ABSTRACT

The deformation of paper and paper products, like that of all polymeric materi­ als, is dependent on the environmental conditions as well as on the time scale of the deformation and the loading pattern. Because paper material is highly hygro­ scopic, the relative humidity of the surroundings, and indeed moisture contact in general, affects the viscoelastic properties of paper to a great extent. Moisture uptake changes the properties of the paper so that although it is a relatively stiff and brittle material at low moisture contents it becomes a very ductile one at high moisture contents. The material properties of paper are strongly affected by process variables in the making of the paper in, for example, the pressing and drying operations; in converting operations such as folding, corrugating, and printing; and during its end use as a packaging or printed material. During all these processes, the paper is subjected to different and complicated loading pat­ terns in terms of both stress levels and the time scale of the load. In order to model the performance of paper under all these conditions, it is first necessary to characterize its mechanical properties according to fundamental principles. Thus testing of paper with regard to its time-dependent behavior under a constant load (creep) or at a constant deformation (stress relaxation) or during cyclical defor­ mation (dynamic behavior) at specific temperatures and relative humidities should provide a better understanding of the behavior of paper subjected to various loading situations.