| This study primarily focused on modifying the kraft pulping process of aspen (Populus tremula) with polyphosphonates as additives in digesters. Cost-effective charge levels of 0.05∼0.20% phosphonates were examined with respect to lignin removal. Addition of phosphonates in cooking resulted in greatly improved delignification and uniformity of alkali consumption, achieving measurable increases in pulp yield. The advantageous effects were particularly significant at reduced H-factors. Small increases in pulp viscosity were also observed. Few obvious advantages were shown in pulp beating or in handsheet strengths, although modified pulp fibers were slightly stronger. Responses of modified pulps to selected elemental chlorine free (ECF) bleaching sequences were considerably improved. In addition to extended delignification, the improved bleachability is attributed to appreciable removal of metal-ions from fibers, especially Ca, Fe, Mn and Zn. The overall influence of phosphonates is comparable or superior to anthraquinone (AQ), and much better than surfactants or chelants.; Qualitative and quantitative analysis of pulps, lignins and polysaccharides by FT-IR, GC-MS and NMR techniques illustrated that phosphonates help remove more lignin while retaining more carbohydrates. Little evidence was found supporting occurrences of reactions between phosphonates and major wood components. Fundamental structures of resulting kraft lignins experienced few important changes. Empirical formulas for kraft lignins were obtained. Residual lignins were enzymatically isolated and characterized by CP/MAS-NMR. Hexeneuronic acid contents on pulps turned out almost unchanged. Molar masses of both acetylated and non-acetylated kraft lignins by HP-SEC adequately revealed that the delignification pattern is identical regardless of phosphonate addition, but is essentially different from the way AQ behaves. The fate of major metals in the overall system was investigated.; Kraft lignin solubility and its solution turbidity evidently increase in the presence of phosphonates less than 300ppm. A mechanism is therefore proposed regarding involved interfacial interactions whereby phosphonates take effect through inherent chelating and dispersing properties, forming washable phosphonate-cations-lignin compounds, reducing metal interference, stabilizing resins, reaching uniform cooking, and restraining re-deposition of dissolved lignins onto fibers. Pilot trials using mill white liquors yielded less noticeable reduction in pulp kappa numbers. Including substantial sodium carbonates in white liquors is likely to impair the performance of phosphonates. |
