| In China, the annual production rate of cellulose acetate is about300,000tons, of which90%is used to produce acetate fiber tow, and the annual consumption of dissolving pulp is about200,000tons. For acetate grade dissolving pulp, the requirements for some key parameters such as a-cellulose content and degree of polymerization (DP) are much higher than that for ordinary viscose grade dissolving pulp. As a matter of fact, there is still no domestic producer of acetate grade dissolving pulp and all of our domestic demands are currently met by importing. In this thesis, eucalyptus globules, birch and cotton linter were used as the raw materials to prepare acetate grade dissolving pulp through in-depth investigation of different pulping and bleaching techniques. New technologies for producing acetate grade dissolving pulps were developed. Important physical and chemical properties of the dissolving pulps produced were thoroughly analyzed and characterized, and their application performance was comprehensively evaluated. Through the work in this thesis the author wishes to offer valuable technical guidance for domestic dissolving pulp manufacturers in their efforts to develop acetate grade dissolving pulp, a high value-added new product.For eucalyptus globules, the prehydrolysis kraft pulping process was investigated. Pretreatment experiments indicated that the prehydrolysis treatment was more effective than alkali treatment in removing pentosan. The difference in their pentosan removal rates was about17%. Under the optimal condition at alkali dosage (in Na2O) of15%, the Kappa number of the unbleached pulp obtained was13.4, the DP was1742, the brightness was45.8%ISO, and the yield of the screened pulp was41.9%. After a two-stage oxygen delignification, followed by XD0ED1ETD2bleaching with total ClO2usage of1.4%, the dissolving pulp obtained had an a-cellulose content of97.6%and a DP of1086. The bleaching yield was88.7%.For birch, a novel prehydrolysis sulfite pulping process was experimented. Under the conditions of total acid dosage of6.5%and combined acid dosage of1.5%, the unbleached pulp produced had a Kappa number of24.2, a DP of1545, and a brightness of72.1%ISO. The yield of screened pulp was40.6%. Considering the existing mill production conditions, CEHETD2bleaching sequence was adopted. The dissolving pulp obtained had a brightness of93.0%ISO, a DP of1032, and an α-cellulose content of98.2%. The bleaching yield was87.9%.For cotton linter, three different cooking methods including Soda process, Soda-H202process and Alkali-oxygen process were investigated separately. The results showed that the Soda-H2O2process was the mildest one, and the unbleached pulp obtained had the highest brightness. The optimized cooking conditions were:alkali dosage at6%and H2O2dosage at3%. Under these conditions, the yield of unbleached pulp was above88%, and the pulp obtained had an a-cellulose content of above96%, a DP of about1900, and a brightness of72-73%ISO. Considering the existing production conditions of domestic cotton linter pulp mills, a two-stage hypochlorite bleaching process was adopted for Soda-H2O2unbleached pulp. When the total chlorine usage was at5.0%, the bleached pulp obtained had a brightness of90.2%ISO and the bleaching yield was92.1%. If a CEH three-stage bleaching process was adopted and when the total chlorine usage was at4.0%, the brightness of the bleached pulp could reach above90.2%ISO. After a supplemental bleaching with hypochlorite or H2O2, the brightness of the pulp could be further increased to93%ISO. When HED bleaching process was adopted and the chlorine dosage was3%during hypochlorite bleaching and the ClO2dosage was1.5%, the brightness of the bleached pulp could reach93.8%ISO. The bleached cotton pulps from all three cooking methods had an a-cellulose content of above98%, some even above99%, indicating that cotton pulp was easier to purify.Analysis of the chemical compositions of the above pulps showed that the properties of the bleached prehydrolysis KP eucalyptus pulp, bleached prehydrolysis SP birch pulp, and the bleached cotton linter pulp produced by Soda-H202cooking in combination with two-stage hypochlorite bleaching could all meet the purchasing specifications of users. The quality of the pulps obtained was generally very close to imported pulps except for some slight difference in a couple parameters.SEM analysis indicated that the surfaces of prehydrolysis KP eucalyptus dissolving pulp fibers was smooth, and the fiber structure appeared more compact, but there was some uneven damage observed and it seemed slightly more than the imported pulp. The surfaces of the prehydrolysis SP birch dissolving pulp fibers were however damaged very seriously, and much more so than that of the imported pulp. Some uneven damage was also observed on the surfaces of the cotton pulp fibers, but the fibers appeared finer, dense and smooth. These morphological characteristics are good indications of their performance differences during the acetification reactions.Results of acetification tests indicated that during the acetification reactions, the eucalyptus dissolving pulp had the highest peak temperature of48.8℃, the shortest heating time of only34min, and the highest rate of temperature increase (1.64℃/min). The rate of temperature increase for birch pulp was1.50℃/min, which was also higher than that for imported pulp (1.46℃/min). Both the eucalyptus and birch lab-made dissolving pulps demonstrated good performance in their acetification reactions. But the cotton dissolving pulp showed the lowest rate of temperature increase (only1.42℃/min) because of the lowest removal rate of impurities during pulping and its densest fiber structure. Compared to the imported pulp, the molecular weight distribution of cellulose acetates made from eucalyptus pulp and cotton pulp were wider, and their solubility in acetone was also poorer. However, the cellulose acetate made from birch pulp had more uniform molecular weight distribution and its solubility in acetone was also better than the imported pulp. |
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