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The impact of ionic strength during kraft cooking on the strength properties of softwood kraft pulp
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0002-8992-3623
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0002-2900-4713
2007 (English)In: Appita journal, ISSN 1038-6807, Vol. 60, no 1, 60-64 p.Article in journal (Refereed) Published
Abstract [en]

A study was undertaken in order to investigate the influence of ionic strength during pulping (measured as sodium ion concentration) on pulp strength (evaluated as tear index vs. tensile index) and on the pulps ability to resist mechanical damage. Sodium chloride was added to the cooking liquor in order to control the ionic strength during the laboratory kraft cooking of soft-wood. The strength properties were compared to a conventional laboratory pulp, pulped at an ionic strength equal to that originating solely from the cooking chemicals added.

It was shown that the ionic strength of the cooking liquor had an impact on pulp strength. Tear index at a certain tensile index decreased at higher ionic strength. The fibre strength, measured as rewetted zero-span tensile index, also decreased. Furthermore, high ionic strength during cooking rendered the fibres more vulnerable to mechanical damage.

Place, publisher, year, edition, pages
2007. Vol. 60, no 1, 60-64 p.
Keyword [en]
carbohydrate composition, ionic strength, kraft pulp, sodium ion concentration, softwood, tear strength, tensile strength, zero-span tensile strength
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-7092ISI: 000243615200011Scopus ID: 2-s2.0-33846323316OAI: oai:DiVA.org:kth-7092DiVA: diva2:12001
Note

QC 20100520

Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2017-12-14Bibliographically approved
In thesis
1. Aspects on Strenght Delivery and Higher Utilisation of the Strength Potential of Kraft Pulp Fibres
Open this publication in new window or tab >>Aspects on Strenght Delivery and Higher Utilisation of the Strength Potential of Kraft Pulp Fibres
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Studies on strength delivery and related fields have so far concentrated on finding the locations in the mill where fibres are damaged and what the damages consist of. However, fibres will invariably encounter mechanical stresses along the fibreline and in this thesis a new concept is introduced; the vulnerability of fibres to mechanical treatment. It is hypothesised that fibres with different properties have different abilities to withstand the mechanical forces they endure as they are discharged from the digester and transported through valves, pumps and various washing and bleaching equipment.

In the thesis, results are presented from trials where pulps with significantly different hemicellulose compositions were high-intensity mixed at pH 13, 70°C and 10% pulp consistency and pulp strength evaluated. By varying alkalinity and temperature, pulps with different carbohydrate composition could be obtained. High alkali concentration and low temperature resulted in high glucomannan content and low xylan content, whereas cooking at low alkali concentration and high temperature rendered a pulp with low glucomannan and high xylan content. The high alkalinity pulp was stronger, determined as tear index at given tensile index. The pulp viscosity was also higher for this pulp. However, when the pulps were subjected to high-intensity mixing, the high alkalinity pulp lost in tear strength and the re-wetted zero-span tensile strength was substantially reduced. The pulp cooked at high alkalinity was thus interpreted as being more vulnerable to mechanical treatment than the pulp obtained by cooking at low alkalinity.

Another pair of pulps was manufactured at high and low sodium ion concentrations, but otherwise with similar chemical charges. The pulp obtained by cooking at low sodium ion concentration became stronger, evaluated as tear index at a given tensile index and the curl index was substantially lower, 8% compared to 12% for the pulp cooked at a high sodium ion concentration. The viscosity was 170 ml/g higher for the pulp manufactured at low sodium ion concentration. When the pulps were subjected to high-intensity mixing, the tear strength of the pulp manufactured at high sodium ion concentration was reduced. The re-wetted zero-span tensile index decreased also after mixing. The pulp obtained by cooking at higher sodium ion concentration was thus interpreted as being more vulnerable to mechanical treatment than the pulp manufactured at lower sodium ion concentration.

In the thesis, two reasons for the low strength delivery of industrially produced pulps compared to laboratory-cooked pulps are put forward. Since the ionic strength of mill cooking liquor systems is much higher than is normally used in laboratory cooking, this can partly explain the difference in strength between mill- and laboratory-cooked pulp. A higher sodium ion concentration was shown in this thesis work to give a pulp of lower strength. Secondly, it is suggested that the difference in retention time of the black liquor in laboratory cooking and continuous mill cooking systems can explain the difference in tensile strength between laboratory-cooked and mill-produced pulp. The black liquor in a continuous digester has a longer retention time in the digester than the chips. This gives a longer time for the dissolved xylan to degrade and, as a consequence, the xylan deposited on the mill pulp fibres will be more degraded than the xylan deposited on the laboratory-cooked pulp fibres.

In the thesis, results are also presented from studies using different strength-enhancing chemicals. The fibre surfaces of bleached never-dried and once-dried pulp were modified by the polyelectrolyte multilayer technique using cationic and anionic starch. Although the pulps absorbed the same amount of starch, the never-dried pulp reached a higher tensile index than the once-dried pulp. When the starch-treated never-dried pulp was dried and reslushed it still had higher tensile index than the never-dried untreated pulp. The starch layers were thus able to counteract part of the hornification effect. The never-dried starch treated pulps were subsequently dried, reslushed and beaten. Pulp with starch layers had a better beatability evaluated as the tensile index obtained after given number of PFI revolutions than dried untreated pulp. Hence, there is a potential to increase the tensile index of market pulp by utilising the polyelectrolyte multilayer technique before drying. Addition of CMC to bleached mill pulp and laboratory-cooked pulp increased the tensile strength to the same degree for both pulps. CMC addition had a straightening effect on the fibres, the shape factor increased and this increased the zero-span tensile strength also.

Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:26
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-4373 (URN)978-91-7178-662-3 (ISBN)
Public defence
2007-05-25, Sal F3, KTH, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20100519Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2010-05-20Bibliographically approved

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Brännvall, ElisabetLindström, Mikael

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