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Aspects on Strenght Delivery and Higher Utilisation of the Strength Potential of Kraft Pulp Fibres
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Wood Chemistry and Pulp Technology.ORCID iD: 0000-0002-8992-3623
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.

Place, publisher, year, edition, pages
2007.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2007:26
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-4373ISBN: 978-91-7178-662-3 (print)OAI: oai:DiVA.org:kth-4373DiVA, id: diva2:12005
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
List of papers
1. A study on the difference in strength between industrially and laboratory-cooked pulp
Open this publication in new window or tab >>A study on the difference in strength between industrially and laboratory-cooked pulp
2006 (English)In: Nordic pulp and paper research journal, ISSN 0283-2631, Vol. 21, no 2, p. 222-226Article in journal (Refereed) Published
Abstract [en]

The tensile strength levels of industrially produced pulp and corresponding laboratory-cooked pulps were investigated. The industrial pulp had a lower tensile strength, which could not be explained by fibre form or fibre strength.

It was concluded that bonding strength was the limiting factor for the tensile strength of the industrial pulp. The industrial pulp, despite of its higher hemicellulose content, had a lower surface charge. The xylan precipitated onto the fibres during the industrial cook was probably more degraded and consequently with lower degree of polymerisation and fewer charged groups.

Place, publisher, year, edition, pages
Stockholm: AB SVENSK PAPPERSTIDNING, 2006
Keywords
KRAFT PULP; SHEET STRENGTH; FIBER STRENGTH; SULFATE PULP; PAPER; QUALITY; JOINT; XYLAN; DRY
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7091 (URN)
Note
QC 20100519Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2010-05-20Bibliographically approved
2. The impact of ionic strength during kraft cooking on the strength properties of softwood kraft pulp
Open this publication in new window or tab >>The impact of ionic strength during kraft cooking on the strength properties of softwood kraft pulp
2007 (English)In: Appita journal, ISSN 1038-6807, Vol. 60, no 1, p. 60-64Article 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.

Keywords
carbohydrate composition, ionic strength, kraft pulp, sodium ion concentration, softwood, tear strength, tensile strength, zero-span tensile strength
National Category
Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-7092 (URN)000243615200011 ()2-s2.0-33846323316 (Scopus ID)
Note

QC 20100520

Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2017-12-14Bibliographically approved
3. The hemicellulose composition of pulp fibres and their ability to endure mechanical treatment
Open this publication in new window or tab >>The hemicellulose composition of pulp fibres and their ability to endure mechanical treatment
(English)In: TAPPI Journal, ISSN 0734-1415Article in journal (Refereed) Accepted
Identifiers
urn:nbn:se:kth:diva-7093 (URN)
Note
QC 20100520Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2017-12-14Bibliographically approved
4. Fibre surface modifications of market pulp by consecutive treatments with cationic and anionic starch
Open this publication in new window or tab >>Fibre surface modifications of market pulp by consecutive treatments with cationic and anionic starch
2007 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 22, no 2, p. 244-248Article in journal (Refereed) Published
Abstract [en]

Bleached softwood kraft pulps were coated with one to three layers of starch, which lead to tensile strength improvement. The strength increase was larger when a never-dried pulp was treated compared to treatment of a once-dried pulp, although equal amounts of starch were adsorbed in both cases. When the never-dried, starch-treated pulp was dried and subsequently reslushed, its tensile strength was higher than that of the never-dried reference pulp. It also required less PFI beating to reach a certain tensile index. Starch-treatment can thereby be a way of improving the tensile strength and beatability of market pulp.

Place, publisher, year, edition, pages
Stockholm: AB SVENSK PAPPERSTIDNING, 2007
Keywords
hornification; kraft pulp; polyelectrolyte multilayers; softwood; tensile strength; starch
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-7094 (URN)10.3183/NPPRJ-2007-22-02-p244-248 (DOI)000248057800014 ()2-s2.0-34547363043 (Scopus ID)
Note
QC 20100520Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2017-12-14Bibliographically approved
5. CMC addition to industrial and laboratory-cooked pulp
Open this publication in new window or tab >>CMC addition to industrial and laboratory-cooked pulp
(English)Manuscript (preprint) (Other academic)
Identifiers
urn:nbn:se:kth:diva-7095 (URN)
Note
QC 20100520Available from: 2007-05-14 Created: 2007-05-14 Last updated: 2010-05-20Bibliographically approved

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