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The influence of a bleaching sequence on the brightness stability of eucalyptus kraft pulp
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0002-2900-4713
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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.

Place, publisher, year, edition, pages
2005. 251-255 p.
Series
Appita Annual Conference, ISSN 1443-5454
Keyword [en]
Bleaching, Discoloration, Forestry
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-8866Scopus ID: 2-s2.0-29344465115OAI: oai:DiVA.org:kth-8866DiVA: diva2:14334
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
In thesis
1. On the importance of oxidizable structures in bleached kraft pulps
Open this publication in new window or tab >>On the importance of oxidizable structures in bleached kraft pulps
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
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:nbn:se:kth:diva-534 (URN)
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

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