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Free Radical Mediated Cellulose Degradation

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Free Radical Mediated Cellulose Degradation ISBN 91-7283-447-1 ISRN KTH/KKE-03/1-SE ISSN 0349-6465 TRITA-KKE-0301 ================================================================== Free Radical Mediated Cellulose Degradation Erik E Johansson Doctoral Thesis Department of Chemistry Nuclear Chemistry Royal Institute of Technology ================================================================== Stockholm, Sweden 2003 Abstract This thesis addresses the mechanisms involved in cellulose degradation in general and Totally Chlorine Free (TCF) bleaching of pulp in particular. The thesis shows that the cellulose degradation during high consistency ozone bleaching is explained by free radical chain reactions. By simulation, it has been shown that the number, weight and viscosity average of liner polymer chain length can be used to calculate the number of random scissions in a linear polymer of any molecular weight distribution, provided that there is a calibrated Mark- Houwink equation. A model describing partial degradation of molecular weight distributions of linear polymers measured with viscometry was developed and verified experimentally. The model predicts viscometric measurement of chemical cellulose degradation by a rapidly reacting reagent to be strongly dependent on cellulose accessibility. The role of free radical reactions in cellulose degradation was studied by varying the amount of ferrous ions and ozone added to the cotton linters. The result was compared to the results obtained from cellulose of lower crystallinity (cellulose beads) by measuring average chain length. When a ferryl ion reacted with cotton linters in the presence of ozone, the very formation of one glycosidic radical was more significant to degradation than the final step of forming one oxidised glycoside. The inefficient degradation observed of the oxidation step is explainable by the amount of accessible glycosides being too small to influence viscometry. The efficient degradation observed in association with the glycosidic radical formation is explained by initiation of free radical chain reactions that are propagated as long as there is ozone in the system. As none of these phenomena were found in the less crystalline cellulose, cellulose structure appears to be important for how free radical mediated cellulose degradation develops. The theory of free radical chain reactions coupled with diffusion suggests a concentric expansion of the chain reactions outwards from the initial site of radical formation during ozonation of carbohydrates. This was confirmed by demonstrating free radical chain reactions spreading from a spot of initiation outwards during ozonation of a filter paper, using a pH- indicator to monitor acid formation. Furthermore, the interior and exterior of cellulose fibres doped with initiator were shown to be permeated by small holes after ozonation. Ethylene glycol was shown to improve the selectivity during ozone bleaching of oxygen bleached kraft pulp at pH 3. Optimal conditions were obtained at pH 3 for 25 wt% ethylene glycol. The influence of ethylene glycol on selectivity is explained by a proportion of the free radical chain reactions being carried by the ethylene glycol instead of the cellulose during ozone bleaching. The observations were summarised in the form of a model where the observed degradations for pulp, bleached pulp and cotton fibres during both ozone bleaching and ethylene glycol assisted ozone bleaching were shown to agree with each other. From γ-irradiation of ozonised aqueous solutions of alcohol, the rate constant of superoxide formation from the peroxyl radical of methanol was estimated to be 10 s-1. Rate constants of the reactions between ozone and alkylperoxyl radicals were determined to be around 104 M-1s-1. The possibility of the reaction between alkylperoxyl radicals and ozone contributing significantly to free radical chain reactions during ozonation of carbohydrates and alcohols could therefore be ruled out. Keywords: Cellulose, degradation, free radical, ozone, selectivity, ethylene glycol, alcohol, bleaching, kraft pulp, cotton linters, delignification, fibre, fibril, crystallinity, ferryl ion, free radical chain reactions, TCF, viscometry, molecular weight distributions, random scissions. Abstrakt Denna avhandling handlar om mekanismerna som är inblandade i cellulosanedbrytning, framförallt den som sker vid Helt KlorFri (TCF) blekning av pappersmassa. Avhandlingen visar att cellulosanedbrytningen som sker under ozonblekning (hög konsistens) förklaras av fria radikalers kedjereaktioner. Genom simulering har det visats att nummer-, vikts- och viskositetsgenomsnitt av linjära polymerkedjors längd kan användas till att beräkna antalet slumpmässiga kedjebrott som