Mechanical properties and structural changes in recycled paper

In this study, a thorough examination of the structural and mechanical properties of recycled paper was made and compared with current theoretical understanding of the phenomenon. For this study pulp freeness and other papermaking conditions were maintained throughout all the cycles. The results showed that mechanical properties of the paper decreased in recycling. The thickness of the fiber and the fiber cell wall remained virtually unchanged; however, the voids between fibers and the overall sheet thickness increased significantly. Mechanical properties of fibers did not deteriorate in recycling. The fiber contact ratio decreased along with the sheet density in recycling. It was observed that modulus of fibers in the paper sheet was dependent on the fiber orientation. The fibers oriented in the MD had much higher modulus than those in the CD. Some of the fibers responded sluggishly in the early part of the stress-strain curve. The frequency of this sluggish response increased in recycling.;Two unique findings were made through quantitative predictions of the elastic moduli of the paper sheets by a micromechanics model. It was discovered that the reduction of the elastic moduli in recycling could not be fully accounted for by the combined effects of lower sheet density and fiber-fiber bonds. It is possible that the sluggish response of the cut-out fibers, in addition to the lower sheet density and fiber-fiber bonds, could have caused the further reduction of the elastic moduli in recycling. It was also discovered that fiber orientation distribution alone could not account for the MD/CD ratio of the elastic moduli. It was discovered that fibers orientated in the MD had much higher elastic modulus than fibers in the CD. This orientation dependence of fiber modulus, probably caused by the significantly greater drying constraints in the MD, might have effectively raised the MD/CD ratio of the paper moduli.;Previous reports in the literature had concluded that the inferior mechanical properties of recycled papers were mainly caused by the reduction of the fiber-fiber bond quality. This study has discovered and clearly demonstrated that even the combined effects of the reduction of fiber-fiber bond and reduction of density could not fully account for the paper sheet moduli reduction. It is likely that the sluggishness of the fiber also had a significant effect on the sheet moduli.