Interactions of time dependent, thermal and hygroscopic characteristics and runnability properties of paper

Abstract

For purposes of design and optimization of industrial processing, many of the physical properties of paper have to be taken into account. The knowledge of the mechanical properties as part of the physical ones must be extended by considering hygroscopic, thermal, optic and electric material aspects. The knowledge on these properties will support the paper processing and converting industry in optimization of their processes and quality of paper.

In previous projects at the Institute of Engineering Design (LMK, Ruhr-University Bochum, Germany) a complete material law was developed and implemented into a finite element system [37]. This model contains also yielding and hardening which are major mechanical properties of paper.

Our ongoing research on the physical properties of paper determines thermal and hygroscopic material behavior. The determination of these physical parameters can be divided into a constructive, an experimental and an analytical part which was performed iteratively.

At first a usual test chamber with temperature control for general material testing was used for determining mechanical properties of paper in the three main directions under all process relevant temperature conditions. In this setup the climate conditions reach the limit for paper testing very soon. Therefore, an individual test chamber was conceived and developed to perform the relevant test environment for the analysis of paper under process relevant thermal and humid climate conditions. Secondly, mechanical experiments were performed in order to find the physical paper behavior, beginning with the mechanical properties of heated and moistured paper. Finally a mathematical formulation of hygro-mechanical behavior of paper was determined.

From our data we found correlations between moisture content and runnability properties. The humidity influence on the elastic modulus and on the strain properties of paper were determined with tensile tests in machine direction (MD) and compression tests in thickness direction (ZD).

Improving this knowledge of general physical material parameters can lead to a realistic mathematical formulation of the physical force-deformation-behavior in paper and paperboard grades in future. Principles, designs and settings of finishing and converting machines can be optimized by using material laws in process analyses, based on this advanced material knowledge. Furthermore, details of the expected runnability properties of established paper and paperboard grades can be determined.