A Study of the Mechanical Properties of Recycled Paper Under Diverse Humidity Environments

Abstract This paper reports on the effect of recycling on the constitutive characteristics of structural paper when it is subjected to a progressively increasing uniaxial tensile loading in the machine direction at various relative humidity levels using the acoustic emissions technique. It was found that at all relative humidities, the recycled material consistently exhibits considerably higher values for stretch and tensile energy absorption (area under the non-linear portion of the stress-strain curves) than the non-recycled paper while maintaining its high strength. The average number of the threshold crossings per hit were found to be generally higher for the recycled as opposed to the non-recycled material. This is indicative of a more “continuous” type of emission, representing more plastic behavior and ductility. It is proposed that the changes in the plastic behavior are associated with the waxy residues which are commonly present in the recycled pulp mix as opposed to the pulp solely composed of virgin cellulosic fibers. As a result, the paper behaves more like a thermoplastic composite. The inelastic properties of the recycled material were found to be pronouncedly different under a relative humidity condition that is believed to correspond to the “equilibrium moisture content” of the material (e.g., 70%). Under such a humidity condition it was found that the magnitude of the “principal amplitude” is lower thus suggesting that a different deformational mechanism is in effect. It is suggested that at such a humidity level moisture interacts with the waxy contents of the recycled paper causing the resin to soften and the material to significantly flow under the applied loading. The magnitudes of the principal counts, energy, duration at failure, as well as the integrated area under the cumulative counts-amplitude distribution curve were found to drastically increase at such a humidity level. As a result, the ductility and the failure toughness of the material increases significantly under such a humidity level. These results re-emphasize the importance and the advantage of recycling to preserve the environment as well as obtain good mechanical integrity under common relative humidity conditions.