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On the importance of oxidizable structures in bleached kraft pulps
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

After cooking, kraft pulps always contain not only residual lignin but also significant amounts of hexenuronic acid and other non-lignin structures oxidizable by permanganate under the standard kappa number determination conditions. These here referred to as false lignin. Like ordinary lignin, the false lignin also consumes bleaching chemicals, thus increasing both the production costs and the environmental impact of bleach plant effluents. The false lignin also has an effect on pulp properties such as brightness stability. This necessitates the development of efficient experimental routines for the determination of false lignin in different types of unbleached and bleached kraft pulps, together with studies of its formation, chemical behaviour, and ultimate fate.

The main aim of this work has been to establish a method for the quantification of various types of oxidizable structures in bleached kraft pulps and to study their impact on pulp quality, particularly, on the brightness stability of pulps bleached in elemental-chlorine-free (ECF) and a totally-chlorine-free (TCF) processes.

Part of this research deals with the relationship between the kappa number and the lignin content in the case of partly oxidized lignins. Spruce and birch kraft pulps processed according to the ODEQP and OQ(OP)Q(PO) bleaching sequences, respectively, have been analyzed. It has been found that the oxidation equivalent of the residual lignin decreases with increasing degree of oxidation along each bleaching sequence. This finding has been further supported by experiments with a number of model compounds. The Ox-Dem kappa number method has been shown to be an accurate means of determining the residual lignin content and of monitoring the efficiency of lignin removal along different bleaching sequences. It has been demonstrated that the kappa number can always be fractioned into partial contributions, the first of which comes from the residual lignin and is measured by the Ox-Dem kappa number, and the second from the false lignin and is given by the difference between the standard kappa number and the Ox-Dem kappa number. The effect of false lignin on the pulp kappa number is most pronounced in unbleached and oxygen-delignified kraft pulps. The extractability of residual and false lignin in different solvents has been investigated. The changes that occurred in the kappa number following different extraction steps have been compared with corresponding changes in the chemical composition and the conclusion has been drawn that the hemicellulose component of a kraft pulp is a major sourse of non-lignin structures contributing to the kappa number.

The influence on the brightness stability of various oxidizable structures, viz.: residual lignin, hexenuronic acid and other non-lignin structures, in spruce, birch and eucalyptus kraft pulps bleached in ECF and TCF type processes was studied. It was demonstrated that the selective removal of all false lignin structures significantly improves the brightness stability. The degree of yellowing was found to be proportional to the content of HexA groups in pulps. It has been shown that 2-furancarboxylic acid, 5-formyl-2furancarboxylic acid and reductic acid are formed during the course of thermal yellowing. The influence of two bleaching sequences, D0(EP)D1 (ECF-type) and Q1(OP)Q2(PO) (TCF)-type, on the content of different oxidizable structures in eucalyptus kraft pulp was studied in relation to the brightness stability of the pulp. It was shown by kappa number fractionation that pulp bleached to full brightness with ECF- and TCF-type sequences contains different amounts of HexA. The most significant discoloration was observed in the case of TCF-bleached pulp having an especially high content of HexA.

The mechanism of the moist (8 % moisture) thermal yellowing of fully bleached kraft pulps was further studied using dissolving pulp impregnated with a set of model compounds representing the most likely HexA degradation products, viz. as 2-furancarboxylic acid (FA), 5-formyl-2-furancarboxylic acid (FFA) and reductic acid (RA), either alone or in combination with Fe(II) or Fe(III) ions. It was found that the latter two acids take part in reactions leading to colour formation whereas 2-furancarboxylic acid does not. The effect of iron ions on the colour formation appears to vary with their oxidation state. The brightness loss caused by either FFA or RA, present in an amounts similar to the content of HexA in industrial pulps, was of the same order of magnitude as that observed in industrial pulps aged under the same conditions. Based on these findings, it is suggested that the overall mechanism of moist thermal yellowing involves several stages, including the degradation of hexenuronic acid and the formation of reactive precursors, such as 5-formyl-2-furancarboxylic acid and reductic acid. The presence of ferrous ions further enhances the discoloration.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , ii, 76 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2005:39
Keyword [en]
bleached pulps, betula, eucalyptus, 5-formyl-2furancarboxylic acid, 2-furancarboxylic acid, hexenuronic acid, kappa number, kraft pulps, oxidation equivalents
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-534OAI: oai:DiVA.org:kth-534DiVA: diva2:14336
Public defence
2005-12-09, Sal F3, Lindstedtsvägen 26, Stockholm, 14:00
Opponent
Supervisors
Note
QC 20101005Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-05Bibliographically approved
List of papers
1. The relationship between kappa number and oxidizable structures in bleached kraft pulps
Open this publication in new window or tab >>The relationship between kappa number and oxidizable structures in bleached kraft pulps
2002 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 28, no 8, 262-266 p.Article in journal (Refereed) Published
Abstract [en]

