Study On Cocrystallization Of Cellulose As A Mechanism And Its Suppression Methods Of Recycled Plant Fibers

Recycled plant fibers as an important fiber source can not only alleviate the paradox between the lack of papermaking raw materials and energy consumption, but also play essential role in keeping up the ecological balance by decreasing the environmental pollution problems. The mechanism and suppression methods for quality decay of recycled plant fibers were studied in this paper. Changes of physical and chemical properties of eucalyptus kraft pulp fiber and paper properties were studied firstly. The microstructure of eucalyptus kraft pulps with very different hemicellulose contents was also studied in order to investigate the presence of hemicellulose influences the properties of recycled fibers. Furthernore, the effects of drying on the microstructure and cocrystallization mechanism of recycled plant fibers were studied. In the meaning time, the resolution of hornification was discussed, from which mechanisms and reasons of it were observed.The results showed that the fiber length did not change significantly during the recycling process. Parts of hemicellulose dissolved after recycling, but the lignin contents were not significant. Water retention value was decreased by 46.0% and crystallinity was increased by 5.58% from the beginning to the fifth reuse, which showed that the arrangement of inter microfiber tend the regularization in hornification and resulted in the decreasing plasticity of fibers and paper strength.There were remarkable differences in both cellulose fibril structure, pore structure and cellulose supermolecular structure between the fiber samples with large differences in cellulose and hemicellulose proportions but similar lignin contents, which were produced by partial removal of the hemicellulose. Average fibril aggregate size (17.9-22.2 nm) increased with decreasing hemicellulose content. The cellulose crystallinity and the cross sectional area of microcrystallinity was increased with decreasing hemicellulose content, but the microcrystallinity size was increased slightly. Fiber with high hemicellulose content has a more porous surface structure. The low and high hemicellulose-containing pulps had very different recycling characteristics, which may be explained by the changes observed at cellulose fiber structure level.The change of hornification was great when the drying time was first 10 min. The lactone in fiber formed after drying, and lactone content increased with the increasing drying time. When the drying time increased from 5 to 20 min, lactone content increased 19.70 mmol/kg. Moreover, lactone content of rewetting fiber was increased further during drying process. Change trend of infrared crystal index was the same as the hornification degree. Drying process had irreversible influence on the microstructure and hornification of recycled plant fibers, because initial swelling properties were not restored after fiber rewetting. Changes in NMR signal strengths indicated that growth of crystalline domains involved cocrystallization rather than accretion of cellulose from noncrystalline domains. A cluster of C-4 signals atδ89.7, assigned to the interiors of crystalline domains, grew at the expense of C-4 signals atδ85.1 andδ84.1, assigned to the well-ordered surfaces of crystalline domains. Meanwhile, LFAD was decreased from 21.5 nm to 18.1 nm after rewetting but increased during second drying, even larger 1.7 nm than that of first drying. The results suggest that cocrystallization might provide a mechanism for quality decay of recycled plant fibers.Based on the studies above, the resolution of hornification was discussed further. Compared the optimization process with the traditional process, recycle properties such as WRV, tensile index, burst index and ring crush index of fibers was increased by 12.9%, 14.7%, 11.0% and 17.9% respectively.