Internal Bond Test (Z-Direction Tensile Test)

For multi-layered materials (duplex paper, coated paper), the internal bond strength between the layers is an important property which determines processability during printing or after coating. During tests to TAPPI T 541 the load must be perpendicular to the surface of the specimen. In order to eliminate the influence of grips or fixtures on the separation of the layers in a test, the tensile force must remain perpendicular to the specimen at failure, making high lateral stiffness and absolute parallelism of the specimen holders a critical requirement. Zwick’s special arrangement for this test incorporates specimen carriers arranged at 90 ° to each other which allow five tests to be performed with a single application.

The specimen carriers are mounted on precision compression platens and are accordingly parallel and stiff in the tensile and transverse directions. The specimen is attached to one specimen carrier using double-sided adhesive tape. Double-sided tape alone is applied to the opposing second holder.

During the test sequence the specimen is initially pressed against the second specimen carrier. Compressive force and duration are freely adjustable. The specimen is then separated in less than half a second in the tensile test. The measured quantity is the tensile strength, related to the surface area of the specimen, giving the tensile stress.

The test device is available in both manual and semi-automatic versions. The semi-automatic version allows a second strip of specimens to be prepared while the first strip is being tested.

The internal bond test offers greatly reduced result variation compared to the Scott bond test, which is used to determine similar paper properties.

Other tests include ply-bond strength to DIN 54516 and the Scott bond test to TAPPI T 403, both of which apply peel loading to the specimen.

A pendulum impact tester is used for the Scott bond test. The test therefore takes into account the high processing speeds usual for this material, but is considerably less reproducible due to internal energy losses through friction and vibration.

Advantages (TAPPI T 541):  

• semi-automatic version enables specimen positioning during test 
• compression and tensile test in one operation 
• compressive force and duration are freely adjustable 
• tests using half the test area are possible for tests on high-strength papers
• no energy measurement – more reliable than Scott bond test.