Evaluation of the stress-strain properties in the thickness direction - particularly for thin and strong papers

Abstract

Abstract The performance of the paper in a number of converting operations such as creasing, bending, printing, and plastic coating put great demands on the mechanical properties in the thickness direction of the material. The knowledge of strength, elastic- and plastic behavior in tension and compression in the thickness direction is needed for a comprehensive description of the performance of the material in these operations. In spite of its importance, very few publications deal with the evaluation of the entire tensile stress-strain curve of paper in the thickness direction. A likely reason for this is the intrinsic difficulty of testing a thin, uneven, porous, fibrous and compressible material such as paper with sufficient precision and testing time efficiency. The z-directional strength test is usually performed by fastening the paper by means of double-adhesive tape to metal platens. The platens are fastened in a testing machine and strained to break. The adhesion of the tape is the limiting factors for how strong papers that can be tested. The tape-based method also is expected to have a lower limit in grammage due to the penetration of the adhesive. The aim of the present publication was to show a procedure how to evaluate the entire stress-elongation curve in the z-direction of papers, using a lamination method for fastening the paper to the metal platens. From this curve the z-strength, z-modulus, z-strain at break, zenergy at break and z-fracture energy could be extracted. Such information is, so far, non-existing in the literature.