The relationship between residual oxidized lignin structures in spruce and birch kraft pulps and their kappa number contribution after each stage in an ODEQP (spruce) and an OQ(OP)Q(PO) (birch) sequence, respectively, has been investigated. Analysis by a modified kappa number method (OxDem kappa number) as well as by determination of permanganate oxidation equivalents on various isolated residual lignin samples revealed that the relationship between the lignin content and kappa number gradually changes as the bleaching sequence proceeds, giving values that differ substantially from that found in unbleached kraft pulp. These effects can be attributed to the successive oxidative fragmentation of aromatic rings and the formation of carboxyl and non-aromatic unsaturated structures in the residual lignin structure. Therefore, a kappa number determination on pulp which has been subjected to an oxidative stage will result in an underestimation of the lignin content.

Keyword
Betula, Bleached pulps, Chlorine free bleaching, Kappa number, Kraft pulps, Lignin content, Multistage process, Oxidation, Picea
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8862 (URN)000177411900003 ()
Note
QC 20101004Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-04Bibliographically approved
2. The distribution of oxidizable structures in ECF- and TCF- bleached kraft pulps
Open this publication in new window or tab >>The distribution of oxidizable structures in ECF- and TCF- bleached kraft pulps
2002 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 17, no 4, 415-419 p.Article in journal (Refereed) Published
Abstract [en]

The kappa number in kraft pulp contains contributions from lignin and from other oxidizable structures denoted as false lignin. The latter can be divided in hexenuronic acid groups and "non-lignin" structures. In this paper, the kappa number units due to the various contributing structures have been quantified by fractionation of the kappa number. Bleached spruce and birch kraft industrial pulps taken after each stage in an ECF (ODEQP) and in a TCF (OQOPQPO) bleaching sequence respectively have been assessed. Possible correlations between the content of false lignin, the pulp origin, the bleaching conditions, and brightness and viscosity values have been sought. It is concluded that the false lignin structures are formed predominantly during the cooking and oxygen delignification stages. The false lignin structures were found to give different responses in the various bleaching stages, but to have no influence on either pulp brightness or viscosity.

Keyword
Bleaching, Chlorine dioxide, Hexenuronic acid, Hydrogen peroxide, Kappa number, Kraft pulps, Lignin, Oxygen, Permanganate consumption
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8863 (URN)10.3183/NPPRJ-2002-17-04-p415-419 (DOI)000180616600010 ()
Note
QC 20101001Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-01Bibliographically approved
3. Extractability and chemical structure of residual and false lignin in kraft pulps
Open this publication in new window or tab >>Extractability and chemical structure of residual and false lignin in kraft pulps
2001 (English)Conference paper, Published paper (Refereed)
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8864 (URN)
Conference
Proc. of the 11th International Symposium on Wood and Pulping Chemistry (ISWPC), Nice, France
Note
QC 20101005Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-10-05Bibliographically approved
4. The influence of various oxidizable structures on the brightness stability of the bleached chemical pulps
Open this publication in new window or tab >>The influence of various oxidizable structures on the brightness stability of the bleached chemical pulps
2006 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 21, no 1, 49-53 p.Article in journal (Refereed) Published
Abstract [en]

The influence on the brightness stability of various oxidizable structures viz.: lignin, hexenuronic acid and other "non-lignin" structures in kraft Pulps was studied. Samples of pulp of different wood species (spruce, birch, eucalyptus) taken from TCF and ECF bleaching sequences were chemically characterised by kappa number fractionation and subjected to accelerated ageing. It was shown that the selective removal of all oxidizable structures of carbohydrate origin present in the pulp significantly improved the brightness stability. The thermal yellowing of the pulps was found to be proportional to the content of hexenuronic acid groups regardless of the pulp origin or bleaching history. Quantitative determination of the changes during ageing showed that 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid are formed in the course of decomposition of hexenuronic acid. The difference between the original amount of hexenuronic acid groups in a pulp sample and the sum of the remaining hexenuronic acid together with the 2-furancarboxylic acid and 5-formyl-2-furancarboxylic acid suggests that the latter are involved in further reactions leading to the formation of chromophoric structures. It was also found that 2,3-dihydroxy-2-cyclopenten-1-one (reductic acid) is formed during the ageing.

