Converting operations can set strong requirements on the mechanical properties of a paper product in the thickness direction. Delamination of paper and paperboard may occur for instance in offset printing, plastic coating processes, corner gluing of corrugated boxes and glued joints in boxes.

 

In some converting operations, such as offset printing, partial delamination cannot be accepted and the local maximum strength cannot be exceeded. In other cases, energy based properties related to the post-peak behaviour of paper are of greater interest. Reliable measurements of the fundamental mechanical properties of a paper (e.g. Z-strength, elastic modulus, energy-to-break and strain-at-break) provide the basis for process and quality control.

 

A new method to measure Z-strength has recently been developed at Innventia. A stiff and strong adhesive with controlled penetration is used to fix the paper to the measuring apparatus. This gives new opportunities to characterize paper materials. The strong, controlled bond enables measurements of a wide range of qualities, particularly strong and thin papers. The complete stress-strain relation is measured in the method, which allows the characterization of both the fundamental mechanical properties and of the post-peak behaviour.