skett under nedbrytning oavsett molekylviktsfördelning, förutsatt att det finns en kalibrerad Mark- Houwinkekvation. En modell som beskriver partiell nedbrytning av linjära polymers molekylviktsfördelningar mätt med viskometri har utvecklats och verifierats experimentellt. Modellen förutsäger att viskometrisk mätning av kemisk cellulosanedbrytning från snabbt reagerande ämnen kommer att vara under stark påverkan av cellulosatillgänglighet. Fria radikalers roll under cellulosanedbrytning studerades genom att variera mängden ferrojoner och ozon som tillsatts bomulls-linters . Resultatet jämfördes med resultat erhållna från cellulosa av lägre kristallinitet (cellulosakulor) genom att mäta genomsnittlig kedjelängd. När ferryljoner reagerade med bomulls-linters i närvaro av ozon var själva bildningen av glykosidradikaler mer signifikant för nedbrytningsresultatet än det slutliga reaktionssteget, bildandet av en oxiderad glykosid. Den oeffektiva nedbrytningen, oxidationsteget, kan förklaras av att mängden tillgänglig cellulosa i provet var för låg för att påverka viskometrin. Den effektiva nedbrytningen, initiationsteget, förklaras av radikala kedjereaktioners verkan, som propageras så länge det finns ozon i systemet. Eftersom inga av dessa fenomen observerades i den mindre kristallina cellulosan, verkar cellulosastruktur vara viktigt för hur kedjereaktioner utvecklar sig. Teori för fria radikala kedjereaktioner kopplad till diffusion för tanken till en koncentrisk expansion av kedjereaktionerna utåt från den ursprungliga radikalbildningsplatsen, under ozoneringen av kolhydrater. Detta bekräftades genom att visa hur fria radikal kedjereaktioner sprider sig från en initieringsplats utåt vid ozonering av ett filterpapper genom att använda pH-indikator för att följa syrabildningen från nedbrytningen. Dessutom uppvisade det yttre och inre av cellulosafibrer som var dopade med initiator, sig vara genomträngda av små hål efter ozonering. Etylenglykol visades förbättra selektiviteten under ozonblekning av syrgasblekt sulfatmassa vid pH 3. Optimala betingelser vid pH 3 rådde vid 25 vikt% etylenglykol. Etylenglykols inverkan på selektiviteten kan förklaras av att en andel av kedjereaktioner har etylenglykol som substrat i stället för cellulosa under blekning Observationerna sammanfattades i form av en modell där de observerade nedbrytningarna för oblekt massa, blekt massa och bomulls-linters under ozonblekning med eller utan etylenglykol visades överenstämma med varandra. Genom γ-bestrålning av ozoniserade vattenlösningar med alkohol, kunde hastighetskonstanten för superoxid-bildning ur metanols peroxylradikal att bestämmas till 10 s-1. Hastighetskonstanter runt 104 M-1s-1 bestämdes för reaktioner mellan ozon och peroxylradikaler. Möjligheten att reaktioner mellan alkylperoxylradikaler och ozon skulle signifikant bidra till kedjereaktioner vid ozonering av kolhydrater och alkoholer kunde därför uteslutas. List of publications This thesis is based on the following publications, referred to in the text by their Roman numerals: Paper I. Johansson, E. E. and Lind, J. (2003). "The general link between random scissions, changes in average chain length and monomer reactions." Paper II. Johansson, E. E. and Lind, J. (2003). "Viscometry of heterogeneously degraded cellulose samples " Paper III. Johansson, E. E. and Lind, J. (2003). "Spreading of free radical reactions in cellulosic materials." Paper IV. Johansson, E. E. and Lind, J. (2003). “Free radical mediated cellulose degradation: Cotton linters during HC pulp ozonation conditions”. J Wood Chem Tech. Submitted. Paper V. Johansson, E. E., Lind, J. and Ljunggren, S. (2000). "Aspects of the chemistry of cellulose degradation and the effect of ethylene glycol during ozone delignification of kraft pulps." Journal of Pulp and Paper Science, 26(7): 239-244. Paper VI. Lind, J., G. Merenyi, Johansson, E. and Brinck, T. (2002). "The reaction of peroxyl radicals with ozone in water." Journal of Physical Chemistry A, 107: 676-681 Some general texts on bleaching and theory in this thesis are more or less been verbatim citations. This concerns especially Ed: Dence, C. W. and Reeve, D., W., “Pulp bleaching. Principles and Practice”, TAPPI Press, 1998 and Gedde U., W., “Polymer Physics”, Chapman & Hall, 1995. This kind of citations is annotated after the section dot, ”.34”, instead of the normal ” 34.” within sentences. Previously published material Johansson, E. E. (2000). Free Radical Mediated Cellulose Degradation During High- Consistency Ozone Bleaching Conditions. Licentiate thesis. Department of Chemistry - Nuclear Chemistry. Stockholm, Royal Institute of Technology. Johansson, E. and Lind, J. (1999). Free radical degradation of fibres during HC pulp ozonation conditions. 10th International Symposium on Wood and Pulping Chemistry, June 7-10 1999, Yokohama, Japan.
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