Keyword
bleaching, kraft pulp, 2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid, hexenuronic acid, reductic acid, thermal yellowing
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8865 (URN)000236358800008 ()2-s2.0-33645678799 (Scopus ID)
Note

Uppdaterad från submitted till published: 20101005. QC 20101005

Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2014-12-17Bibliographically approved
5. The influence of a bleaching sequence on the brightness stability of eucalyptus kraft pulp
Open this publication in new window or tab >>The influence of a bleaching sequence on the brightness stability of eucalyptus kraft pulp
2005 (English)In: Appita Annual Conference, 2005, 251-255 p.Conference paper, Published paper (Refereed)
Abstract [en]

In the present research the influence of two bleaching sequences, D 0(EP)D1 (ECF-type) and Q1(OP)Q2(PO) (TCP-type), on the content of different oxidizable structures and metal ions in eucalyptus kraft pulp was studied in relation to the brightness stability of the pulp. It was shown that pulps bleached to full brightness with ECF- and TCF-type sequences contain differing amounts of oxidizable structures of carbohydrate origin, such as hexenuronic acid (HexA). These structures play a dominant role in the thermal yellowing of bleached kraft pulps and largely determine the yellowing kinetics. The most significant discoloration is observed for TCF-bleached pulps having an especially high content of HexA. The discoloration does not appear to be related to the presence of metal ions in pulps. There is a correlation between the HexA content and brightness stability of the bleached pulps. This correlation is due to reactions where HexA acts as a precursor in the formation of coloured compounds or intermediates which are then involved in the color formation. Furoic structures representing the main acid degradation products of HexA are suggested as the actual intermediates that cause yellowing by forming colored compounds.

Series
Appita Annual Conference, ISSN 1443-5454
Keyword
Bleaching, Discoloration, Forestry
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8866 (URN)2-s2.0-29344465115 (Scopus ID)
Conference
59th Appita Annual Conference and Exhibition, incorporating the 13th ISWFPC: International Symposium on Wood, Fibre and Pulping Chemistry; Auckland, New Zealand, 16-19 May 2005
Note

QC 20141202

Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2017-03-24Bibliographically approved
6. On the reaction mechanism of the thermal yellowing of bleached chemical pulps
Open this publication in new window or tab >>On the reaction mechanism of the thermal yellowing of bleached chemical pulps
2006 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 21, no 2, 188-192 p.Article in journal (Refereed) Published
Abstract [en]

The mechanism of moist thermal yellowing of fully bleached chemical pulps was studied using dissolving pulp impregnated with different types of degradation products from hexenuronic acid, viz. 2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid and 2,3-dihydroxy-2-cyclopenten-l-one (reductic acid) either alone or in combination with Fe2+ or Fe3+ ions. It was found that the two latter acids take part in reactions leading to colour formation whereas 2-furancarboxylic acid does not. The effect of iron ions on the colour formation appears to depend on their oxidation state. The brightness loss caused by either 5-formyl-2-furancarboxylic acid or reductic acid, present in an amount similar to the content of hexenuronic acid in industrial pulps, was of the same order of magnitude as that observed for industrial pulps aged under the same conditions. Based on these findings, the overall mechanism of thermal yellowing is suggested to involve several stages, including the degradation of hexenuronic acid and the formation of reactive precursors, such as 5-formyl-2-furancarboxylic acid or reductic acid, which subsequently take part in the yellowing reactions. The presence of ferrous ions further enhances the discoloration.

Keyword
2, 3-dihydroxy-2-cyclopenten-1-one (reductic acid), 2-furancarboxylic acid, 5-formyl-2-furancarboxylic acid, Chemical pulps, Hexenuronic acid, Reaction mechanism, Thermal yellowing
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-8867 (URN)10.3183/NPPRJ-2006-21-02-p188-192 (DOI)000238726200004 ()2-s2.0-33745615826 (Scopus ID)
Note
QC 20100924. Uppdaterad från Submitted till Published (20100924).Available from: 2005-12-02 Created: 2005-12-02 Last updated: 2010-09-24Bibliographically approved

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