| 1. |
- Esteves, Claudia, et al.
(författare)
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Differences and similarities between kraft and oxygen delignification of softwood fibers : effects on mechanical properties
- 2021
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Ingår i: Cellulose. - : Springer Science and Business Media B.V.. - 0969-0239 .- 1572-882X. ; 28, s. 3775-3788
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Tidskriftsartikel (refereegranskat)abstract
- Charged groups in pulp have been shown to enhance the tensile strength of the paper produced from the pulp. Oxygen delignification introduces charged groups and it is of interest to determine how the delignification should be distributed between the cooking and the oxygen stage with respect to mechanical properties. A number of unbleached kraft cooked and oxygen delignified pulps within a wide kappa number range were produced and refined, and the effects of the refining on the morphology and mechanical properties were studied. The WRV correlated with the fiber charge and at a given fiber charge, kraft cooked and oxygen delignified pulps had the same WRV development in refining, although they had significantly different kappa numbers. The tensile strength development during refining depends on the fiber rigidity which is affected by the lignin content, the fiber charge and the chemical and mechanical processes used. Refining increased the curl of the kraft cooked pulps and decreased the curl of oxygen delignified pulps, irrespective of kappa number. A greater increase in tensile strength was seen for the pulps with a higher fiber charge and WRV, probably because of the greater degree of fibrillation achieved in the beating process. Despite the greater fiber deformation in the oxygen delignified pulps, the strength can be increased by a larger amount of charged groups and a greater swelling of the fibers. Graphic abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
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| 2. |
- Esteves, Claudia, et al.
(författare)
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Differences and similarities between kraft and oxygen delignification of softwood fibers : effects on chemical and physical properties
- 2021
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Ingår i: Cellulose. - : Springer Science and Business Media B.V.. - 0969-0239 .- 1572-882X. ; 28:5, s. 3149-3167
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Tidskriftsartikel (refereegranskat)abstract
- The fiber properties after oxygen delignification and kraft pulping were studied by looking into the chemical characteristics and morphology. The effect of the two processes on the fibers was evaluated and compared over a wider kappa number range (from 62 down to15). Wide-angle X-ray scattering, nuclear magnetic resonance and fiber saturation point were used to characterize the fiber network structure. Fiber morphology and fiber dislocations were evaluated by an optical image analysis. The total and surface fiber charges were studied by conductometric and polyelectrolyte titrations. The fiber wall supramolecular structure, such as crystallinity, size of fibril aggregates, pore size and pore volume, were similar for the two processes. The selectivity, in terms of carbohydrate yield, was equal for kraft cooking and oxygen delignification, but the selectivity in terms of viscosity loss per amount of delignification is poorer for oxygen delignification. Clearly more fiber deformations (2–6% units in curl index) in the fibers after oxygen delignification were seen. Introduction of curl depended on the physical state of the fibers, i.e. liberated or in wood matrix. In the pulping stage, the fiber continue to be supported by neighboring fibers, as the delignified chips maintain their form. However, in the subsequent oxygen stage the fibers enter in the form of pulp (liberated fibers), which makes them more susceptible to changes in fiber form. Graphic abstract: [Figure not available: see fulltext.] © 2021, The Author(s).
