Characterization of sulfite pulp and its conversion to super-water absorbent material via grafting with methyl acrylate.

摘要

The focus of the thesis includes (1) to characterize the fundamental differences between sulfite and kraft pulps, (2) to develop a process to produce super-water absorbent material from sulfite pulp based on the methyl acrylate grafting method. To determine the differences in the strength properties of unbleached kraft and bisulfite pulps, the surface structure of the fibers at various degree of beating was investigated by atomic force microscope (AFM) and scanning electron microscope (SEM). The results indicate that the surface of the unbeaten kraft pulp fibers showed the appearance of primary wall (P), whereas the majority of the unbeaten bisulfite pulp fibers revealed the fibrillar structure of outer secondary wall (S1). For all beating degrees, fibers were heterogeneously beaten along their length, and heavily fibrillated area or beated area alternates with less fibrillated area irrespective of the pulp type. Beating was found to have more uniform action along the fiber length, in the form of longitudinal shrinkage folds, for bisulfite pulp fibers in comparison to the kraft pulp fibers. In addition to that high damage sites were relatively frequent for bisulfite pulp fibers in comparison to the kraft pulp fibers. Extensive disruption of the S2 wall took place at the high damage sites for bisulfite pulp fibers. However, kraft pulp fibers did not show high damage to the S2 wall layer.;The distribution of the polymer chains grafted at low and medium fiber consistency during grafting was investigated in the fiber matrix. It was found that the distribution of poly (methyl acrylate) is affected by the pulp consistency during grafting. At a medium consistency condition the outer region of the fiber structure had a higher polymer concentration than the inner region. On the other hand, at a low consistency condition, grafting occurred uniformly across the fiber wall structure.;The major contribution to the knowledge in this thesis work is that sulfite pulp is weaker than kraft pulp because of the frequent damage sites along the fiber length. On grafting PMA on bleached sulfite pulp, WRV was found to be higher for graft-copolymers obtained at medium-consistency condition than that of low-consistency condition. Hydrolysis of the graft-copolymers obtained at medium consistency condition was found to be diffusion controlled and fiber consistency during grafting affects the distribution of the grafted polymer chains.;High water absorbency for graft copolymers obtained at medium-consistency condition is of great significance on industrial scale as pulp and paper industry is looking forward to operate at medium-consistency condition because of the environment problem.;Super water absorbent hydrogels of wood cellulose fibers were prepared by graft copolymerization of poly (methyl acrylate) (PMA) onto softwood sulfite pulp fibers using a free radical initiator followed by alkaline hydrolysis. Ceric ammonium nitrate (CAN) was used as the free radical initiator. Effects of various parameters such as fiber concentration, monomer/pulp (M/Pulp) ratio, CAN concentration and reaction time on the grafting yield and on other grafting parameters were investigated. The graft conversion was the same from low to medium fiber consistency. The amount of initiator required was found to be independent of fiber consistency to achieve the maximum grafting yield. After hydrolysis, the water retention value of the grafted fibers obtained at a medium fiber consistency was higher than that of the grafted fibers obtained at a low fiber consistency. The effect of process variables such as hydrolysis reaction time, temperature and NaOH concentration during alkaline hydrolysis was carefully determined. The degree of hydrolysis was estimated based on the determination of carboxylate group contents. The material loss during hydrolysis followed a first order reaction mechanism. The differences in the behaviour of the grafted fibers during hydrolysis were explained based on their structural differences. At a lower M/pulp ratio, the grafted polymer chains were shorter and their structure was less compact; while at a higher M/pulp ratio, the grafted polymer chains were longer and their structure was much more compact. Consequently, longer polymer chains provide resistance for the hydrolysis reactions and need a longer reaction time and higher NaOH concentration as well as higher reaction temperature to reach well swollen state, and thus complete hydrolysis.