, the specimen is at first pressed against the two specimen holders. Compressive loads, as well as pressing time are freely adjustable. After the compressive phase, the tensile test is performed within half a second.

The test result is the tensile strength, which is the maximum tensile load, related to the sample’s surface area. The z-tensile test rig is available as a manual version, as well as a semi-automatic one. With the semiautomatic test rig, a second paper sample can be prepared during the time while testing the first sample set of 5 specimens.

Compared to Scott bond tests, the z-tensile test has proved to have much less result dispersion.

Similar tests: ply bond strength to DIN 54516 and Scott Bond Test to TAPPI T 403. In these tests, the specimen is subjected to peel loading. The Scott Bond Test is performed with a pendulum impact tester. This takes into consideration the high processing speeds of this material.

The advantages (TAPPI T 541):

• Quick placement of the specimen by using a semiautomatic machine even while running a test series
• Pressure step and z-tensile test without specimen handling
• Compressive strength and compression time are freely adjustable
• Use of only half the standard area for tests on high strength papers