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| 3. |
- Esteves, Claudia, 1989-
(författare)
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Pulp strength enhancement by oxygen delignification
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Oxygen delignification is widely used in the pulp and paper industry as a part of delignification process between the kraft cook and bleaching. However, its potential has not been fully utilized. Rather than an intermediate process between cooking and bleaching, oxygen delignification is a strong oxidizing agent with powerful effects on the pulp properties. In this work, the hypothesis that oxygen delignification has the potential to improve the pulp mechanical properties was investigated. Several pulps were produced by either kraft cooking or kraft cooking combined with a subsequent oxygen delignification stage to a similar kappa number and their properties were analyzed and compared. This methodology assessed the real oxidative potential of oxygen on the final fiber properties. Total fiber charge, pulp mechanical properties, fiber morphology, swellability and fiber nanostructure, were studied.The major part of this research investigated the relationship between the carboxylic acid groups, seen as total fiber charge, and the mechanical strength of the paper. The total fiber charge was evaluated by conductometric titration and correlated with the pulp swellability and mechanical properties. It was demonstrated that oxygen delignification could significantly increase the charge content and the swelling of the pulp when an extended oxygen delignification (i.e, higher delignification degree) was used. In addition, the tensile index of the sheets increased when the fiber charge after oxygen delignification was sufficiently high. The swelling of the different pulps was investigated by Schopper-Riegler degree (SR°), water retention value (WRV) and fiber saturation point (FSP). It was determined that the higher the fiber charge, the higher the swelling ability, regardless of the lignin content. High alkali impregnation was utilized in this study due to its potential to increase cooking yield. The yield was compared to kraft pulp cooked with standard and high alkali impregnation, followed by oxygen delignification and bleaching. It was observed that the increase in yield was preserved in both unit processes, i.e., after oxygen delignification and after bleaching. During this work, pulp properties such as fiber morphology and fiber nanostructure were also important properties that were studied following each unit process and refining step. Oxygen-delignified pulps presented higher fiber deformation when compared to the kraft-cooked pulps. However, even with higher fiber deformation, oxygen-delignified pulps showed higher mechanical strength, contradicting previous reports that claimed lower pulp strength for oxygen-delignified pulps, due to fiber deformation. Additionally, it was found that fiber deformation tends to increase with PFI-refining for kraft-cooked pulps, while for oxygen and bleached pulps it tends to decrease. Fiber nanostructure was additionally studied by X-ray scattering, and the results obtained from pulp delignification by kraft and kraft followed by oxygen delignification were compared. This thesis highlights the benefits of increasing fiber charge by performing an extended oxygen delignification after a reduced kraft cooking. The results indicate that when oxygen-delignified pulps achieve 80 % higher fiber charge than kraft-cooked pulps at a similar kappa number, the pulp tensile index can be improved by up to 18 %. The oxidation reactions that occur during the oxygen delignification lead to a significant increase in the carboxylic acid groups in the fibers which increases the fiber's swelling ability and improves the refining process efficiency. The combination of those effects results in a higher tensile index and lower refining energy required. However, to obtain mechanical improvement, the oxygen delignification must be sufficiently long (extended). Therefore, it is believed that an extended oxygen delignification will yield a more uniform distribution of the charged groups in the fibers, which will increase the fiber swelling and fiber flexibility leading to a more efficient refining process and stronger fiber bonding structure in the paper.
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| 4. |
- Esteves, Cláudia S. V. G., 1989-, et al.
(författare)
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Evaluating the Potential to Modify Pulp and Paper Properties through Oxygen Delignification
- 2020
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 5:23, s. 13703-13711
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Tidskriftsartikel (refereegranskat)abstract
- The potential to modify pulp and paper properties by oxygen delignification was assessed by looking beyond the ordinary purpose of oxygen delignification. Pulps with the same kappa number were obtained by both pulping and the combination of pulping and oxygen delignification, and the mechanical and chemical properties were compared. The oxidation of pulp components leads to an increase in carboxylic acid groups in the fibers, resulting in a large influence on fiber swelling, seen as an increase in the water retention value and fiber saturation point. The introduction of charged groups appears to replace some of the morphological changes caused by refining and enhance the strength of fiber–fiber joints, generating pulps with better refinability and higher tensile strength. Oxygen delignification was able to improve the tensile index with 6% at the same sheet density and less refining energy, when the amount of total fiber charges was higher than 140 μekv/g.
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| 5. |
- Esteves, Claudia, et al.
(författare)
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The effects of high alkali impregnation and oxygen delignification of softwood kraft pulps on the yield and mechanical properties
- 2022
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Ingår i: Nordic Pulp & Paper Research Journal. - : De Gruyter Open Ltd. - 0283-2631 .- 2000-0669. ; 37:2, s. 223-231
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Tidskriftsartikel (refereegranskat)abstract
- This study investigated whether the yield improvement after high alkali impregnation (HAI) is maintained after oxygen delignification and whether the potential of oxygen delignification to increase the mechanical properties is affected by high alkali impregnation. The yield improvement achieved by high alkali impregnation (1 %) was preserved after oxygen delignification, particularly of glucomannan. The total fiber charge and swelling increased after oxygen delignification regardless of the type of impregnation in the cooking step. The tensile index improvement obtained by oxygen delignification was retained if this was preceded by high alkali impregnation. The stiffness index was higher and elongation slightly lower after HAI impregnation than after a standard (REF) impregnation. Fibers obtained through high alkali impregnation seem to be slightly less deformed and slightly wider than fibers obtained after a standard impregnation.
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| 6. |
- Esteves, Claudia, et al.
(författare)
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The impact of bleaching on the yield of softwood kraft pulps obtained by high alkali impregnation : Bleaching and high alkali impregnation impact
- 2022
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Ingår i: Nordic Pulp & Paper Research Journal. - : De Gruyter Open Ltd. - 0283-2631 .- 2000-0669. ; 37:4, s. 593-608
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Tidskriftsartikel (refereegranskat)abstract
- High alkali impregnation (HAI) increases the total yield of softwood pulps following kraft cooking. This yield improvement is also maintained after oxygen delignification. This study evaluates how bleaching with either chlorine dioxide or hydrogen peroxide affects the final yield of samples obtained with standard and HAI. The chemical composition, viscosity, brightness, mechanical and morphological properties were studied. Compared to cooking after standard impregnation the yield improvement achieved by HAI was preserved in both types of bleaching sequences (2 % units for chlorine dioxide and 4 % units for hydrogen peroxide). The introduction of charged groups into the cellulose fibers was higher with hydrogen peroxide bleaching than with chlorine dioxide however, no significant impact was seen on the swelling or mechanical properties. The brightness was higher for the pulps bleached with chlorine dioxide compared with hydrogen peroxide. Hydrogen peroxide bleaching resulted in similar brightness development for both standard and HAI. Fibers bleached with chlorine dioxide had the highest curl index (16-17 %) compared to the fibers bleached with hydrogen peroxide (15 %). © 2022 the author(s)
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| 7. |
- Abitbol, Tiffany, et al.
(författare)
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Isolation of Mixed Compositions of Cellulose Nanocrystals, Microcrystalline Cellulose, and Lignin Nanoparticles from Wood Pulps
- 2023
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Ingår i: ACS Omega. - : American Chemical Society (ACS). - 2470-1343. ; 8:24, s. 21474-21484
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Tidskriftsartikel (refereegranskat)abstract
- From a circular economyperspective, one-pot strategies for theisolation of cellulose nanomaterials at a high yield and with multifunctionalproperties are attractive. Here, the effects of lignin content (bleachedvs unbleached softwood kraft pulp) and sulfuric acid concentrationon the properties of crystalline lignocellulose isolates and theirfilms are explored. Hydrolysis at 58 wt % sulfuric acid resulted inboth cellulose nanocrystals (CNCs) and microcrystalline celluloseat a relatively high yield (>55%), whereas hydrolysis at 64 wt% gaveCNCs at a lower yield (<20%). CNCs from 58 wt % hydrolysis weremore polydisperse and had a higher average aspect ratio (1.5-2x),a lower surface charge (2x), and a higher shear viscosity (100-1000x).Hydrolysis of unbleached pulp additionally yielded spherical nanoparticles(NPs) that were <50 nm in diameter and identified as lignin bynanoscale Fourier transform infrared spectroscopy and IR imaging.Chiral nematic self-organization was observed in films from CNCs isolatedat 64 wt % but not from the more heterogeneous CNC qualities producedat 58 wt %. All films degraded to some extent under simulated sunlighttrials, but these effects were less pronounced in lignin-NP-containingfilms, suggesting a protective feature, but the hemicellulose contentand CNC crystallinity may be implicated as well. Finally, heterogeneousCNC compositions obtained at a high yield and with improved resourceefficiency are suggested for specific nanocellulose uses, for instance,as thickeners or reinforcing fillers, representing a step toward thedevelopment of application-tailored CNC grades.
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| 8. |
- Bengtsson, Andreas
(författare)
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Biobased carbon fibers from solution spun lignocellulosic precursors
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbon fibers (CFs) have excellent mechanical properties and a low density, making themattractive as a reinforcing fiber in composites. The use of CFs is limited to high-end applications,since they are produced from an expensive fossil-based precursor via an energy-intensivemanufacturing process, explaining the need for cheaper CFs from renewables. CFs can be madefrom strong cellulosic precursors, but the low carbon content of cellulose results in a lowconversion yield, and thus an expensive CF. Lignin has a higher carbon content than cellulose butCFs from melt spun lignin precursors have presented challenges, since these precursors have a lowstrength and are difficult to convert to CF in a realistic conversion time.In the present work, CFs from solution spun precursors consisting of blends of softwood kraftlignin and cellulose have been developed. The lignin-cellulose precursors (up to 70% lignin) wereprepared with air-gap spinning and wet spinning, using an ionic liquid and a water-based solventsystem for co-dissolution, respectively. Co-processing of cellulose and lignin was beneficial as theformer made the precursor strong and easy to handle, whereas the latter gave a higher conversionyield than precursors based solely on cellulose. The precursors were converted to CFs via bothbatchwise and continuous conversion, using industrially relevant times (< 2 h), with a yield up to45 wt% after incorporation of a flame retardant.These CFs have a moderate Young’s modulus and tensile strength up to 75–77 GPa and 1.2 GPa,respectively, i.e. similar to the values for CFs from fossil-based isotropic pitch and they can thusbe classified as general-grade CFs. These biobased CFs have a disordered turbostratic graphitestructure, and their tensile properties are affected by the precursor structure, the conversionconditions, and the final diameter. These CFs can potentially be used as a sustainable componentin non-structural and semi-structural applications.
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| 9. |
- Bengtsson, Andreas, et al.
(författare)
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Carbon Fibers from Wet-Spun Cellulose-Lignin Precursors Using the Cold Alkali Process
- 2022
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Ingår i: FIBERS. - : MDPI AG. - 2079-6439. ; 10:12
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, there has been extensive research into the development of cheaper and more sustainable carbon fiber (CF) precursors, and air-gap-spun cellulose-lignin precursors have gained considerable attention where ionic liquids have been used for the co-dissolution of cellulose and lignin. However, ionic liquids are expensive and difficult to recycle. In the present work, an aqueous solvent system, cold alkali, was used to prepare cellulose-lignin CF precursors by wet spinning solutions containing co-dissolved dissolving-grade kraft pulp and softwood kraft lignin. Precursors containing up to 30 wt% lignin were successfully spun using two different coagulation bath compositions, where one of them introduced a flame retardant into the precursor to increase the CF conversion yield. The precursors were converted to CFs via batchwise and continuous conversion. The precursor and conversion conditions had a significant effect on the conversion yield (12-44 wt%), the Young's modulus (33-77 GPa), and the tensile strength (0.48-1.17 GPa), while the precursor morphology was preserved. Structural characterization of the precursors and CFs showed that a more oriented and crystalline precursor gave a more ordered CF structure with higher tensile properties. The continuous conversion trials highlighted the importance of tension control to increase the mechanical properties of the CFs.
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| 10. |
- Brännvall, Elisabet, 1961-, et al.
(författare)
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Analysis of entrapped and free liquor to gain new insights into kraft pulping
- 2021
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Ingår i: Cellulose. - : Springer Science and Business Media B.V.. - 0969-0239 .- 1572-882X. ; 28, s. 2403-2418
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Tidskriftsartikel (refereegranskat)abstract
- Most of our knowledge on kraft pulping comes from studies on dissolved lignin in the freely drainable black liquor and isolated residual lignin in pulp. However, entrapped liquor in the delignified chips has been shown to differ significantly from the free liquor. The present study has compared three liquor fractions: free, lumen and fiber wall liquor. The free liquor was obtained by draining the delignified chips, the lumen liquor was separated by centrifugation and the fiber wall liquor by subsequent leaching. The liquor in the fiber wall had the lowest concentration of lignin and hydrosulfide ions and the highest concentration of monovalent cations. The dissolved lignin in the fiber wall liquor had the highest molar mass and the highest content of xylan. The highest concentration of dissolved lignin was in the liquor filling the lumen cavities. The lignin in the free liquor had the lowest molar mass and the lowest content of lignin structures containing β-O-4 linkages and aliphatic hydroxyl groups. The lowest mass transfer rate of dissolved lignin was from the lumen liquor to the free liquor probably restricted by the tortuosity of the chip.
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